//! Git support for package fetching.
//!
//! This is not intended to support all features of Git: it is limited to the
//! basic functionality needed to clone a repository for the purpose of fetching
//! a package.

const std = @import("std");
const Io = std.Io;
const mem = std.mem;
const testing = std.testing;
const Allocator = mem.Allocator;
const Sha1 = std.crypto.hash.Sha1;
const Sha256 = std.crypto.hash.sha2.Sha256;
const assert = std.debug.assert;

/// The ID of a Git object.
pub const Oid = union(Format) {
    sha1: [Sha1.digest_length]u8,
    sha256: [Sha256.digest_length]u8,

    pub const max_formatted_length = len: {
        var max: usize = 0;
        for (std.enums.values(Format)) |f| {
            max = @max(max, f.formattedLength());
        }
        break :len max;
    };

    pub const Format = enum {
        sha1,
        sha256,

        pub fn byteLength(f: Format) usize {
            return switch (f) {
                .sha1 => Sha1.digest_length,
                .sha256 => Sha256.digest_length,
            };
        }

        pub fn formattedLength(f: Format) usize {
            return 2 * f.byteLength();
        }
    };

    const Hasher = union(Format) {
        sha1: Sha1,
        sha256: Sha256,

        fn init(oid_format: Format) Hasher {
            return switch (oid_format) {
                .sha1 => .{ .sha1 = Sha1.init(.{}) },
                .sha256 => .{ .sha256 = Sha256.init(.{}) },
            };
        }

        // Must be public for use from HashedReader and HashedWriter.
        pub fn update(hasher: *Hasher, b: []const u8) void {
            switch (hasher.*) {
                inline else => |*inner| inner.update(b),
            }
        }

        fn finalResult(hasher: *Hasher) Oid {
            return switch (hasher.*) {
                inline else => |*inner, tag| @unionInit(Oid, @tagName(tag), inner.finalResult()),
            };
        }
    };

    const Hashing = union(Format) {
        sha1: Io.Writer.Hashing(Sha1),
        sha256: Io.Writer.Hashing(Sha256),

        fn init(oid_format: Format, buffer: []u8) Hashing {
            return switch (oid_format) {
                .sha1 => .{ .sha1 = .init(buffer) },
                .sha256 => .{ .sha256 = .init(buffer) },
            };
        }

        fn writer(h: *@This()) *Io.Writer {
            return switch (h.*) {
                inline else => |*inner| &inner.writer,
            };
        }

        fn final(h: *@This()) Oid {
            switch (h.*) {
                inline else => |*inner, tag| {
                    inner.writer.flush() catch unreachable; // hashers cannot fail
                    return @unionInit(Oid, @tagName(tag), inner.hasher.finalResult());
                },
            }
        }
    };

    pub fn fromBytes(oid_format: Format, bytes: []const u8) Oid {
        assert(bytes.len == oid_format.byteLength());
        return switch (oid_format) {
            inline else => |tag| @unionInit(Oid, @tagName(tag), bytes[0..comptime tag.byteLength()].*),
        };
    }

    pub fn readBytes(oid_format: Format, reader: *Io.Reader) !Oid {
        return switch (oid_format) {
            inline else => |tag| @unionInit(Oid, @tagName(tag), (try reader.takeArray(tag.byteLength())).*),
        };
    }

    pub fn parse(oid_format: Format, s: []const u8) error{InvalidOid}!Oid {
        switch (oid_format) {
            inline else => |tag| {
                if (s.len != tag.formattedLength()) return error.InvalidOid;
                var bytes: [tag.byteLength()]u8 = undefined;
                for (&bytes, 0..) |*b, i| {
                    b.* = std.fmt.parseUnsigned(u8, s[2 * i ..][0..2], 16) catch return error.InvalidOid;
                }
                return @unionInit(Oid, @tagName(tag), bytes);
            },
        }
    }

    test parse {
        try testing.expectEqualSlices(
            u8,
            &.{ 0xCE, 0x91, 0x9C, 0xCF, 0x45, 0x95, 0x18, 0x56, 0xA7, 0x62, 0xFF, 0xDB, 0x8E, 0xF8, 0x50, 0x30, 0x1C, 0xD8, 0xC5, 0x88 },
            &(try parse(.sha1, "ce919ccf45951856a762ffdb8ef850301cd8c588")).sha1,
        );
        try testing.expectError(error.InvalidOid, parse(.sha256, "ce919ccf45951856a762ffdb8ef850301cd8c588"));
        try testing.expectError(error.InvalidOid, parse(.sha1, "7f444a92bd4572ee4a28b2c63059924a9ca1829138553ef3e7c41ee159afae7a"));
        try testing.expectEqualSlices(
            u8,
            &.{ 0x7F, 0x44, 0x4A, 0x92, 0xBD, 0x45, 0x72, 0xEE, 0x4A, 0x28, 0xB2, 0xC6, 0x30, 0x59, 0x92, 0x4A, 0x9C, 0xA1, 0x82, 0x91, 0x38, 0x55, 0x3E, 0xF3, 0xE7, 0xC4, 0x1E, 0xE1, 0x59, 0xAF, 0xAE, 0x7A },
            &(try parse(.sha256, "7f444a92bd4572ee4a28b2c63059924a9ca1829138553ef3e7c41ee159afae7a")).sha256,
        );
        try testing.expectError(error.InvalidOid, parse(.sha1, "ce919ccf"));
        try testing.expectError(error.InvalidOid, parse(.sha256, "ce919ccf"));
        try testing.expectError(error.InvalidOid, parse(.sha1, "master"));
        try testing.expectError(error.InvalidOid, parse(.sha256, "master"));
        try testing.expectError(error.InvalidOid, parse(.sha1, "HEAD"));
        try testing.expectError(error.InvalidOid, parse(.sha256, "HEAD"));
    }

    pub fn parseAny(s: []const u8) error{InvalidOid}!Oid {
        return for (std.enums.values(Format)) |f| {
            if (s.len == f.formattedLength()) break parse(f, s);
        } else error.InvalidOid;
    }

    pub fn format(oid: Oid, writer: *Io.Writer) Io.Writer.Error!void {
        try writer.print("{x}", .{oid.slice()});
    }

    pub fn slice(oid: *const Oid) []const u8 {
        return switch (oid.*) {
            inline else => |*bytes| bytes,
        };
    }
};

pub const Diagnostics = struct {
    allocator: Allocator,
    errors: std.ArrayList(Error) = .empty,

    pub const Error = union(enum) {
        unable_to_create_sym_link: struct {
            code: anyerror,
            file_name: []const u8,
            link_name: []const u8,
        },
        unable_to_create_file: struct {
            code: anyerror,
            file_name: []const u8,
        },
    };

    pub fn deinit(d: *Diagnostics) void {
        for (d.errors.items) |item| {
            switch (item) {
                .unable_to_create_sym_link => |info| {
                    d.allocator.free(info.file_name);
                    d.allocator.free(info.link_name);
                },
                .unable_to_create_file => |info| {
                    d.allocator.free(info.file_name);
                },
            }
        }
        d.errors.deinit(d.allocator);
        d.* = undefined;
    }
};

pub const Repository = struct {
    odb: Odb,

    pub fn init(
        repo: *Repository,
        allocator: Allocator,
        format: Oid.Format,
        pack_file: *Io.File.Reader,
        index_file: *Io.File.Reader,
    ) !void {
        repo.* = .{ .odb = undefined };
        try repo.odb.init(allocator, format, pack_file, index_file);
    }

    pub fn deinit(repository: *Repository) void {
        repository.odb.deinit();
        repository.* = undefined;
    }

    /// Checks out the repository at `commit_oid` to `worktree`.
    pub fn checkout(
        repository: *Repository,
        io: Io,
        worktree: Io.Dir,
        commit_oid: Oid,
        diagnostics: *Diagnostics,
    ) !void {
        try repository.odb.seekOid(commit_oid);
        const tree_oid = tree_oid: {
            const commit_object = try repository.odb.readObject();
            if (commit_object.type != .commit) return error.NotACommit;
            break :tree_oid try getCommitTree(repository.odb.format, commit_object.data);
        };
        try repository.checkoutTree(io, worktree, tree_oid, "", diagnostics);
    }

    /// Checks out the tree at `tree_oid` to `worktree`.
    fn checkoutTree(
        repository: *Repository,
        io: Io,
        dir: Io.Dir,
        tree_oid: Oid,
        current_path: []const u8,
        diagnostics: *Diagnostics,
    ) !void {
        try repository.odb.seekOid(tree_oid);
        const tree_object = try repository.odb.readObject();
        if (tree_object.type != .tree) return error.NotATree;
        // The tree object may be evicted from the object cache while we're
        // iterating over it, so we can make a defensive copy here to make sure
        // it remains valid until we're done with it
        const tree_data = try repository.odb.allocator.dupe(u8, tree_object.data);
        defer repository.odb.allocator.free(tree_data);

        var tree_iter: TreeIterator = .{
            .format = repository.odb.format,
            .data = tree_data,
            .pos = 0,
        };
        while (try tree_iter.next()) |entry| {
            switch (entry.type) {
                .directory => {
                    try dir.createDir(io, entry.name, .default_dir);
                    var subdir = try dir.openDir(io, entry.name, .{});
                    defer subdir.close(io);
                    const sub_path = try std.fs.path.join(repository.odb.allocator, &.{ current_path, entry.name });
                    defer repository.odb.allocator.free(sub_path);
                    try repository.checkoutTree(io, subdir, entry.oid, sub_path, diagnostics);
                },
                .file => {
                    try repository.odb.seekOid(entry.oid);
                    const file_object = try repository.odb.readObject();
                    if (file_object.type != .blob) return error.InvalidFile;
                    var file = dir.createFile(io, entry.name, .{ .exclusive = true }) catch |e| {
                        const file_name = try std.fs.path.join(diagnostics.allocator, &.{ current_path, entry.name });
                        errdefer diagnostics.allocator.free(file_name);
                        try diagnostics.errors.append(diagnostics.allocator, .{ .unable_to_create_file = .{
                            .code = e,
                            .file_name = file_name,
                        } });
                        continue;
                    };
                    defer file.close(io);
                    try file.writePositionalAll(io, file_object.data, 0);
                },
                .symlink => {
                    try repository.odb.seekOid(entry.oid);
                    const symlink_object = try repository.odb.readObject();
                    if (symlink_object.type != .blob) return error.InvalidFile;
                    const link_name = symlink_object.data;
                    dir.symLink(io, link_name, entry.name, .{}) catch |e| {
                        const file_name = try std.fs.path.join(diagnostics.allocator, &.{ current_path, entry.name });
                        errdefer diagnostics.allocator.free(file_name);
                        const link_name_dup = try diagnostics.allocator.dupe(u8, link_name);
                        errdefer diagnostics.allocator.free(link_name_dup);
                        try diagnostics.errors.append(diagnostics.allocator, .{ .unable_to_create_sym_link = .{
                            .code = e,
                            .file_name = file_name,
                            .link_name = link_name_dup,
                        } });
                    };
                },
                .gitlink => {
                    // Consistent with git archive behavior, create the directory but
                    // do nothing else
                    try dir.createDir(io, entry.name, .default_dir);
                },
            }
        }
    }

    /// Returns the ID of the tree associated with the given commit (provided as
    /// raw object data).
    fn getCommitTree(format: Oid.Format, commit_data: []const u8) !Oid {
        if (!mem.startsWith(u8, commit_data, "tree ") or
            commit_data.len < "tree ".len + format.formattedLength() + "\n".len or
            commit_data["tree ".len + format.formattedLength()] != '\n')
        {
            return error.InvalidCommit;
        }
        return try .parse(format, commit_data["tree ".len..][0..format.formattedLength()]);
    }

    const TreeIterator = struct {
        format: Oid.Format,
        data: []const u8,
        pos: usize,

        const Entry = struct {
            type: Type,
            executable: bool,
            name: [:0]const u8,
            oid: Oid,

            const Type = enum(u4) {
                directory = 0o4,
                file = 0o10,
                symlink = 0o12,
                gitlink = 0o16,
            };
        };

        fn next(iterator: *TreeIterator) !?Entry {
            if (iterator.pos == iterator.data.len) return null;

            const mode_end = mem.findScalarPos(u8, iterator.data, iterator.pos, ' ') orelse return error.InvalidTree;
            const mode: packed struct {
                permission: u9,
                unused: u3,
                type: u4,
            } = @bitCast(std.fmt.parseUnsigned(u16, iterator.data[iterator.pos..mode_end], 8) catch return error.InvalidTree);
            const @"type" = std.enums.fromInt(Entry.Type, mode.type) orelse return error.InvalidTree;
            const executable = switch (mode.permission) {
                0 => if (@"type" == .file) return error.InvalidTree else false,
                0o644 => if (@"type" != .file) return error.InvalidTree else false,
                0o755 => if (@"type" != .file) return error.InvalidTree else true,
                else => return error.InvalidTree,
            };
            iterator.pos = mode_end + 1;

            const name_end = mem.findScalarPos(u8, iterator.data, iterator.pos, 0) orelse return error.InvalidTree;
            const name = iterator.data[iterator.pos..name_end :0];
            iterator.pos = name_end + 1;

            const oid_length = iterator.format.byteLength();
            if (iterator.pos + oid_length > iterator.data.len) return error.InvalidTree;
            const oid: Oid = .fromBytes(iterator.format, iterator.data[iterator.pos..][0..oid_length]);
            iterator.pos += oid_length;

            return .{ .type = @"type", .executable = executable, .name = name, .oid = oid };
        }
    };
};

/// A Git object database backed by a packfile. A packfile index is also used
/// for efficient access to objects in the packfile.
///
/// The format of the packfile and its associated index are documented in
/// [pack-format](https://git-scm.com/docs/pack-format).
const Odb = struct {
    format: Oid.Format,
    pack_file: *Io.File.Reader,
    index_header: IndexHeader,
    index_file: *Io.File.Reader,
    cache: ObjectCache = .{},
    allocator: Allocator,

    /// Initializes the database from open pack and index files.
    fn init(
        odb: *Odb,
        allocator: Allocator,
        format: Oid.Format,
        pack_file: *Io.File.Reader,
        index_file: *Io.File.Reader,
    ) !void {
        try pack_file.seekTo(0);
        try index_file.seekTo(0);
        odb.* = .{
            .format = format,
            .pack_file = pack_file,
            .index_header = undefined,
            .index_file = index_file,
            .allocator = allocator,
        };
        try odb.index_header.read(&index_file.interface);
    }

    fn deinit(odb: *Odb) void {
        odb.cache.deinit(odb.allocator);
        odb.* = undefined;
    }

    /// Reads the object at the current position in the database.
    fn readObject(odb: *Odb) !Object {
        var base_offset = odb.pack_file.logicalPos();
        var base_header: EntryHeader = undefined;
        var delta_offsets: std.ArrayList(u64) = .empty;
        defer delta_offsets.deinit(odb.allocator);
        const base_object = while (true) {
            if (odb.cache.get(base_offset)) |base_object| break base_object;

            base_header = try EntryHeader.read(odb.format, &odb.pack_file.interface);
            switch (base_header) {
                .ofs_delta => |ofs_delta| {
                    try delta_offsets.append(odb.allocator, base_offset);
                    base_offset = std.math.sub(u64, base_offset, ofs_delta.offset) catch return error.InvalidFormat;
                    try odb.pack_file.seekTo(base_offset);
                },
                .ref_delta => |ref_delta| {
                    try delta_offsets.append(odb.allocator, base_offset);
                    try odb.seekOid(ref_delta.base_object);
                    base_offset = odb.pack_file.logicalPos();
                },
                else => {
                    const base_data = try readObjectRaw(odb.allocator, &odb.pack_file.interface, base_header.uncompressedLength());
                    errdefer odb.allocator.free(base_data);
                    const base_object: Object = .{ .type = base_header.objectType(), .data = base_data };
                    try odb.cache.put(odb.allocator, base_offset, base_object);
                    break base_object;
                },
            }
        };

        const base_data = try resolveDeltaChain(
            odb.allocator,
            odb.format,
            odb.pack_file,
            base_object,
            delta_offsets.items,
            &odb.cache,
        );

        return .{ .type = base_object.type, .data = base_data };
    }

    /// Seeks to the beginning of the object with the given ID.
    fn seekOid(odb: *Odb, oid: Oid) !void {
        const oid_length = odb.format.byteLength();
        const key = oid.slice()[0];
        var start_index = if (key > 0) odb.index_header.fan_out_table[key - 1] else 0;
        var end_index = odb.index_header.fan_out_table[key];
        const found_index = while (start_index < end_index) {
            const mid_index = start_index + (end_index - start_index) / 2;
            try odb.index_file.seekTo(IndexHeader.size + mid_index * oid_length);
            const mid_oid = try Oid.readBytes(odb.format, &odb.index_file.interface);
            switch (mem.order(u8, mid_oid.slice(), oid.slice())) {
                .lt => start_index = mid_index + 1,
                .gt => end_index = mid_index,
                .eq => break mid_index,
            }
        } else return error.ObjectNotFound;

        const n_objects = odb.index_header.fan_out_table[255];
        const offset_values_start = IndexHeader.size + n_objects * (oid_length + 4);
        try odb.index_file.seekTo(offset_values_start + found_index * 4);
        const l1_offset: packed struct { value: u31, big: bool } = @bitCast(try odb.index_file.interface.takeInt(u32, .big));
        const pack_offset = pack_offset: {
            if (l1_offset.big) {
                const l2_offset_values_start = offset_values_start + n_objects * 4;
                try odb.index_file.seekTo(l2_offset_values_start + l1_offset.value * 4);
                break :pack_offset try odb.index_file.interface.takeInt(u64, .big);
            } else {
                break :pack_offset l1_offset.value;
            }
        };

        try odb.pack_file.seekTo(pack_offset);
    }
};

const Object = struct {
    type: Type,
    data: []const u8,

    const Type = enum {
        commit,
        tree,
        blob,
        tag,
    };
};

/// A cache for object data.
///
/// The purpose of this cache is to speed up resolution of deltas by caching the
/// results of resolving delta objects, while maintaining a maximum cache size
/// to avoid excessive memory usage. If the total size of the objects in the
/// cache exceeds the maximum, the cache will begin evicting the least recently
/// used objects: when resolving delta chains, the most recently used objects
/// will likely be more helpful as they will be further along in the chain
/// (skipping earlier reconstruction steps).
///
/// Object data stored in the cache is managed by the cache. It should not be
/// freed by the caller at any point after inserting it into the cache. Any
/// objects remaining in the cache will be freed when the cache itself is freed.
const ObjectCache = struct {
    objects: std.AutoHashMapUnmanaged(u64, CacheEntry) = .empty,
    lru_nodes: std.DoublyLinkedList = .{},
    lru_nodes_len: usize = 0,
    byte_size: usize = 0,

    const max_byte_size = 128 * 1024 * 1024; // 128MiB
    /// A list of offsets stored in the cache, with the most recently used
    /// entries at the end.
    const LruListNode = struct {
        data: u64,
        node: std.DoublyLinkedList.Node,
    };
    const CacheEntry = struct { object: Object, lru_node: *LruListNode };

    fn deinit(cache: *ObjectCache, allocator: Allocator) void {
        var object_iterator = cache.objects.iterator();
        while (object_iterator.next()) |object| {
            allocator.free(object.value_ptr.object.data);
            allocator.destroy(object.value_ptr.lru_node);
        }
        cache.objects.deinit(allocator);
        cache.* = undefined;
    }

    /// Gets an object from the cache, moving it to the most recently used
    /// position if it is present.
    fn get(cache: *ObjectCache, offset: u64) ?Object {
        if (cache.objects.get(offset)) |entry| {
            cache.lru_nodes.remove(&entry.lru_node.node);
            cache.lru_nodes.append(&entry.lru_node.node);
            return entry.object;
        } else {
            return null;
        }
    }

    /// Puts an object in the cache, possibly evicting older entries if the
    /// cache exceeds its maximum size. Note that, although old objects may
    /// be evicted, the object just added to the cache with this function
    /// will not be evicted before the next call to `put` or `deinit` even if
    /// it exceeds the maximum cache size.
    fn put(cache: *ObjectCache, allocator: Allocator, offset: u64, object: Object) !void {
        const lru_node = try allocator.create(LruListNode);
        errdefer allocator.destroy(lru_node);
        lru_node.data = offset;

        const gop = try cache.objects.getOrPut(allocator, offset);
        if (gop.found_existing) {
            cache.byte_size -= gop.value_ptr.object.data.len;
            cache.lru_nodes.remove(&gop.value_ptr.lru_node.node);
            cache.lru_nodes_len -= 1;
            allocator.destroy(gop.value_ptr.lru_node);
            allocator.free(gop.value_ptr.object.data);
        }
        gop.value_ptr.* = .{ .object = object, .lru_node = lru_node };
        cache.byte_size += object.data.len;
        cache.lru_nodes.append(&lru_node.node);
        cache.lru_nodes_len += 1;

        while (cache.byte_size > max_byte_size and cache.lru_nodes_len > 1) {
            // The > 1 check is to make sure that we don't evict the most
            // recently added node, even if it by itself happens to exceed the
            // maximum size of the cache.
            const evict_node: *LruListNode = @alignCast(@fieldParentPtr("node", cache.lru_nodes.popFirst().?));
            cache.lru_nodes_len -= 1;
            const evict_offset = evict_node.data;
            allocator.destroy(evict_node);
            const evict_object = cache.objects.get(evict_offset).?.object;
            cache.byte_size -= evict_object.data.len;
            allocator.free(evict_object.data);
            _ = cache.objects.remove(evict_offset);
        }
    }
};

/// A single pkt-line in the Git protocol.
///
/// The format of a pkt-line is documented in
/// [protocol-common](https://git-scm.com/docs/protocol-common). The special
/// meanings of the delimiter and response-end packets are documented in
/// [protocol-v2](https://git-scm.com/docs/protocol-v2).
pub const Packet = union(enum) {
    flush,
    delimiter,
    response_end,
    data: []const u8,

    pub const max_data_length = 65516;

    /// Reads a packet in pkt-line format.
    fn read(reader: *Io.Reader) !Packet {
        const packet: Packet = try .peek(reader);
        switch (packet) {
            .data => |data| reader.toss(data.len),
            else => {},
        }
        return packet;
    }

    /// Consumes the header of a pkt-line packet and reads any associated data
    /// into the reader's buffer, but does not consume the data.
    fn peek(reader: *Io.Reader) !Packet {
        const length = std.fmt.parseUnsigned(u16, try reader.take(4), 16) catch return error.InvalidPacket;
        switch (length) {
            0 => return .flush,
            1 => return .delimiter,
            2 => return .response_end,
            3 => return error.InvalidPacket,
            else => if (length - 4 > max_data_length) return error.InvalidPacket,
        }
        return .{ .data = try reader.peek(length - 4) };
    }

    /// Writes a packet in pkt-line format.
    fn write(packet: Packet, writer: *Io.Writer) !void {
        switch (packet) {
            .flush => try writer.writeAll("0000"),
            .delimiter => try writer.writeAll("0001"),
            .response_end => try writer.writeAll("0002"),
            .data => |data| {
                assert(data.len <= max_data_length);
                try writer.print("{x:0>4}", .{data.len + 4});
                try writer.writeAll(data);
            },
        }
    }

    /// Returns the normalized form of textual packet data, stripping any
    /// trailing '\n'.
    ///
    /// As documented in
    /// [protocol-common](https://git-scm.com/docs/protocol-common#_pkt_line_format),
    /// non-binary (textual) pkt-line data should contain a trailing '\n', but
    /// is not required to do so (implementations must support both forms).
    fn normalizeText(data: []const u8) []const u8 {
        return if (mem.endsWith(u8, data, "\n"))
            data[0 .. data.len - 1]
        else
            data;
    }
};

/// A client session for the Git protocol, currently limited to an HTTP(S)
/// transport. Only protocol version 2 is supported, as documented in
/// [protocol-v2](https://git-scm.com/docs/protocol-v2).
pub const Session = struct {
    transport: *std.http.Client,
    location: Location,
    supports_agent: bool,
    supports_shallow: bool,
    object_format: Oid.Format,
    arena: Allocator,

    const agent = "zig/" ++ @import("builtin").zig_version_string;
    const agent_capability = std.fmt.comptimePrint("agent={s}\n", .{agent});

    /// Initializes a client session and discovers the capabilities of the
    /// server for optimal transport.
    pub fn init(
        arena: Allocator,
        transport: *std.http.Client,
        uri: std.Uri,
        /// Asserted to be at least `Packet.max_data_length`
        response_buffer: []u8,
    ) !Session {
        assert(response_buffer.len >= Packet.max_data_length);
        var session: Session = .{
            .transport = transport,
            .location = try .init(arena, uri),
            .supports_agent = false,
            .supports_shallow = false,
            .object_format = .sha1,
            .arena = arena,
        };
        var capability_iterator: CapabilityIterator = undefined;
        try session.getCapabilities(&capability_iterator, response_buffer);
        defer capability_iterator.deinit();
        while (try capability_iterator.next()) |capability| {
            if (mem.eql(u8, capability.key, "agent")) {
                session.supports_agent = true;
            } else if (mem.eql(u8, capability.key, "fetch")) {
                var feature_iterator = mem.splitScalar(u8, capability.value orelse continue, ' ');
                while (feature_iterator.next()) |feature| {
                    if (mem.eql(u8, feature, "shallow")) {
                        session.supports_shallow = true;
                    }
                }
            } else if (mem.eql(u8, capability.key, "object-format")) {
                if (std.meta.stringToEnum(Oid.Format, capability.value orelse continue)) |format| {
                    session.object_format = format;
                }
            }
        }
        return session;
    }

    /// An owned `std.Uri` representing the location of the server (base URI).
    const Location = struct {
        uri: std.Uri,

        fn init(arena: Allocator, uri: std.Uri) !Location {
            const scheme = try arena.dupe(u8, uri.scheme);
            const user = if (uri.user) |user| try std.fmt.allocPrint(arena, "{f}", .{
                std.fmt.alt(user, .formatUser),
            }) else null;
            const password = if (uri.password) |password| try std.fmt.allocPrint(arena, "{f}", .{
                std.fmt.alt(password, .formatPassword),
            }) else null;
            const host = if (uri.host) |host| try std.fmt.allocPrint(arena, "{f}", .{
                std.fmt.alt(host, .formatHost),
            }) else null;
            const path = try std.fmt.allocPrint(arena, "{f}", .{
                std.fmt.alt(uri.path, .formatPath),
            });
            // The query and fragment are not used as part of the base server URI.
            return .{
                .uri = .{
                    .scheme = scheme,
                    .user = if (user) |s| .{ .percent_encoded = s } else null,
                    .password = if (password) |s| .{ .percent_encoded = s } else null,
                    .host = if (host) |s| .{ .percent_encoded = s } else null,
                    .port = uri.port,
                    .path = .{ .percent_encoded = path },
                },
            };
        }
    };

    /// Returns an iterator over capabilities supported by the server.
    ///
    /// The `session.location` is updated if the server returns a redirect, so
    /// that subsequent session functions do not need to handle redirects.
    fn getCapabilities(session: *Session, it: *CapabilityIterator, response_buffer: []u8) !void {
        const arena = session.arena;
        assert(response_buffer.len >= Packet.max_data_length);
        var info_refs_uri = session.location.uri;
        {
            const session_uri_path = try std.fmt.allocPrint(arena, "{f}", .{
                std.fmt.alt(session.location.uri.path, .formatPath),
            });
            info_refs_uri.path = .{ .percent_encoded = try std.fs.path.resolvePosix(arena, &.{
                "/", session_uri_path, "info/refs",
            }) };
        }
        info_refs_uri.query = .{ .percent_encoded = "service=git-upload-pack" };
        info_refs_uri.fragment = null;

        const max_redirects = 3;
        it.* = .{
            .request = try session.transport.request(.GET, info_refs_uri, .{
                .redirect_behavior = .init(max_redirects),
                .extra_headers = &.{
                    .{ .name = "Git-Protocol", .value = "version=2" },
                },
            }),
            .reader = undefined,
            .decompress = undefined,
        };
        errdefer it.deinit();
        const request = &it.request;
        try request.sendBodiless();

        var redirect_buffer: [1024]u8 = undefined;
        var response = try request.receiveHead(&redirect_buffer);
        if (response.head.status != .ok) return error.ProtocolError;
        const any_redirects_occurred = request.redirect_behavior.remaining() < max_redirects;
        if (any_redirects_occurred) {
            const request_uri_path = try std.fmt.allocPrint(arena, "{f}", .{
                std.fmt.alt(request.uri.path, .formatPath),
            });
            if (!mem.endsWith(u8, request_uri_path, "/info/refs")) return error.UnparseableRedirect;
            var new_uri = request.uri;
            new_uri.path = .{ .percent_encoded = request_uri_path[0 .. request_uri_path.len - "/info/refs".len] };
            session.location = try .init(arena, new_uri);
        }

        const decompress_buffer = try arena.alloc(u8, response.head.content_encoding.minBufferCapacity());
        it.reader = response.readerDecompressing(response_buffer, &it.decompress, decompress_buffer);
        var state: enum { response_start, response_content } = .response_start;
        while (true) {
            // Some Git servers (at least GitHub) include an additional
            // '# service=git-upload-pack' informative response before sending
            // the expected 'version 2' packet and capability information.
            // This is not universal: SourceHut, for example, does not do this.
            // Thus, we need to skip any such useless additional responses
            // before we get the one we're actually looking for. The responses
            // will be delimited by flush packets.
            const packet = Packet.read(it.reader) catch |err| switch (err) {
                error.EndOfStream => return error.UnsupportedProtocol, // 'version 2' packet not found
                else => |e| return e,
            };
            switch (packet) {
                .flush => state = .response_start,
                .data => |data| switch (state) {
                    .response_start => if (mem.eql(u8, Packet.normalizeText(data), "version 2")) {
                        return;
                    } else {
                        state = .response_content;
                    },
                    else => {},
                },
                else => return error.UnexpectedPacket,
            }
        }
    }

    const CapabilityIterator = struct {
        request: std.http.Client.Request,
        reader: *Io.Reader,
        decompress: std.http.Decompress,

        const Capability = struct {
            key: []const u8,
            value: ?[]const u8 = null,

            fn parse(data: []const u8) Capability {
                return if (mem.findScalar(u8, data, '=')) |separator_pos|
                    .{ .key = data[0..separator_pos], .value = data[separator_pos + 1 ..] }
                else
                    .{ .key = data };
            }
        };

        fn deinit(it: *CapabilityIterator) void {
            it.request.deinit();
            it.* = undefined;
        }

        fn next(it: *CapabilityIterator) !?Capability {
            switch (try Packet.read(it.reader)) {
                .flush => return null,
                .data => |data| return Capability.parse(Packet.normalizeText(data)),
                else => return error.UnexpectedPacket,
            }
        }
    };

    const ListRefsOptions = struct {
        /// The ref prefixes (if any) to use to filter the refs available on the
        /// server. Note that the client must still check the returned refs
        /// against its desired filters itself: the server is not required to
        /// respect these prefix filters and may return other refs as well.
        ref_prefixes: []const []const u8 = &.{},
        /// Whether to include symref targets for returned symbolic refs.
        include_symrefs: bool = false,
        /// Whether to include the peeled object ID for returned tag refs.
        include_peeled: bool = false,
        /// Asserted to be at least `Packet.max_data_length`.
        buffer: []u8,
    };

    /// Returns an iterator over refs known to the server.
    pub fn listRefs(session: Session, it: *RefIterator, options: ListRefsOptions) !void {
        const arena = session.arena;
        assert(options.buffer.len >= Packet.max_data_length);
        var upload_pack_uri = session.location.uri;
        {
            const session_uri_path = try std.fmt.allocPrint(arena, "{f}", .{
                std.fmt.alt(session.location.uri.path, .formatPath),
            });
            upload_pack_uri.path = .{ .percent_encoded = try std.fs.path.resolvePosix(arena, &.{ "/", session_uri_path, "git-upload-pack" }) };
        }
        upload_pack_uri.query = null;
        upload_pack_uri.fragment = null;

        var body: Io.Writer = .fixed(options.buffer);
        try Packet.write(.{ .data = "command=ls-refs\n" }, &body);
        if (session.supports_agent) {
            try Packet.write(.{ .data = agent_capability }, &body);
        }
        {
            const object_format_packet = try std.fmt.allocPrint(arena, "object-format={t}\n", .{
                session.object_format,
            });
            try Packet.write(.{ .data = object_format_packet }, &body);
        }
        try Packet.write(.delimiter, &body);
        for (options.ref_prefixes) |ref_prefix| {
            const ref_prefix_packet = try std.fmt.allocPrint(arena, "ref-prefix {s}\n", .{ref_prefix});
            try Packet.write(.{ .data = ref_prefix_packet }, &body);
        }
        if (options.include_symrefs) {
            try Packet.write(.{ .data = "symrefs\n" }, &body);
        }
        if (options.include_peeled) {
            try Packet.write(.{ .data = "peel\n" }, &body);
        }
        try Packet.write(.flush, &body);

        it.* = .{
            .request = try session.transport.request(.POST, upload_pack_uri, .{
                .redirect_behavior = .unhandled,
                .extra_headers = &.{
                    .{ .name = "Content-Type", .value = "application/x-git-upload-pack-request" },
                    .{ .name = "Git-Protocol", .value = "version=2" },
                },
            }),
            .reader = undefined,
            .format = session.object_format,
            .decompress = undefined,
        };
        const request = &it.request;
        errdefer request.deinit();
        try request.sendBodyComplete(body.buffered());

        var response = try request.receiveHead(options.buffer);
        if (response.head.status != .ok) return error.ProtocolError;
        const decompress_buffer = try arena.alloc(u8, response.head.content_encoding.minBufferCapacity());
        it.reader = response.readerDecompressing(options.buffer, &it.decompress, decompress_buffer);
    }

    pub const RefIterator = struct {
        format: Oid.Format,
        request: std.http.Client.Request,
        reader: *Io.Reader,
        decompress: std.http.Decompress,

        pub const Ref = struct {
            oid: Oid,
            name: []const u8,
            symref_target: ?[]const u8,
            peeled: ?Oid,
        };

        pub fn deinit(iterator: *RefIterator) void {
            iterator.request.deinit();
            iterator.* = undefined;
        }

        pub fn next(it: *RefIterator) !?Ref {
            switch (try Packet.read(it.reader)) {
                .flush => return null,
                .data => |data| {
                    const ref_data = Packet.normalizeText(data);
                    const oid_sep_pos = mem.findScalar(u8, ref_data, ' ') orelse return error.InvalidRefPacket;
                    const oid = Oid.parse(it.format, data[0..oid_sep_pos]) catch return error.InvalidRefPacket;

                    const name_sep_pos = mem.findScalarPos(u8, ref_data, oid_sep_pos + 1, ' ') orelse ref_data.len;
                    const name = ref_data[oid_sep_pos + 1 .. name_sep_pos];

                    var symref_target: ?[]const u8 = null;
                    var peeled: ?Oid = null;
                    var last_sep_pos = name_sep_pos;
                    while (last_sep_pos < ref_data.len) {
                        const next_sep_pos = mem.findScalarPos(u8, ref_data, last_sep_pos + 1, ' ') orelse ref_data.len;
                        const attribute = ref_data[last_sep_pos + 1 .. next_sep_pos];
                        if (mem.startsWith(u8, attribute, "symref-target:")) {
                            symref_target = attribute["symref-target:".len..];
                        } else if (mem.startsWith(u8, attribute, "peeled:")) {
                            peeled = Oid.parse(it.format, attribute["peeled:".len..]) catch return error.InvalidRefPacket;
                        }
                        last_sep_pos = next_sep_pos;
                    }

                    return .{ .oid = oid, .name = name, .symref_target = symref_target, .peeled = peeled };
                },
                else => return error.UnexpectedPacket,
            }
        }
    };

    /// Fetches the given refs from the server. A shallow fetch (depth 1) is
    /// performed if the server supports it.
    pub fn fetch(
        session: Session,
        fs: *FetchStream,
        wants: []const []const u8,
        /// Asserted to be at least `Packet.max_data_length`.
        response_buffer: []u8,
    ) !void {
        const arena = session.arena;
        assert(response_buffer.len >= Packet.max_data_length);
        var upload_pack_uri = session.location.uri;
        {
            const session_uri_path = try std.fmt.allocPrint(arena, "{f}", .{
                std.fmt.alt(session.location.uri.path, .formatPath),
            });
            upload_pack_uri.path = .{ .percent_encoded = try std.fs.path.resolvePosix(arena, &.{ "/", session_uri_path, "git-upload-pack" }) };
        }
        upload_pack_uri.query = null;
        upload_pack_uri.fragment = null;

        var body: Io.Writer = .fixed(response_buffer);
        try Packet.write(.{ .data = "command=fetch\n" }, &body);
        if (session.supports_agent) {
            try Packet.write(.{ .data = agent_capability }, &body);
        }
        {
            const object_format_packet = try std.fmt.allocPrint(arena, "object-format={s}\n", .{@tagName(session.object_format)});
            try Packet.write(.{ .data = object_format_packet }, &body);
        }
        try Packet.write(.delimiter, &body);
        // Our packfile parser supports the OFS_DELTA object type
        try Packet.write(.{ .data = "ofs-delta\n" }, &body);
        // We do not currently convey server progress information to the user
        try Packet.write(.{ .data = "no-progress\n" }, &body);
        if (session.supports_shallow) {
            try Packet.write(.{ .data = "deepen 1\n" }, &body);
        }
        for (wants) |want| {
            var buf: [Packet.max_data_length]u8 = undefined;
            const arg = std.mem.print(&buf, "want {s}\n", .{want}) catch unreachable;
            try Packet.write(.{ .data = arg }, &body);
        }
        try Packet.write(.{ .data = "done\n" }, &body);
        try Packet.write(.flush, &body);

        fs.* = .{
            .request = try session.transport.request(.POST, upload_pack_uri, .{
                .redirect_behavior = .not_allowed,
                .extra_headers = &.{
                    .{ .name = "Content-Type", .value = "application/x-git-upload-pack-request" },
                    .{ .name = "Git-Protocol", .value = "version=2" },
                },
            }),
            .input = undefined,
            .reader = undefined,
            .remaining_len = undefined,
            .decompress = undefined,
        };
        const request = &fs.request;
        errdefer request.deinit();

        try request.sendBodyComplete(body.buffered());

        var response = try request.receiveHead(&.{});
        if (response.head.status != .ok) return error.ProtocolError;

        const decompress_buffer = try arena.alloc(u8, response.head.content_encoding.minBufferCapacity());
        const reader = response.readerDecompressing(response_buffer, &fs.decompress, decompress_buffer);
        // We are not interested in any of the sections of the returned fetch
        // data other than the packfile section, since we aren't doing anything
        // complex like ref negotiation (this is a fresh clone).
        var state: enum { section_start, section_content } = .section_start;
        while (true) {
            const packet = try Packet.read(reader);
            switch (state) {
                .section_start => switch (packet) {
                    .data => |data| if (mem.eql(u8, Packet.normalizeText(data), "packfile")) {
                        fs.input = reader;
                        fs.reader = .{
                            .buffer = &.{},
                            .vtable = &.{ .stream = FetchStream.stream },
                            .seek = 0,
                            .end = 0,
                        };
                        fs.remaining_len = 0;
                        return;
                    } else {
                        state = .section_content;
                    },
                    else => return error.UnexpectedPacket,
                },
                .section_content => switch (packet) {
                    .delimiter => state = .section_start,
                    .data => {},
                    else => return error.UnexpectedPacket,
                },
            }
        }
    }

    pub const FetchStream = struct {
        request: std.http.Client.Request,
        input: *Io.Reader,
        reader: Io.Reader,
        err: ?Error = null,
        remaining_len: usize,
        decompress: std.http.Decompress,

        pub fn deinit(fs: *FetchStream) void {
            fs.request.deinit();
        }

        pub const Error = error{
            InvalidPacket,
            ProtocolError,
            UnexpectedPacket,
            WriteFailed,
            ReadFailed,
            EndOfStream,
        };

        const StreamCode = enum(u8) {
            pack_data = 1,
            progress = 2,
            fatal_error = 3,
            _,
        };

        pub fn stream(r: *Io.Reader, w: *Io.Writer, limit: Io.Limit) Io.Reader.StreamError!usize {
            const fs: *FetchStream = @alignCast(@fieldParentPtr("reader", r));
            const input = fs.input;
            if (fs.remaining_len == 0) {
                while (true) {
                    switch (Packet.peek(input) catch |err| {
                        fs.err = err;
                        return error.ReadFailed;
                    }) {
                        .flush => return error.EndOfStream,
                        .data => |data| switch (@as(StreamCode, @fromBackingInt(@intCast(data[0])))) {
                            .pack_data => {
                                input.toss(1);
                                fs.remaining_len = data.len - 1;
                                break;
                            },
                            .fatal_error => {
                                fs.err = error.ProtocolError;
                                return error.ReadFailed;
                            },
                            else => {
                                input.toss(data.len);
                            },
                        },
                        else => {
                            fs.err = error.UnexpectedPacket;
                            return error.ReadFailed;
                        },
                    }
                }
            }
            const buf = limit.slice(try w.writableSliceGreedy(1));
            const n = @min(buf.len, fs.remaining_len);
            try input.readSliceAll(buf[0..n]);
            w.advance(n);
            fs.remaining_len -= n;
            return n;
        }
    };
};

const PackHeader = struct {
    total_objects: u32,

    const signature = "PACK";
    const supported_version = 2;

    fn read(reader: *Io.Reader) !PackHeader {
        const actual_signature = reader.take(4) catch |e| switch (e) {
            error.EndOfStream => return error.InvalidHeader,
            else => |other| return other,
        };
        if (!mem.eql(u8, actual_signature, signature)) return error.InvalidHeader;
        const version = reader.takeInt(u32, .big) catch |e| switch (e) {
            error.EndOfStream => return error.InvalidHeader,
            else => |other| return other,
        };
        if (version != supported_version) return error.UnsupportedVersion;
        const total_objects = reader.takeInt(u32, .big) catch |e| switch (e) {
            error.EndOfStream => return error.InvalidHeader,
            else => |other| return other,
        };
        return .{ .total_objects = total_objects };
    }
};

const EntryHeader = union(Type) {
    commit: Undeltified,
    tree: Undeltified,
    blob: Undeltified,
    tag: Undeltified,
    ofs_delta: OfsDelta,
    ref_delta: RefDelta,

    const Type = enum(u3) {
        commit = 1,
        tree = 2,
        blob = 3,
        tag = 4,
        ofs_delta = 6,
        ref_delta = 7,
    };

    const Undeltified = struct {
        uncompressed_length: u64,
    };

    const OfsDelta = struct {
        offset: u64,
        uncompressed_length: u64,
    };

    const RefDelta = struct {
        base_object: Oid,
        uncompressed_length: u64,
    };

    fn objectType(header: EntryHeader) Object.Type {
        return switch (header) {
            inline .commit, .tree, .blob, .tag => |_, tag| @field(Object.Type, @tagName(tag)),
            else => unreachable,
        };
    }

    fn uncompressedLength(header: EntryHeader) u64 {
        return switch (header) {
            inline else => |entry| entry.uncompressed_length,
        };
    }

    fn read(format: Oid.Format, reader: *Io.Reader) !EntryHeader {
        const InitialByte = packed struct { len: u4, type: u3, has_next: bool };
        const initial: InitialByte = @bitCast(reader.takeByte() catch |e| switch (e) {
            error.EndOfStream => return error.InvalidFormat,
            else => |other| return other,
        });
        const rest_len = if (initial.has_next) try reader.takeLeb128(u64) else 0;
        var uncompressed_length: u64 = initial.len;
        uncompressed_length |= std.math.shlExact(u64, rest_len, 4) catch return error.InvalidFormat;
        const @"type" = std.enums.fromInt(EntryHeader.Type, initial.type) orelse return error.InvalidFormat;
        return switch (@"type") {
            inline .commit, .tree, .blob, .tag => |tag| @unionInit(EntryHeader, @tagName(tag), .{
                .uncompressed_length = uncompressed_length,
            }),
            .ofs_delta => .{ .ofs_delta = .{
                .offset = try readOffsetVarInt(reader),
                .uncompressed_length = uncompressed_length,
            } },
            .ref_delta => .{ .ref_delta = .{
                .base_object = Oid.readBytes(format, reader) catch |e| switch (e) {
                    error.EndOfStream => return error.InvalidFormat,
                    else => |other| return other,
                },
                .uncompressed_length = uncompressed_length,
            } },
        };
    }
};

fn readOffsetVarInt(r: *Io.Reader) !u64 {
    const Byte = packed struct { value: u7, has_next: bool };
    var b: Byte = @bitCast(try r.takeByte());
    var value: u64 = b.value;
    while (b.has_next) {
        b = @bitCast(try r.takeByte());
        value = std.math.shlExact(u64, value + 1, 7) catch return error.InvalidFormat;
        value |= b.value;
    }
    return value;
}

const IndexHeader = struct {
    fan_out_table: [256]u32,

    const signature = "\xFFtOc";
    const supported_version = 2;
    const size = 4 + 4 + @sizeOf([256]u32);

    fn read(index_header: *IndexHeader, reader: *Io.Reader) !void {
        const sig = try reader.take(4);
        if (!mem.eql(u8, sig, signature)) return error.InvalidHeader;
        const version = try reader.takeInt(u32, .big);
        if (version != supported_version) return error.UnsupportedVersion;
        try reader.readSliceEndian(u32, &index_header.fan_out_table, .big);
    }
};

const IndexEntry = struct {
    offset: u64,
    crc32: u32,
};

/// Writes out a version 2 index for the given packfile, as documented in
/// [pack-format](https://git-scm.com/docs/pack-format).
pub fn indexPack(
    allocator: Allocator,
    format: Oid.Format,
    pack: *Io.File.Reader,
    index_writer: *Io.File.Writer,
) !void {
    try pack.seekTo(0);

    var index_entries: std.AutoHashMapUnmanaged(Oid, IndexEntry) = .empty;
    defer index_entries.deinit(allocator);
    var pending_deltas: std.ArrayList(IndexEntry) = .empty;
    defer pending_deltas.deinit(allocator);

    const pack_checksum = try indexPackFirstPass(allocator, format, pack, &index_entries, &pending_deltas);

    var cache: ObjectCache = .{};
    defer cache.deinit(allocator);
    var remaining_deltas = pending_deltas.items.len;
    while (remaining_deltas > 0) {
        var i: usize = remaining_deltas;
        while (i > 0) {
            i -= 1;
            const delta = pending_deltas.items[i];
            if (try indexPackHashDelta(allocator, format, pack, delta, index_entries, &cache)) |oid| {
                try index_entries.put(allocator, oid, delta);
                _ = pending_deltas.swapRemove(i);
            }
        }
        if (pending_deltas.items.len == remaining_deltas) return error.IncompletePack;
        remaining_deltas = pending_deltas.items.len;
    }

    var oids: std.ArrayList(Oid) = .empty;
    defer oids.deinit(allocator);
    try oids.ensureTotalCapacityPrecise(allocator, index_entries.count());
    var index_entries_iter = index_entries.iterator();
    while (index_entries_iter.next()) |entry| {
        oids.appendAssumeCapacity(entry.key_ptr.*);
    }
    mem.sortUnstable(Oid, oids.items, {}, struct {
        fn lessThan(_: void, o1: Oid, o2: Oid) bool {
            return mem.lessThan(u8, o1.slice(), o2.slice());
        }
    }.lessThan);

    var fan_out_table: [256]u32 = undefined;
    var count: u32 = 0;
    var fan_out_index: u8 = 0;
    for (oids.items) |oid| {
        const key = oid.slice()[0];
        if (key > fan_out_index) {
            @memset(fan_out_table[fan_out_index..key], count);
            fan_out_index = key;
        }
        count += 1;
    }
    @memset(fan_out_table[fan_out_index..], count);

    var index_hashed_writer = Io.Writer.hashed(&index_writer.interface, Oid.Hasher.init(format), &.{});
    const writer = &index_hashed_writer.writer;
    try writer.writeAll(IndexHeader.signature);
    try writer.writeInt(u32, IndexHeader.supported_version, .big);
    for (fan_out_table) |fan_out_entry| {
        try writer.writeInt(u32, fan_out_entry, .big);
    }

    for (oids.items) |oid| {
        try writer.writeAll(oid.slice());
    }

    for (oids.items) |oid| {
        try writer.writeInt(u32, index_entries.get(oid).?.crc32, .big);
    }

    var big_offsets: std.ArrayList(u64) = .empty;
    defer big_offsets.deinit(allocator);
    for (oids.items) |oid| {
        const offset = index_entries.get(oid).?.offset;
        if (offset <= std.math.maxInt(u31)) {
            try writer.writeInt(u32, @intCast(offset), .big);
        } else {
            const index = big_offsets.items.len;
            try big_offsets.append(allocator, offset);
            try writer.writeInt(u32, @as(u32, @intCast(index)) | (1 << 31), .big);
        }
    }
    for (big_offsets.items) |offset| {
        try writer.writeInt(u64, offset, .big);
    }

    try writer.writeAll(pack_checksum.slice());
    const index_checksum = index_hashed_writer.hasher.finalResult();
    try index_writer.interface.writeAll(index_checksum.slice());
    try index_writer.end();
}

/// Performs the first pass over the packfile data for index construction.
/// This will index all non-delta objects, queue delta objects for further
/// processing, and return the pack checksum (which is part of the index
/// format).
fn indexPackFirstPass(
    allocator: Allocator,
    format: Oid.Format,
    pack: *Io.File.Reader,
    index_entries: *std.AutoHashMapUnmanaged(Oid, IndexEntry),
    pending_deltas: *std.ArrayList(IndexEntry),
) !Oid {
    var flate_buffer: [std.compress.flate.max_window_len]u8 = undefined;
    var pack_buffer: [2048]u8 = undefined; // Reasonably large buffer for file system.
    var pack_hashed = pack.interface.hashed(Oid.Hasher.init(format), &pack_buffer);

    const pack_header = try PackHeader.read(&pack_hashed.reader);

    for (0..pack_header.total_objects) |_| {
        const entry_offset = pack.logicalPos() - pack_hashed.reader.bufferedLen();
        const entry_header = try EntryHeader.read(format, &pack_hashed.reader);
        switch (entry_header) {
            .commit, .tree, .blob, .tag => |object| {
                var entry_decompress: std.compress.flate.Decompress = .init(&pack_hashed.reader, .zlib, &.{});
                var oid_hasher: Oid.Hashing = .init(format, &flate_buffer);
                const oid_hasher_w = oid_hasher.writer();
                // The object header is not included in the pack data but is
                // part of the object's ID
                try oid_hasher_w.print("{t} {d}\x00", .{ entry_header, object.uncompressed_length });
                const n = try entry_decompress.reader.streamRemaining(oid_hasher_w);
                if (n != object.uncompressed_length) return error.InvalidObject;
                const oid = oid_hasher.final();
                if (!skip_checksums) @compileError("TODO");
                try index_entries.put(allocator, oid, .{
                    .offset = entry_offset,
                    .crc32 = 0,
                });
            },
            inline .ofs_delta, .ref_delta => |delta| {
                var entry_decompress: std.compress.flate.Decompress = .init(&pack_hashed.reader, .zlib, &flate_buffer);
                const n = try entry_decompress.reader.discardRemaining();
                if (n != delta.uncompressed_length) return error.InvalidObject;
                if (!skip_checksums) @compileError("TODO");
                try pending_deltas.append(allocator, .{
                    .offset = entry_offset,
                    .crc32 = 0,
                });
            },
        }
    }

    if (!skip_checksums) @compileError("TODO");
    return pack_hashed.hasher.finalResult();
}

/// Attempts to determine the final object ID of the given deltified object.
/// May return null if this is not yet possible (if the delta is a ref-based
/// delta and we do not yet know the offset of the base object).
fn indexPackHashDelta(
    allocator: Allocator,
    format: Oid.Format,
    pack: *Io.File.Reader,
    delta: IndexEntry,
    index_entries: std.AutoHashMapUnmanaged(Oid, IndexEntry),
    cache: *ObjectCache,
) !?Oid {
    // Figure out the chain of deltas to resolve
    var base_offset = delta.offset;
    var base_header: EntryHeader = undefined;
    var delta_offsets: std.ArrayList(u64) = .empty;
    defer delta_offsets.deinit(allocator);
    const base_object = while (true) {
        if (cache.get(base_offset)) |base_object| break base_object;

        try pack.seekTo(base_offset);
        base_header = try EntryHeader.read(format, &pack.interface);
        switch (base_header) {
            .ofs_delta => |ofs_delta| {
                try delta_offsets.append(allocator, base_offset);
                base_offset = std.math.sub(u64, base_offset, ofs_delta.offset) catch return error.InvalidObject;
            },
            .ref_delta => |ref_delta| {
                try delta_offsets.append(allocator, base_offset);
                base_offset = (index_entries.get(ref_delta.base_object) orelse return null).offset;
            },
            else => {
                const base_data = try readObjectRaw(allocator, &pack.interface, base_header.uncompressedLength());
                errdefer allocator.free(base_data);
                const base_object: Object = .{ .type = base_header.objectType(), .data = base_data };
                try cache.put(allocator, base_offset, base_object);
                break base_object;
            },
        }
    };

    const base_data = try resolveDeltaChain(allocator, format, pack, base_object, delta_offsets.items, cache);

    var entry_hasher_buffer: [64]u8 = undefined;
    var entry_hasher: Oid.Hashing = .init(format, &entry_hasher_buffer);
    const entry_hasher_w = entry_hasher.writer();
    // Writes to hashers cannot fail.
    entry_hasher_w.print("{t} {d}\x00", .{ base_object.type, base_data.len }) catch unreachable;
    entry_hasher_w.writeAll(base_data) catch unreachable;
    return entry_hasher.final();
}

/// Resolves a chain of deltas, returning the final base object data. `pack` is
/// assumed to be looking at the start of the object data for the base object of
/// the chain, and will then apply the deltas in `delta_offsets` in reverse order
/// to obtain the final object.
fn resolveDeltaChain(
    allocator: Allocator,
    format: Oid.Format,
    pack: *Io.File.Reader,
    base_object: Object,
    delta_offsets: []const u64,
    cache: *ObjectCache,
) ![]const u8 {
    var base_data = base_object.data;
    var i: usize = delta_offsets.len;
    while (i > 0) {
        i -= 1;

        const delta_offset = delta_offsets[i];
        try pack.seekTo(delta_offset);
        const delta_header = try EntryHeader.read(format, &pack.interface);
        const delta_data = try readObjectRaw(allocator, &pack.interface, delta_header.uncompressedLength());
        defer allocator.free(delta_data);
        var delta_reader: Io.Reader = .fixed(delta_data);
        _ = try delta_reader.takeLeb128(u64); // base object size
        const expanded_size = try delta_reader.takeLeb128(u64);

        const expanded_alloc_size = std.math.cast(usize, expanded_size) orelse return error.ObjectTooLarge;
        const expanded_data = try allocator.alloc(u8, expanded_alloc_size);
        errdefer allocator.free(expanded_data);
        var expanded_delta_stream: Io.Writer = .fixed(expanded_data);
        try expandDelta(base_data, &delta_reader, &expanded_delta_stream);
        if (expanded_delta_stream.end != expanded_size) return error.InvalidObject;

        try cache.put(allocator, delta_offset, .{ .type = base_object.type, .data = expanded_data });
        base_data = expanded_data;
    }
    return base_data;
}

/// Reads the complete contents of an object from `reader`. This function may
/// read more bytes than required from `reader`, so the reader position after
/// returning is not reliable.
fn readObjectRaw(allocator: Allocator, reader: *Io.Reader, size: u64) ![]u8 {
    const alloc_size = std.math.cast(usize, size) orelse return error.ObjectTooLarge;
    var aw: Io.Writer.Allocating = .init(allocator);
    try aw.ensureTotalCapacity(alloc_size + std.compress.flate.max_window_len);
    defer aw.deinit();
    var decompress: std.compress.flate.Decompress = .init(reader, .zlib, &.{});
    try decompress.reader.streamExact(&aw.writer, alloc_size);
    return aw.toOwnedSlice();
}

/// Expands delta data from `delta_reader` to `writer`.
///
/// The format of the delta data is documented in
/// [pack-format](https://git-scm.com/docs/pack-format).
fn expandDelta(base_object: []const u8, delta_reader: *Io.Reader, writer: *Io.Writer) !void {
    while (true) {
        const inst: packed struct { value: u7, copy: bool } = @bitCast(delta_reader.takeByte() catch |e| switch (e) {
            error.EndOfStream => return,
            else => |other| return other,
        });
        if (inst.copy) {
            const available: packed struct {
                offset1: bool,
                offset2: bool,
                offset3: bool,
                offset4: bool,
                size1: bool,
                size2: bool,
                size3: bool,
            } = @bitCast(inst.value);
            const offset_parts: packed struct { offset1: u8, offset2: u8, offset3: u8, offset4: u8 } = .{
                .offset1 = if (available.offset1) try delta_reader.takeByte() else 0,
                .offset2 = if (available.offset2) try delta_reader.takeByte() else 0,
                .offset3 = if (available.offset3) try delta_reader.takeByte() else 0,
                .offset4 = if (available.offset4) try delta_reader.takeByte() else 0,
            };
            const base_offset: u32 = @bitCast(offset_parts);
            const size_parts: packed struct { size1: u8, size2: u8, size3: u8 } = .{
                .size1 = if (available.size1) try delta_reader.takeByte() else 0,
                .size2 = if (available.size2) try delta_reader.takeByte() else 0,
                .size3 = if (available.size3) try delta_reader.takeByte() else 0,
            };
            var size: u24 = @bitCast(size_parts);
            if (size == 0) size = 0x10000;
            try writer.writeAll(base_object[base_offset..][0..size]);
        } else if (inst.value != 0) {
            try delta_reader.streamExact(writer, inst.value);
        } else {
            return error.InvalidDeltaInstruction;
        }
    }
}

/// Runs the packfile indexing and checkout test.
///
/// The two testrepo repositories under testdata contain identical commit
/// histories and contents.
///
/// To verify the contents of the packfiles using Git alone, run the
/// following commands in an empty directory:
///
/// 1. `git init --object-format=(sha1|sha256)`
/// 2. `git unpack-objects <path/to/testrepo.pack`
/// 3. `git fsck` - will print one "dangling commit":
///    - SHA-1: `dd582c0720819ab7130b103635bd7271b9fd4feb`
///    - SHA-256: `7f444a92bd4572ee4a28b2c63059924a9ca1829138553ef3e7c41ee159afae7a`
/// 4. `git checkout $commit`
fn runRepositoryTest(io: Io, comptime format: Oid.Format, head_commit: []const u8) !void {
    const testrepo_pack = @embedFile("git/testdata/testrepo-" ++ @tagName(format) ++ ".pack");

    var git_dir = testing.tmpDir(.{});
    defer git_dir.cleanup();
    var pack_file = try git_dir.dir.createFile(io, "testrepo.pack", .{ .read = true });
    defer pack_file.close(io);
    try pack_file.writeStreamingAll(io, testrepo_pack);

    var pack_file_buffer: [2000]u8 = undefined;
    var pack_file_reader = pack_file.reader(io, &pack_file_buffer);

    var index_file = try git_dir.dir.createFile(io, "testrepo.idx", .{ .read = true });
    defer index_file.close(io);
    var index_file_buffer: [2000]u8 = undefined;
    var index_file_writer = index_file.writer(io, &index_file_buffer);
    try indexPack(testing.allocator, format, &pack_file_reader, &index_file_writer);

    // Arbitrary size limit on files read while checking the repository contents
    // (all files in the test repo are known to be smaller than this)
    const max_file_size = 8192;

    if (!skip_checksums) {
        const index_file_data = try git_dir.dir.readFileAlloc(io, "testrepo.idx", testing.allocator, .limited(max_file_size));
        defer testing.allocator.free(index_file_data);
        // testrepo.idx is generated by Git. The index created by this file should
        // match it exactly. Running `git verify-pack -v testrepo.pack` can verify
        // this.
        const testrepo_idx = @embedFile("git/testdata/testrepo-" ++ @tagName(format) ++ ".idx");
        try testing.expectEqualSlices(u8, testrepo_idx, index_file_data);
    }

    var index_file_reader = index_file.reader(io, &index_file_buffer);
    var repository: Repository = undefined;
    try repository.init(testing.allocator, format, &pack_file_reader, &index_file_reader);
    defer repository.deinit();

    var worktree = testing.tmpDir(.{ .iterate = true });
    defer worktree.cleanup();

    const commit_id = try Oid.parse(format, head_commit);

    var diagnostics: Diagnostics = .{ .allocator = testing.allocator };
    defer diagnostics.deinit();
    try repository.checkout(io, worktree.dir, commit_id, &diagnostics);
    try testing.expect(diagnostics.errors.items.len == 0);

    const expected_files: []const []const u8 = &.{
        "dir/file",
        "dir/subdir/file",
        "dir/subdir/file2",
        "dir2/file",
        "dir3/file",
        "dir3/file2",
        "file",
        "file2",
        "file3",
        "file4",
        "file5",
        "file6",
        "file7",
        "file8",
        "file9",
    };
    var actual_files: std.ArrayList([]u8) = .empty;
    defer actual_files.deinit(testing.allocator);
    defer for (actual_files.items) |file| testing.allocator.free(file);
    var walker = try worktree.dir.walk(testing.allocator);
    defer walker.deinit();
    while (try walker.next(io)) |entry| {
        if (entry.kind != .file) continue;
        const path = try testing.allocator.dupe(u8, entry.path);
        errdefer testing.allocator.free(path);
        mem.replaceScalar(u8, path, std.fs.path.sep, '/');
        try actual_files.append(testing.allocator, path);
    }
    mem.sortUnstable([]u8, actual_files.items, {}, struct {
        fn lessThan(_: void, a: []u8, b: []u8) bool {
            return mem.lessThan(u8, a, b);
        }
    }.lessThan);
    try testing.expectEqualDeep(expected_files, actual_files.items);

    const expected_file_contents =
        \\revision 1
        \\revision 2
        \\revision 4
        \\revision 5
        \\revision 7
        \\revision 8
        \\revision 9
        \\revision 10
        \\revision 12
        \\revision 13
        \\revision 14
        \\revision 18
        \\revision 19
        \\
    ;
    const actual_file_contents = try worktree.dir.readFileAlloc(io, "file", testing.allocator, .limited(max_file_size));
    defer testing.allocator.free(actual_file_contents);
    try testing.expectEqualStrings(expected_file_contents, actual_file_contents);
}

/// Checksum calculation is useful for troubleshooting and debugging, but it's
/// redundant since the package manager already does content hashing at the
/// end. Let's save time by not doing that work, but, I left a cookie crumb
/// trail here if you want to restore the functionality for tinkering purposes.
const skip_checksums = true;

test "SHA-1 packfile indexing and checkout" {
    try runRepositoryTest(std.testing.io, .sha1, "dd582c0720819ab7130b103635bd7271b9fd4feb");
}

test "SHA-256 packfile indexing and checkout" {
    try runRepositoryTest(std.testing.io, .sha256, "7f444a92bd4572ee4a28b2c63059924a9ca1829138553ef3e7c41ee159afae7a");
}

/// Checks out a commit of a packfile. Intended for experimenting with and
/// benchmarking possible optimizations to the indexing and checkout behavior.
pub fn main() !void {
    const allocator = std.heap.smp_allocator;

    var threaded: Io.Threaded = .init(allocator, .{});
    defer threaded.deinit();
    const io = threaded.io();

    const args = try std.process.argsAlloc(allocator);
    defer std.process.argsFree(allocator, args);
    if (args.len != 5) {
        return error.InvalidArguments; // Arguments: format packfile commit worktree
    }

    const format = std.meta.stringToEnum(Oid.Format, args[1]) orelse return error.InvalidFormat;

    var pack_file = try Io.Dir.cwd().openFile(io, args[2], .{});
    defer pack_file.close(io);
    var pack_file_buffer: [4096]u8 = undefined;
    var pack_file_reader = pack_file.reader(io, &pack_file_buffer);

    const commit = try Oid.parse(format, args[3]);
    var worktree = try Io.Dir.cwd().createDirPathOpen(io, args[4], .{});
    defer worktree.close(io);

    var git_dir = try worktree.createDirPathOpen(io, ".git", .{});
    defer git_dir.close(io);

    std.debug.print("Starting index...\n", .{});
    var index_file = try git_dir.createFile(io, "idx", .{ .read = true });
    defer index_file.close(io);
    var index_file_buffer: [4096]u8 = undefined;
    var index_file_writer = index_file.writer(io, &index_file_buffer);
    try indexPack(allocator, format, &pack_file_reader, &index_file_writer);

    std.debug.print("Starting checkout...\n", .{});
    var index_file_reader = index_file.reader(io, &index_file_buffer);
    var repository: Repository = undefined;
    try repository.init(allocator, format, &pack_file_reader, &index_file_reader);
    defer repository.deinit();
    var diagnostics: Diagnostics = .{ .allocator = allocator };
    defer diagnostics.deinit();
    try repository.checkout(io, worktree, commit, &diagnostics);

    for (diagnostics.errors.items) |err| {
        std.debug.print("Diagnostic: {}\n", .{err});
    }
}
