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Artifact 7062d8abec47e7d1889293510a55a5d6d2d1e2a0fb880904fdd204f02cdde78f:

//! The Fossil zlib container codec — the single implementation of the §4.1
//! compressed container shared by every layer that stores or transmits one.
//!
//! draft-fossil-sync-protocol-00 §4.1 L889-919: a container is a 4-byte
//! big-endian pre-compression input length followed by a zlib stream [RFC1950]
//! (2-byte header, DEFLATE body [RFC1951], 4-byte Adler-32 trailer). It is
//! neither raw DEFLATE nor gzip. Ordinary compression uses the zlib default
//! level; the inline body-login concatenation compresses at DEFLATE level 9;
//! receivers MUST be level-independent (§4.1 L916-918). The 4-byte field is
//! an unsigned 32-bit output-capacity value: a decode that finishes within
//! that capacity is accepted and returns the actual decoded bytes, which may
//! be fewer than declared; "bytes after a successfully decoded zlib stream are
//! ignored." The artifact hash names the UNCOMPRESSED bytes (§4.1 L1206-1208),
//! so storage compression never affects an artifact's address.
//!
//! This is a LEAF module: it imports only `std` and sits below `logstore`,
//! `repostate`, and `sync`, which each previously carried their own cited copy
//! of this framing. It is the sole home for the §4.1 codec so the container
//! rules can only ever be fixed in one place.

const std = @import("std");
const flate = std.compress.flate;

/// §4.1 L916-918 — the two encode levels the protocol names: the zlib
/// default for ordinary single-input containers, and DEFLATE level 9 for the
/// inline body-login concatenation. Decode is level-independent, so this
/// choice never crosses the wire beyond the bytes it produces.
pub const Level = enum { default, best };

fn compressOptions(level: Level) flate.Compress.Options {
    return switch (level) {
        .default => .level_6,
        .best => .level_9,
    };
}

/// `Compress.init` asserts its output writer already has more than 8 bytes of
/// buffer capacity; `Writer.Allocating` starts empty, so the encoder pre-sizes
/// before calling it. 64 is a cited headroom over that `>8` floor, comfortably
/// covering the 4-byte length prefix plus the 2-byte zlib header written first.
const compress_out_min: usize = 64;

/// A container's length field is an unsigned 32-bit value (§4.1 L909-911), so
/// input longer than this cannot be framed and is rejected before any work.
pub const EncodeError = error{ OutOfMemory, InputTooLarge };

/// Every §4.1 decode-side rejection: a header shorter than the 4-byte length
/// prefix, a declared capacity over the caller's `max_output` bound or a stream
/// that overruns its declared capacity (both `OutputTooLarge`), and a malformed
/// zlib stream (`InvalidStream`).
pub const DecodeError = error{ OutOfMemory, TruncatedHeader, InvalidStream, OutputTooLarge };

/// §4.1 L889-892, L1204-1205 — wrap `bytes` in the §4.1 container: a 4-byte
/// big-endian pre-compression length, then an RFC1950 zlib stream at `level`.
/// The sender MUST encode the actual input length (§4.1 L892), which the
/// postcondition below asserts.
pub fn encode(gpa: std.mem.Allocator, bytes: []const u8, level: Level) EncodeError![]u8 {
    if (bytes.len > std.math.maxInt(u32)) return error.InputTooLarge;

    var aw = std.Io.Writer.Allocating.init(gpa);
    defer aw.deinit();
    aw.ensureUnusedCapacity(compress_out_min) catch return error.OutOfMemory;
    aw.writer.writeInt(u32, @intCast(bytes.len), .big) catch return error.OutOfMemory;

    const window = gpa.alloc(u8, flate.max_window_len) catch return error.OutOfMemory;
    defer gpa.free(window);

    var c = flate.Compress.init(&aw.writer, window, .zlib, compressOptions(level)) catch return error.OutOfMemory;
    c.writer.writeAll(bytes) catch return error.OutOfMemory;
    c.finish() catch return error.OutOfMemory;

    const out = try aw.toOwnedSlice();
    std.debug.assert(std.mem.readInt(u32, out[0..4], .big) == bytes.len); // §4.1 L892 — actual length encoded
    return out;
}

/// The first `want` plaintext bytes of a §4.1 container, or fewer when the
/// artifact itself is shorter. Inflates only far enough to produce them and
/// abandons the stream, so the cost is the zlib header plus one block rather
/// than the whole artifact.
///
/// This is a CLASSIFIER's read, never a content read: the bytes it returns are
/// a prefix, so nothing may hash them, store them, or hand them to a parser
/// that would treat a truncated artifact as a whole one. `decode` remains the
/// only way to obtain an artifact.
///
/// Truncation is why this cannot enforce `decode`'s capacity postcondition: a
/// stream stopped early has no declared-length identity to check against. It
/// still rejects a header too short to carry the length prefix, and still
/// bounds its own iteration the way `decode` does.
pub fn decodePrefix(gpa: std.mem.Allocator, wire: []const u8, want: usize) DecodeError![]u8 {
    if (wire.len < 4) return error.TruncatedHeader;
    const declared = std.mem.readInt(u32, wire[0..4], .big);
    const cap: usize = @min(want, @as(usize, declared));
    if (cap == 0) return gpa.alloc(u8, 0) catch return error.OutOfMemory;

    var fr = std.Io.Reader.fixed(wire[4..]);
    const window = gpa.alloc(u8, flate.max_window_len) catch return error.OutOfMemory;
    defer gpa.free(window);
    var dc = flate.Decompress.init(&fr, .zlib, window);

    var aw = std.Io.Writer.Allocating.init(gpa);
    defer aw.deinit();

    // Same pump and the same bounded-iteration argument as `decode`; only the
    // ceiling differs, and a short read is a legitimate result here rather
    // than the truncation `decode` must reject.
    var total: usize = 0;
    const max_iters: usize = wire.len + cap + 64;
    var iters: usize = 0;
    while (total < cap and iters < max_iters) : (iters += 1) {
        const n = dc.reader.stream(&aw.writer, .limited(cap - total)) catch |err| switch (err) {
            error.EndOfStream => break,
            error.WriteFailed => return error.OutOfMemory,
            error.ReadFailed => return error.InvalidStream,
        };
        total += n;
    }
    if (iters >= max_iters) return error.InvalidStream;
    return aw.toOwnedSlice();
}

/// §4.1 L901-914 — decode a container produced by `encode`. The length field
/// is a capacity, not an exact length: a decode that finishes within that
/// capacity is accepted and returns the actual decoded byte count, which may be
/// less than declared. `max_output` bounds the declared length (rejecting a
/// lying header before any decompression work happens) and the actual stream is
/// pumped no further than `declared + 1` bytes, so a truncated-length lie
/// cannot drive unbounded allocation; the `+1` is what detects a stream that
/// overruns its declared capacity. Decode is compression-level-independent
/// (§4.1 L916-918).
pub fn decode(gpa: std.mem.Allocator, wire: []const u8, max_output: usize) DecodeError![]u8 {
    if (wire.len < 4) return error.TruncatedHeader;
    const declared = std.mem.readInt(u32, wire[0..4], .big);
    if (declared > max_output) return error.OutputTooLarge;

    var fr = std.Io.Reader.fixed(wire[4..]);
    const window = gpa.alloc(u8, flate.max_window_len) catch return error.OutOfMemory;
    defer gpa.free(window);
    var dc = flate.Decompress.init(&fr, .zlib, window);

    var aw = std.Io.Writer.Allocating.init(gpa);
    defer aw.deinit();

    const cap: usize = @as(usize, declared) + 1;
    var total: usize = 0;
    // `stream()` may legitimately return 0 progress on a call that only does
    // internal bookkeeping (e.g. parsing a block header) without yet emitting
    // output bytes, so 0 is not itself a completion signal — only
    // `error.EndOfStream` or reaching `cap` ends the pump. The iteration count
    // is still explicitly bounded (input size plus the output cap plus
    // headroom) so a stdlib regression that stalled forever would surface as
    // `InvalidStream` rather than hang.
    const max_iters: usize = wire.len + cap + 64;
    var iters: usize = 0;
    while (total < cap and iters < max_iters) : (iters += 1) {
        const n = dc.reader.stream(&aw.writer, .limited(cap - total)) catch |err| switch (err) {
            error.EndOfStream => break,
            error.WriteFailed => return error.OutOfMemory,
            error.ReadFailed => return error.InvalidStream,
        };
        total += n;
    }
    if (iters >= max_iters) return error.InvalidStream;

    if (total > declared) return error.OutputTooLarge; // §4.1 L901-909: must fit within capacity
    const out = try aw.toOwnedSlice();
    std.debug.assert(out.len == total and out.len <= declared); // postcondition: accounting + capacity ceiling
    return out;
}