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//! Minimal ZIP writer: stored (uncompressed) entries, central directory, EOCD.
//!
//! Entries are stored uncompressed (method 0). Zig 0.15.2's std deflate
//! *compressor* is unfinished (`std.compress.flate.Compress` panics / loops),
//! so we avoid it entirely; stored entries are fully ZIP/EPUB compliant. The
//! cost is larger files, acceptable for docs committed to the repo.
//!
//! Output is byte-for-byte reproducible: a fixed DOS timestamp is used for every
//! entry (no wall clock), so identical inputs produce identical archives and git
//! only sees a diff when the underlying docs actually change.
const std = @import("std");
const assert = std.debug.assert;
const Crc32 = std.hash.crc.Crc32;
/// Upper bound on entries in a single archive (NASA Power-of-10: bounded loops).
pub const max_entries = 64;
/// Fixed DOS date/time for reproducibility (2021-01-01 00:00:00).
const dos_time: u16 = 0;
const dos_date: u16 = (41 << 9) | (1 << 5) | 1; // (year-1980)<<9 | month<<5 | day
const sig_local = std.zip.local_file_header_sig;
const sig_central = std.zip.central_file_header_sig;
const sig_end = std.zip.end_record_sig;
const method_store: u16 = @intFromEnum(std.zip.CompressionMethod.store);
const Entry = struct {
name: []const u8,
crc: u32,
comp_size: u32,
uncomp_size: u32,
method: u16,
offset: u32,
};
pub const Writer = struct {
allocator: std.mem.Allocator,
bytes: std.ArrayList(u8),
entries: std.ArrayList(Entry),
pub fn init(allocator: std.mem.Allocator) Writer {
return .{
.allocator = allocator,
.bytes = .empty,
.entries = .empty,
};
}
pub fn deinit(w: *Writer) void {
w.bytes.deinit(w.allocator);
w.entries.deinit(w.allocator);
}
/// Add an entry stored uncompressed (method 0).
pub fn addStored(w: *Writer, name: []const u8, data: []const u8) !void {
assert(name.len > 0);
assert(w.entries.items.len < max_entries);
const crc = Crc32.hash(data);
try w.writeLocal(name, crc, @intCast(data.len), @intCast(data.len), method_store);
try w.bytes.appendSlice(w.allocator, data);
}
/// Append central directory + end-of-central-directory; return archive bytes.
/// Caller owns the returned slice.
pub fn finish(w: *Writer) ![]u8 {
assert(w.entries.items.len > 0);
const cd_offset: u32 = @intCast(w.bytes.items.len);
for (w.entries.items) |e| {
try w.appendBytes(&sig_central);
try w.appendInt(u16, 20); // version made by
try w.appendInt(u16, 20); // version needed
try w.appendInt(u16, 0); // flags
try w.appendInt(u16, e.method);
try w.appendInt(u16, dos_time);
try w.appendInt(u16, dos_date);
try w.appendInt(u32, e.crc);
try w.appendInt(u32, e.comp_size);
try w.appendInt(u32, e.uncomp_size);
try w.appendInt(u16, @intCast(e.name.len));
try w.appendInt(u16, 0); // extra len
try w.appendInt(u16, 0); // comment len
try w.appendInt(u16, 0); // disk number start
try w.appendInt(u16, 0); // internal attrs
try w.appendInt(u32, 0); // external attrs
try w.appendInt(u32, e.offset);
try w.appendBytes(e.name);
}
const cd_size: u32 = @as(u32, @intCast(w.bytes.items.len)) - cd_offset;
const count: u16 = @intCast(w.entries.items.len);
try w.appendBytes(&sig_end);
try w.appendInt(u16, 0); // disk number
try w.appendInt(u16, 0); // cd start disk
try w.appendInt(u16, count); // records this disk
try w.appendInt(u16, count); // total records
try w.appendInt(u32, cd_size);
try w.appendInt(u32, cd_offset);
try w.appendInt(u16, 0); // comment len
return w.bytes.toOwnedSlice(w.allocator);
}
fn writeLocal(w: *Writer, name: []const u8, crc: u32, comp_size: u32, uncomp_size: u32, method: u16) !void {
const offset: u32 = @intCast(w.bytes.items.len);
assert(w.entries.items.len == 0 or offset > w.entries.items[w.entries.items.len - 1].offset);
try w.appendBytes(&sig_local);
try w.appendInt(u16, 20); // version needed
try w.appendInt(u16, 0); // flags
try w.appendInt(u16, method);
try w.appendInt(u16, dos_time);
try w.appendInt(u16, dos_date);
try w.appendInt(u32, crc);
try w.appendInt(u32, comp_size);
try w.appendInt(u32, uncomp_size);
try w.appendInt(u16, @intCast(name.len));
try w.appendInt(u16, 0); // extra len
try w.appendBytes(name);
try w.entries.append(w.allocator, .{
.name = name,
.crc = crc,
.comp_size = comp_size,
.uncomp_size = uncomp_size,
.method = method,
.offset = offset,
});
}
fn appendBytes(w: *Writer, data: []const u8) !void {
try w.bytes.appendSlice(w.allocator, data);
}
fn appendInt(w: *Writer, comptime T: type, value: T) !void {
var buf: [@sizeOf(T)]u8 = undefined;
std.mem.writeInt(T, &buf, value, .little);
try w.bytes.appendSlice(w.allocator, &buf);
}
};
/// Write `archive` to a temp dir, extract it with the std.zip reader, and return
/// the extracted contents of `name`. Exercises a full ZIP round-trip.
fn extractOne(gpa: std.mem.Allocator, dir: std.fs.Dir, archive: []const u8, name: []const u8) ![]u8 {
try dir.writeFile(.{ .sub_path = "out.zip", .data = archive });
var file = try dir.openFile("out.zip", .{});
defer file.close();
var buf: [4096]u8 = undefined;
var fr = file.reader(&buf);
try std.zip.extract(dir, &fr, .{});
return dir.readFileAlloc(gpa, name, 1 << 20);
}
test "stored entry round-trips via std.zip reader" {
const gpa = std.testing.allocator;
var w = Writer.init(gpa);
defer w.deinit();
const payload = "hello, epub";
try w.addStored("a.txt", payload);
const archive = try w.finish();
defer gpa.free(archive);
try std.testing.expect(std.mem.startsWith(u8, archive, "PK"));
var tmp = std.testing.tmpDir(.{});
defer tmp.cleanup();
const got = try extractOne(gpa, tmp.dir, archive, "a.txt");
defer gpa.free(got);
try std.testing.expectEqualStrings(payload, got);
}
test "multiple entries round-trip in nested paths" {
const gpa = std.testing.allocator;
var w = Writer.init(gpa);
defer w.deinit();
try w.addStored("first.txt", "one");
try w.addStored("dir/second.txt", "A" ** 1000);
const archive = try w.finish();
defer gpa.free(archive);
var tmp = std.testing.tmpDir(.{});
defer tmp.cleanup();
const got = try extractOne(gpa, tmp.dir, archive, "dir/second.txt");
defer gpa.free(got);
try std.testing.expectEqualStrings("A" ** 1000, got);
}
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