outputHeader

    fn outputHeader(
        h: *Huffman,
        freqs: *const [257]u16,
        buf: *CodesBuf,
        bytes: u16,
        eos: bool,
    ) Writer.Error!?struct {
        codes: *const [257]u16,
        bits: *const [257]u4,
    } {
        assert(freqs[256] == 1);
        const dyn_codes_bitsize, _ = huffman.build(
            freqs,
            buf.dyn_codes[0..257],
            buf.dyn_bits[0..257],
            15,
            true,
        );

        var clen_values: [258]u8 = undefined;
        var clen_extra: [258]u8 = undefined;
        var clen_freqs: [19]u16 = @splat(0);
        const clen_len, const clen_extra_bitsize = buildClen(
            &buf.dyn_bits,
            &clen_values,
            &clen_extra,
            &clen_freqs,
        );

        var clen_codes: [19]u16 = undefined;
        var clen_bits: [19]u4 = @splat(0);
        const clen_codes_bitsize, _ = huffman.build(
            &clen_freqs,
            &clen_codes,
            &clen_bits,
            7,
            false,
        );
        const hclen = clenHlen(clen_freqs);

        const dynamic_bitsize = @as(u32, 14) +
            (4 + @as(u6, hclen)) * 3 + clen_codes_bitsize + clen_extra_bitsize +
            dyn_codes_bitsize;
        const fixed_bitsize = n: {
            const freq7 = 1; // eos
            var freq9: u16 = 0;
            for (freqs[144..256]) |f| freq9 += f;
            const freq8: u16 = bytes - freq9;
            break :n @as(u32, freq7) * 7 + @as(u32, freq8) * 8 + @as(u32, freq9) * 9;
        };
        const stored_bitsize = n: {
            const stored_align_bits = -%(h.bit_writer.buffered_n +% 3);
            break :n stored_align_bits + @as(u32, 32) + @as(u32, bytes) * 8;
        };

        if (stored_bitsize <= @min(dynamic_bitsize, fixed_bitsize)) {
            try h.bit_writer.write(BlockHeader.int(.{ .kind = .stored, .final = eos }), 3);
            try h.bit_writer.output.rebase(0, 5);
            h.bit_writer.byteAlign();
            h.bit_writer.output.writeInt(u16, bytes, .little) catch unreachable;
            h.bit_writer.output.writeInt(u16, ~bytes, .little) catch unreachable;
            return null;
        }

        if (fixed_bitsize <= dynamic_bitsize) {
            try h.bit_writer.write(BlockHeader.int(.{ .final = eos, .kind = .fixed }), 3);
            return .{
                .codes = token.fixed_lit_codes[0..257],
                .bits = token.fixed_lit_bits[0..257],
            };
        } else {
            try h.bit_writer.write(BlockHeader.Dynamic.int(.{
                .regular = .{ .final = eos, .kind = .dynamic },
                .hlit = 0,
                .hdist = 0,
                .hclen = hclen,
            }), 17);
            try h.bit_writer.writeClen(
                hclen,
                clen_values[0..clen_len],
                clen_extra[0..clen_len],
                clen_codes,
                clen_bits,
            );
            return .{ .codes = buf.dyn_codes[0..257], .bits = buf.dyn_bits[0..257] };
        }
    }