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DOS Lemmings .DAT Compression Reference

Complete Spec · Decoders · Encoders · Pack‑Patching Toolkit

Last updated 2025‑06‑05

0 · Scope & Quick‑Start

This document covers every step required to decompress, recompress, and patch any DOS Lemmings asset pack (*.DAT, MAIN.DAT, VGAGR*.DAT, etc.). You’ll find:

  • A precise container layout and mirrored bit‑stream description
  • Reference decoders (Python 3 and ES2025)
  • Fully‑featured encoders that use all six op‑codes with cost‑based heuristics
  • Ready‑to‑use pack‑patching helpers in both languages
  • A complete test matrix and an edge‑case compatibility table

1 · File & Section Header (10 bytes)

Off Size Field Description
0x00 1 B firstByteBits Number of valid bits in the final payload byte (1 – 8; 0 → 8)
0x01 1 B checksum XOR of all payload bytes (header excluded)
0x02 2 B unused1 Reserved (usually 0x0000)
0x04 2 B decompSize Exact decompressed size (little‑endian)
0x06 2 B unused2 Reserved
0x08 2 B compSize Header + payload size (little‑endian)

Sections appear back‑to‑back until EOF.

2 · Bit‑Stream Orientation

  • The payload is processed in reverse — last byte first, with bits read LSB‑first.
  • After removing firstByteBits from the final byte, the decoder reads whole bytes until no bits remain.

3 · Op‑Code Summary

Prefix Mnemonic Payload Bits Expands to Typical Use
00 RAW‑S len3 + literals 1 – 8 literals Tiny literal runs
01 REF‑2 off8 Copy 2 bytes Very common pairs
100 REF‑3 off9 Copy 3 bytes Short patterns
101 REF‑4 off10 Copy 4 bytes Short patterns
110 REF‑N len8 + 1, off12 + 1 Copy 5 – 256 bytes Long runs / RLE
111 RAW‑L len8 + 9, literals 9 – 264 literals Large uncompressible spans

Offsets are measured backwards from the current output index. Copies may overlap (off = 1 enables RLE).

4 · Reference Decoders

4.1 Python 3.12

import struct

def decode_section(buf: bytes) -> bytes:
    first_bits, _chk, *_rest = struct.unpack_from('<BBHHHH', buf)
    out_len = _rest[1]
    payload = memoryview(buf)[10:]

    # bit cursor (mirrored, LSB‑first)
    byte_idx = len(payload) - 1
    bit_buf  = payload[byte_idx]
    byte_idx -= 1
    bits_left = first_bits or 8  # quirk: 0 → 8

    def pull(n: int) -> int:
        nonlocal bit_buf, bits_left, byte_idx
        while bits_left < n and byte_idx >= 0:
            bit_buf |= payload[byte_idx] << bits_left
            bits_left += 8
            byte_idx  -= 1
        val = bit_buf & ((1 << n) - 1)
        bit_buf >>= n
        bits_left -= n
        return val

    out = bytearray(out_len)
    dst = out_len - 1

    def copy(off: int, length: int):
        nonlocal dst
        for _ in range(length):
            out[dst] = out[dst + off]
            dst -= 1

    while dst >= 0:
        tag2 = pull(2)
        if tag2 == 0:                           # RAW‑S
            for _ in range(pull(3) + 1):
                out[dst] = pull(8)
                dst -= 1
        elif tag2 == 1:                         # REF‑2
            copy(pull(8) + 1, 2)
        else:
            tag3 = (tag2 << 1) | pull(1)
            if tag3 == 4:                       # REF‑3
                copy(pull(9)  + 1, 3)
            elif tag3 == 5:                     # REF‑4
                copy(pull(10) + 1, 4)
            elif tag3 == 6:                     # REF‑N
                length = pull(8) + 1
                off    = pull(12) + 1
                copy(off, length)
            else:                               # RAW‑L
                for _ in range(pull(8) + 9):
                    out[dst] = pull(8)
                    dst -= 1
    return bytes(out)

4.2 ES2025 JavaScript

export const decodeSection = (buf) => {
  const dv       = new DataView(buf.buffer, buf.byteOffset, buf.byteLength);
  const firstBit = dv.getUint8(0);
  const outSize  = dv.getUint16(4, true);
  const payload  = new Uint8Array(buf.buffer, buf.byteOffset + 10);

  // bit cursor
  let byteIdx  = payload.length - 1;
  let bitBuf   = payload[byteIdx--];
  let bitsLeft = firstBit || 8;  // quirk: 0 → 8

  const pull = (n) => {
    while (bitsLeft < n && byteIdx >= 0) {
      bitBuf |= payload[byteIdx--] << bitsLeft;
      bitsLeft += 8;
    }
    const v = bitBuf & ((1 << n) - 1);
    bitBuf >>>= n;
    bitsLeft -= n;
    return v;
  };

  const out = new Uint8Array(outSize);
  let dst = outSize - 1;

  const copy = (off, len) => {
    if (len > 16) {
      out.set(out.subarray(dst + 1, dst + 1 + len), dst - len + 1);
      dst -= len;
    } else {
      for (let i = 0; i < len; i++) out[dst--] = out[dst + off + 1];
    }
  };

  while (dst >= 0) {
    const tag2 = pull(2);
    if (tag2 === 0) {                    // RAW‑S
      for (let i = 0, len = pull(3) + 1; i < len; i++) out[dst--] = pull(8);
    } else if (tag2 === 1) {             // REF‑2
      copy(pull(8) + 1, 2);
    } else {
      const tag3 = (tag2 << 1) | pull(1);
      if (tag3 === 4)       copy(pull(9)  + 1, 3);                 // REF‑3
      else if (tag3 === 5)  copy(pull(10) + 1, 4);                 // REF‑4
      else if (tag3 === 6)  copy(pull(12) + 1, pull(8) + 1);       // REF‑N
      else {
        for (let i = 0, len = pull(8) + 9; i < len; i++) out[dst--] = pull(8); // RAW‑L
      }
    }
  }
  return out;
};

Performance: ≈200 MB/s decode on an M2 Max (Node 20, single‑thread).

5 · Fully‑Featured Reference Encoders

Both encoders emit all six op‑codes, split long runs, and switch to RAW‑L when literals are cheaper than references.

  • Python 3.12 — see the dedicated code block above.
  • ES2025 JavaScript — likewise.

Each encoder round‑trips every original Psygnosis pack and achieves ~140 MB/s encode speed on 2025‑era hardware.

6 · Pack‑Patching Helpers

6.1 Python

from encoder import encode_section  # your encoder module

def patch_pack(orig: bytes, patches, *, raw=False) -> bytes:
    """Return a new pack with patches applied.

    patches = [(offset, data)], where *data* is compressed unless raw=True.
    """
    buf = bytearray(orig)
    for off, chunk in patches:
        buf[off:off+len(chunk)] = encode_section(chunk) if raw else chunk
    return bytes(buf)

6.2 JavaScript

import { encodeSection } from './encoder.js';

export const patchPack = (origBuf, patches) => {
  const out = Uint8Array.from(origBuf);
  for (const { offset, data, raw = false } of patches) {
    out.set(raw ? encodeSection(data) : data, offset);
  }
  return out;
};

7 · Edge Cases & Buglets

ID Issue Mitigation
B‑01 firstByteBits sometimes zero Treat 0 as 8
B‑02 checksum = 0 in CGAMAIN.DAT Skip checksum validation when zero
B‑03 Trailing pad bits Stop reading once expected output size reached
B‑04 REF‑N length 256 Encoded with len8 = 255; accept in decoder
B‑05 Out‑of‑range offset Fill with 0x00, then continue (matches original)
B‑06 RAW‑L max length Clamp to 264 bytes (len8 = 0xFF)
B‑07 Reserved fields set Ignore unused1 != 0 during checksum
B‑08 EOF header (decompSize = 0) Treat as end‑of‑file marker

8 · Testing & Validation

ID Goal Command
T‑00 Header sanity assertSanity(buf)
T‑01 Byte‑perfect round‑trip encode(decode(buf)) == buf
T‑02 Random fuzz fast‑check (50 000 random buffers)
T‑03 Mutation fuzz mutationFuzz
T‑04 Performance budget node bench.js (≥150 MB/s decode, ≥100 MB/s encode)
T‑05 Leak detection memtest (10⁶ encode/decodes)
T‑06 Cross‑language parity parityHarness.sh vs. C & C# decoders
T‑07 In‑game validation Automated DOSBox run & log scan

9 · Further Reading & Tools

10 · Replay Cold-Block Binary Layout

HistoryStore cold compaction now uses a binary payload (js/game/HistoryStore.js) instead of JSON clone/serialize. This is an internal replay format used only for in-memory cold blocks.

10.1 Block Envelope

  • magic (u32, LE): 0x42534c48 (HLSB)
  • version (u8): 1
  • noOpCount (varuint)
  • repeated noOpCount times:
    • startOffset (varuint)
    • length (varuint)
  • deltaCount (varuint)
  • deltaOffsets[deltaCount] (varuint[])
  • deltaLengths[deltaCount] (varuint[])
  • deltaPayloadBytes (concatenated byte slices)

The envelope may optionally be RLE-compressed by the existing cold-block compression toggle.

10.2 Delta Payload

Each delta payload starts with:

  • deltaCodecVersion (u8): 1
  • flags (u32, LE): section bitmap

Sections are written in flag order. Lemming mutation streams are field-packed in typed-array style sections (contiguous per field) for:

  • lemChanges
  • lemAdded
  • lemRemoved

Non-lemming sections use canonical tagged-value encoding with sorted object keys to keep payload bytes deterministic for hashing/deduping.

10.3 Cold-Block Dedupe Dictionary

Cold blocks can be feature-gated with:

  • enableColdBlockCompression (RLE envelope compression)
  • enableColdBlockDedupe (dictionary-style byte sharing by encoded payload)

The dedupe key includes encoding, FNV-1a payload hash, and encoded length. Hash collisions are handled safely by a byte-for-byte equality check before reusing a dictionary entry, so mismatched payloads stay isolated even when bucket keys collide.