pure_zstd

A zstd decompressor in plain Dart, with no native dependency.

It exists because every zstd binding on pub.dev is a wrapper over libzstd, and an app that reads block-compressed elevation tiles on a phone would rather not carry a native library per platform to do it. Decompression only. There is no compressor and there is no plan for one.

This is a feasibility spike, not a finished library. It decodes the real data it was written for, correctly and completely, and it is roughly four times slower than native zstd. See Speed below before depending on it.

Using it

import 'package:pure_zstd/pure_zstd.dart';

final plain = zstdDecode(frame);

// Decoding many frames: hold the decoder, which keeps its entropy tables and
// scratch buffers between calls and then allocates only the output.
final decoder = ZstdDecoder();
for (final frame in frames) {
  use(decoder.decode(frame));
}

A frame that declares no content size needs expectedSize, because the output is allocated whole rather than grown.

What it implements

RFC 8878 is the specification. Of it:

Part of the format	State
Frame header, single segment and windowed	Decoded. The window descriptor is skipped, since the output is allocated from the content size.
Raw, RLE and compressed blocks	All three.
Multi-block frames	Yes, including a later block reusing an earlier one's tables and recent offsets.
Raw, RLE, compressed and treeless literals	All four, in one-stream and four-stream form.
Huffman weights	Both the direct nibble form and the FSE-coded form.
Sequence tables	Predefined, RLE, FSE-compressed and repeat, for all three of literal length, match length and offset.
Repeated offsets	Yes, including the no-literals shift.
Content checksum	Verified. XXH64 over the output, compared with the stored low 32 bits. Hashed only when the frame carries one.
Dictionaries	Refused.
Skippable frames, several frames in one buffer	Refused. Reading one frame is the whole API, and input left over after it is an error.
Streaming, or decoding part of a frame	Not offered.

It targets native Dart and not the web: the literals header reads a five-byte field with a 32-bit shift, and the bit reader carries 56 bits in an int. Both are exact on the VM and on AOT, and neither is on JavaScript.

Defects found and closed, 2026-09-21

Two audits, each finding with a reproducer. All five are fixed.

One decision is still open, and it is not a defect in this package: hgtz.py writes no checksum, so nothing in a .hgtz is covered by the verification below. Making it write one costs four bytes a block and a re-transcode of every tile. It is narrower than it sounds, because the downloader verifies a SHA-256 over the whole container before it keeps the download, so only corruption at rest on the phone is uncovered.

1. Frame_Content_Size sized the output allocation with no ceiling. A 16-byte frame can ask for 4 EiB. One bit flipped in the frame descriptor of a real tile gives RangeError or OutOfMemoryError, and neither is catchable as ZstdException, so the isolate dies. This is what the skipped Window_Descriptor was for: RFC 8878 §3.1.1.1.2 allows a decoder to refuse a frame asking for more memory than it will give. Require the declared size to be under a stated ceiling, and have the .hgtz reader pass the exact height * width * 2 it expects. ZstdDecoder now takes a maxOutputSize, 64 MiB by default, and a caller passing expectedSize gets an exact check against the frame's own figure instead.
2. fseDecodeInterleaved wrote three symbols in a pass that checked room for two, so a crafted Huffman weight stream indexed _weights[256]. The reproducer is 53 bytes, and random mutation of a real tile found it independently.
3. Several frames in one buffer decoded to the first and returned silently. Nothing compared the cursor with the input length, so a second frame, a trailing skippable frame and four bytes of junk were all accepted and ignored. A frame claiming a checksum could also supply none, because the four bytes were stepped over rather than read.
4. RLE blocks skipped the block-size ceiling, so 97 bytes of input returned 44 MB of output that a caller cannot tell from a good tile.
5. The content checksum was parsed and never verified. Over 600,000 mutations of real block data, 34% decoded into wrong bytes with no error raised, because a damaged match or literal length usually still produces plausible output of the right length. Every other guard catches a frame that cannot be parsed; this is the only one that catches a frame that parses and is wrong.

Over-permissive against libzstd, with no wrong output, so lower priority: a compressed block over blockSizeMax is accepted, the Huffman weight table is read with maxSymbol 255 where libzstd uses 12, and Regenerated_Size is capped at a flat 128 KiB rather than min(Window_Size, 128 KiB).

Correctness

The reference is pyzstd, which wraps libzstd. tool/oracle.py extracts every block of a .hgtz container as its own frame, decompresses it with pyzstd, hands it to tool/verify.dart, and compares byte for byte.

./tool/oracle.py path/to/srtm1z/N39W105.hgtz
./tool/oracle.py --every 1700 path/to/srtm1z path/to/srtm3z

As of 2026-09-21 that is 32 real containers, 2,615 blocks and 0.3 GB of output, all identical, spanning both the one and three arc-second sets, every thinned column count from 3601 down to 361, and latitudes from 88 S to 80 N.

dart test needs no Python. It decodes the five blocks of the container in test/fixtures/, whose every sample is a known function of its position, so a wrong decode cannot pass.

Speed

Measured 2026-09-21 on this eight-core Linux box, dart compile exe, Dart 3.12.2. One 256 by 256 block is 131,072 bytes out. The frames are read into memory first, so only decoding is timed, and the figure includes allocating the output buffer because the API allocates it.

Block set	pure_zstd, AOT	pyzstd (libzstd)
Colorado, N39W105, 196 blocks, median	0.66 ms	0.18 ms
Alaska, N60W160, 98 blocks, median	0.94 ms	not measured
One block, repeated 1000 times	0.85 ms	0.22 ms
113 blocks, a 360 degree sweep	67 ms	19 ms

That is 140 to 200 MB/s of output. The JIT is about 25% slower than AOT, so measure with dart compile exe.

The cost is almost all in the sequence loop rather than in Huffman literals. A block of real terrain carries around 1,700 literals and 14,400 sequences, so the hot path is three FSE state updates, six bit reads and a nine-byte match copy, about 46 ns per sequence against native's dozen or so. The headroom left is bounds checks and the byte-at-a-time match copy; widening that copy to 64-bit words, the way libzstd does, is the next thing to try.

Run it yourself:

dart compile exe benchmark/bench.dart -o /tmp/bench && /tmp/bench

Licence

Not chosen yet.

Measured on the target device, 2026-09-21

Sixteen real 256x256 blocks lifted out of path/to/srtm1z/N39W105.hgtz, 131,072 bytes of output each, the same blocks in every row so the numbers are comparable. Median of 960 timed calls after 320 warm-up calls.

where	mode	median	MB/s	113-block sweep
Pixel 10 Pro XL	profile, AOT	0.414 ms	302	47 ms
Pixel 10 Pro XL	debug, JIT	0.540 ms	231	61 ms
this laptop	AOT	0.917 ms	136	104 ms
this laptop	JIT	1.125 ms	111	127 ms

The phone is about twice as fast as the laptop at this, which is why the desktop figures in the commit message read worse than reality.

Native, for scale: pyzstd on the laptop is 0.175 ms on comparable blocks. That is a Python C extension rather than Dart FFI, so take it as an indication. No native-on-phone figure was taken.

Do not benchmark against test/fixtures/N51E000.hgtz. Its samples are a synthetic pattern that compresses to almost no sequences, and decode cost tracks sequence count, so it reads about six times too fast. That mistake was made once already.