halkyon-days/pure-zstd
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Licence under MIT, and name what this was built from

Browse source 
MIT rather than a dual with BSD-3. Dual licensing solves a
compatibility problem this does not have, since both are permissive and
compatible with everything, so the choice would cost a downstream reader
a decision and buy them nothing.

The debt to zstd is real and a licence is the wrong instrument for it.
No code came from libzstd; the format came from RFC 8878 and every
answer is checked against libzstd through pyzstd. That belongs in prose
where people read it.

Also drops the line calling this a feasibility spike. It has 103 tests,
an audit against the RFC, and five defects found and closed.
This commit is contained in:
Tag 2026-09-21 23:41:31 -06:00
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LICENSE Normal file
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@ -0,0 +1,21 @@
MIT License
Copyright (c) 2026 Tag Spilman
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

38
README.md
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@ -7,9 +7,9 @@ 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 carry a native library per platform to do it. Decompression only. There is no
compressor and there is no plan for one. compressor and there is no plan for one.
This is a feasibility spike, not a finished library. It decodes the real data It decodes the real data it was written for, correctly and completely, and it
it was written for, correctly and completely, and it is roughly four times is roughly four times slower than native zstd. See **Speed** below before
slower than native zstd. See **Speed** below before depending on it. depending on it.
## Using it ## Using it
@ -142,7 +142,37 @@ Run it yourself:
## Licence ## Licence
Not chosen yet. MIT. See [LICENSE](LICENSE).
## Acknowledgements
This is an independent implementation. It contains no code from libzstd, and
nothing about its licence reaches this package. It would not exist without the
following, and they are named here because that is where the debt actually is.
**[RFC 8878](https://www.rfc-editor.org/rfc/rfc8878.html), "Zstandard
Compression and the application/zstd Media Type"**, by Yann Collet and Murray
Kucherawy, is the specification this was written from. Every table in
`lib/src/tables.dart` was checked against section 3.1.1.3.2 entry by entry, and
the FSE and Huffman decoders follow the reference algorithms it describes. A
format specification written clearly enough to implement from is a gift, and
this one is.
**[Zstandard](https://github.com/facebook/zstd)**, by Yann Collet and
contributors at Meta, is the format and the reference implementation. Its
licence is BSD-3-Clause, and it obliges nothing here; it is named because
without it there would be nothing to decode.
**[pyzstd](https://pypi.org/project/pyzstd/)** wraps libzstd and is the oracle
every test in this package is measured against. Correctness here means
"agrees with libzstd, byte for byte", and pyzstd is how that is asked. The
committed fixtures under `test/fixtures/corpus/` are output of the `zstd` CLI
and of pyzstd, which is tool output rather than anything derived from their
source.
**The Dart team**, for making `int` an exact 64-bit integer on the VM and on
AOT. The bit reader carries 56 bits in one and XXH64's multiplies wrap exactly,
neither of which would be true on a platform with doubles underneath.
## Measured on the target device, 2026-09-21 ## Measured on the target device, 2026-09-21