# FFmpeg source snapshot

Verbatim subset of FFmpeg source pinned for use as reference
implementations of the VP9 8×8 inverse DCT (Phase 1 target of
`daedalus-fourier`). See `../../docs/phase2.md §2` and `§5` for
the rationale.

## Upstream pin

- **Repository**: https://github.com/FFmpeg/FFmpeg
- **Tag**: `n7.1.3` (matches `libavcodec61 8:7.1.3-0+deb13u1+rpt1`
  shipping in Debian Trixie on the dev host `hertz`)
- **Annotated tag object**: `0a9a757e96fdf053697084bbd1f620edeac9d084`
- **Commit object (tag target)**: `f46e514491172d15bd74b4abb1814cd2f05a763e`
- **Snapshot fetched**: 2026-05-18 (UTC), via
  `https://raw.githubusercontent.com/FFmpeg/FFmpeg/n7.1.3/<path>`

## Files in this snapshot

All files are byte-for-byte copies of the upstream source at the
tagged commit, no modifications.

| Path | Lines | Bytes | SHA-256 |
|---|---|---|---|
| `libavcodec/vp9dsp_template.c` | 2578 | 89045 | `41b21f667a6c497b620aa1637d8269badc45d1ac7e621d694441c5bf39356e4f` |
| `libavcodec/aarch64/vp9itxfm_neon.S` | 1580 | 63534 | `82ee3ceed4735c63576bafdcee28e2215652743ade55a9eab46a16d9530369f6` |
| `libavcodec/aarch64/vp9lpf_neon.S` | 1334 | — | `384e49e7a6e838d9e38aedc00838ed4aebfa6c5bdb343ecaf23ef639bc10fbb7` |
| `libavcodec/aarch64/vp9mc_neon.S` | 665 | — | `6b1d50f9821742584fdd47758057f810644aff3a008faaa774ff5b9cac4d1fef` |
| `libavcodec/aarch64/h264idct_neon.S` | 415 | 16269 | `963ffe5f31b5a6a422e13b0d394cf5630126927abfb23aa214f7cbe83d60683f` — H.264 IDCT 4×4/8×8/DC NEON kernels for cycle 6+ |
| `libavcodec/aarch64/h264dsp_neon.S` | 1076 | — | `978e076f0020e688b40c6dd827708c3d53e17c64a99fd0052e43d983536ce638` — H.264 in-loop deblock + weight/biweight kernels for cycle 8+ |
| `libavcodec/vp9_subpel_filters_table.c` | — | — | hand-extracted from `libavcodec/vp9dsp.c` at same n7.1.3 pin — provides `ff_vp9_subpel_filters` for `vp9mc_neon.S` to link against without dragging in vp9dsp.c's full init machinery |
| `libavcodec/aarch64/neon.S` | 173 | 7496 | `72d36ce6c3fcc5e53de869cfe10fda16225ebe580c32891bccc240a30a85a538` |
| `libavutil/aarch64/asm.S` | 260 | 8069 | `c0d03143b1bc5a9e358222d08d2d449d595271844fe7a3dc23bffb91abe8b0e3` |
| `COPYING.LGPLv2.1` | 502 | — | `b634ab5640e258563c536e658cad87080553df6f34f62269a21d554844e58bfe` |

Verify with:

```sh
( cd external/ffmpeg-snapshot && sha256sum -c <<'EOF'
41b21f667a6c497b620aa1637d8269badc45d1ac7e621d694441c5bf39356e4f  libavcodec/vp9dsp_template.c
82ee3ceed4735c63576bafdcee28e2215652743ade55a9eab46a16d9530369f6  libavcodec/aarch64/vp9itxfm_neon.S
72d36ce6c3fcc5e53de869cfe10fda16225ebe580c32891bccc240a30a85a538  libavcodec/aarch64/neon.S
c0d03143b1bc5a9e358222d08d2d449d595271844fe7a3dc23bffb91abe8b0e3  libavutil/aarch64/asm.S
b634ab5640e258563c536e658cad87080553df6f34f62269a21d554844e58bfe  COPYING.LGPLv2.1
EOF
)
```

## License

LGPL-2.1-or-later. See `COPYING.LGPLv2.1`. Original copyright
holders include the FFmpeg authors and Google Inc. (2016) for
the aarch64 NEON paths. The snapshot inherits FFmpeg's license
in full.

## Why each file is in this snapshot

- `libavcodec/vp9dsp_template.c` — contains `idct_idct_8x8_add_c`,
  the bit-exact C reference for the Phase 1 kernel under test (M1).
- `libavcodec/aarch64/vp9itxfm_neon.S` — contains
  `ff_vp9_idct_idct_8x8_add_neon`, the NEON throughput baseline
  (M3). Also defines `idct8`, `dmbutterfly0`, `dmbutterfly`,
  `dmbutterfly_l`, `butterfly_8h`, and the `idct_coeffs` constant
  table.
- `libavcodec/aarch64/neon.S` — defines `transpose_8x8H` used by
  `vp9itxfm_neon.S`.
- `libavutil/aarch64/asm.S` — defines `function`, `endfunc`,
  `movrel`, `const`, `endconst`, and other assembly preamble
  macros required to assemble the above NEON files.

## Re-vendoring procedure

If the upstream pin needs to change (e.g., hertz updates to a
newer libavcodec):

```sh
TAG=nX.Y.Z
BASE=https://raw.githubusercontent.com/FFmpeg/FFmpeg/$TAG
cd external/ffmpeg-snapshot
for f in libavcodec/vp9dsp_template.c \
         libavcodec/aarch64/vp9itxfm_neon.S \
         libavcodec/aarch64/neon.S \
         libavutil/aarch64/asm.S \
         COPYING.LGPLv2.1; do
  curl -sSf -o "$f" "$BASE/$f"
done
sha256sum libavcodec/vp9dsp_template.c \
          libavcodec/aarch64/vp9itxfm_neon.S \
          libavcodec/aarch64/neon.S \
          libavutil/aarch64/asm.S \
          COPYING.LGPLv2.1
# update this PROVENANCE.md with the new tag, commit hash, and hashes
```

After re-vendoring, re-run the bit-exact gate (M1) and throughput
baseline (M3) — both can shift across FFmpeg versions even when
the VP9 spec doesn't change (e.g., NEON micro-optimizations).