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h264: expose chroma DC 2x2 Hadamard as public API

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PR #23 added the Hadamard as a test-only spec reference; this PR
promotes it to a public symbol in src/ so consumers (the eventual
marfrit-packages substitution-arc patch 0011) can link against it.

  New: void daedalus_h264_chroma_dc_hadamard_2x2(int16_t c[4]);
       — operates in-place on 4 int16, no QP-dependent scaling
       (caller composes that themselves per §8.5.11.2).

The src/ implementation is byte-for-byte identical to the test-only
ref in tests/h264_chroma_dc_hadamard_ref.c (kept as a separate
spec-validation copy).  A new "public API parity" test case verifies
the two produce identical output for a non-trivial input.

Pure CPU primitive — no substrate-dispatch wrapper because the work
is 4 adds + 4 subs; the substrate machinery would cost more than
the kernel itself.

Verified on hertz:

  $ ./build/test_chroma_dc_hadamard
    all-uniform 5                    PASS
    col gradient [0,10,0,10]         PASS
    row gradient [0,0,10,10]         PASS
    anti-diagonal [10,0,0,10]        PASS
    asymmetric [1,2,3,4]             PASS
    sign-alternating [-5,5,-5,5]     PASS
    double-Hadamard = 4*orig         PASS
    public API parity vs _ref        PASS

  ALL chroma DC Hadamard tests PASS

  $ nm -g build/libdaedalus_core.a | grep chroma_dc_hadamard
  0000000000000000 T daedalus_h264_chroma_dc_hadamard_2x2

Unblocks marfrit-packages 0011 (substituting
H264DSPContext.chroma_dc_dequant_idct, which composes the Hadamard
+ qmul scaling).
This commit is contained in:
Claude (noether)
2026-05-25 13:32:01 +02:00
parent b21b35c74b
commit 1f07f3cd70
4 changed files with 71 additions and 0 deletions
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include/daedalus.h
+15
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@@ -544,6 +544,21 @@ DECLARE_QPEL_AVG(avg_mc33)
#undef DECLARE_QPEL_AVG
/* -------------------------------------------------------------------
* H.264 chroma DC 2x2 Hadamard pre-pass (per H.264 §8.5.11.1).
*
* Operates in-place on 4 int16 (the DC coefficients of an MB's
* chroma 4x4 AC blocks). Pure CPU primitive — no substrate
* dispatch wrapper because the work is 4 adds + 4 subs. Callers
* compose with QP-dependent scaling themselves; the scale shape
* varies by slice/PPS chroma_qp offset context.
*
* Bit-exact validated against tests/h264_chroma_dc_hadamard_ref.c
* (7-case spec-derived test suite including the H·H = 4·I algebraic
* invariant; see PR #23).
* ----------------------------------------------------------------- */
void daedalus_h264_chroma_dc_hadamard_2x2(int16_t c[4]);
/* -------------------------------------------------------------------
* Recipe query — what does the API recommend for each kernel?
* ----------------------------------------------------------------- */