h264: expose chroma DC 2x2 Hadamard as public API

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).
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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+/* SPDX-License-Identifier: BSD-2-Clause */
+/*
+ * H.264 chroma DC 2x2 Hadamard pre-pass (public, in-tree CPU).
+ *
+ * The 4 DC coefficients of an MB's chroma 4x4 AC blocks go through
+ * this 2x2 Hadamard before quant-scaling and re-injection into the
+ * AC blocks' [0,0] coefficient.  Algorithm per H.264 §8.5.11.1.
+ *
+ * Pure CPU primitive — there's 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 and shouldn't be baked into the kernel).
+ *
+ * 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).  Same algorithm; this is the public
+ * src-tree copy.
+ */
+#include "daedalus.h"
+
+#include <stdint.h>
+
+void daedalus_h264_chroma_dc_hadamard_2x2(int16_t c[4])
+{
+    int t0 = c[0] + c[1];
+    int t1 = c[0] - c[1];
+    int t2 = c[2] + c[3];
+    int t3 = c[2] - c[3];
+
+    c[0] = (int16_t)(t0 + t2);   /* f[0,0] = sum of all 4   */
+    c[1] = (int16_t)(t1 + t3);   /* f[0,1] = col-difference */
+    c[2] = (int16_t)(t0 - t2);   /* f[1,0] = row-difference */
+    c[3] = (int16_t)(t1 - t3);   /* f[1,1] = anti-diagonal  */
+}