[FFmpeg-devel] [PATCH] avcodec/lagarith: use VLC for probe code length

[Paul B Mahol <onemda@gmail.com>]

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Patch Attached.

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From cd774229dcf32cb44455031663b4357f0597b4c0 Mon Sep 17 00:00:00 2001
From: Paul B Mahol <onemda@gmail.com>
Date: Tue, 12 Sep 2023 01:29:55 +0200
Subject: [PATCH] avcodec/lagarith: use VLC for prob code length

Signed-off-by: Paul B Mahol <onemda@gmail.com>
---
 libavcodec/lagarith.c | 57 ++++++++++++++++++++++++++++++-------------
 1 file changed, 40 insertions(+), 17 deletions(-)

diff --git a/libavcodec/lagarith.c b/libavcodec/lagarith.c
index ebc1f7613a..31cad907c0 100644
--- a/libavcodec/lagarith.c
+++ b/libavcodec/lagarith.c
@@ -27,6 +27,9 @@
 
 #include <inttypes.h>
 
+#include "libavutil/mem_internal.h"
+#include "libavutil/thread.h"
+
 #include "avcodec.h"
 #include "codec_internal.h"
 #include "get_bits.h"
@@ -35,6 +38,8 @@
 #include "lossless_videodsp.h"
 #include "thread.h"
 
+#define VLC_BITS 11
+
 enum LagarithFrameType {
     FRAME_RAW           = 1,    /**< uncompressed */
     FRAME_U_RGB24       = 2,    /**< unaligned RGB24 */
@@ -56,6 +61,35 @@ typedef struct LagarithContext {
     int zeros_rem;              /**< number of zero bytes remaining to output */
 } LagarithContext;
 
+static VLC lag_tab;
+
+static const uint8_t lag_bits[] = {
+    7, 7, 7, 2, 7, 3, 4, 5, 6, 7, 7, 7, 7, 7, 6, 7, 4, 5, 7, 7, 7, 7,
+    5, 6, 7, 7, 7, 7, 7, 7, 6, 7, 7, 7, 7, 7, 6, 7, 7, 7, 7, 7, 7, 7,
+    7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+};
+
+static const uint8_t lag_codes[] = {
+    0x00, 0x01, 0x02, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x04, 0x05,
+    0x08, 0x09, 0x0A, 0x0B, 0x0B, 0x0B, 0x0B, 0x10, 0x11, 0x12, 0x13,
+    0x13, 0x13, 0x14, 0x15, 0x20, 0x21, 0x22, 0x23, 0x23, 0x24, 0x25,
+    0x28, 0x29, 0x2A, 0x2B, 0x2B, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45,
+    0x48, 0x49, 0x4A, 0x4B, 0x50, 0x51, 0x52, 0x53, 0x54, 0x55,
+};
+
+static const uint8_t lag_symbols[] = {
+    -1, 20, 12, 0, 12, 1, 2, 4, 7, 7, 28, 4, 25, 17,
+    10, 17, 3, 6, 2, 23, 15, 15, 5, 9, 10, 31, 1, 22,
+    14, 14, 8, 9, 30, 6, 27, 19, 11, 19, 0, 21, 13, 13,
+    8, 29, 5, 26, 18, 18, 3, 24, 16, 16, 11, 32,
+};
+
+static av_cold void lag_init_static_data(void)
+{
+    VLC_INIT_SPARSE_STATIC(&lag_tab, VLC_BITS, FF_ARRAY_ELEMS(lag_bits),
+                           lag_bits, 1, 1, lag_codes, 1, 1, lag_symbols, 1, 1, 2048);
+}
+
 /**
  * Compute the 52-bit mantissa of 1/(double)denom.
  * This crazy format uses floats in an entropy coder and we have to match x86
@@ -101,23 +135,10 @@ static uint8_t lag_calc_zero_run(int8_t x)
 
 static int lag_decode_prob(GetBitContext *gb, uint32_t *value)
 {
-    static const uint8_t series[] = { 1, 2, 3, 5, 8, 13, 21 };
-    int i;
-    int bit     = 0;
-    int bits    = 0;
-    int prevbit = 0;
-    unsigned val;
-
-    for (i = 0; i < 7; i++) {
-        if (prevbit && bit)
-            break;
-        prevbit = bit;
-        bit = get_bits1(gb);
-        if (bit && !prevbit)
-            bits += series[i];
-    }
-    bits--;
-    if (bits < 0 || bits > 31) {
+    unsigned val, bits;
+
+    bits = get_vlc2(gb, lag_tab.table, VLC_BITS, 3);
+    if (bits > 31) {
         *value = 0;
         return AVERROR_INVALIDDATA;
     } else if (bits == 0) {
@@ -720,10 +741,12 @@ static int lag_decode_frame(AVCodecContext *avctx, AVFrame *p,
 
 static av_cold int lag_decode_init(AVCodecContext *avctx)
 {
+    static AVOnce init_static_once = AV_ONCE_INIT;
     LagarithContext *l = avctx->priv_data;
     l->avctx = avctx;
 
     ff_llviddsp_init(&l->llviddsp);
+    ff_thread_once(&init_static_once, lag_init_static_data);
 
     return 0;
 }
-- 
2.39.1


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