Reduce the size of av1_prob_cost[]

Only half of it was necessary.

Change-Id: I0b5fc9ae6a17f5d812e10ee903a12f23f1377d8e

av1/encoder/cost.c

@@ -13,29 +13,17 @@
 #include "av1/encoder/cost.h"
 #include "av1/common/entropy.h"
 
-/* round(-log2(i/256.) * (1 << AV1_PROB_COST_SHIFT))
-   Begins with a bogus entry for simpler addressing. */
-const uint16_t av1_prob_cost[256] = {
-  4096, 4096, 3584, 3284, 3072, 2907, 2772, 2659, 2560, 2473, 2395, 2325, 2260,
-  2201, 2147, 2096, 2048, 2003, 1961, 1921, 1883, 1847, 1813, 1780, 1748, 1718,
-  1689, 1661, 1635, 1609, 1584, 1559, 1536, 1513, 1491, 1470, 1449, 1429, 1409,
-  1390, 1371, 1353, 1335, 1318, 1301, 1284, 1268, 1252, 1236, 1221, 1206, 1192,
-  1177, 1163, 1149, 1136, 1123, 1110, 1097, 1084, 1072, 1059, 1047, 1036, 1024,
-  1013, 1001, 990,  979,  968,  958,  947,  937,  927,  917,  907,  897,  887,
-  878,  868,  859,  850,  841,  832,  823,  814,  806,  797,  789,  780,  772,
-  764,  756,  748,  740,  732,  724,  717,  709,  702,  694,  687,  680,  673,
-  665,  658,  651,  644,  637,  631,  624,  617,  611,  604,  598,  591,  585,
-  578,  572,  566,  560,  554,  547,  541,  535,  530,  524,  518,  512,  506,
-  501,  495,  489,  484,  478,  473,  467,  462,  456,  451,  446,  441,  435,
-  430,  425,  420,  415,  410,  405,  400,  395,  390,  385,  380,  375,  371,
-  366,  361,  356,  352,  347,  343,  338,  333,  329,  324,  320,  316,  311,
-  307,  302,  298,  294,  289,  285,  281,  277,  273,  268,  264,  260,  256,
-  252,  248,  244,  240,  236,  232,  228,  224,  220,  216,  212,  209,  205,
-  201,  197,  194,  190,  186,  182,  179,  175,  171,  168,  164,  161,  157,
-  153,  150,  146,  143,  139,  136,  132,  129,  125,  122,  119,  115,  112,
-  109,  105,  102,  99,   95,   92,   89,   86,   82,   79,   76,   73,   70,
-  66,   63,   60,   57,   54,   51,   48,   45,   42,   38,   35,   32,   29,
-  26,   23,   20,   18,   15,   12,   9,    6,    3
+// round(-log2(i/256.) * (1 << AV1_PROB_COST_SHIFT)); i = 128~255.
+const uint16_t av1_prob_cost[128] = {
+  512, 506, 501, 495, 489, 484, 478, 473, 467, 462, 456, 451, 446, 441, 435,
+  430, 425, 420, 415, 410, 405, 400, 395, 390, 385, 380, 375, 371, 366, 361,
+  356, 352, 347, 343, 338, 333, 329, 324, 320, 316, 311, 307, 302, 298, 294,
+  289, 285, 281, 277, 273, 268, 264, 260, 256, 252, 248, 244, 240, 236, 232,
+  228, 224, 220, 216, 212, 209, 205, 201, 197, 194, 190, 186, 182, 179, 175,
+  171, 168, 164, 161, 157, 153, 150, 146, 143, 139, 136, 132, 129, 125, 122,
+  119, 115, 112, 109, 105, 102, 99,  95,  92,  89,  86,  82,  79,  76,  73,
+  70,  66,  63,  60,  57,  54,  51,  48,  45,  42,  38,  35,  32,  29,  26,
+  23,  20,  18,  15,  12,  9,   6,   3,
 };
 
 void av1_cost_tokens_from_cdf(int *costs, const aom_cdf_prob *cdf,

av1/encoder/cost.h

@@ -19,7 +19,7 @@
 extern "C" {
 #endif
 
-extern const uint16_t av1_prob_cost[256];
+extern const uint16_t av1_prob_cost[128];
 
 // The factor to scale from cost in bits to cost in av1_prob_cost units.
 #define AV1_PROB_COST_SHIFT 9
@@ -32,8 +32,9 @@ extern const uint16_t av1_prob_cost[256];
 static INLINE int av1_cost_symbol(aom_cdf_prob p15) {
   assert(0 < p15 && p15 < CDF_PROB_TOP);
   const int shift = CDF_PROB_BITS - 1 - get_msb(p15);
-  return av1_prob_cost[get_prob(p15 << shift, CDF_PROB_TOP)] +
-         av1_cost_literal(shift);
+  const int prob = get_prob(p15 << shift, CDF_PROB_TOP);
+  assert(prob >= 128);
+  return av1_prob_cost[prob - 128] + av1_cost_literal(shift);
 }
 
 void av1_cost_tokens_from_cdf(int *costs, const aom_cdf_prob *cdf,