static int update_prob(VP56RangeCoder *c, int p) { static const int inv_map_table[254] = { 7, 20, 33, 46, 59, 72, 85, 98, 111, 124, 137, 150, 163, 176, 189, 202, 215, 228, 241, 254, 1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, }; int d; /* This code is trying to do a differential probability update. For a * current probability A in the range [1, 255], the difference to a new * probability of any value can be expressed differentially as 1-A,255-A * where some part of this (absolute range) exists both in positive as * well as the negative part, whereas another part only exists in one * half. We're trying to code this shared part differentially, i.e. * times two where the value of the lowest bit specifies the sign, and * the single part is then coded on top of this. This absolute difference * then again has a value of [0,254], but a bigger value in this range * indicates that we're further away from the original value A, so we * can code this as a VLC code, since higher values are increasingly * unlikely. The first 20 values in inv_map_table[] allow 'cheap, rough' * updates vs. the 'fine, exact' updates further down the range, which * adds one extra dimension to this differential update model. */ if (!vp8_rac_get(c)) { d = vp8_rac_get_uint(c, 4) + 0; } else if (!vp8_rac_get(c)) { d = vp8_rac_get_uint(c, 4) + 16; } else if (!vp8_rac_get(c)) { d = vp8_rac_get_uint(c, 5) + 32; } else { d = vp8_rac_get_uint(c, 7); if (d >= 65) d = (d << 1) - 65 + vp8_rac_get(c); d += 64; } return p <= 128 ? 1 + inv_recenter_nonneg(inv_map_table[d], p - 1) : 255 - inv_recenter_nonneg(inv_map_table[d], 255 - p); }