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);

}
