ref: 790dd3538cf7d06bbd3112f3299b6a7c438bb2b4
dir: /src/msac.c/
/* * Copyright © 2018, VideoLAN and dav1d authors * Copyright © 2018, Two Orioles, LLC * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are met: * * 1. Redistributions of source code must retain the above copyright notice, this * list of conditions and the following disclaimer. * * 2. Redistributions in binary form must reproduce the above copyright notice, * this list of conditions and the following disclaimer in the documentation * and/or other materials provided with the distribution. * * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */ #include "config.h" #include <assert.h> #include <limits.h> #include "common/intops.h" #include "src/msac.h" #define EC_MIN_PROB 4 // must be <= (1<<EC_PROB_SHIFT)/16 #define EC_WIN_SIZE (sizeof(ec_win) << 3) static inline void ctx_refill(MsacContext *s) { const uint8_t *buf_pos = s->buf_pos; const uint8_t *buf_end = s->buf_end; int c = EC_WIN_SIZE - s->cnt - 24; ec_win dif = s->dif; while (c >= 0 && buf_pos < buf_end) { dif ^= ((ec_win)*buf_pos++) << c; c -= 8; } s->dif = dif; s->cnt = EC_WIN_SIZE - c - 24; s->buf_pos = buf_pos; } /* Takes updated dif and range values, renormalizes them so that * 32768 <= rng < 65536 (reading more bytes from the stream into dif if * necessary), and stores them back in the decoder context. * dif: The new value of dif. * rng: The new value of the range. */ static inline void ctx_norm(MsacContext *s, ec_win dif, uint32_t rng) { const uint16_t d = 15 - (31 ^ clz(rng)); assert(rng <= 65535U); s->cnt -= d; s->dif = ((dif + 1) << d) - 1; /* Shift in 1s in the LSBs */ s->rng = rng << d; if (s->cnt < 0) ctx_refill(s); } /* Decodes a symbol given an inverse cumulative distribution function (CDF) * table in Q15. */ unsigned msac_decode_symbol(MsacContext *const s, const uint16_t *const cdf, const unsigned n_symbols) { ec_win u, v = s->rng, r = s->rng >> 8; const ec_win c = s->dif >> (EC_WIN_SIZE - 16); unsigned ret = 0; assert(!cdf[n_symbols - 1]); do { u = v; v = r * (cdf[ret++] >> EC_PROB_SHIFT); v >>= 7 - EC_PROB_SHIFT; v += EC_MIN_PROB * (n_symbols - ret); } while (c < v); assert(u <= s->rng); ctx_norm(s, s->dif - (v << (EC_WIN_SIZE - 16)), u - v); return ret - 1; } /* Decode a single binary value. * f: The probability that the bit is one * Return: The value decoded (0 or 1). */ unsigned msac_decode_bool(MsacContext *const s, const unsigned f) { ec_win v, vw, dif = s->dif; uint16_t r = s->rng; unsigned ret; assert((dif >> (EC_WIN_SIZE - 16)) < r); v = ((r >> 8) * f >> (7 - EC_PROB_SHIFT)) + EC_MIN_PROB; vw = v << (EC_WIN_SIZE - 16); ret = dif >= vw; dif -= ret*vw; v += ret*(r - 2*v); ctx_norm(s, dif, v); return !ret; } unsigned msac_decode_bools(MsacContext *const c, const unsigned l) { int v = 0; for (int n = (int) l - 1; n >= 0; n--) v = (v << 1) | msac_decode_bool(c, EC_BOOL_EPROB); return v; } int msac_decode_subexp(MsacContext *const c, const int ref, const int n, const unsigned k) { int i = 0; int a = 0; int b = k; while ((2 << b) < n) { if (!msac_decode_bool(c, EC_BOOL_EPROB)) break; b = k + i++; a = (1 << b); } const unsigned v = msac_decode_bools(c, b) + a; return ref * 2 <= n ? inv_recenter(ref, v) : n - 1 - inv_recenter(n - 1 - ref, v); } int msac_decode_uniform(MsacContext *const c, const unsigned n) { assert(n > 0); const int l = ulog2(n) + 1; assert(l > 1); const unsigned m = (1 << l) - n; const unsigned v = msac_decode_bools(c, l - 1); return v < m ? v : (v << 1) - m + msac_decode_bool(c, EC_BOOL_EPROB); } static void update_cdf(uint16_t *const cdf, const unsigned val, const unsigned n_symbols) { const unsigned count = cdf[n_symbols]; const int rate = ((count >> 4) | 4) + (n_symbols > 3); unsigned i; for (i = 0; i < val; i++) cdf[i] += (32768 - cdf[i]) >> rate; for (; i < n_symbols - 1; i++) cdf[i] -= cdf[i] >> rate; cdf[n_symbols] = count + (count < 32); } unsigned msac_decode_symbol_adapt(MsacContext *const c, uint16_t *const cdf, const unsigned n_symbols) { const unsigned val = msac_decode_symbol(c, cdf, n_symbols); if(c->allow_update_cdf) update_cdf(cdf, val, n_symbols); return val; } unsigned msac_decode_bool_adapt(MsacContext *const c, uint16_t *const cdf) { const unsigned bit = msac_decode_bool(c, *cdf >> EC_PROB_SHIFT); if(c->allow_update_cdf){ // update_cdf() specialized for boolean CDFs const unsigned count = cdf[1]; const int rate = (count >> 4) | 4; if (bit) { cdf[0] += (32768 - cdf[0]) >> rate; } else { cdf[0] -= cdf[0] >> rate; } cdf[1] = count + (count < 32); } return bit; } void msac_init(MsacContext *const s, const uint8_t *const data, const size_t sz, const int disable_cdf_update_flag) { s->buf_pos = data; s->buf_end = data + sz; s->dif = ((ec_win)1 << (EC_WIN_SIZE - 1)) - 1; s->rng = 0x8000; s->cnt = -15; s->allow_update_cdf = !disable_cdf_update_flag; ctx_refill(s); }