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+/*
+ * Copyright (c) 2001-2016, Alliance for Open Media. All rights reserved
+ *
+ * This source code is subject to the terms of the BSD 2 Clause License and
+ * the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
+ * was not distributed with this source code in the LICENSE file, you can
+ * obtain it at www.aomedia.org/license/software. If the Alliance for Open
+ * Media Patent License 1.0 was not distributed with this source code in the
+ * PATENTS file, you can obtain it at www.aomedia.org/license/patent.
+ */
+
+#include <assert.h>
+#include "aom_dsp/entdec.h"
+#include "aom_dsp/prob.h"
+
+/*A range decoder.
+ This is an entropy decoder based upon \cite{Mar79}, which is itself a
+ rediscovery of the FIFO arithmetic code introduced by \cite{Pas76}.
+ It is very similar to arithmetic encoding, except that encoding is done with
+ digits in any base, instead of with bits, and so it is faster when using
+ larger bases (i.e.: a byte).
+ The author claims an average waste of $\frac{1}{2}\log_b(2b)$ bits, where $b$
+ is the base, longer than the theoretical optimum, but to my knowledge there
+ is no published justification for this claim.
+ This only seems true when using near-infinite precision arithmetic so that
+ the process is carried out with no rounding errors.
+
+ An excellent description of implementation details is available at
+ http://www.arturocampos.com/ac_range.html
+ A recent work \cite{MNW98} which proposes several changes to arithmetic
+ encoding for efficiency actually re-discovers many of the principles
+ behind range encoding, and presents a good theoretical analysis of them.
+
+ End of stream is handled by writing out the smallest number of bits that
+ ensures that the stream will be correctly decoded regardless of the value of
+ any subsequent bits.
+ od_ec_dec_tell() can be used to determine how many bits were needed to decode
+ all the symbols thus far; other data can be packed in the remaining bits of
+ the input buffer.
+ @PHDTHESIS{Pas76,
+ author="Richard Clark Pasco",
+ title="Source coding algorithms for fast data compression",
+ school="Dept. of Electrical Engineering, Stanford University",
+ address="Stanford, CA",
+ month=May,
+ year=1976,
+ URL="http://www.richpasco.org/scaffdc.pdf"
+ }
+ @INPROCEEDINGS{Mar79,
+ author="Martin, G.N.N.",
+ title="Range encoding: an algorithm for removing redundancy from a digitised
+ message",
+ booktitle="Video & Data Recording Conference",
+ year=1979,
+ address="Southampton",
+ month=Jul,
+ URL="http://www.compressconsult.com/rangecoder/rngcod.pdf.gz"
+ }
+ @ARTICLE{MNW98,
+ author="Alistair Moffat and Radford Neal and Ian H. Witten",
+ title="Arithmetic Coding Revisited",
+ journal="{ACM} Transactions on Information Systems",
+ year=1998,
+ volume=16,
+ number=3,
+ pages="256--294",
+ month=Jul,
+ URL="http://researchcommons.waikato.ac.nz/bitstream/handle/10289/78/content.pdf"
+ }*/
+
+/*This is meant to be a large, positive constant that can still be efficiently
+ loaded as an immediate (on platforms like ARM, for example).
+ Even relatively modest values like 100 would work fine.*/
+#define OD_EC_LOTS_OF_BITS (0x4000)
+
+/*The return value of od_ec_dec_tell does not change across an od_ec_dec_refill
+ call.*/
+static void od_ec_dec_refill(od_ec_dec *dec) {
+ int s;
+ od_ec_window dif;
+ int16_t cnt;
+ const unsigned char *bptr;
+ const unsigned char *end;
+ dif = dec->dif;
+ cnt = dec->cnt;
+ bptr = dec->bptr;
+ end = dec->end;
+ s = OD_EC_WINDOW_SIZE - 9 - (cnt + 15);
+ for (; s >= 0 && bptr < end; s -= 8, bptr++) {
+ assert(s <= OD_EC_WINDOW_SIZE - 8);
+ dif ^= (od_ec_window)bptr[0] << s;
+ cnt += 8;
+ }
+ if (bptr >= end) {
+ dec->tell_offs += OD_EC_LOTS_OF_BITS - cnt;
+ cnt = OD_EC_LOTS_OF_BITS;
+ }
+ dec->dif = dif;
+ dec->cnt = cnt;
+ dec->bptr = bptr;
+}
+
+/*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.
+ ret: The value to return.
+ Return: ret.
+ This allows the compiler to jump to this function via a tail-call.*/
+static int od_ec_dec_normalize(od_ec_dec *dec, od_ec_window dif, unsigned rng,
+ int ret) {
+ int d;
+ assert(rng <= 65535U);
+ // The number of leading zeros in the 16-bit binary representation of rng.
+ d = 16 - OD_ILOG_NZ(rng);
+ dec->cnt -= d;
+ /*This is equivalent to shifting in 1's instead of 0's.*/
+ dec->dif = ((dif + 1) << d) - 1;
+ dec->rng = rng << d;
+ if (dec->cnt < 0) od_ec_dec_refill(dec);
+ return ret;
+}
+
+/*Initializes the decoder.
+ buf: The input buffer to use.
+ Return: 0 on success, or a negative value on error.*/
+void od_ec_dec_init(od_ec_dec *dec, const unsigned char *buf,
+ uint32_t storage) {
+ dec->buf = buf;
+ dec->tell_offs = 10 - (OD_EC_WINDOW_SIZE - 8);
+ dec->end = buf + storage;
+ dec->bptr = buf;
+ dec->dif = ((od_ec_window)1 << (OD_EC_WINDOW_SIZE - 1)) - 1;
+ dec->rng = 0x8000;
+ dec->cnt = -15;
+ dec->error = 0;
+ od_ec_dec_refill(dec);
+}
+
+/*Decode a single binary value.
+ f: The probability that the bit is one, scaled by 32768.
+ Return: The value decoded (0 or 1).*/
+int od_ec_decode_bool_q15(od_ec_dec *dec, unsigned f) {
+ od_ec_window dif;
+ od_ec_window vw;
+ unsigned r;
+ unsigned r_new;
+ unsigned v;
+ int ret;
+ assert(0 < f);
+ assert(f < 32768U);
+ dif = dec->dif;
+ r = dec->rng;
+ assert(dif >> (OD_EC_WINDOW_SIZE - 16) < r);
+ assert(32768U <= r);
+ v = ((r >> 8) * (uint32_t)(f >> EC_PROB_SHIFT) >> (7 - EC_PROB_SHIFT));
+ v += EC_MIN_PROB;
+ vw = (od_ec_window)v << (OD_EC_WINDOW_SIZE - 16);
+ ret = 1;
+ r_new = v;
+ if (dif >= vw) {
+ r_new = r - v;
+ dif -= vw;
+ ret = 0;
+ }
+ return od_ec_dec_normalize(dec, dif, r_new, ret);
+}
+
+/*Decodes a symbol given an inverse cumulative distribution function (CDF)
+ table in Q15.
+ icdf: CDF_PROB_TOP minus the CDF, such that symbol s falls in the range
+ [s > 0 ? (CDF_PROB_TOP - icdf[s - 1]) : 0, CDF_PROB_TOP - icdf[s]).
+ The values must be monotonically non-increasing, and icdf[nsyms - 1]
+ must be 0.
+ nsyms: The number of symbols in the alphabet.
+ This should be at most 16.
+ Return: The decoded symbol s.*/
+int od_ec_decode_cdf_q15(od_ec_dec *dec, const uint16_t *icdf, int nsyms) {
+ od_ec_window dif;
+ unsigned r;
+ unsigned c;
+ unsigned u;
+ unsigned v;
+ int ret;
+ (void)nsyms;
+ dif = dec->dif;
+ r = dec->rng;
+ const int N = nsyms - 1;
+
+ assert(dif >> (OD_EC_WINDOW_SIZE - 16) < r);
+ assert(icdf[nsyms - 1] == OD_ICDF(CDF_PROB_TOP));
+ assert(32768U <= r);
+ assert(7 - EC_PROB_SHIFT - CDF_SHIFT >= 0);
+ c = (unsigned)(dif >> (OD_EC_WINDOW_SIZE - 16));
+ v = r;
+ ret = -1;
+ do {
+ u = v;
+ v = ((r >> 8) * (uint32_t)(icdf[++ret] >> EC_PROB_SHIFT) >>
+ (7 - EC_PROB_SHIFT - CDF_SHIFT));
+ v += EC_MIN_PROB * (N - ret);
+ } while (c < v);
+ assert(v < u);
+ assert(u <= r);
+ r = u - v;
+ dif -= (od_ec_window)v << (OD_EC_WINDOW_SIZE - 16);
+ return od_ec_dec_normalize(dec, dif, r, ret);
+}
+
+/*Returns the number of bits "used" by the decoded symbols so far.
+ This same number can be computed in either the encoder or the decoder, and is
+ suitable for making coding decisions.
+ Return: The number of bits.
+ This will always be slightly larger than the exact value (e.g., all
+ rounding error is in the positive direction).*/
+int od_ec_dec_tell(const od_ec_dec *dec) {
+ return (int)((dec->bptr - dec->buf) * 8 - dec->cnt + dec->tell_offs);
+}
+
+/*Returns the number of bits "used" by the decoded symbols so far.
+ This same number can be computed in either the encoder or the decoder, and is
+ suitable for making coding decisions.
+ Return: The number of bits scaled by 2**OD_BITRES.
+ This will always be slightly larger than the exact value (e.g., all
+ rounding error is in the positive direction).*/
+uint32_t od_ec_dec_tell_frac(const od_ec_dec *dec) {
+ return od_ec_tell_frac(od_ec_dec_tell(dec), dec->rng);
+}