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Diffstat (limited to 'third_party/aom/aom_dsp/entdec.c')
| -rw-r--r-- | third_party/aom/aom_dsp/entdec.c | 247 |
1 files changed, 247 insertions, 0 deletions
diff --git a/third_party/aom/aom_dsp/entdec.c b/third_party/aom/aom_dsp/entdec.c new file mode 100644 index 0000000000..5bbcddae08 --- /dev/null +++ b/third_party/aom/aom_dsp/entdec.c @@ -0,0 +1,247 @@ +/* + * 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++) { + /*Each time a byte is inserted into the window (dif), bptr advances and cnt + is incremented by 8, so the total number of consumed bits (the return + value of od_ec_dec_tell) does not change.*/ + assert(s <= OD_EC_WINDOW_SIZE - 8); + dif ^= (od_ec_window)bptr[0] << s; + cnt += 8; + } + if (bptr >= end) { + /*We've reached the end of the buffer. It is perfectly valid for us to need + to fill the window with additional bits past the end of the buffer (and + this happens in normal operation). These bits should all just be taken + as zero. But we cannot increment bptr past 'end' (this is undefined + behavior), so we start to increment dec->tell_offs. We also don't want + to keep testing bptr against 'end', so we set cnt to OD_EC_LOTS_OF_BITS + and adjust dec->tell_offs so that the total number of unconsumed bits in + the window (dec->cnt - dec->tell_offs) does not change. This effectively + puts lots of zero bits into the window, and means we won't try to refill + it from the buffer for a very long time (at which point we'll put lots + of zero bits into the window again).*/ + 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); + /*d bits in dec->dif are consumed.*/ + 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. + storage: The size in bytes of the input buffer.*/ +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; + 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 >= 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)); + 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) { + /*There is a window of bits stored in dec->dif. The difference + (dec->bptr - dec->buf) tells us how many bytes have been read into this + window. The difference (dec->cnt - dec->tell_offs) tells us how many of + the bits in that window remain unconsumed.*/ + 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); +} |