diff options
Diffstat (limited to 'jbig2dec/jbig2_arith.c')
| -rw-r--r-- | jbig2dec/jbig2_arith.c | 98 |
1 files changed, 35 insertions, 63 deletions
diff --git a/jbig2dec/jbig2_arith.c b/jbig2dec/jbig2_arith.c index 6416fad6..04566337 100644 --- a/jbig2dec/jbig2_arith.c +++ b/jbig2dec/jbig2_arith.c @@ -42,21 +42,16 @@ struct _Jbig2ArithState { int offset; }; -#undef SOFTWARE_CONVENTION - /* - A note on the "software conventions". - - Previously, I had misinterpreted the spec, and had thought that the - spec's description of the "software convention" was wrong. Now I - believe that this code is both correct and matches the spec, with - SOFTWARE_CONVENTION defined or not. Thanks to William Rucklidge for - the clarification. - - In any case, my benchmarking indicates no speed difference at all. - Therefore, for now we will just use the normative version. - - */ + Previous versions of this code had a #define to allow + us to choose between using the revised arithmetic decoding + specified in the 'Software Convention' section of the spec. + Back to back tests showed that the 'Software Convention' + version was indeed slightly faster. We therefore enable it + by default. We also strip the option out, because a) it + makes the code harder to read, and b) such things are an + invitation to bitrot. +*/ static void jbig2_arith_bytein(Jbig2ArithState *as) @@ -66,6 +61,23 @@ jbig2_arith_bytein(Jbig2ArithState *as) /* invariant: as->next_word_bytes > 0 */ + /* This code confused me no end when I first read it, so a quick note + * to save others (and future me's) from being similarly confused. + * 'next_word' does indeed contain 'next_word_bytes' of valid data + * (always starting at the most significant byte). The confusing + * thing is that the first byte has always already been read. + * i.e. it serves only as an indication that the last byte we read + * was FF or not. + * + * The jbig2 bytestream uses FF bytes, followed by a byte > 0x8F as + * marker bytes. These never occur in normal streams of arithmetic + * encoding, so meeting one terminates the stream (with an infinite + * series of 1 bits). + * + * If we meet an FF byte, we return it as normal. We just 'remember' + * that fact for the next byte we read. + */ + /* Figure G.3 */ B = (byte)((as->next_word >> 24) & 0xFF); if (B == 0xFF) { @@ -83,9 +95,6 @@ jbig2_arith_bytein(Jbig2ArithState *as) #ifdef JBIG2_DEBUG_ARITH fprintf(stderr, "read %02x (aa)\n", B); #endif -#ifndef SOFTWARE_CONVENTION - as->C += 0xFF00; -#endif as->CT = 8; as->next_word = 0xFF000000 | (as->next_word >> 8); as->next_word_bytes = 4; @@ -94,11 +103,7 @@ jbig2_arith_bytein(Jbig2ArithState *as) #ifdef JBIG2_DEBUG_ARITH fprintf(stderr, "read %02x (a)\n", B); #endif -#ifdef SOFTWARE_CONVENTION as->C += 0xFE00 - (B1 << 9); -#else - as->C += B1 << 9; -#endif as->CT = 7; } } else { @@ -107,9 +112,6 @@ jbig2_arith_bytein(Jbig2ArithState *as) #ifdef JBIG2_DEBUG_ARITH fprintf(stderr, "read %02x (ba)\n", B); #endif -#ifndef SOFTWARE_CONVENTION - as->C += 0xFF00; -#endif as->CT = 8; } else { as->next_word_bytes--; @@ -118,11 +120,7 @@ jbig2_arith_bytein(Jbig2ArithState *as) fprintf(stderr, "read %02x (b)\n", B); #endif -#ifdef SOFTWARE_CONVENTION as->C += 0xFE00 - (B1 << 9); -#else - as->C += (B1 << 9); -#endif as->CT = 7; } } @@ -141,11 +139,7 @@ jbig2_arith_bytein(Jbig2ArithState *as) as->next_word_bytes = new_bytes; } B = (byte)((as->next_word >> 24) & 0xFF); -#ifdef SOFTWARE_CONVENTION as->C += 0xFF00 - (B << 8); -#else - as->C += (B << 8); -#endif } } @@ -172,11 +166,7 @@ jbig2_arith_new(Jbig2Ctx *ctx, Jbig2WordStream *ws) result->offset = new_bytes; /* Figure E.20 */ -#ifdef SOFTWARE_CONVENTION result->C = (~(result->next_word >> 8)) & 0xFF0000; -#else - result->C = (result->next_word >> 8) & 0xFF0000; -#endif jbig2_arith_bytein(result); result->C <<= 7; @@ -195,7 +185,7 @@ typedef struct { byte lps_xor; /* lps_xor = index ^ NLPS ^ (SWITCH << 7) */ } Jbig2ArithQe; -const Jbig2ArithQe jbig2_arith_Qe[MAX_QE_ARRAY_SIZE] = { +static const Jbig2ArithQe jbig2_arith_Qe[MAX_QE_ARRAY_SIZE] = { {0x5601, 1 ^ 0, 1 ^ 0 ^ 0x80}, {0x3401, 2 ^ 1, 6 ^ 1}, {0x1801, 3 ^ 2, 9 ^ 2}, @@ -258,34 +248,22 @@ jbig2_arith_renormd(Jbig2ArithState *as) } while ((as->A & 0x8000) == 0); } -bool -jbig2_arith_decode(Jbig2ArithState *as, Jbig2ArithCx *pcx, int *code) +int +jbig2_arith_decode(Jbig2ArithState *as, Jbig2ArithCx *pcx) { Jbig2ArithCx cx = *pcx; const Jbig2ArithQe *pqe; unsigned int index = cx & 0x7f; bool D; - if (index >= MAX_QE_ARRAY_SIZE) { - *code = -1; - return 0; - } else { - pqe = &jbig2_arith_Qe[index]; - } + if (index >= MAX_QE_ARRAY_SIZE) + return -1; /* Error */ + + pqe = &jbig2_arith_Qe[index]; /* Figure E.15 */ as->A -= pqe->Qe; - if ( -#ifdef SOFTWARE_CONVENTION - /* Note: I do not think this is correct. See above. */ - (as->C >> 16) < as->A -#else - !((as->C >> 16) < pqe->Qe) -#endif - ) { -#ifndef SOFTWARE_CONVENTION - as->C -= pqe->Qe << 16; -#endif + if ((as->C >> 16) < as->A) { if ((as->A & 0x8000) == 0) { /* MPS_EXCHANGE, Figure E.16 */ if (as->A < pqe->Qe) { @@ -296,16 +274,12 @@ jbig2_arith_decode(Jbig2ArithState *as, Jbig2ArithCx *pcx, int *code) *pcx ^= pqe->mps_xor; } jbig2_arith_renormd(as); - *code = 0; return D; } else { - *code = 0; return cx >> 7; } } else { -#ifdef SOFTWARE_CONVENTION as->C -= (as->A) << 16; -#endif /* LPS_EXCHANGE, Figure E.17 */ if (as->A < pqe->Qe) { as->A = pqe->Qe; @@ -317,7 +291,6 @@ jbig2_arith_decode(Jbig2ArithState *as, Jbig2ArithCx *pcx, int *code) *pcx ^= pqe->lps_xor; } jbig2_arith_renormd(as); - *code = 0; return D; } } @@ -378,7 +351,6 @@ main(int argc, char **argv) Jbig2ArithState *as; int i; Jbig2ArithCx cx = 0; - int code; ctx = jbig2_ctx_new(NULL, 0, NULL, NULL, NULL); @@ -394,7 +366,7 @@ main(int argc, char **argv) #else (void) #endif - jbig2_arith_decode(as, &cx, &code); + jbig2_arith_decode(as, &cx); #ifdef JBIG2_DEBUG_ARITH fprintf(stderr, "%3d: D = %d, ", i, D); |