diff options
| -rw-r--r-- | lib/bunzip.c | 29 |
1 files changed, 16 insertions, 13 deletions
diff --git a/lib/bunzip.c b/lib/bunzip.c index 5cdc4528..5c3b842b 100644 --- a/lib/bunzip.c +++ b/lib/bunzip.c @@ -117,7 +117,7 @@ static unsigned int get_bits(bunzip_data *bd, char bits_wanted) return bits; } -// Decompress a block of text to into intermediate buffer +// Decompress a block of text to intermediate buffer int read_bunzip_data(bunzip_data *bd) { struct group_data *hufGroup; @@ -171,7 +171,8 @@ int read_bunzip_data(bunzip_data *bd) // nSelectors: Every GROUP_SIZE many symbols we select a new huffman coding // group. Read in the group selector list, which is stored as MTF encoded - // bit runs. + // bit runs. (MTF = Move To Front. Every time a symbol occurs it's moved + // to the front of the table, so it has a shorter encoding next time.) if (!(nSelectors = get_bits(bd, 15))) return RETVAL_DATA_ERROR; for (i=0; i<groupCount; i++) mtfSymbol[i] = i; for (i=0; i<nSelectors; i++) { @@ -180,7 +181,7 @@ int read_bunzip_data(bunzip_data *bd) for(j=0;get_bits(bd,1);j++) if (j>=groupCount) return RETVAL_DATA_ERROR; - // Decode MTF to get the next selector + // Decode MTF to get the next selector, and move it to the front. uc = mtfSymbol[j]; memmove(mtfSymbol+1, mtfSymbol, j); mtfSymbol[0] = selectors[i] = uc; @@ -221,7 +222,7 @@ int read_bunzip_data(bunzip_data *bd) * * To use these, keep reading bits until value <= limit[bitcount] or * you've read over 20 bits (error). Then the decoded symbol - * equals permute[hufcode_value-base[hufcode_bitcount]]. + * equals permute[hufcode_value - base[hufcode_bitcount]]. */ hufGroup = bd->groups+j; hufGroup->minLen = minLen; @@ -230,25 +231,25 @@ int read_bunzip_data(bunzip_data *bd) // Note that minLen can't be smaller than 1, so we adjust the base // and limit array pointers so we're not always wasting the first // entry. We do this again when using them (during symbol decoding). - base=hufGroup->base-1; - limit=hufGroup->limit-1; + base = hufGroup->base-1; + limit = hufGroup->limit-1; // Calculate permute[] pp = 0; - for (i=minLen; i<=maxLen; i++) - for (t=0;t<symCount;t++) - if (length[t]==i) hufGroup->permute[pp++] = t; + for (i = minLen; i <= maxLen; i++) + for (t = 0; t < symCount; t++) + if (length[t] == i) hufGroup->permute[pp++] = t; // Count cumulative symbols coded for at each bit length - for (i=minLen; i<=maxLen; i++) temp[i] = limit[i] = 0; - for (i=0; i<symCount; i++) temp[length[i]]++; + for (i = minLen; i <= maxLen; i++) temp[i] = limit[i] = 0; + for (i = 0; i < symCount; i++) temp[length[i]]++; /* Calculate limit[] (the largest symbol-coding value at each bit * length, which is (previous limit<<1)+symbols at this level), and * base[] (number of symbols to ignore at each bit length, which is * limit-cumulative count of symbols coded for already). */ pp = t = 0; - for (i=minLen; i<maxLen; i++) { + for (i = minLen; i < maxLen; i++) { pp += temp[i]; limit[i] = pp-1; pp <<= 1; @@ -266,7 +267,9 @@ int read_bunzip_data(bunzip_data *bd) memset(byteCount, 0, 256*sizeof(int)); for(i=0; i<256; i++) mtfSymbol[i] = (unsigned char)i; - // Loop through compressed symbols + // Loop through compressed symbols. This is the first "tight inner loop" + // that needs to be micro-optimized for speed. (This one fills out dbuf[] + // linearly, staying in cache more, so isn't as limited by DRAM access.) runPos = dbufCount = symCount = selector = 0; for (;;) { |