zstd_opt: minor speed optimization

Calculate reference log2sums only once per serie of sequence
(as opposed to once per sequence)

Also: improved code comments

lib/compress/zstd_compress_internal.h

@@ -72,26 +72,27 @@ typedef struct {
 } ZSTD_optimal_t;
 
 typedef struct {
-    U32* litFreq;
-    U32* litLengthFreq;
-    U32* matchLengthFreq;
-    U32* offCodeFreq;
-    ZSTD_match_t* matchTable;
-    ZSTD_optimal_t* priceTable;
+    /* All tables are allocated inside cctx->workspace by ZSTD_resetCCtx_internal() */
+    U32* litFreq;               /* table of literals statistics, of size 256 */
+    U32* litLengthFreq;         /* table of litLength statistics, of size (MaxLL+1) */
+    U32* matchLengthFreq;       /* table of matchLength statistics, of size (MaxML+1) */
+    U32* offCodeFreq;           /* table of offCode statistics, of size (MaxOff+1) */
+    ZSTD_match_t* matchTable;   /* list of found matches, of size ZSTD_OPT_NUM+1 */
+    ZSTD_optimal_t* priceTable; /* All positions tracked by optimal parser, of size ZSTD_OPT_NUM+1 */
 
     U32  litSum;                 /* nb of literals */
     U32  litLengthSum;           /* nb of litLength codes */
     U32  matchLengthSum;         /* nb of matchLength codes */
-    U32  matchSum;               /* one argument to calculate `factor` */
+    U32  matchSum;               /* a strange argument used in calculating `factor` */
     U32  offCodeSum;             /* nb of offset codes */
     /* begin updated by ZSTD_setLog2Prices */
     U32  log2litSum;             /* pow2 to compare log2(litfreq) to */
     U32  log2litLengthSum;       /* pow2 to compare log2(llfreq) to */
     U32  log2matchLengthSum;     /* pow2 to compare log2(mlfreq) to */
     U32  log2offCodeSum;         /* pow2 to compare log2(offreq) to */
-    U32  factor;                 /* added to calculate ZSTD_getPrice() (but why?) */
+    U32  factor;                 /* fixed cost added when calculating ZSTD_getPrice() (but why ? seems to favor less sequences) */
     /* end : updated by ZSTD_setLog2Prices */
-    U32  staticPrices;           /* prices follow a static cost structure, statistics are irrelevant */
+    U32  staticPrices;           /* prices follow a pre-defined cost structure, statistics are irrelevant */
     U32  cachedPrice;
     U32  cachedLitLength;
     const BYTE* cachedLiterals;