xdiff/xprepare: skip classification

xdiff performs "classification" of records (xdl_classify_record()),
replacing hashes (xrecord_t.ha) with a unique identifier of the
record/line and building a hash table (xrecord_t.rhash) of records. This
is then used to "cleanup" records (xdl_cleanup_records()).

We don't need any of that in histogram diff, so we omit calls to these
functions. We also skip allocating memory to the hash table, rhash, as
it is no longer used.

This gives us a small boost in performance.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

xdiff/xprepare.c

@@ -154,11 +154,15 @@ static int xdl_prepare_ctx(mmfile_t *mf, long narec, xpparam_t const *xpp,
 	if (!(recs = (xrecord_t **) xdl_malloc(narec * sizeof(xrecord_t *))))
 		goto abort;
 
-	hbits = xdl_hashbits((unsigned int) narec);
-	hsize = 1 << hbits;
-	if (!(rhash = (xrecord_t **) xdl_malloc(hsize * sizeof(xrecord_t *))))
-		goto abort;
-	memset(rhash, 0, hsize * sizeof(xrecord_t *));
+	if (xpp->flags & XDF_HISTOGRAM_DIFF)
+		hbits = hsize = 0;
+	else {
+		hbits = xdl_hashbits((unsigned int) narec);
+		hsize = 1 << hbits;
+		if (!(rhash = (xrecord_t **) xdl_malloc(hsize * sizeof(xrecord_t *))))
+			goto abort;
+		memset(rhash, 0, hsize * sizeof(xrecord_t *));
+	}
 
 	nrec = 0;
 	if ((cur = blk = xdl_mmfile_first(mf, &bsize)) != NULL) {
@@ -183,7 +187,8 @@ static int xdl_prepare_ctx(mmfile_t *mf, long narec, xpparam_t const *xpp,
 			crec->ha = hav;
 			recs[nrec++] = crec;
 
-			if (xdl_classify_record(cf, rhash, hbits, crec) < 0)
+			if (!(xpp->flags & XDF_HISTOGRAM_DIFF) &&
+				xdl_classify_record(cf, rhash, hbits, crec) < 0)
 				goto abort;
 		}
 	}
@@ -240,7 +245,8 @@ int xdl_prepare_env(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
 	enl1 = xdl_guess_lines(mf1) + 1;
 	enl2 = xdl_guess_lines(mf2) + 1;
 
-	if (xdl_init_classifier(&cf, enl1 + enl2 + 1, xpp->flags) < 0) {
+	if (!(xpp->flags & XDF_HISTOGRAM_DIFF) &&
+		xdl_init_classifier(&cf, enl1 + enl2 + 1, xpp->flags) < 0) {
 
 		return -1;
 	}
@@ -257,9 +263,11 @@ int xdl_prepare_env(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
 		return -1;
 	}
 
-	xdl_free_classifier(&cf);
+	if (!(xpp->flags & XDF_HISTOGRAM_DIFF))
+		xdl_free_classifier(&cf);
 
 	if (!(xpp->flags & XDF_PATIENCE_DIFF) &&
+			!(xpp->flags & XDF_HISTOGRAM_DIFF) &&
 			xdl_optimize_ctxs(&xe->xdf1, &xe->xdf2) < 0) {
 
 		xdl_free_ctx(&xe->xdf2);