combine-diff: refactor built-in xdiff interface.

This refactors the line-by-line callback mechanism used in
combine-diff so that other programs can reuse it more easily.

Signed-off-by: Junio C Hamano <junkio@cox.net>

xdiff-interface.c

@@ -0,0 +1,58 @@
+#include "cache.h"
+#include "xdiff-interface.h"
+
+static void consume_one(void *priv_, char *s, unsigned long size)
+{
+	struct xdiff_emit_state *priv = priv_;
+	char *ep;
+	while (size) {
+		unsigned long this_size;
+		ep = memchr(s, '\n', size);
+		this_size = (ep == NULL) ? size : (ep - s + 1);
+		priv->consume(priv, s, this_size);
+		size -= this_size;
+		s += this_size;
+	}
+}
+
+int xdiff_outf(void *priv_, mmbuffer_t *mb, int nbuf)
+{
+	struct xdiff_emit_state *priv = priv_;
+	int i;
+
+	for (i = 0; i < nbuf; i++) {
+		if (mb[i].ptr[mb[i].size-1] != '\n') {
+			/* Incomplete line */
+			priv->remainder = realloc(priv->remainder,
+						  priv->remainder_size +
+						  mb[i].size);
+			memcpy(priv->remainder + priv->remainder_size,
+			       mb[i].ptr, mb[i].size);
+			priv->remainder_size += mb[i].size;
+			continue;
+		}
+
+		/* we have a complete line */
+		if (!priv->remainder) {
+			consume_one(priv, mb[i].ptr, mb[i].size);
+			continue;
+		}
+		priv->remainder = realloc(priv->remainder,
+					  priv->remainder_size +
+					  mb[i].size);
+		memcpy(priv->remainder + priv->remainder_size,
+		       mb[i].ptr, mb[i].size);
+		consume_one(priv, priv->remainder,
+			    priv->remainder_size + mb[i].size);
+		free(priv->remainder);
+		priv->remainder = NULL;
+		priv->remainder_size = 0;
+	}
+	if (priv->remainder) {
+		consume_one(priv, priv->remainder, priv->remainder_size);
+		free(priv->remainder);
+		priv->remainder = NULL;
+		priv->remainder_size = 0;
+	}
+	return 0;
+}