xdiff: generate "anti-diffs" aka what is common to two files

From: Linus Torvalds <torvalds@osdl.org>
Date: Jun 29, 2006, 04:57 UTC
Message-ID: <Pine.LNX.4.64.0606282149060.12404@g5.osdl.org>

This fairly trivial patch adds a new XDL_EMIT_xxx flag to tell libxdiff that we don't want to generate the _diff_ between two files, we want to see the lines that are _common_ to two files.

So when you set XDL_EMIT_COMMON, xdl_diff() will do everything exactly like it used to do, but the output records it generates just contain the lines that aren't part of the diff.

This is for doing things like generating the common base case for a file that was added in both branches.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---

Davide, I say it's a "fairly trivial patch", but quite frankly, I'm winging it. I'm not that comfortable with the libxdiff internal workings, so while this seems to work for me and make sense, can you please take a quick look.

Junio: the patch that actually _uses_ this comes next. It's based on my 
previous "throw-away" example patches, if you want me to base it on 
something else, please holler.

diff --git a/xdiff/xdiff.h b/xdiff/xdiff.h
index 2ce10b4..c9f8178 100644
--- a/xdiff/xdiff.h
+++ b/xdiff/xdiff.h
@@ -39,6 +39,7 @@ #define XDL_PATCH_MODEMASK ((1 << 8) - 1
 #define XDL_PATCH_IGNOREBSPACE (1 << 8)
 
 #define XDL_EMIT_FUNCNAMES (1 << 0)
+#define XDL_EMIT_COMMON (1 << 1)
 
 #define XDL_MMB_READONLY (1 << 0)
 
diff --git a/xdiff/xemit.c b/xdiff/xemit.c
index ad5bfb1..3604ca1 100644
--- a/xdiff/xemit.c
+++ b/xdiff/xemit.c
@@ -100,6 +100,21 @@ static void xdl_find_func(xdfile_t *xf, 
 }
 
 
+int xdl_emit_common(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
+		    xdemitconf_t const *xecfg) {
+	xdfile_t *xdf = &xe->xdf1;
+	const char *rchg = xdf->rchg;
+	long ix;
+
+	for (ix = 0; ix < xdf->nrec; ix++) {
+		if (rchg[ix])
+			continue;
+		if (xdl_emit_record(xdf, ix, "", ecb))
+			return -1;
+	}
+	return 0;
+}		
+
 int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 		  xdemitconf_t const *xecfg) {
 	long s1, s2, e1, e2, lctx;
@@ -107,6 +122,9 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange
 	char funcbuf[40];
 	long funclen = 0;
 
+	if (xecfg->flags & XDL_EMIT_COMMON)
+		return xdl_emit_common(xe, xscr, ecb, xecfg);
+
 	for (xch = xche = xscr; xch; xch = xche->next) {
 		xche = xdl_get_hunk(xch, xecfg);