[PATCH 3/9] diff: avoid generating unused hunk header lines

[Jeff King <peff@peff.net>]

Some callers of xdi_diff_outf() do not look at the generated hunk header
lines at all. By plugging in a no-op hunk callback, this tells xdiff not
to even bother formatting them.

This patch introduces a stock no-op callback and uses it with a few
callers whose line callbacks explicitly ignore hunk headers (because
they look only for +/- lines).

Signed-off-by: Jeff King <peff@peff.net>
---
 diff.c             | 4 ++--
 diffcore-pickaxe.c | 3 ++-
 xdiff-interface.c  | 6 ++++++
 xdiff-interface.h  | 6 ++++++
 4 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/diff.c b/diff.c
index 07be5879e5..d84356e007 100644
--- a/diff.c
+++ b/diff.c
@@ -3637,8 +3637,8 @@ static void builtin_diffstat(const char *name_a, const char *name_b,
 		xpp.anchors_nr = o->anchors_nr;
 		xecfg.ctxlen = o->context;
 		xecfg.interhunkctxlen = o->interhunkcontext;
-		if (xdi_diff_outf(&mf1, &mf2, NULL, diffstat_consume,
-				  diffstat, &xpp, &xecfg))
+		if (xdi_diff_outf(&mf1, &mf2, discard_hunk_line,
+				  diffstat_consume, diffstat, &xpp, &xecfg))
 			die("unable to generate diffstat for %s", one->path);
 	}
 
diff --git a/diffcore-pickaxe.c b/diffcore-pickaxe.c
index 58df35670a..69fc55ea1e 100644
--- a/diffcore-pickaxe.c
+++ b/diffcore-pickaxe.c
@@ -62,7 +62,8 @@ static int diff_grep(mmfile_t *one, mmfile_t *two,
 	ecbdata.hit = 0;
 	xecfg.ctxlen = o->context;
 	xecfg.interhunkctxlen = o->interhunkcontext;
-	if (xdi_diff_outf(one, two, NULL, diffgrep_consume, &ecbdata, &xpp, &xecfg))
+	if (xdi_diff_outf(one, two, discard_hunk_line, diffgrep_consume,
+			  &ecbdata, &xpp, &xecfg))
 		return 0;
 	return ecbdata.hit;
 }
diff --git a/xdiff-interface.c b/xdiff-interface.c
index 16d37ce6be..2622981d97 100644
--- a/xdiff-interface.c
+++ b/xdiff-interface.c
@@ -157,6 +157,12 @@ int xdi_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp, xdemitconf_t co
 	return xdl_diff(&a, &b, xpp, xecfg, xecb);
 }
 
+void discard_hunk_line(void *priv,
+		       long ob, long on, long nb, long nn,
+		       const char *func, long funclen)
+{
+}
+
 int xdi_diff_outf(mmfile_t *mf1, mmfile_t *mf2,
 		  xdiff_emit_hunk_fn hunk_fn,
 		  xdiff_emit_line_fn line_fn,
diff --git a/xdiff-interface.h b/xdiff-interface.h
index 2dbe2feb19..7b0ccbdd1d 100644
--- a/xdiff-interface.h
+++ b/xdiff-interface.h
@@ -35,6 +35,12 @@ extern void xdiff_clear_find_func(xdemitconf_t *xecfg);
 extern int git_xmerge_config(const char *var, const char *value, void *cb);
 extern int git_xmerge_style;
 
+/*
+ * Can be used as a no-op hunk_fn for xdi_diff_outf(), since a NULL
+ * one just sends the hunk line to the line_fn callback).
+ */
+void discard_hunk_line(void *, long, long, long, long, const char *, long);
+
 /*
  * Compare the strings l1 with l2 which are of size s1 and s2 respectively.
  * Returns 1 if the strings are deemed equal, 0 otherwise.
-- 
2.19.1.1336.g081079ac04