diff: avoid repeated scanning while looking for funcname

For each hunk, xdl_find_func searches the preimage for a function name
until the beginning of the file. If the file does not contain any
function names, this search has complexity O(n^2) in the number of
hunks n.

Instead, inline xdl_find_func() and keep track of up to which line we
have scanned already and the contents of the last funcname line that
we have found.

Noticed and a different approach proposed by Clemens Buchacher.
This alternative solution was done by René Scharfe.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/xdiff/xemit.c b/xdiff/xemit.c
index c4bedf0d1ce1252563d7f36da7d846f5943343b0..277e2eec5b4cced5882f37ac42af7f22efe92902 100644 (file)
--- a/xdiff/xemit.c
+++ b/xdiff/xemit.c
@@ -85,27 +85,6 @@ static long def_ff(const char *rec, long len, char *buf, long sz, void *priv)
        return -1;
 }
 
-static void xdl_find_func(xdfile_t *xf, long i, char *buf, long sz, long *ll,
-               find_func_t ff, void *ff_priv) {
-
-       /*
-        * Be quite stupid about this for now.  Find a line in the old file
-        * before the start of the hunk (and context) which starts with a
-        * plausible character.
-        */
-
-       const char *rec;
-       long len;
-
-       while (i-- > 0) {
-               len = xdl_get_rec(xf, i, &rec);
-               if ((*ll = ff(rec, len, buf, sz, ff_priv)) >= 0)
-                       return;
-       }
-       *ll = 0;
-}
-
-
 static int xdl_emit_common(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
                            xdemitconf_t const *xecfg) {
        xdfile_t *xdf = &xe->xdf1;
@@ -127,6 +106,7 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
        xdchange_t *xch, *xche;
        char funcbuf[80];
        long funclen = 0;
+       long funclineprev = -1;
        find_func_t ff = xecfg->find_func ?  xecfg->find_func : def_ff;
 
        if (xecfg->flags & XDL_EMIT_COMMON)
@@ -150,9 +130,19 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
                 */
 
                if (xecfg->flags & XDL_EMIT_FUNCNAMES) {
-                       xdl_find_func(&xe->xdf1, s1, funcbuf,
-                                     sizeof(funcbuf), &funclen,
-                                     ff, xecfg->find_func_priv);
+                       long l;
+                       for (l = s1 - 1; l >= 0 && l > funclineprev; l--) {
+                               const char *rec;
+                               long reclen = xdl_get_rec(&xe->xdf1, l, &rec);
+                               long newfunclen = ff(rec, reclen, funcbuf,
+                                                    sizeof(funcbuf),
+                                                    xecfg->find_func_priv);
+                               if (newfunclen >= 0) {
+                                       funclen = newfunclen;
+                                       break;
+                               }
+                       }
+                       funclineprev = s1 - 1;
                }
                if (xdl_emit_hunk_hdr(s1 + 1, e1 - s1, s2 + 1, e2 - s2,
                                      funcbuf, funclen, ecb) < 0)