xdiff: add xdl_merge()

This new function implements the functionality of RCS merge, but
in-memory. It returns < 0 on error, otherwise the number of conflicts.

Finding the conflicting lines can be a very expensive task. You can
control the eagerness of this algorithm:

- a level value of 0 means that all overlapping changes are treated
  as conflicts,
- a value of 1 means that if the overlapping changes are identical,
  it is not treated as a conflict.
- If you set level to 2, overlapping changes will be analyzed, so that
  almost identical changes will not result in huge conflicts. Rather,
  only the conflicting lines will be shown inside conflict markers.

With each increasing level, the algorithm gets slower, but more accurate.
Note that the code for level 2 depends on the simple definition of
mmfile_t specific to git, and therefore it will be harder to port that
to LibXDiff.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -723,7 +723,8 @@ $(DIFF_OBJS): diffcore.h
 $(LIB_FILE): $(LIB_OBJS)
 	rm -f $@ && $(AR) rcs $@ $(LIB_OBJS)
 
-XDIFF_OBJS=xdiff/xdiffi.o xdiff/xprepare.o xdiff/xutils.o xdiff/xemit.o
+XDIFF_OBJS=xdiff/xdiffi.o xdiff/xprepare.o xdiff/xutils.o xdiff/xemit.o \
+	xdiff/xmerge.o
 $(XDIFF_OBJS): xdiff/xinclude.h xdiff/xmacros.h xdiff/xdiff.h xdiff/xtypes.h \
 	xdiff/xutils.h xdiff/xprepare.h xdiff/xdiffi.h xdiff/xemit.h
 

xdiff/xdiff.h

@@ -49,6 +49,9 @@ extern "C" {
 #define XDL_BDOP_CPY 2
 #define XDL_BDOP_INSB 3
 
+#define XDL_MERGE_MINIMAL 0
+#define XDL_MERGE_EAGER 1
+#define XDL_MERGE_ZEALOUS 2
 
 typedef struct s_mmfile {
 	char *ptr;
@@ -90,6 +93,10 @@ long xdl_mmfile_size(mmfile_t *mmf);
 int xdl_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
 	     xdemitconf_t const *xecfg, xdemitcb_t *ecb);
 
+int xdl_merge(mmfile_t *orig, mmfile_t *mf1, const char *name1,
+		mmfile_t *mf2, const char *name2,
+		xpparam_t const *xpp, int level, mmbuffer_t *result);
+
 #ifdef __cplusplus
 }
 #endif /* #ifdef __cplusplus */

xdiff/xdiffi.c

@@ -45,7 +45,6 @@ static long xdl_split(unsigned long const *ha1, long off1, long lim1,
 		      long *kvdf, long *kvdb, int need_min, xdpsplit_t *spl,
 		      xdalgoenv_t *xenv);
 static xdchange_t *xdl_add_change(xdchange_t *xscr, long i1, long i2, long chg1, long chg2);
-static int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags);
 
 
 
@@ -397,7 +396,7 @@ static xdchange_t *xdl_add_change(xdchange_t *xscr, long i1, long i2, long chg1,
 }
 
 
-static int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
+int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
 	long ix, ixo, ixs, ixref, grpsiz, nrec = xdf->nrec;
 	char *rchg = xdf->rchg, *rchgo = xdfo->rchg;
 	xrecord_t **recs = xdf->recs;

xdiff/xdiffi.h

@@ -50,6 +50,7 @@ int xdl_recs_cmp(diffdata_t *dd1, long off1, long lim1,
 		 long *kvdf, long *kvdb, int need_min, xdalgoenv_t *xenv);
 int xdl_do_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
 		xdfenv_t *xe);
+int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags);
 int xdl_build_script(xdfenv_t *xe, xdchange_t **xscr);
 void xdl_free_script(xdchange_t *xscr);
 int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,