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merge3: clamp aligned lines to be within the file.

--- a/sys/src/cmd/diff/merge3.c

+++ b/sys/src/cmd/diff/merge3.c

@@ -3,6 +3,9 @@

 #include <bio.h>

 #include "diff.h"

+#define min(a, b) ((a)<(b)?(a):(b))

+#define max(a, b) ((a)>(b)?(a):(b))

+

 static int

 changecmp(void *a, void *b)

 {

@@ -118,29 +121,31 @@

 		}

 		if(lc != nil && rc != nil && overlaps(lx, ly, rx, ry)){

 			/*

-			 * align the edges of the chunks

+			 * align the edges of the chunks, expanding them

+			 * so that when we compare for sameness, we are

+			 * comparing same-sized chunks.

 			 */

 			if(lc->oldx < rc->oldx){

 				x = lc->newx;

 				δ = rc->oldx - lc->oldx;

-				rc->oldx -= δ;

-				rc->newx -= δ;

+				rc->oldx = max(rc->oldx-δ, 1);

+				rc->newx = max(rc->newx-δ, 1);

 			}else{

 				x = rc->newx;

 				δ = lc->oldx - rc->oldx;

-				lc->oldx -= δ;

-				lc->newx -= δ;

+				lc->oldx = max(lc->oldx-δ, 1);

+				lc->newx = max(lc->newx-δ, 1);

 			}

 			if(lc->oldy > rc->oldy){

 				y = lc->newy;

 				δ = lc->oldy - rc->oldy;

-				rc->oldy += δ;

-				rc->newy += δ;

+				rc->oldy = min(rc->oldy+δ, r->len[0]);

+				rc->newy = min(rc->newy+δ, r->len[1]);

 			}else{

 				y = rc->newy;

 				δ = rc->oldy - lc->oldy;

-				lc->oldy += δ;

-				lc->newy += δ;

+				lc->oldy = min(lc->oldy+δ, l->len[0]);

+				lc->newy = min(lc->newy+δ, l->len[1]);

 			}

 			if(same(l, lc, r, rc)){

 				fetch(l, l->ixold, ln, x-1, l->input[0], "");

@@ -150,7 +155,7 @@

 				Bprint(&stdout, "<<<<<<<<<< %s\n", l->file2);

 				fetch(l, l->ixnew, lc->newx, lc->newy, l->input[1], "");

 				Bprint(&stdout, "========== original\n");

-				fetch(l, l->ixold, x, y, l->input[0], "");

+				fetch(l, l->ixold, lc->oldx, lc->oldy, l->input[0], "");

 				Bprint(&stdout, "========== %s\n", r->file2);

 				fetch(r, r->ixnew, rc->newx, rc->newy, r->input[1], "");

 				Bprint(&stdout, ">>>>>>>>>>\n");

-- 

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