diff 172dc0005aa16dea8451bf2bcef7f3eb04df2c04 uncommitted
--- a/sys/src/cmd/patch.c
+++ b/sys/src/cmd/patch.c
@@ -146,11 +146,19 @@
 	if(strncmp(s, "@@ -", 4) != 0)
 		return -1;
 	e = s + 4;
-	h->oldln = strtol(e, &e, 10);
-	h->oldcnt = 1;
+	if(reverse){
+		h->newln = strtol(e, &e, 10);
+		h->newcnt = 1;
+	}else{
+		h->oldln = strtol(e, &e, 10);
+		h->oldcnt = 1;
+	}
 	if(*e == ','){
 		e++;
-		h->oldcnt = strtol(e, &e, 10);
+		if(reverse)
+			h->newcnt = strtol(e, &e, 10);
+		else
+			h->oldcnt = strtol(e, &e, 10);
 	}
 	while(*e == ' ' || *e == '\t')
 		e++;
@@ -157,13 +165,21 @@
 	if(*e != '+')
 		return -1;
 	e++;
-	h->newln = strtol(e, &e, 10);
+	if(reverse){
+		h->oldln = strtol(e, &e, 10);
+		h->oldcnt = 1;
+	}else{
+		h->newln = strtol(e, &e, 10);
+		h->newcnt = 1;
+	}
 	if(e == s)
 		return -1;
-	h->newcnt = 1;
 	if(*e == ','){
 		e++;
-		h->newcnt = strtol(e, &e, 10);
+		if(reverse)
+			h->oldcnt = strtol(e, &e, 10);
+		else
+			h->newcnt = strtol(e, &e, 10);
 	}
 	if(e == s || *e != ' ')
 		return -1;
@@ -298,8 +314,10 @@
 
 	while(1){
 		if((ln = readline(f, &lnum)) == nil){
-			if(oldcnt != h.oldcnt || newcnt != h.newcnt)
-				sysfatal("%s:%d: malformed hunk: mismatched counts", name, lnum);
+			if(oldcnt != h.oldcnt)
+				sysfatal("%s:%d: malformed hunk: mismatched -hunk size %d != %d", name, lnum, oldcnt, h.oldcnt);
+			if(newcnt != h.newcnt)
+				sysfatal("%s:%d: malformed hunk: mismatched +hunk size %d != %d", name, lnum, newcnt, h.newcnt);
 			addhunk(p, &h);
 			break;
 		}
@@ -309,11 +327,17 @@
 			goto out;
 		case '-':
 			addold(&h, ln);
-			oldcnt++;
+			if(reverse)
+				newcnt++;
+			else
+				oldcnt++;
 			break;
 		case '+':
 			addnew(&h, ln);
-			newcnt++;
+			if(reverse)
+				oldcnt++;
+			else
+				newcnt++;
 			break;
 		case '\n':
 			addold(&h, " \n");