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diff: support unified diff via -u

The format produced by `diff -u` is inferior to that
produced by `diff -c`, but it's what ape/patch and
unix patch expect, so it's useful to generate it.

This patch adds `diff -u`.

--- a/sys/man/1/diff

+++ b/sys/man/1/diff

@@ -4,7 +4,7 @@

 .SH SYNOPSIS

 .B diff

 [

-.B -abcefmnrw

+.B -abcefmnruw

 ] 

 .I file1 ... file2

 .SH DESCRIPTION

@@ -142,6 +142,24 @@

 The

 .B -a

 flag displays the entire file as context.

+.PP

+The

+.B -u

+option provides a unix-compatible unified diff.

+This format is similar to that provided by

+.BR -c .

+However, the

+.L +

+and

+.L -

+prefixes are not separated from the rest of the line by spaces,

+and the file header is in the following format:

+.IP

+.EX

+--- filename.old

++++ filename.new

+@@ -line,len +line,len @@

+.EE

 .PP

 Except in rare circumstances,

 .I diff

--- a/sys/src/cmd/diff/diff.h

+++ b/sys/src/cmd/diff/diff.h

@@ -22,5 +22,6 @@

 void panic(int, char *, ...);

 void check(Biobuf *, Biobuf *);

 void change(int, int, int, int);

+void fileheader(void);

 void flushchanges(void);

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

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

@@ -303,6 +303,7 @@

 		break;

 	case 'c':

 	case 'a':

+	case 'u':

 		if(nchanges%1024 == 0)

 			changes = erealloc(changes, (nchanges+1024)*sizeof(changes[0]));

 		ch = &changes[nchanges++];

@@ -339,6 +340,15 @@

 }

 void

+fileheader(void)

+{

+	if(mode != 'u')

+		return;

+	Bprint(&stdout, "--- %s\n", file1);

+	Bprint(&stdout, "+++ %s\n", file2);

+}

+

+void

 flushchanges(void)

 {

 	int a, b, c, d, at;

@@ -368,20 +378,24 @@

 			d = len[1];

 			j = nchanges;

 		}

-		Bprint(&stdout, "%s:", file1);

-		range(a, b, ",");

-		Bprint(&stdout, " - ");

-		Bprint(&stdout, "%s:", file2);

-		range(c, d, ",");

-		Bputc(&stdout, '\n');

+		if(mode == 'u'){

+			Bprint(&stdout, "@@ -%d,%d +%d,%d @@\n", a, b-a+1, c, d-c+1);

+		}else{

+			Bprint(&stdout, "%s:", file1);

+			range(a, b, ",");

+			Bprint(&stdout, " - ");

+			Bprint(&stdout, "%s:", file2);

+			range(c, d, ",");

+			Bputc(&stdout, '\n');

+		}

 		at = a;

 		for(; i<j; i++){

-			fetch(ixold, at, changes[i].a-1, input[0], "  ");

-			fetch(ixold, changes[i].a, changes[i].b, input[0], "- ");

-			fetch(ixnew, changes[i].c, changes[i].d, input[1], "+ ");

+			fetch(ixold, at, changes[i].a-1, input[0], mode == 'u' ? " " : "  ");

+			fetch(ixold, changes[i].a, changes[i].b, input[0], mode == 'u' ? "-" : "- ");

+			fetch(ixnew, changes[i].c, changes[i].d, input[1], mode == 'u' ? "+" : "- ");

 			at = changes[i].b+1;

 		}

-		fetch(ixold, at, b, input[0], "  ");

+		fetch(ixold, at, b, input[0], mode == 'u' ? " " : "  ");

 	}

 	nchanges = 0;

 }

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

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

@@ -285,6 +285,7 @@

 	m = len[0];

 	J[0] = 0;

 	J[m+1] = len[1]+1;

+	fileheader();

 	if (mode != 'e') {

 		for (i0 = 1; i0 <= m; i0 = i1+1) {

 			while (i0 <= m && J[i0] == J[i0-1]+1)

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

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

@@ -192,6 +192,7 @@

 			case 'n':

 			case 'c':

 			case 'a':

+			case 'u':

 				mode = *p;

 				break;