fmt: support '*' for width or precision

R=r
CC=golang-dev
https://golang.org/cl/2237044

src/pkg/fmt/doc.go

@@ -47,7 +47,9 @@
 	number of places after the decimal, if appropriate.  The
 	format %6.2f prints 123.45. The width of a field is the number
 	of Unicode code points in the string. This differs from C's printf where
-	the field width is the number of bytes.
+	the field width is the number of bytes.  Either or both of the
+	flags may be replaced with the character '*', causing their values
+	to be obtained from the next operand, which must be of type int.
 
 	Other flags:
 		+	always print a sign for numeric values

src/pkg/fmt/fmt_test.go

@@ -605,3 +605,45 @@ func TestFormatterPrintln(t *testing.T) {
 		t.Errorf("Sprintf wrong with Formatter: expected %q got %q\n", expect, s)
 	}
 }
+
+func args(a ...interface{}) []interface{} { return a }
+
+type starTest struct {
+	fmt string
+	in  []interface{}
+	out string
+}
+
+var startests = []starTest{
+	starTest{"%*d", args(4, 42), "  42"},
+	starTest{"%.*d", args(4, 42), "0042"},
+	starTest{"%*.*d", args(8, 4, 42), "    0042"},
+	starTest{"%0*d", args(4, 42), "0042"},
+	starTest{"%-*d", args(4, 42), "42  "},
+
+	// erroneous
+	starTest{"%*d", args(nil, 42), "%(badwidth)42"},
+	starTest{"%.*d", args(nil, 42), "%(badprec)42"},
+	starTest{"%*d", args(5, "foo"), "%d(string=  foo)"},
+	starTest{"%*% %d", args(20, 5), "% 5"},
+	starTest{"%*", args(4), "%(badwidth)%*(int=4)"},
+	starTest{"%*d", args(int32(4), 42), "%(badwidth)42"},
+}
+
+// TODO: there's no conversion from []T to ...T, but we can fake it.  These
+// functions do the faking.  We index the table by the length of the param list.
+var sprintf = []func(string, []interface{}) string{
+	0: func(f string, i []interface{}) string { return Sprintf(f) },
+	1: func(f string, i []interface{}) string { return Sprintf(f, i[0]) },
+	2: func(f string, i []interface{}) string { return Sprintf(f, i[0], i[1]) },
+	3: func(f string, i []interface{}) string { return Sprintf(f, i[0], i[1], i[2]) },
+}
+
+func TestWidthAndPrecision(t *testing.T) {
+	for _, tt := range startests {
+		s := sprintf[len(tt.in)](tt.fmt, tt.in)
+		if s != tt.out {
+			t.Errorf("got %q expected %q", s, tt.out)
+		}
+	}
+}

src/pkg/fmt/print.go

@@ -24,6 +24,8 @@ var (
 	extraBytes      = []byte("?(extra ")
 	irparenBytes    = []byte("i)")
 	bytesBytes      = []byte("[]byte{")
+	widthBytes      = []byte("%(badwidth)")
+	precBytes       = []byte("%(badprec)")
 )
 
 // State represents the printer state passed to custom formatters.
@@ -782,6 +784,16 @@ BigSwitch:
 	return false
 }
 
+// intFromArg gets the fieldnumth element of a. On return, isInt reports whether the argument has type int.
+func intFromArg(a []interface{}, end, i, fieldnum int) (num int, isInt bool, newi, newfieldnum int) {
+	newi, newfieldnum = end, fieldnum
+	if i < end && fieldnum < len(a) {
+		num, isInt = a[fieldnum].(int)
+		newi, newfieldnum = i+1, fieldnum+1
+	}
+	return
+}
+
 func (p *pp) doPrintf(format string, a []interface{}) {
 	end := len(format) - 1
 	fieldnum := 0 // we process one field per non-trivial format
@@ -816,11 +828,25 @@ func (p *pp) doPrintf(format string, a []interface{}) {
 				break F
 			}
 		}
-		// do we have 20 (width)?
-		p.fmt.wid, p.fmt.widPresent, i = parsenum(format, i, end)
-		// do we have .20 (precision)?
+		// do we have width?
+		if format[i] == '*' {
+			p.fmt.wid, p.fmt.widPresent, i, fieldnum = intFromArg(a, end, i, fieldnum)
+			if !p.fmt.widPresent {
+				p.buf.Write(widthBytes)
+			}
+		} else {
+			p.fmt.wid, p.fmt.widPresent, i = parsenum(format, i, end)
+		}
+		// do we have precision?
 		if i < end && format[i] == '.' {
-			p.fmt.prec, p.fmt.precPresent, i = parsenum(format, i+1, end)
+			if format[i+1] == '*' {
+				p.fmt.prec, p.fmt.precPresent, i, fieldnum = intFromArg(a, end, i+1, fieldnum)
+				if !p.fmt.precPresent {
+					p.buf.Write(precBytes)
+				}
+			} else {
+				p.fmt.prec, p.fmt.precPresent, i = parsenum(format, i+1, end)
+			}
 		}
 		c, w = utf8.DecodeRuneInString(format[i:])
 		i += w

src/pkg/fmt/scan_test.go

@@ -289,8 +289,6 @@ var s, t string
 var c complex
 var x, y Xs
 
-func args(a ...interface{}) []interface{} { return a }
-
 var multiTests = []ScanfMultiTest{
 	ScanfMultiTest{"", "", nil, nil, ""},
 	ScanfMultiTest{"%d", "23", args(&i), args(23), ""},