big: support %v and # modifier, better handling of unknown formats

R=r
CC=golang-dev
https://golang.org/cl/4536065

src/pkg/big/int.go

@@ -309,11 +309,13 @@ func (x *Int) Cmp(y *Int) (r int) {
 
 
 func (x *Int) String() string {
-	s := ""
-	if x.neg {
-		s = "-"
+	switch {
+	case x == nil:
+		return "<nil>"
+	case x.neg:
+		return "-" + x.abs.decimalString()
 	}
-	return s + x.abs.decimalString()
+	return x.abs.decimalString()
 }
 
 
@@ -323,7 +325,7 @@ func charset(ch int) string {
 		return lowercaseDigits[0:2]
 	case 'o':
 		return lowercaseDigits[0:8]
-	case 'd':
+	case 'd', 'v':
 		return lowercaseDigits[0:10]
 	case 'x':
 		return lowercaseDigits[0:16]
@@ -339,14 +341,36 @@ func charset(ch int) string {
 // (lowercase hexadecimal), and 'X' (uppercase hexadecimal).
 //
 func (x *Int) Format(s fmt.State, ch int) {
-	if x == nil {
+	cs := charset(ch)
+
+	// special cases
+	switch {
+	case cs == "":
+		// unknown format
+		fmt.Fprintf(s, "%%!%c(big.Int=%s)", ch, x.String())
+		return
+	case x == nil:
 		fmt.Fprint(s, "<nil>")
 		return
 	}
+
+	// determine format
+	format := "%s"
+	if s.Flag('#') {
+		switch ch {
+		case 'o':
+			format = "0%s"
+		case 'x':
+			format = "0x%s"
+		case 'X':
+			format = "0X%s"
+		}
+	}
 	if x.neg {
-		fmt.Fprint(s, "-")
+		format = "-" + format
 	}
-	fmt.Fprint(s, x.abs.string(charset(ch)))
+
+	fmt.Fprintf(s, format, x.abs.string(cs))
 }
 
 

src/pkg/big/int_test.go

@@ -348,6 +348,55 @@ func TestSetString(t *testing.T) {
 }
 
 
+var formatTests = []struct {
+	input  string
+	format string
+	output string
+}{
+	{"<nil>", "%x", "<nil>"},
+	{"<nil>", "%#x", "<nil>"},
+	{"<nil>", "%#y", "%!y(big.Int=<nil>)"},
+
+	{"10", "%b", "1010"},
+	{"10", "%o", "12"},
+	{"10", "%d", "10"},
+	{"10", "%v", "10"},
+	{"10", "%x", "a"},
+	{"10", "%X", "A"},
+	{"-10", "%X", "-A"},
+	{"10", "%y", "%!y(big.Int=10)"},
+	{"-10", "%y", "%!y(big.Int=-10)"},
+
+	{"10", "%#b", "1010"},
+	{"10", "%#o", "012"},
+	{"10", "%#d", "10"},
+	{"10", "%#v", "10"},
+	{"10", "%#x", "0xa"},
+	{"10", "%#X", "0XA"},
+	{"-10", "%#X", "-0XA"},
+	{"10", "%#y", "%!y(big.Int=10)"},
+	{"-10", "%#y", "%!y(big.Int=-10)"},
+}
+
+
+func TestFormat(t *testing.T) {
+	for i, test := range formatTests {
+		var x *Int
+		if test.input != "<nil>" {
+			var ok bool
+			x, ok = new(Int).SetString(test.input, 0)
+			if !ok {
+				t.Errorf("#%d failed reading input %s", i, test.input)
+			}
+		}
+		output := fmt.Sprintf(test.format, x)
+		if output != test.output {
+			t.Errorf("#%d got %s; want %s", i, output, test.output)
+		}
+	}
+}
+
+
 // Examples from the Go Language Spec, section "Arithmetic operators"
 var divisionSignsTests = []struct {
 	x, y int64
@@ -984,6 +1033,7 @@ func testBitFunSelf(t *testing.T, msg string, f bitFun, x, y *Int, exp string) {
 	}
 }
 
+
 func altBit(x *Int, i int) uint {
 	z := new(Int).Rsh(x, uint(i))
 	z = z.And(z, NewInt(1))
@@ -993,6 +1043,7 @@ func altBit(x *Int, i int) uint {
 	return 0
 }
 
+
 func altSetBit(z *Int, x *Int, i int, b uint) *Int {
 	one := NewInt(1)
 	m := one.Lsh(one, uint(i))
@@ -1005,6 +1056,7 @@ func altSetBit(z *Int, x *Int, i int, b uint) *Int {
 	panic("set bit is not 0 or 1")
 }
 
+
 func testBitset(t *testing.T, x *Int) {
 	n := x.BitLen()
 	z := new(Int).Set(x)
@@ -1042,6 +1094,7 @@ func testBitset(t *testing.T, x *Int) {
 	}
 }
 
+
 var bitsetTests = []struct {
 	x string
 	i int
@@ -1061,6 +1114,7 @@ var bitsetTests = []struct {
 	{"-0x2000000000000000000000000001", 110, 1},
 }
 
+
 func TestBitSet(t *testing.T) {
 	for _, test := range bitwiseTests {
 		x := new(Int)
@@ -1081,6 +1135,7 @@ func TestBitSet(t *testing.T) {
 	}
 }
 
+
 func BenchmarkBitset(b *testing.B) {
 	z := new(Int)
 	z.SetBit(z, 512, 1)
@@ -1091,6 +1146,7 @@ func BenchmarkBitset(b *testing.B) {
 	}
 }
 
+
 func BenchmarkBitsetNeg(b *testing.B) {
 	z := NewInt(-1)
 	z.SetBit(z, 512, 0)
@@ -1101,6 +1157,7 @@ func BenchmarkBitsetNeg(b *testing.B) {
 	}
 }
 
+
 func BenchmarkBitsetOrig(b *testing.B) {
 	z := new(Int)
 	altSetBit(z, z, 512, 1)
@@ -1111,6 +1168,7 @@ func BenchmarkBitsetOrig(b *testing.B) {
 	}
 }
 
+
 func BenchmarkBitsetNegOrig(b *testing.B) {
 	z := NewInt(-1)
 	altSetBit(z, z, 512, 0)
@@ -1121,6 +1179,7 @@ func BenchmarkBitsetNegOrig(b *testing.B) {
 	}
 }
 
+
 func TestBitwise(t *testing.T) {
 	x := new(Int)
 	y := new(Int)
@@ -1155,6 +1214,7 @@ var notTests = []struct {
 	},
 }
 
+
 func TestNot(t *testing.T) {
 	in := new(Int)
 	out := new(Int)
@@ -1183,6 +1243,7 @@ var modInverseTests = []struct {
 	{"239487239847", "2410312426921032588552076022197566074856950548502459942654116941958108831682612228890093858261341614673227141477904012196503648957050582631942730706805009223062734745341073406696246014589361659774041027169249453200378729434170325843778659198143763193776859869524088940195577346119843545301547043747207749969763750084308926339295559968882457872412993810129130294592999947926365264059284647209730384947211681434464714438488520940127459844288859336526896320919633919"},
 }
 
+
 func TestModInverse(t *testing.T) {
 	var element, prime Int
 	one := NewInt(1)

src/pkg/big/nat.go

@@ -677,13 +677,14 @@ const (
 
 // string converts x to a string using digits from a charset; a digit with
 // value d is represented by charset[d]. The conversion base is determined
-// by len(charset). If len(charset) < 2, the result is "<illegal base>".
+// by len(charset), which must be >= 2.
 func (x nat) string(charset string) string {
 	base := len(charset)
 
+	// special cases
 	switch {
 	case base < 2:
-		return "<illegal base>"
+		panic("illegal base")
 	case len(x) == 0:
 		return string(charset[0])
 	}