math/big: implement Float.Scan, type assert fmt interfaces to enforce docs

Implements Float.Scan which satisfies fmt.Scanner interface.
Also enforces docs' interface implementation claims with compile time
type assertions, that is:
+ Float always implements fmt.Formatter and fmt.Scanner
+ Int always implements fmt.Formatter and fmt.Scanner
+ Rat always implements fmt.Formatter
which will ensure that the API claims are strictly matched.

Also note that Float.Scan doesn't handle ±Inf.

Fixes #17391

Change-Id: I3d3dfbe7f602066975c7a7794fe25b4c645440ce
Reviewed-on: https://go-review.googlesource.com/30723
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/example_test.go

@@ -51,6 +51,19 @@ func ExampleInt_Scan() {
 	// Output: 18446744073709551617
 }
 
+func ExampleFloat_Scan() {
+	// The Scan function is rarely used directly;
+	// the fmt package recognizes it as an implementation of fmt.Scanner.
+	f := new(big.Float)
+	_, err := fmt.Sscan("1.19282e99", f)
+	if err != nil {
+		log.Println("error scanning value:", err)
+	} else {
+		fmt.Println(f)
+	}
+	// Output: 1.19282e+99
+}
+
 // This example demonstrates how to use big.Int to compute the smallest
 // Fibonacci number with 100 decimal digits and to test whether it is prime.
 func Example_fibonacci() {

src/math/big/floatconv.go

@@ -12,6 +12,8 @@ import (
 	"strings"
 )
 
+var floatZero Float
+
 // SetString sets z to the value of s and returns z and a boolean indicating
 // success. s must be a floating-point number of the same format as accepted
 // by Parse, with base argument 0. The entire string (not just a prefix) must
@@ -276,3 +278,16 @@ func (z *Float) Parse(s string, base int) (f *Float, b int, err error) {
 func ParseFloat(s string, base int, prec uint, mode RoundingMode) (f *Float, b int, err error) {
 	return new(Float).SetPrec(prec).SetMode(mode).Parse(s, base)
 }
+
+var _ fmt.Scanner = &floatZero // *Float must implement fmt.Scanner
+
+// Scan is a support routine for fmt.Scanner; it sets z to the value of
+// the scanned number. It accepts formats whose verbs are supported by
+// fmt.Scan for floating point values, which are:
+// 'b' (binary), 'e', 'E', 'f', 'F', 'g' and 'G'.
+// Scan doesn't handle ±Inf.
+func (z *Float) Scan(s fmt.ScanState, ch rune) error {
+	s.SkipSpace()
+	_, _, err := z.scan(byteReader{s}, 0)
+	return err
+}

src/math/big/floatconv_test.go

@@ -5,6 +5,7 @@
 package big
 
 import (
+	"bytes"
 	"fmt"
 	"math"
 	"strconv"
@@ -665,3 +666,54 @@ func BenchmarkParseFloatLargeExp(b *testing.B) {
 		}
 	}
 }
+
+func TestFloatScan(t *testing.T) {
+	var floatScanTests = []struct {
+		input     string
+		format    string
+		output    string
+		remaining int
+		wantErr   bool
+	}{
+		0: {"10.0", "%f", "10", 0, false},
+		1: {"23.98+2.0", "%v", "23.98", 4, false},
+		2: {"-1+1", "%v", "-1", 2, false},
+		3: {" 00000", "%v", "0", 0, false},
+		4: {"-123456p-78", "%b", "-4.084816388e-19", 0, false},
+		5: {"+123", "%b", "123", 0, false},
+		6: {"-1.234e+56", "%e", "-1.234e+56", 0, false},
+		7: {"-1.234E-56", "%E", "-1.234e-56", 0, false},
+		8: {"-1.234e+567", "%g", "-1.234e+567", 0, false},
+		9: {"+1234567891011.234", "%G", "1.234567891e+12", 0, false},
+
+		// Scan doesn't handle ±Inf.
+		10: {"Inf", "%v", "", 3, true},
+		11: {"-Inf", "%v", "", 3, true},
+		12: {"-Inf", "%v", "", 3, true},
+	}
+
+	var buf bytes.Buffer
+	for i, test := range floatScanTests {
+		x := new(Float)
+		buf.Reset()
+		buf.WriteString(test.input)
+		_, err := fmt.Fscanf(&buf, test.format, x)
+		if test.wantErr {
+			if err == nil {
+				t.Errorf("#%d want non-nil err", i)
+			}
+			continue
+		}
+
+		if err != nil {
+			t.Errorf("#%d error: %s", i, err)
+		}
+
+		if x.String() != test.output {
+			t.Errorf("#%d got %s; want %s", i, x.String(), test.output)
+		}
+		if buf.Len() != test.remaining {
+			t.Errorf("#%d got %d bytes remaining; want %d", i, buf.Len(), test.remaining)
+		}
+	}
+}

src/math/big/ftoa.go

@@ -376,6 +376,8 @@ func min(x, y int) int {
 	return y
 }
 
+var _ fmt.Formatter = &floatZero // *Float must implement fmt.Formatter
+
 // Format implements fmt.Formatter. It accepts all the regular
 // formats for floating-point numbers ('b', 'e', 'E', 'f', 'F',
 // 'g', 'G') as well as 'p' and 'v'. See (*Float).Text for the

src/math/big/intconv.go

@@ -52,6 +52,8 @@ func writeMultiple(s fmt.State, text string, count int) {
 	}
 }
 
+var _ fmt.Formatter = intOne // *Int must implement fmt.Formatter
+
 // Format implements fmt.Formatter. It accepts the formats
 // 'b' (binary), 'o' (octal), 'd' (decimal), 'x' (lowercase
 // hexadecimal), and 'X' (uppercase hexadecimal).
@@ -223,6 +225,8 @@ func (r byteReader) UnreadByte() error {
 	return r.UnreadRune()
 }
 
+var _ fmt.Scanner = intOne // *Int must implement fmt.Scanner
+
 // Scan is a support routine for fmt.Scanner; it sets z to the value of
 // the scanned number. It accepts the formats 'b' (binary), 'o' (octal),
 // 'd' (decimal), 'x' (lowercase hexadecimal), and 'X' (uppercase hexadecimal).

src/math/big/ratconv.go

@@ -18,6 +18,9 @@ func ratTok(ch rune) bool {
 	return strings.ContainsRune("+-/0123456789.eE", ch)
 }
 
+var ratZero Rat
+var _ fmt.Scanner = &ratZero // *Rat must implement fmt.Scanner
+
 // Scan is a support routine for fmt.Scanner. It accepts the formats
 // 'e', 'E', 'f', 'F', 'g', 'G', and 'v'. All formats are equivalent.
 func (z *Rat) Scan(s fmt.ScanState, ch rune) error {