Fix encoding format for float32 and float64

encode.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding"
 	"io"
+	"math"
 	"reflect"
 	"strconv"
 	"sync"
@@ -245,11 +246,29 @@ func (e *Encoder) encodeUint64(v uint64) {
 }
 
 func (e *Encoder) encodeFloat32(v float32) {
-	e.buf = strconv.AppendFloat(e.buf, float64(v), 'f', -1, 32)
+	f64 := float64(v)
+	abs := math.Abs(f64)
+	fmt := byte('f')
+	// Note: Must use float32 comparisons for underlying float32 value to get precise cutoffs right.
+	if abs != 0 {
+		f32 := float32(abs)
+		if f32 < 1e-6 || f32 >= 1e21 {
+			fmt = 'e'
+		}
+	}
+	e.buf = strconv.AppendFloat(e.buf, f64, fmt, -1, 32)
 }
 
 func (e *Encoder) encodeFloat64(v float64) {
-	e.buf = strconv.AppendFloat(e.buf, v, 'f', -1, 64)
+	abs := math.Abs(v)
+	fmt := byte('f')
+	// Note: Must use float32 comparisons for underlying float32 value to get precise cutoffs right.
+	if abs != 0 {
+		if abs < 1e-6 || abs >= 1e21 {
+			fmt = 'e'
+		}
+	}
+	e.buf = strconv.AppendFloat(e.buf, v, fmt, -1, 64)
 }
 
 func (e *Encoder) encodeBool(v bool) {

encode_test.go

@@ -3,7 +3,11 @@ package json_test
 import (
 	"errors"
 	"fmt"
+	"log"
+	"math"
 	"reflect"
+	"regexp"
+	"strconv"
 	"testing"
 	"time"
 
@@ -862,3 +866,110 @@ func TestMarshalerError(t *testing.T) {
 		}
 	}
 }
+
+var re = regexp.MustCompile
+
+// syntactic checks on form of marshaled floating point numbers.
+var badFloatREs = []*regexp.Regexp{
+	re(`p`),                     // no binary exponential notation
+	re(`^\+`),                   // no leading + sign
+	re(`^-?0[^.]`),              // no unnecessary leading zeros
+	re(`^-?\.`),                 // leading zero required before decimal point
+	re(`\.(e|$)`),               // no trailing decimal
+	re(`\.[0-9]+0(e|$)`),        // no trailing zero in fraction
+	re(`^-?(0|[0-9]{2,})\..*e`), // exponential notation must have normalized mantissa
+	re(`e[0-9]`),                // positive exponent must be signed
+	//re(`e[+-]0`),                // exponent must not have leading zeros
+	re(`e-[1-6]$`),             // not tiny enough for exponential notation
+	re(`e+(.|1.|20)$`),         // not big enough for exponential notation
+	re(`^-?0\.0000000`),        // too tiny, should use exponential notation
+	re(`^-?[0-9]{22}`),         // too big, should use exponential notation
+	re(`[1-9][0-9]{16}[1-9]`),  // too many significant digits in integer
+	re(`[1-9][0-9.]{17}[1-9]`), // too many significant digits in decimal
+	// below here for float32 only
+	re(`[1-9][0-9]{8}[1-9]`),  // too many significant digits in integer
+	re(`[1-9][0-9.]{9}[1-9]`), // too many significant digits in decimal
+}
+
+func TestMarshalFloat(t *testing.T) {
+	t.Parallel()
+	nfail := 0
+	test := func(f float64, bits int) {
+		vf := interface{}(f)
+		if bits == 32 {
+			f = float64(float32(f)) // round
+			vf = float32(f)
+		}
+		bout, err := json.Marshal(vf)
+		if err != nil {
+			t.Errorf("Marshal(%T(%g)): %v", vf, vf, err)
+			nfail++
+			return
+		}
+		out := string(bout)
+
+		// result must convert back to the same float
+		g, err := strconv.ParseFloat(out, bits)
+		if err != nil {
+			t.Errorf("Marshal(%T(%g)) = %q, cannot parse back: %v", vf, vf, out, err)
+			nfail++
+			return
+		}
+		if f != g || fmt.Sprint(f) != fmt.Sprint(g) { // fmt.Sprint handles ±0
+			t.Errorf("Marshal(%T(%g)) = %q (is %g, not %g)", vf, vf, out, float32(g), vf)
+			nfail++
+			return
+		}
+
+		bad := badFloatREs
+		if bits == 64 {
+			bad = bad[:len(bad)-2]
+		}
+		for _, re := range bad {
+			if re.MatchString(out) {
+				t.Errorf("Marshal(%T(%g)) = %q, must not match /%s/", vf, vf, out, re)
+				nfail++
+				return
+			}
+		}
+	}
+
+	var (
+		bigger  = math.Inf(+1)
+		smaller = math.Inf(-1)
+	)
+
+	var digits = "1.2345678901234567890123"
+	for i := len(digits); i >= 2; i-- {
+		if testing.Short() && i < len(digits)-4 {
+			break
+		}
+		for exp := -30; exp <= 30; exp++ {
+			for _, sign := range "+-" {
+				for bits := 32; bits <= 64; bits += 32 {
+					s := fmt.Sprintf("%c%se%d", sign, digits[:i], exp)
+					f, err := strconv.ParseFloat(s, bits)
+					if err != nil {
+						log.Fatal(err)
+					}
+					next := math.Nextafter
+					if bits == 32 {
+						next = func(g, h float64) float64 {
+							return float64(math.Nextafter32(float32(g), float32(h)))
+						}
+					}
+					test(f, bits)
+					test(next(f, bigger), bits)
+					test(next(f, smaller), bits)
+					if nfail > 50 {
+						t.Fatalf("stopping test early")
+					}
+				}
+			}
+		}
+	}
+	test(0, 64)
+	test(math.Copysign(0, -1), 64)
+	test(0, 32)
+	test(math.Copysign(0, -1), 32)
+}