fastfloat: add fuzzy test from #22 (comment)

fastfloat/parse.go

@@ -96,8 +96,10 @@ func ParseInt64BestEffort(s string) int64 {
 }
 
 // Exact powers of 10.
+//
+// This works faster than math.Pow10, since it avoids additional multiplication.
 var float64pow10 = [...]float64{
-	1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9, 1e10, 1e11, 1e12, 1e13, 1e14, 1e15, 1e16, 1e17, 1e18,
+	1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9, 1e10, 1e11, 1e12, 1e13, 1e14, 1e15, 1e16,
 }
 
 // ParseBestEffort parses floating-point number s.
@@ -169,8 +171,8 @@ func ParseBestEffort(s string) float64 {
 			if s[i] >= '0' && s[i] <= '9' {
 				d = d*10 + uint64(s[i]-'0')
 				i++
-				if i-j > 18 {
-					// The mantissa may be out of range.  Fall back to standard parsing.
+				if i-j >= uint(len(float64pow10)) {
+					// The mantissa is out of range. Fall back to standard parsing.
 					f, err := strconv.ParseFloat(s, 64)
 					if err != nil && !math.IsInf(f, 0) {
 						return 0
@@ -185,7 +187,7 @@ func ParseBestEffort(s string) float64 {
 			return 0
 		}
 		// Convert the entire mantissa to a float at once to avoid rounding errors.
-		f = float64(d) / float64pow10[int(i-k)]
+		f = float64(d) / float64pow10[i-k]
 		if i >= uint(len(s)) {
 			// Fast path - parsed fractional number.
 			if minus {

fastfloat/parse_test.go

@@ -2,6 +2,8 @@ package fastfloat
 
 import (
 	"math"
+	"math/rand"
+	"strconv"
 	"testing"
 )
 
@@ -189,3 +191,19 @@ func TestParseBestEffort(t *testing.T) {
 	f("naN", math.NaN())
 	f("NaN", math.NaN())
 }
+
+func TestParseBestEffortFuzz(t *testing.T) {
+	r := rand.New(rand.NewSource(0))
+	for i := 0; i < 100000; i++ {
+		f := r.Float64()
+		s := strconv.FormatFloat(f, 'g', -1, 64)
+		numExpected, err := strconv.ParseFloat(s, 64)
+		if err != nil {
+			t.Fatalf("unexpected error when parsing %q: %s", s, err)
+		}
+		num := ParseBestEffort(s)
+		if num != numExpected {
+			t.Fatalf("unexpected number parsed from %q; got %g; want %g", s, num, numExpected)
+		}
+	}
+}

fastfloat/parse_timing_test.go

@@ -16,7 +16,7 @@ func BenchmarkParseInt64BestEffort(b *testing.B) {
 }
 
 func BenchmarkParseBestEffort(b *testing.B) {
-	for _, s := range []string{"0", "12", "12345", "1234567890", "1234.45678", "1234e45", "12.34e-34", "12345.123456789012", "12345.1234567890123"} {
+	for _, s := range []string{"0", "12", "12345", "1234567890", "1234.45678", "1234e45", "12.34e-34", "12345.1234567890", "12345.12345678901"} {
 		b.Run(s, func(b *testing.B) {
 			benchmarkParseBestEffort(b, s)
 		})

go.mod

@@ -1,3 +1,3 @@
 module github.com/valyala/fastjson
 
-go 1.13
+go 1.12