improve fuzz testing and fix actual crash

fpconv/dtoa.go

@@ -51,7 +51,7 @@ func minv(a, b int) int {
 
 func Dtoa(d float64) string {
 	var (
-		dest   [24]rune
+		dest   [25]rune // Note C has 24, which is broken
 		digits [18]rune
 
 		str_len int = 0

fpconv/fpconv_test.go

@@ -69,4 +69,8 @@ func TestFormatFloat(t *testing.T) {
 	eq(t, "3.5999999999999996", a)
 	a += 1.2
 	eq(t, "4.8", a)
+
+	// fuzz cases
+	eq(t, "386804001288986340000000", 3.8680400128898634e+23)   // not a fuzz case...
+	eq(t, "-386804001288986340000000", -3.8680400128898634e+23) // ...but this one crashed
 }

fpconv/fuzz_test.go

@@ -1,10 +1,10 @@
-//go:build !go1.14
-// +build !go1.14
+//go:build go1.16
+// +build go1.16
 
 package fpconv
 
 import (
-	"strings"
+	"strconv"
 	"testing"
 )
 
@@ -19,11 +19,12 @@ func FuzzDtoa(f *testing.F) {
 		if s == "" {
 			t.Errorf("empty result")
 		}
-		if strings.Count(s, ".") > 1 {
-			t.Errorf("too many .s")
+		n, err := strconv.ParseFloat(s, 64)
+		if err != nil {
+			t.Errorf("parse failed: %s", err)
 		}
-		if strings.Count(s, "e") > 1 {
-			t.Errorf("too many e's")
+		if n != orig {
+			t.Errorf("changed %f -> %f", n, orig)
 		}
 	})
 }