Add fuzzing for Comma

comma_fuzz_test.go

@@ -0,0 +1,58 @@
+//go:build go1.18
+// +build go1.18
+
+package humanize_test
+
+import (
+	"math"
+	"strconv"
+	"strings"
+	"testing"
+
+	"github.com/dustin/go-humanize"
+)
+
+func FuzzComma(f *testing.F) {
+	f.Add(int64(0))
+	f.Add(int64(-1))
+	f.Add(int64(10))
+	f.Add(int64(100))
+	f.Add(int64(1000))
+	f.Add(int64(-1000))
+	f.Add(int64(math.MaxInt64))
+	f.Add(int64(math.MaxInt64) - 1)
+	f.Add(int64(math.MinInt64))
+	f.Add(int64(math.MinInt64) + 1)
+
+	f.Fuzz(func(t *testing.T, v int64) {
+		got := humanize.Comma(v)
+		gotNoCommas := strings.ReplaceAll(got, ",", "")
+		expected := strconv.FormatInt(v, 10)
+		if gotNoCommas != expected {
+			t.Fatalf("%d: got %q, expected %q", v, got, expected)
+		}
+
+		if v < 0 {
+			if got[0] != '-' {
+				t.Fatalf("%d: got: %q", v, got)
+			}
+			// Remove sign
+			got = got[1:]
+		}
+		// Check that commas are located every 3 digits
+		l := len(got)
+		for i := l - 1; i >= 0; i-- {
+			var ok bool
+			if (l-1-i)%4 == 3 {
+				ok = got[i] == ','
+			} else {
+				ok = got[i] >= '0' && got[i] <= '9'
+			}
+			if !ok {
+				t.Log(l - 1 - i)
+				t.Log((l - 1 - i) % 4)
+				t.Fatalf("%d: got: %q", v, got)
+			}
+		}
+	})
+}