feat: add Format

README.md

@@ -74,7 +74,6 @@ package main
 import (
 	"encoding/json"
 	"fmt"
-	"io"
 	"os"
 
 	"github.com/go-playground/sensitive"
@@ -84,10 +83,10 @@ func init() {
 	// override default Formatter
 	sensitive.FormatStringFn = func(s sensitive.String, f fmt.State, c rune) {
 		switch c {
-		case 's':
-			_, _ = io.WriteString(f, "redacted")
+		default:
+		        sensitive.Format(f, c, "redacted")
 		case 'v':
-			_, _ = io.WriteString(f, string(s)[:4]+"*******")
+		        sensitive.Format(f, c, string(s)[:4]+"*******")
 		}
 	}
 }

_examples/custom/main.go

@@ -3,7 +3,6 @@ package main
 import (
 	"encoding/json"
 	"fmt"
-	"io"
 	"os"
 
 	"github.com/go-playground/sensitive"
@@ -13,10 +12,10 @@ func init() {
 	// override default Formatter
 	sensitive.FormatStringFn = func(s sensitive.String, f fmt.State, c rune) {
 		switch c {
-		case 's':
-			_, _ = io.WriteString(f, "redacted")
+		default:
+			sensitive.Format(f, c, "redacted")
 		case 'v':
-			_, _ = io.WriteString(f, string(s)[:4]+"*******")
+			sensitive.Format(f, c, string(s)[:4]+"*******")
 		}
 	}
 }

format.go

@@ -0,0 +1,39 @@
+package sensitive
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+// Format outputs value accordingly to formatting options.
+//
+// It is useful in case you'll redefine some Format<type>Fn to output
+// redacted value using formatting applied to original value.
+//
+//     sensitive.FormatStringFn = func(s sensitive.String, f fmt.State, c rune) {
+//         sensitive.Format(f, c, "REDACTED")
+//     }
+//     sensitive.FormatBytesFn = func(s sensitive.Bytes, f fmt.State, c rune) {
+//         sensitive.Format(f, c, []byte{0xDE, 0xFA, 0xCE})
+//     }
+func Format(f fmt.State, c rune, value interface{}) {
+	const flags = "+-# 0"
+	var format strings.Builder
+	format.Grow(8)
+	format.WriteRune('%')
+	for _, c := range flags {
+		if f.Flag(int(c)) {
+			format.WriteRune(c)
+		}
+	}
+	if wid, ok := f.Width(); ok {
+		format.WriteString(strconv.Itoa(wid))
+	}
+	if prec, ok := f.Precision(); ok {
+		format.WriteRune('.')
+		format.WriteString(strconv.Itoa(prec))
+	}
+	format.WriteRune(c)
+	fmt.Fprintf(f, format.String(), value)
+}

format_test.go

@@ -0,0 +1,38 @@
+package sensitive
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestFormat(t *testing.T) {
+	oldFn := FormatStringFn
+	defer func() {
+		FormatStringFn = oldFn
+	}()
+	FormatStringFn = func(s String, f fmt.State, c rune) {
+		Format(f, c, string(s))
+	}
+
+	tests := []struct {
+		formatting string
+	}{
+		{"%s"},
+		{"%q"},
+		{"%10s"},
+		{"%.3[1]q"},
+		{"%#-10v"},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.formatting, func(t *testing.T) {
+			assert := require.New(t)
+			want := fmt.Sprintf(tc.formatting, "value")
+			result := fmt.Sprintf(tc.formatting, String("value"))
+			assert.Equal(want, result)
+		})
+	}
+}

string_test.go

@@ -131,7 +131,7 @@ func TestStringCustomFormatFn(t *testing.T) {
 		FormatStringFn = oldFn
 	}()
 	FormatStringFn = func(s String, f fmt.State, c rune) {
-		_, _ = f.Write([]byte("blah"))
+		Format(f, c, "blah")
 	}
 
 	value := String("value")