1 files changed, 67 insertions, 0 deletions
diff --git a/lib/util/fmt.go b/lib/util/fmt.go
new file mode 100644
index 0000000..af7404c
--- /dev/null
+++ b/lib/util/fmt.go
@@ -0,0 +1,67 @@
+package util
+
+import (
+	"fmt"
+	"strings"
+)
+
+// FmtStateString returns the fmt.Printf string that produced a given
+// fmt.State and verb.
+func FmtStateString(st fmt.State, verb rune) string {
+	var ret strings.Builder
+	ret.WriteByte('%')
+	for _, flag := range []int{'-', '+', '#', ' ', '0'} {
+		if st.Flag(flag) {
+			ret.WriteByte(byte(flag))
+		}
+	}
+	if width, ok := st.Width(); ok {
+		fmt.Fprintf(&ret, "%v", width)
+	}
+	if prec, ok := st.Precision(); ok {
+		if prec == 0 {
+			ret.WriteByte('.')
+		} else {
+			fmt.Fprintf(&ret, ".%v", prec)
+		}
+	}
+	ret.WriteRune(verb)
+	return ret.String()
+}
+
+// FormatByteArrayStringer is function for helping to implement
+// fmt.Formatter for []byte or [n]byte types that have a custom string
+// representation.  Use it like:
+//
+//     type MyType [16]byte
+//
+//     func (val MyType) String() string {
+//         …
+//     }
+//
+//     func (val MyType) Format(f fmt.State, verb rune) {
+//         util.FormatByteArrayStringer(val, val[:], f, verb)
+//     }
+func FormatByteArrayStringer(
+	obj interface {
+		fmt.Stringer
+		fmt.Formatter
+	},
+	objBytes []byte,
+	f fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		if !f.Flag('#') {
+			FormatByteArrayStringer(obj, objBytes, f, 's') // as a string
+		} else {
+			byteStr := fmt.Sprintf("%#v", objBytes)
+			objType := fmt.Sprintf("%T", obj)
+			objStr := objType + strings.TrimPrefix(byteStr, "[]byte")
+			fmt.Fprintf(f, FmtStateString(f, 's'), objStr)
+		}
+	case 's', 'q': // string
+		fmt.Fprintf(f, FmtStateString(f, verb), obj.String())
+	default:
+		fmt.Fprintf(f, FmtStateString(f, verb), objBytes)
+	}
+}