cmd/streamdata: y/n prompt helper

This returns a boolean value with fallback.

Signed-off-by: Yonah <contrib@gensokyo.uk>

cmd/streamdata/prompt.go

@@ -126,6 +126,40 @@ invalid:
 	return v, true
 }
 
+// promptBool prompts the user for a y/n response until a valid response or a
+// single newline character is read.
+func promptBool(
+	br *bufio.Reader,
+	p string,
+	fallback bool,
+) (bool, bool) {
+	if fallback {
+		p += "[Y/n] "
+	} else {
+		p += "[y/N] "
+	}
+
+invalid:
+	s, ok := prompt(br, p)
+	if !ok {
+		return false, false
+	}
+
+	switch strings.ToLower(strings.TrimSpace(s)) {
+	default:
+		goto invalid
+
+	case "":
+		return fallback, true
+
+	case "y":
+		return true, true
+
+	case "n":
+		return false, true
+	}
+}
+
 // require panics with an error if ok is false, and returns v otherwise.
 func require[T any](v T, ok bool) T {
 	if !ok {