[clap-v3-utils] Add `try_get_word_count` (#411)

clap-v3-utils/src/keygen/mnemonic.rs

@@ -25,16 +25,32 @@ pub const NO_PASSPHRASE_ARG: ArgConstant<'static> = ArgConstant {
     help: "Do not prompt for a BIP39 passphrase",
 };
 
+// The constant `POSSIBLE_WORD_COUNTS` and function `try_get_word_count` must always be updated in
+// sync
+const POSSIBLE_WORD_COUNTS: &[&str] = &["12", "15", "18", "21", "24"];
 pub fn word_count_arg<'a>() -> Arg<'a> {
     Arg::new(WORD_COUNT_ARG.name)
         .long(WORD_COUNT_ARG.long)
-        .value_parser(PossibleValuesParser::new(["12", "15", "18", "21", "24"]))
+        .value_parser(PossibleValuesParser::new(POSSIBLE_WORD_COUNTS))
         .default_value("12")
         .value_name("NUMBER")
         .takes_value(true)
         .help(WORD_COUNT_ARG.help)
 }
 
+pub fn try_get_word_count(matches: &ArgMatches) -> Result<Option<usize>, Box<dyn error::Error>> {
+    Ok(matches
+        .try_get_one::<String>(WORD_COUNT_ARG.name)?
+        .map(|count| match count.as_str() {
+            "12" => 12,
+            "15" => 15,
+            "18" => 18,
+            "21" => 21,
+            "24" => 24,
+            _ => unreachable!(),
+        }))
+}
+
 pub fn language_arg<'a>() -> Arg<'a> {
     Arg::new(LANGUAGE_ARG.name)
         .long(LANGUAGE_ARG.long)