passing numeric-start should imply using numeric suffixes

src/main.rs

@@ -42,9 +42,9 @@ struct Options {
     #[argh(switch, short = 'd')]
     numeric: bool,
 
-    /// use numeric suffixes starting with 0, not alphabetic
-    #[argh(option, long = "numeric-start", default = "0")]
-    numeric_start: usize,
+    /// use numeric suffixes starting with this value
+    #[argh(option, long = "numeric-start")]
+    numeric_start: Option<usize>,
 
     /// filename to read from, or "-" for stdin (default "-")
     #[argh(positional, default = "String::from(\"-\")")]
@@ -94,6 +94,7 @@ impl Iterator for AlphabeticSuffixGenerator {
 fn try_main() -> Result<(), Box<dyn Error>> {
     let opts: Options = argh::from_env();
 
+    // open source file
     let input: Box<dyn Read> = match opts.filename.as_str() {
         "-" => Box::new(io::stdin()),
         path => match File::open(path) {
@@ -101,22 +102,30 @@ fn try_main() -> Result<(), Box<dyn Error>> {
             Ok(file) => Box::new(file),
         },
     };
+    let mut reader = BufReader::new(input);
 
+    // build prefix format
     let prefix = match opts.prefix.as_str() {
         "" => String::from(""),
         _ => opts.prefix + "_",
     };
 
-    let suffix_gen: Box<dyn Iterator<Item = String>> = if opts.numeric {
+    // use numeric suffixes if --numeric or --numeric-start are passed
+    let use_numeric = opts.numeric || opts.numeric_start.is_some();
+
+    let suffix_gen: Box<dyn Iterator<Item = String>> = if use_numeric {
+        let numeric_start = match opts.numeric_start {
+            Some(num) => num,
+            None => 0,
+        };
         Box::new(
-            (opts.numeric_start..(opts.numeric_start + opts.count)).map(|num| format!("{}", num)),
+            (numeric_start..(numeric_start + opts.count)).map(|num| format!("{}", num)),
         )
     } else {
         Box::new(AlphabeticSuffixGenerator::new(opts.count))
     };
 
     let mut eof = false;
-    let mut reader = BufReader::new(input);
     for suffix in suffix_gen {
         let mut out_file = File::create(format!("{}{}", prefix, suffix))?;
         let mut line = String::new();