Works

codex-rs/cli/examples/fibonacci.rs

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+//! Print the Fibonacci sequence.
+//!
+//! Usage:
+//!   cargo run -p codex-cli --example fibonacci -- [COUNT]
+//!
+//! If COUNT is omitted, the first 10 numbers are printed.
+
+use std::env;
+use std::process;
+
+fn fibonacci(count: usize) -> Vec<u128> {
+    let mut seq = Vec::with_capacity(count);
+    if count == 0 {
+        return seq;
+    }
+    // Start with 0, 1
+    let mut a: u128 = 0;
+    let mut b: u128 = 1;
+    for _ in 0..count {
+        seq.push(a);
+        let next = a.saturating_add(b);
+        a = b;
+        b = next;
+    }
+    seq
+}
+
+fn parse_count_arg() -> Result<usize, String> {
+    let mut args = env::args().skip(1);
+    match args.next() {
+        None => Ok(10), // default
+        Some(s) => s
+            .parse::<usize>()
+            .map_err(|_| format!("Invalid COUNT: '{}' (expected a non-negative integer)", s)),
+    }
+}
+
+fn main() {
+    let count = match parse_count_arg() {
+        Ok(n) => n,
+        Err(e) => {
+            eprintln!(
+                "{}\nUsage: cargo run -p codex-cli --example fibonacci -- [COUNT]",
+                e
+            );
+            process::exit(2);
+        }
+    };
+
+    for n in fibonacci(count) {
+        println!("{}", n);
+    }
+}