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//@ run-pass
#![expect(incomplete_features)]
#![feature(explicit_tail_calls)]
/// A very unnecessarily complicated "implementation" of the Collatz conjecture.
/// Returns the number of steps to reach `1`.
///
/// This is just a test for tail calls, which involves multiple functions calling each other.
///
/// Panics if `x == 0`.
const fn collatz(x: u32) -> u32 {
assert!(x > 0);
const fn switch(x: u32, steps: u32) -> u32 {
match x {
1 => steps,
_ if x & 1 == 0 => become div2(x, steps + 1),
_ => become mul3plus1(x, steps + 1),
}
}
const fn div2(x: u32, steps: u32) -> u32 {
become switch(x >> 1, steps)
}
const fn mul3plus1(x: u32, steps: u32) -> u32 {
become switch(3 * x + 1, steps)
}
switch(x, 0)
}
const ASSERTS: () = {
assert!(collatz(1) == 0);
assert!(collatz(2) == 1);
assert!(collatz(3) == 7);
assert!(collatz(4) == 2);
assert!(collatz(6171) == 261);
};
fn main() {
let _ = ASSERTS;
}
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