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