Sandwich layer validator (Rust)
#[derive(Debug, PartialEq, Eq)]
enum Layer {
Bread,
Cheese,
Ham,
Cucumber,
Other(&'static str),
}
fn validate_layers(layers: &[Layer]) -> Result<(), String> {
// Helper to find index of first occurrence
let pos = |item: Layer| -> Option<usize> {
layers.iter().position(|l| *l == item)
};
// Cheese must exist
let cheese_idx = pos(Layer::Cheese)
.ok_or_else(|| "Invalid sandwich: missing cheese".to_string())?;
// Ham must be above (i.e., appear after) cheese
if let Some(ham_idx) = pos(Layer::Ham) {
if ham_idx < cheese_idx {
return Err("Awkward: ham is below cheese; ham should be on top of cheese".into());
}
}
// Cucumber must be on top of cheese (i.e., after cheese).
if let Some(cuke_idx) = pos(Layer::Cucumber) {
if cuke_idx < cheese_idx {
return Err("Invalid: cucumber is below cheese; cucumber must be on top of cheese".into());
}
}
// If both ham and cucumber exist, order among them is allowed as long as both are after cheese.
Ok(())
}
fn main() {
let good = vec![
Layer::Bread,
Layer::Cheese,
Layer::Ham,
Layer::Cucumber,
Layer::Bread,
];
let bad_ham = vec![
Layer::Bread,
Layer::Ham,
Layer::Cheese,
Layer::Bread,
];
let bad_cuke = vec![
Layer::Bread,
Layer::Cucumber,
Layer::Cheese,
Layer::Bread,
];
for (name, s) in &[("good", &good), ("bad_ham", &bad_ham), ("bad_cuke", &bad_cuke)] {
match validate_layers(s) {
Ok(()) => println!("{}: valid sandwich", name),
Err(e) => println!("{}: {}", name, e),
}
}
}
Notes:
Cheese must exist.
Ham and cucumber, if present, must appear after the cheese (i.e., on top of it).
Order between ham and cucumber is allowed (both on top of cheese).
