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[doubling] get #200

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Merged
ia7ck merged 2 commits into from
Feb 7, 2026
Merged

[doubling] get #200

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2447a16
max_steps = 0 で使うことはなさそう
ia7ck Feb 7, 2026
ae663fd
get()
ia7ck Feb 7, 2026
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38 changes: 32 additions & 6 deletions libs/doubling/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -77,11 +77,9 @@ where
where
F: Fn(usize) -> Transition<V>,
{
let log2_max_steps = if max_steps == 0 {
0
} else {
max_steps.ilog2() as usize
};
assert!(max_steps > 0);

let log2_max_steps = max_steps.ilog2() as usize;

Comment on lines +80 to 83
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Copilot AI Feb 7, 2026

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Doubling::new previously supported max_steps == 0 (and fold(start, 0, ...) still works), but this change introduces an unconditional panic for that input. If this crate is consumed externally, this is an API-breaking behavior change; consider keeping the old handling (treat 0 as a valid value) or documenting the new precondition clearly (e.g., in rustdoc / changelog) and making tests reflect the intended contract.

Suggested change
assert!(max_steps > 0);
let log2_max_steps = max_steps.ilog2() as usize;
let log2_max_steps = if max_steps == 0 {
0
} else {
max_steps.ilog2() as usize
};

Copilot uses AI. Check for mistakes.
let mut transitions = Vec::with_capacity(n_state * (log2_max_steps + 1));
for i in 0..n_state {
Expand Down Expand Up @@ -112,6 +110,15 @@ where
}
}

/// 状態`start`から長さ`pow(2, k)`の遷移を返します。
pub fn get(&self, start: usize, k: usize) -> &Transition<V> {
assert!(start < self.n_state);
assert!(k <= self.log2_max_steps);

let offset = self.n_state * k;
&self.transitions[offset + start]
}

/// 状態`start`から`step`回の遷移、初期値`init`から始めて`f`で畳みこんだ結果を返します。
pub fn fold<A, F>(&self, start: usize, step: usize, init: A, mut f: F) -> A
where
Expand Down Expand Up @@ -190,6 +197,25 @@ mod tests {
);
}

#[test]
fn test_get() {
let n = 3;
let to = vec![1, 2, 0];
let doubling = Doubling::new(n, 100, |i| Transition::new(to[i], Sum(1)));

let t = doubling.get(0, 0);
assert_eq!(t.value, Sum(1));

let t = doubling.get(0, 1);
assert_eq!(t.value, Sum(2));

let t = doubling.get(0, 2);
assert_eq!(t.value, Sum(4));

let t = doubling.get(0, 3);
assert_eq!(t.value, Sum(8));
}

impl Value for String {
fn op(&self, other: &Self) -> Self {
format!("{}{}", self, other)
Expand All @@ -199,7 +225,7 @@ mod tests {
proptest! {
#[test]
fn test_fold_associativity(
(n_state, max_steps, nexts, values, start, step1, step2) in (1_usize..=10, 0_usize..=100)
(n_state, max_steps, nexts, values, start, step1, step2) in (1_usize..=10, 1_usize..=100)
.prop_flat_map(|(n_state, max_steps)| {
(
Just(n_state),
Expand Down