Auto merge of #162 - est31:add_checked_pow, r=cuviper
Add checked_pow function Implements a `checked_pow` function which does the same as `pow`, just with overflow checks. And, similar to #152 and #153, the function uses references instead of cloning. Adds a little macro to spare code repetition. Its scoped to the function so nothing gets polluted.
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d1adf45389
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src/lib.rs
42
src/lib.rs
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// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
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// Copyright 2014-2016 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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@ -164,6 +164,46 @@ pub fn pow<T: Clone + One + Mul<T, Output = T>>(mut base: T, mut exp: usize) ->
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acc
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}
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/// Raises a value to the power of exp, returning `None` if an overflow occured.
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///
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/// Otherwise same as the `pow` function.
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///
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/// # Example
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///
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/// ```rust
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/// use num;
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///
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/// assert_eq!(num::checked_pow(2i8, 4), Some(16));
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/// assert_eq!(num::checked_pow(7i8, 8), None);
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/// assert_eq!(num::checked_pow(7u32, 8), Some(5_764_801));
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/// ```
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#[inline]
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pub fn checked_pow<T: Clone + One + CheckedMul>(mut base: T, mut exp: usize) -> Option<T> {
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if exp == 0 { return Some(T::one()) }
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macro_rules! optry {
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( $ expr : expr ) => {
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if let Some(val) = $expr { val } else { return None }
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}
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}
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while exp & 1 == 0 {
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base = optry!(base.checked_mul(&base));
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exp >>= 1;
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}
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if exp == 1 { return Some(base) }
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let mut acc = base.clone();
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while exp > 1 {
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exp >>= 1;
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base = optry!(base.checked_mul(&base));
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if exp & 1 == 1 {
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acc = optry!(acc.checked_mul(&base));
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}
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}
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Some(acc)
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}
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#[cfg(test)]
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fn hash<T: hash::Hash>(x: &T) -> u64 {
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use std::hash::Hasher;
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