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Added powm (power modulo) to the std::num module. #11735

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Added powm (power modulo) to the std::num module. #11735

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5fe86bb
Added powm (power modulo) to the std::num module.
lilac Jan 22, 2014
b4a2bf7
Rename powm arguments and add more tests.
lilac Jan 23, 2014
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31 changes: 31 additions & 0 deletions src/libstd/num/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -329,6 +329,28 @@ pub fn pow<T: One + Mul<T, T>>(mut base: T, mut exp: uint) -> T {
}
}

/// Raises a value to the power of p modulo d, using exponentiation by squaring.
///
/// # Example
///
/// ```rust
/// use std::num;
///
/// assert_eq!(num::powm(2, 4, 3), 1);
/// ```
#[inline]
pub fn powm<T: One + Mul<T, T> + Rem<T, T>>(a: T, mut p: uint, d: T) -> T {
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It would be good to have an explicit check for d == 0, to make the error messages nicer.

let mut r: T = One::one();
let mut n: T = a % d;
while p > 0 {
if (p & 1) == 1 {
r = r * n % d;
}
n = n * n % d;
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Do these associate correctly? (Maybe better to write as (r * n) % d and (n * n) % d just to be clear?)

p = p >> 1;
}
r
}
/// Raise a number to a power.
///
/// # Example
Expand Down Expand Up @@ -1670,6 +1692,15 @@ mod tests {
assert_pow!((8.5, 5 ) => 44370.53125);
assert_pow!((2u64, 50) => 1125899906842624);
}

#[test]
fn test_powm() {
assert_eq!(4, powm(3, 2, 5));
assert_eq!(4, powm(2, 10, 10));
assert_eq!(1, powm(3, 16, 2));
assert_eq!(2, powm(2, 3, 3));
assert_eq!(1, powm(2, 64, 3));
}
}


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