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pub fn pow(
m: usize,
x: usize,
exp: usize,
) -> usize {
if exp == 0 {
return 1;
}
let mut y = pow(m, x, exp >> 1);
y *= y;
y %= m;
if exp & 1 == 1 {
y *= x;
y %= m;
}
y
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test() {
let m = 1_000_000_007;
let inv_2 = (m + 1) >> 1;
assert_eq!(pow(m, 2, m - 2), inv_2);
}
}