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