binary_exponentiation.sf

#!/usr/bin/ruby

# Generic implementation of the Binary Exponentiation algorithm.

# See also:
#   https://en.wikipedia.org/wiki/Exponentiation_by_squaring

func binary_exp(r,x,n,f) {

    for bit in (n.as_bin.flip.chars) {
        r = f(x, r) if bit
        x = f(x, x)
    }

    return r
}

say binary_exp(1, 3, 42, {|a,b| a * b })  # 3^42
say binary_exp(0, 3, 42, {|a,b| a + b })  # 3*42