021 Amicable numbers.sf

#!/usr/bin/ruby

# Daniel "Trizen" Șuteu
# https://github.com/trizen

# For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110; therefore d(220) = 284.
# The proper divisors of 284 are 1, 2, 4, 71 and 142; so d(284) = 220.
# Evaluate the sum of all the amicable numbers under 10000.

# https://projecteuler.net/problem=21

# Runtime: 0.417s

var sum = 0
var mem = Hash()

for i in (2 ..^ 10000) {

    var d = i.sigma-i

    if (mem.exists(d)) {

        var n = mem{d}
        if (n.sigma-n == d) {
            sum += d+n
            say "#{d} #{n}"
        }

        mem.delete(d)
    }
    elsif (i != d) {
        mem{i} = d
    }
}

say "Sum: #{sum}"