challenge-147/abigail/bc/ch-2.bc


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#!/usr/bin/bc

#
# See https://theweeklychallenge.org/blog/perl-weekly-challenge-147
#

#
# Run as: bc ch-2.bc
#

define is_pentagonal (p) {
    auto l, h, m
    l = l
    h = high
    while (l <= h) {
        m = (l + h) / 2
        if (pentagon [m] == p) {return 1}
        if (pentagon [m] <  p) {l = m + 1} else {h = m - 1}
    }
    return 0
}

while (!done) {
    p = p + n + n + n + 1
    n = n + 1
    high = high + 1

    pentagon [high] = p

    for (i = 1; i <= high && pentagon [i] + pentagon [i] < p && !done; i ++) {
        seen = pentagon [i]
        
        if (is_pentagonal (p - seen) && is_pentagonal (p - seen - seen)) {
            print seen, " "
            p - seen
            done = 1
        }
    }
}

halt