almost_prime_numbers_from_factor_set.sf

#!/usr/bin/ruby

# Daniel "Trizen" Șuteu
# Date: 06 June 2021
# https://github.com/trizen

# Generate all the k-almost prime numbers <= n, using a given set of primes.

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

func almost_prime_numbers(n, k, primes, callback, squarefree = false) {

    var sqf = (squarefree ? 1 : 0)

    var factors = primes.sort.uniq
    var factors_end = factors.end

    if (k == 0) {
        callback(1)
        return nil
    }

    func (m, k, i=0) {

        if (k == 1) {

            var L = idiv(n,m)

            for j in (i..factors_end) {
                with (factors[j]) {|q|
                    q > L && break
                    callback(m*q)
                }
            }

            return nil
        }

        var L = idiv(n,m).iroot(k)

        for j in (i..factors_end) {
            with (factors[j]) { |q|
                q > L && break
                __FUNC__(m*q, k-1, j + sqf)
            }
        }
    }(1, k)

    return nil
}

var limit = 1e4
var primes = [2,3,5]

say "\n:: Generating k-almost primes <= #{limit} using P = #{primes}:\n"

for k in (0 .. limit.ilog2) {
    say ("#{'%2d' % k}: ", gather { almost_prime_numbers(limit, k, primes, {|n| take(n) }) }.sort)
}

__END__

:: Generating k-almost primes <= 10000 using P = [2, 3, 5]:

 0: [1]
 1: [2, 3, 5]
 2: [4, 6, 9, 10, 15, 25]
 3: [8, 12, 18, 20, 27, 30, 45, 50, 75, 125]
 4: [16, 24, 36, 40, 54, 60, 81, 90, 100, 135, 150, 225, 250, 375, 625]
 5: [32, 48, 72, 80, 108, 120, 162, 180, 200, 243, 270, 300, 405, 450, 500, 675, 750, 1125, 1250, 1875, 3125]
 6: [64, 96, 144, 160, 216, 240, 324, 360, 400, 486, 540, 600, 729, 810, 900, 1000, 1215, 1350, 1500, 2025, 2250, 2500, 3375, 3750, 5625, 6250, 9375]
 7: [128, 192, 288, 320, 432, 480, 648, 720, 800, 972, 1080, 1200, 1458, 1620, 1800, 2000, 2187, 2430, 2700, 3000, 3645, 4050, 4500, 5000, 6075, 6750, 7500]
 8: [256, 384, 576, 640, 864, 960, 1296, 1440, 1600, 1944, 2160, 2400, 2916, 3240, 3600, 4000, 4374, 4860, 5400, 6000, 6561, 7290, 8100, 9000, 10000]
 9: [512, 768, 1152, 1280, 1728, 1920, 2592, 2880, 3200, 3888, 4320, 4800, 5832, 6480, 7200, 8000, 8748, 9720]
10: [1024, 1536, 2304, 2560, 3456, 3840, 5184, 5760, 6400, 7776, 8640, 9600]
11: [2048, 3072, 4608, 5120, 6912, 7680]
12: [4096, 6144, 9216]
13: [8192]