Project Euler

problem_3.js

function find(start, finish, qualifier) {
  var index;

  for (index = start; start <= finish; index++) {
    if (qualifier(index)) {
      return index;
    }
  }
}

function primeFactors(number) {
  var factors, factor;

  if (1 === number) {
    return [];
  } else {
    factor = find(2, number, function (potentialFactor) {
      return 0 === number % potentialFactor;
    });

    return primeFactors(number / factor).concat(factor);
  }
}

function largestPrimeFactor(number) {
  return primeFactors(number)[0];
}

console.log(largestPrimeFactor(13195));
console.log(largestPrimeFactor(600851475143));

problem_4.rb

# A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
#
# Find the largest palindrome made from the product of two 3-digit numbers.

def assert(boolean)
  raise 'assertion failed' unless boolean
end

def palindrome?(stringable)
  string = String(stringable)
  string == string.reverse
end

assert palindrome?('otto')
assert palindrome?(1)
assert palindrome?(101)
assert palindrome?(9009)

puts (100..999).
  map { |i| (100..999).map { |j| i * j } }.
  flatten.
  select { |n| palindrome?(n) }.
  sort.
  last

problem_5.rb

# 2520 is the smallest number that can be divided by each of the numbers from 1 to 10 without any remainder.
#
# What is the smallest positive number that is evenly divisible by all of the numbers from 1 to 20?

# n = (1..999999999).find { |i| (1..20).all? { |j| 0 == i % j } }
# puts n

primes = [1, 2, 3, 5, 7, 11, 13, 17, 19]
non_primes = [4, 6, 8, 10, 12, 14, 15, 18, 20]

prime_product = primes.reduce { |a, e| a * e }
puts prime_product * non_primes[0] * non_primes[1]