Verse GCF

main.js

define({
  'test greatest common factor of primes is 1'() {
    assert(gcf(1, 3), is, 1)
    assert(gcf(7, 3), is, 1)
    assert(gcf(7, 57), is, 1)
  },
  
  'test gcf of 0 and 0 is NaN'() {
    assert(gcf(0, 0), isNaN)
  },
  
  'test gcf of 0 and x is x'() {
    assert(gcf(0, 1), is, 1)
    assert(gcf(0, 22), is, 22)
    assert(gcf(23, 0), is, 23)
  },
  
  'test gcf of x and x is x'() {
    assert(gcf(3, 3), is, 3)
    assert(gcf(129, 129), is, 129)
  },
  
  'test greatest common factor does not care about order'() {
    assert(gcf(3, 1), is, 1)
    assert(gcf(3, 7), is, 1)
  },
  
  'test gcf when one number is a factor of the other'() {
    assert(gcf(3, 51), is, 3)
  },
  
  'test gcf when neither number is a factor of the other'() {
    assert(gcf(98, 22), is, 2)
    assert(gcf(12, 9), is, 3)
    assert(gcf(12, 8), is, 4)
    assert(gcf(42, 18), is, 6)
    assert(gcf(123, 45), is, 3)
    assert(gcf(96, 20), is, 4)
  },
  
  gcf(a, b) {
    if (a === 0 && b === 0) return NaN
    if (a === 0) return b
    if (b === 0) return a
    let lesser = Math.min(a, b)
    let greater = Math.max(a, b)
    let remainder = greater % lesser
    switch (remainder) {
      case 0:
        // `lesser` is a factor of `greater`.
        // Since the GCF cannot be > `lesser`, it is `lesser`
        return lesser
      default:
        // `lesser` is not a factor of `greater`.
        // The GCF must be a factor of all of `lesser`, `greater`,
        // and `remainder`. Since `remainder` < `lesser`, we can
        // reduce the problem to finding the GCF of `remainder`
        // and `lesser`.
        return gcf(remainder, lesser)
    }
  },
})