Even Fib Numbers

fib-even.js

/**
 * Project Euler, Problem 2
 * https://projecteuler.net/problem=2
 *
 * Each new term in the Fibonacci sequence is generated by adding the previous
 * two terms. By starting with 1 and 2, the first 10 terms will be:
 *
 *    1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
 *
 * By considering the terms in the Fibonacci sequence whose values do not
 * exceed four million, find the sum of the even-valued terms.
 */

// max fibonacci value is 4 million for this
const MAX_FIB_VALUE = 4000000

/**
 * fibEven takes a value (capped at 4 million), and sums the total of all even
 * fibonacci values that are less than that value.
 *
 *  e.g fibEven(10) === 10
 *
 * @param {Number} maxValue - an integer, representing the maximum value
 *    of the fibonacci value to compute until
 * @returns {Number} - the sum of the even values, defaulting to 0 in any error case
 */
function fibEven(maxValue) {

  // because its JavaScript, anything can happen
  if (isNaN(maxValue)) return 0;

  // clamp the value
  if (maxValue > MAX_FIB_VALUE) maxValue = MAX_FIB_VALUE;

  // fibonacci things
  let nMinusOne = 1;
  let nMinusTwo = 0;
  let fibValue = 0;

  // return value
  let evenSum = 0;

  // loop time (better perf than recursion)
  while (fibValue <= maxValue) {
    // if this is an even fibonacci value, add it to our sum
    if (fibValue % 2 === 0) evenSum += fibValue;

    // reassign the fibonacci values
    nMinusTwo = nMinusOne;
    nMinusOne = fibValue;

    // add up the next fibonacci value
    fibValue = nMinusOne + nMinusTwo;
  }

  return evenSum;
}

module.exports = { fibEven };

test.js

// this file assumes you're using mocha
var assert = require('assert');

var { fibEven } = require('./fib-even');

const MAX_EVEN_VALUE = 4613732;

describe('Testing the fib even values', function() {
  it('Tests to make sure fibEven() returns the correct value', function() {
    let testCases = [
      // some basic test cases
      {
        maxValue: 0,
        sumValue: 0,
      },
      {
        maxValue: 2,
        sumValue: 2,
      },
      {
        maxValue: 10,
        sumValue: 10,
      },
      {
        maxValue: 100,
        sumValue: 44,
      },
      {
        maxValue: 1000,
        sumValue: 798,
      },


      // lets push the envelope
      // (i wonder where that saying comes from)
      {
        maxValue: 4000000,
        sumValue: MAX_EVEN_VALUE,
      },
      {
        maxValue: 4000000000,
        sumValue: MAX_EVEN_VALUE,
      },
      {
        maxValue: Infinity,
        sumValue: MAX_EVEN_VALUE,
      },
      {
        maxValue: Math.pow(10, 1000),
        sumValue: MAX_EVEN_VALUE,
      },

      // corner cases
      {
        maxValue: undefined,
        sumValue: 0,
      },
      {
        maxValue: null,
        sumValue: 0,
      },
      {
        maxValue: -1,
        sumValue: 0,
      },
      {
        maxValue: 1.2345,
        sumValue: 0,
      },
      {
        maxValue: "abc123",
        sumValue: 0,
      },
      {
        maxValue: ["ha", "ha", "ha"],
        sumValue: 0,
      },
      {
        maxValue: { foo: 'bar' },
        sumValue: 0,
      },
      {
        maxValue: function() {}, // why not
        sumValue: 0,
      }
    ];

    for (var testCase of testCases) {
      // get the sum value
      const actualSumValue = fibEven(testCase.maxValue);

      // assert the values
      assert.equal(
        actualSumValue,
        testCase.sumValue,
        `Sum values do not match for max value: ${testCase.maxValue}`
      );
    }
  })
})