4-11 Patrick's solution

4-11 Patrick's solution

// var assert = require('assert');
var assert = require('chai').assert;

describe('The findMaxProfit function', function() {

  function findMaxProfit(stockPrices) {

    // make sure we have at least 2 prices
    if (stockPrices.length < 2) {
        throw new Error('Getting a profit requires at least 2 prices');
    }

    // we'll greedily update minPrice and maxProfit, so we initialize
    // them to the first price and the first possible profit
    var minPrice = stockPrices[0];
    var maxProfit = stockPrices[1] - stockPrices[0];

    // start at the second (index 1) time
    // we can't sell at the first time, since we must buy first,
    // and we can't buy and sell at the same time!
    // if we started at index 0, we'd try to buy /and/ sell at time 0.
    // this would give a profit of 0, which is a problem if our
    // maxProfit is supposed to be /negative/--we'd return 0!
    for (var i = 1; i < stockPrices.length; i++) {
        var currentPrice = stockPrices[i];

        // see what our profit would be if we bought at the
        // min price and sold at the current price
        var potentialProfit = currentPrice - minPrice;

        // update maxProfit if we can do better
        maxProfit = Math.max(maxProfit, potentialProfit);

        // update minPrice so it's always
        // the lowest price we've seen so far
        minPrice = Math.min(minPrice, currentPrice);
    }

    return maxProfit;
  }

  it('should return the max profit for an array of stock prices', function(){
    var arr = [9, 6, 7, 5, 10, 8];
    assert.equal(findMaxProfit(arr), 5);
  })

  it('should be return a negative profit if values decrease over time', function () {
    var arr = [10, 9, 8, 7, 6, 5]
    assert.isBelow(findMaxProfit(arr), 0)
  });

  it('should throw an error if input array length is less than 2', function () {
    var arr = [1];
    assert.throws(function(){return findMaxProfit(arr)}, Error, 'Getting a profit requires at least 2 prices');
  });

});