Yesterday, I solved the puzzle from day 1 of this year’s Advent of Code challenge as a way of demonstrating how to get started with Node.js and ES6. Today, we’ll solve the second challenge, which will give us an opportunity to learn a couple more ES6 features – destructuring and generator functions.

### Parsing the Input

The puzzle solving framework we created yesterday already reads the puzzle input from a text file and loads it into an array of strings. But today’s puzzle gave us tab separated numbers on each line, so we need to convert every row from a tab separated string `"4\t3\t7"` to an array of numbers `[4,3,7]`. We can do that by mapping the input array like this.

``````function solve(input, part) {
const spreadsheet = input.map(r => r.split('\t').map(s => Number(s)))
// ...
``````

### Min and Max

The “checksum” we need to calculate for the spreadsheet involves summing values for each row. For part 1 the value we need to find is the difference between the largest and smallest values on each row. What we need is a helpful `max` function that given an array like `[2,5,3]` will return 5.

Now it is possible to do this with clever use of the built in  `Math.max` function.

``````Math.max.apply(null,[2,5,3]) // returns 5
``````

But that’s a little bit ugly, and actually I’ve already created my own `min` and `max` functions that operate on an array (or iterable) in my utils.js module. I created these utils when solving the 2015 problems in JavaScript as preparation for this year’s challenge.

So we need access to the `min` and `max` functions from the utils module and this gives us an opportunity to get our first look at ES6 destructuring.

To import the utils module we could do this:

``````const utils = require('../../utils/utils')
``````

That would mean we’d need to say `utils.max` and `utils.min` every time we wanted to use these helper functions. So we might do this to let us just use `min` and `max`:

``````const utils = require('../../utils/utils');
const max = utils.max;
const min = utils.min;
``````

What destructuring lets us do is simplify that code down into one line:

``````const {max,min} = require('../../utils/utils');
``````

It’s nice and succinct and it has one other benefit here. Utils contains lots of functions, and by taking this approach I advertise which functions from utils I actually need. This would also help with “tree shaking” if we were trying to minimise a module bundle.

Now we have `min` and `max` helpers that work on arrays we can solve the part 1 challenge. I’ve defined a `checksum` function that takes the spreadsheet (an array of rows which are arrays of integers) and a function to calculate the value for that row (`rowFn`). Like yesterday, we use `reduce` to add the values together.

``````function checksum(spreadsheet, rowFn) {
.reduce((a,b) => a+b);
}
``````

We can call this `checksum` method making use of `min` and `max` to solve part 1:

``````function solve(input, part) {
const spreadsheet = input.map(r => r.split('\t').map(s => Number(s)))
if (part === 1) {
return checksum(spreadsheet, r => max(r) - min(r));
}
}
``````

### Combinations

For part 2, the calculation for each row changed. We needed to find the pair of numbers that were evenly divisible and return the result of dividing them. A simplistic approach to solving this problem would be to loop through and check every pair like this:

``````function findDivisors(row) {
for (let x = 0; x < row.length; x++) {
for (let y = x+1; y < row.length; y++) {
if (row[x]%row[y] === 0) {
return row[x] / row[y];
}
if (row[y]%row[x] === 0) {
return row[y] / row[x];
}
}
}
throw new Error("no divisors found in row");
}
``````

That allows us to solve part 2 like this:

``````return checksum(spreadsheet, findDivisors);
``````

So that’s nice and easy, but I think we can improve on the `findDivisors` function. It has two distinct responsibilities. First it is iterating through all pairs of numbers in the row. And then it is testing those pairs to see if they are divisible. What if we had a helper function that could return all pairs from an array? We can use another ES6 feature to help us with that:

### Generator Functions

If you’re a C# programmer who enjoys using LINQ, you probably know the benefit of functions that return an `IEnumerable<T>` using the `yield` keyword. It can be much more efficient than returning a whole array as the consumer might not need to iterate the whole way through.

In ES6, you can create generator functions, which work very similarly. You declare them with `function*`, and you use the `yield` keyword to return each element in the sequence. Here’s my function that returns all pairs (both ways round) as two element arrays. So the array `[a,b,c]` would return a sequence of `[a,b] [a,c] [b,a], [b,c], [c,a], [c,b]` :

``````function* allPairs(arr) {
for (let x = 0; x < arr.length; x++) {
for (let y = 0; y < arr.length; y++) {
if (x !== y) yield [arr[x],arr[y]];
}
}
}
``````

The way you consume the output of these functions is with the for…of loop we discussed yesterday. And we can combine this with the destructuring syntax. Since each element in the sequence is a two element array, I can destructure it into the `a` and `b` variables using this syntax:

``````function findDivisors(row) {
for (let[a,b] of allPairs(row)) {
if(a%b === 0) return a/b;
}
throw new Error("no divisors found in row");
}
``````

### Combining Map and Reduce

As we discussed yesterday, there is a fine balance between conciseness and readability in code.  Code that is too verbose is hard to follow, but so is code that has been “code golfed”. However, I think that the `checksum` function which is just a `map` and a `reduce` could be simplified slightly from:

``````function checksum(spreadsheet, rowFn) {
to a form where we just use `reduce`. We do need to pass in 0 as the initial value as the values in the arrays are not numbers so the `rowFn` must be applied to each one:
``````const checksum = (spreadsheet, rowFn) => spreadsheet.reduce((a,b) => a+rowFn(b),0);