four-in-a-row.js

function getFourInARow(numbers) {
  var xs = partitionEvery(4, numbers);
  var transducer = compose(
    mapping(calcProduct),
    updateIn("maxProduct", max2),
    updateIn("avg", avg2(xs.length))
  );
  var result = xs.reduce(transducer(id), { maxProduct: 0, avg: 0 });
  return result.maxProduct - result.avg
}

// the magic...
function compose(...fs) {
  return fs.reduce(function(f, g) {
    return function(input) {
      return f(g(input));
    };
  });
}

function mapping(f) {
  return function(task) {
    return function(acc, x) {
      return task(acc, f(x));
    };
  };
}

function updateIn(field, f) {
  return function(task) {
    return function(acc, x) {
      acc[field] = f(acc[field], x);
      return task(acc, x);
    };
  };
}

function id(x) { return x; }
function max2(x, y) { return x >= y ? x : y; }
function avg2(total) {
  return function(x, y) {
    if (x === 0)
      return y / total;
    else
      return x + y / total;
  }
}
function multiply2(a, b) { return a * parseInt(b); }
function calcProduct(xs) { return xs.split("").reduce(multiply2); }

function partitionEvery(n, numbers) {
  var adjacents = [];
  while (true) {
    if (numbers.length <= 4) {
      adjacents.push(numbers);
      break;
    } else {
      adjacents.push(numbers.slice(0, n));
      numbers = numbers.slice(1);
    }
  }
  return adjacents;
}

// console.log(getFourInARow("12345678987654321")); // =>2592