weasel with a Y combinator

weasel-Y.js

(function () {
  "use strict";

  const goal = "METHINKS IT IS LIKE A WEASEL";
  const size = 100;
  const rate = 0.04;
  const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ ";

  var range = function (count) {
    return new Array(count).join(' ').split(' ');
  };

  var randGene = function () {
    return alphabet[Math.floor(Math.random() * alphabet.length)];
  };

  var getScore = function (weasel) {
    return weasel.split('').reduce(function (out, next, i) {
      return out + Number(next === goal[i]);
    }, 0);
  };

  var createChild = function (parent) {
    return parent.split('').map(function (c) {
      return Math.random() < rate ? randGene() : c;
    }).join('');
  };

  // Y combinator
  // see http://matt.might.net/articles/implementation-of-recursive-fixed-point-y-combinator-in-javascript-for-memoization/
  var Y = function (F) {
    return (function (x) {
      return F(function (y) {
        return (x(x))(y)
      });
    })(function (x) {
      return F(function (y) {
        return (x(x))(y)
      });
    });
  };

  var algo = Y(function (step) {
    return function (acc) {
      var curState = acc.slice(-1)[0];
      if (curState.bestWeasel === goal) {
        return acc;
      }

      var litter = range(size).map(function () {
        return createChild(curState.bestWeasel);
      });

      var sorted = litter.sort(function (a, b) {
        return getScore(b) - getScore(a);
      });

      var newBestWeasel = sorted[0];

      return step(acc.concat({
        bestWeasel: newBestWeasel,
        score: getScore(newBestWeasel),
        generation: curState.generation + 1
      }));
    };
  });

  var initWeasel = range(goal.length).map(randGene).join('');
  var initialState = [{
    bestWeasel: initWeasel,
    score: getScore(initWeasel),
    generation: 0
  }];
  var result = algo(initialState);

  result.forEach(function (state) {
    console.log(state.generation, state.bestWeasel, state.score);
  });
})();