Create elo snippet

snippets/elo.md

@@ -0,0 +1,25 @@
+### elo
+
+Computes the new ratings between two opponents using the Elo rating formula. It takes an array
+of two pre-ratings and returns an array containing two post-ratings. The winner's rating is be the first element of the array.
+
+Use `Math.pow()` and math operators to compute the expected score (chance of winning) of each opponent
+and compute the change in rating for each. Omit the second argument to use the default K-factor of
+32, or supply a custom K-factor value.
+
+```js
+const elo = ([a, b], kFactor = 32) => {
+  const expectedScore = (self, opponent) => 1 / (1 + Math.pow(10, (opponent - self) / 400));
+  const [eA, eB] = [expectedScore(a, b), expectedScore(b, a)];
+  const newRating = (rating, index) =>
+    rating + kFactor * ((index === 0 ? 1 : 0) - (index === 0 ? eA : eB));
+  return [newRating(a, 0), newRating(b, 1)];
+};
+```
+
+```js
+elo([1200, 1200]); // [1216, 1184]
+elo([1000, 2000]); // [1031.8991261061358, 1968.1008738938642]
+elo([1500, 1000]); // [1501.7036868864648, 998.2963131135352]
+elo([1200, 1200], 64); // [1232, 1168]
+```

tag_database

@@ -29,6 +29,7 @@ distinctValuesOfArray:array
 dropElements:array
 dropRight:array
 elementIsVisibleInViewport:browser
+elo:math
 escapeHTML:string
 escapeRegExp:string
 everyNth:array