25 lines
1013 B
Markdown
25 lines
1013 B
Markdown
### elo
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Computes the new ratings between two opponents using the [Elo rating system](https://en.wikipedia.org/wiki/Elo_rating_system). It takes an array
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of two pre-ratings and returns an array containing two post-ratings.
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The winner's rating is the first element of the array.
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Use `Math.pow()` and math operators to compute the expected score (chance of winning) of each opponent
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and compute the new rating for each. Omit the second argument to use the default K-factor of
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32, or supply a custom K-factor value.
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```js
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const elo = ([a, b], kFactor = 32) => {
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const expectedScore = (self, opponent) => 1 / (1 + Math.pow(10, (opponent - self) / 400));
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const newRating = (rating, i) => rating + kFactor * (i - expectedScore(i ? a : b, i ? b : a));
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return [newRating(a, 1), newRating(b, 0)];
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};
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```
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```js
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elo([1200, 1200]); // [1216, 1184]
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elo([1000, 2000]); // [1031.8991261061358, 1968.1008738938642]
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elo([1500, 1000]); // [1501.7036868864648, 998.2963131135352]
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elo([1200, 1200], 64); // [1232, 1168]
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```
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