Fix elo calculations

2026-07-23 04:26:57 +00:00 · 2023-09-15 13:03:42 +02:00
parent cf010855b7
commit 7e37999d43
2 changed files with 93 additions and 23 deletions
@@ -0,0 +1,86 @@
+import { clamp, last } from "lodash";
+
+// Fide uses teh following table to establish the chance of winning based on the difference in elo.
+// https://handbook.fide.com/chapter/B022017
+
+const differenceToProbability: [
+  start: number,
+  end: number,
+  winChance: number,
+][] = [
+  [0, 3, 0.5],
+  [4, 10, 0.51],
+  [11, 17, 0.52],
+  [18, 25, 0.53],
+  [26, 32, 0.54],
+  [33, 39, 0.55],
+  [40, 46, 0.56],
+  [47, 53, 0.57],
+  [54, 61, 0.58],
+  [62, 68, 0.59],
+  [69, 76, 0.6],
+  [77, 83, 0.61],
+  [84, 91, 0.62],
+  [92, 98, 0.63],
+  [99, 106, 0.64],
+  [107, 113, 0.65],
+  [114, 121, 0.66],
+  [122, 129, 0.67],
+  [130, 137, 0.68],
+  [138, 145, 0.69],
+  [146, 153, 0.7],
+  [154, 162, 0.71],
+  [163, 170, 0.72],
+  [171, 179, 0.73],
+  [180, 188, 0.74],
+  [189, 197, 0.75],
+  [198, 206, 0.76],
+  [207, 215, 0.77],
+  [216, 225, 0.78],
+  [226, 235, 0.79],
+  [236, 245, 0.8],
+  [246, 256, 0.81],
+  [257, 267, 0.82],
+  [268, 278, 0.83],
+  [279, 290, 0.84],
+  [291, 302, 0.85],
+  [303, 315, 0.86],
+  [316, 328, 0.87],
+  [329, 344, 0.88],
+  [345, 357, 0.89],
+  [358, 374, 0.9],
+  [375, 391, 0.91],
+  [392, 411, 0.92],
+  [412, 432, 0.93],
+  [433, 456, 0.94],
+  [457, 484, 0.95],
+  [485, 517, 0.96],
+  [518, 559, 0.97],
+  [560, 619, 0.98],
+  [620, 735, 0.99],
+  [736, 1000, 1],
+];
+
+export const getNewRating = (
+  rating: number,
+  opponentRating: number,
+  result: 1 | 0 | 0.5,
+  kFactor: number,
+) => {
+  const differenceInElo = clamp(opponentRating - rating, -400, 400);
+  const tableRow = (differenceToProbability.find(
+    ([start, end]) =>
+      Math.abs(differenceInElo) >= start && Math.abs(differenceInElo) <= end,
+  ) ?? last(differenceToProbability)!)[2];
+
+  const probabilityOfWinning = differenceInElo < 0 ? tableRow : 1 - tableRow;
+  const delta =
+    Math.round(
+      kFactor * (result - probabilityOfWinning + Number.EPSILON) * 100,
+    ) / 100;
+
+  return {
+    delta,
+    newRating: Math.round((rating + delta + Number.EPSILON) * 100) / 100,
+  };
+};