Ranking System v1

src/calculateRank.js

@@ -1,19 +1,5 @@
-// https://stackoverflow.com/a/5263759/10629172
-function normalcdf(mean, sigma, to) {
-  var z = (to - mean) / Math.sqrt(2 * sigma * sigma);
-  var t = 1 / (1 + 0.3275911 * Math.abs(z));
-  var a1 = 0.254829592;
-  var a2 = -0.284496736;
-  var a3 = 1.421413741;
-  var a4 = -1.453152027;
-  var a5 = 1.061405429;
-  var erf =
-    1 - ((((a5 * t + a4) * t + a3) * t + a2) * t + a1) * t * Math.exp(-z * z);
-  var sign = 1;
-  if (z < 0) {
-    sign = -1;
-  }
-  return (1 / 2) * (1 + sign * erf);
+function expsf(lambda, x) {
+  return Math.exp(-lambda * x);
 }
 
 function calculateRank({
@@ -25,65 +11,41 @@ function calculateRank({
   issues,
   stargazers,
 }) {
-  const COMMITS_OFFSET = 1.65;
-  const CONTRIBS_OFFSET = 1.65;
-  const ISSUES_OFFSET = 1;
-  const STARS_OFFSET = 0.75;
-  const PRS_OFFSET = 0.5;
-  const FOLLOWERS_OFFSET = 0.45;
-  const REPO_OFFSET = 1;
-
-  const ALL_OFFSETS =
-    CONTRIBS_OFFSET +
-    ISSUES_OFFSET +
-    STARS_OFFSET +
-    PRS_OFFSET +
-    FOLLOWERS_OFFSET +
-    REPO_OFFSET;
+  // value of statistics should be equal to 1 / <average number per user> (see https://en.wikipedia.org/wiki/Exponential_distribution#Mean,_variance,_moments_and_median)
+  const COMMITS_LAMBDA = 1 / 10000;
+  const CONTRIBS_LAMBDA = 1 / 1000;
+  const ISSUES_LAMBDA = 1 / 100;
+  const STARS_LAMBDA = 1 / 400;
+  const PRS_LAMBDA = 1 / 300;
+  const FOLLOWERS_LAMBDA = 1 / 100;
+  const REPO_LAMBDA = 1 / 20;
 
   const RANK_S_VALUE = 1;
   const RANK_DOUBLE_A_VALUE = 25;
   const RANK_A2_VALUE = 45;
   const RANK_A3_VALUE = 60;
   const RANK_B_VALUE = 100;
 
-  const TOTAL_VALUES =
-    RANK_S_VALUE + RANK_A2_VALUE + RANK_A3_VALUE + RANK_B_VALUE;
-
-  // prettier-ignore
-  const score = (
-    totalCommits * COMMITS_OFFSET +
-    contributions * CONTRIBS_OFFSET +
-    issues * ISSUES_OFFSET +
-    stargazers * STARS_OFFSET +
-    prs * PRS_OFFSET +
-    followers * FOLLOWERS_OFFSET + 
-    totalRepos * REPO_OFFSET 
-  ) / 100;
-
-  const normalizedScore = normalcdf(score, TOTAL_VALUES, ALL_OFFSETS) * 100;
+  const normalizedScore = 700 / (
+      1 / expsf(COMMITS_LAMBDA, totalCommits) +
+      1 / expsf(CONTRIBS_LAMBDA, contributions) +
+      1 / expsf(ISSUES_LAMBDA, issues) +
+      1 / expsf(STARS_LAMBDA, stargazers) +
+      1 / expsf(PRS_LAMBDA, prs) +
+      1 / expsf(FOLLOWERS_LAMBDA, followers) +
+      1 / expsf(REPO_LAMBDA, totalRepos))
 
   let level = "";
 
   if (normalizedScore < RANK_S_VALUE) {
     level = "S+";
-  }
-  if (
-    normalizedScore >= RANK_S_VALUE &&
-    normalizedScore < RANK_DOUBLE_A_VALUE
-  ) {
+  } else if (normalizedScore < RANK_DOUBLE_A_VALUE) {
     level = "S";
-  }
-  if (
-    normalizedScore >= RANK_DOUBLE_A_VALUE &&
-    normalizedScore < RANK_A2_VALUE
-  ) {
+  } else if (normalizedScore < RANK_A2_VALUE) {
     level = "A++";
-  }
-  if (normalizedScore >= RANK_A2_VALUE && normalizedScore < RANK_A3_VALUE) {
+  } else if (normalizedScore < RANK_A3_VALUE) {
     level = "A+";
-  }
-  if (normalizedScore >= RANK_A3_VALUE && normalizedScore < RANK_B_VALUE) {
+  } else {
     level = "B+";
   }
 

tests/calculateRank.test.js

@@ -13,6 +13,34 @@ describe("Test calculateRank", () => {
         prs: 300,
         issues: 200,
       }),
-    ).toStrictEqual({ level: "A+", score: 49.16605417270399 });
+    ).toStrictEqual({ level: "A++", score: 37.035320318128086 });
+  });
+
+  it("new user gets B+ rank", () => {
+    expect(
+      calculateRank({
+        totalCommits: 0,
+        totalRepos: 0,
+        followers: 0,
+        contributions: 0,
+        stargazers: 0,
+        prs: 0,
+        issues: 0,
+      }),
+    ).toStrictEqual({ level: "B+", score: 100 });
+  });
+
+  it("Linus Torvalds gets S+ rank", () => {
+    expect(
+      calculateRank({
+        totalCommits: 2700,
+        totalRepos: 2,
+        followers: 132000,
+        contributions: 2700,
+        stargazers: 109100,
+        prs: 71,
+        issues: 2,
+      }),
+    ).toStrictEqual({ level: "S+", score: 0 });
   });
 });