core: calc difficulty after each submission

packages/hydrooj/src/lib/difficulty.ts

@@ -0,0 +1,46 @@
+const _CACHE_INFO = {
+    last_s: 0.0,
+    last_y: 0,
+    values: [0.0],
+};
+
+function _LOGP(x: number) {
+    const sqrt_2_pi = 2.506628274631000502415765284811; // Sqrt[Pi]
+    return (2 * Math.exp(-1.0 * (Math.log(x) ** 2) * 2)) / x / sqrt_2_pi;
+}
+
+function _integrate_ensure_cache(y: number) {
+    let last_y = _CACHE_INFO.last_y;
+    if (y <= last_y) return _CACHE_INFO;
+    let s = _CACHE_INFO.last_s;
+    const dx = 0.1;
+    const dT = 2;
+    let x0 = (last_y / dT) * dx;
+    while (y > last_y) {
+        x0 += dx;
+        s += _LOGP(x0) * dx;
+        for (let i = 1; i <= dT; i++) _CACHE_INFO.values.push(s);
+        last_y += dT;
+    }
+    _CACHE_INFO.last_y = last_y;
+    _CACHE_INFO.last_s = s;
+    return _CACHE_INFO;
+}
+
+_integrate_ensure_cache(1000000);
+
+function _integrate(y: number) {
+    _integrate_ensure_cache(y);
+    return _CACHE_INFO.values[y];
+}
+
+function difficultyAlgorithm(nSubmit: number, nAccept: number) {
+    if (!nSubmit) return null;
+    const s = _integrate(nSubmit);
+    const ac_rate = nAccept / nSubmit;
+    const ans = Math.round(10.0 - 1.30 * s * 10.0 * ac_rate);
+    return Math.max(ans, 1);
+}
+
+export = difficultyAlgorithm;
+global.Hydro.lib.difficulty = difficultyAlgorithm;