Store and use seed for randomness

src/Types.tsx

@@ -249,6 +249,7 @@ export interface ScenarioType {
 }
 
 export interface GameType {
+  seed: number;
   difficulty: DifficultyType;
   scenarioId: number;
   location: LocationType;

src/helpers/Math.tsx

@@ -7,7 +7,7 @@ export function getIntersectionX(
   line2StartX: number,
   line2StartY: number,
   line2EndX: number,
-  line2EndY: number,
+  line2EndY: number
 ) {
   const denominator =
     (line2EndY - line2StartY) * (line1EndX - line1StartX) -
@@ -22,9 +22,32 @@ export function getIntersectionX(
   return line1StartX + (numerator1 / denominator) * (line1EndX - line1StartX);
 }
 
+// https://stackoverflow.com/questions/521295/seeding-the-random-number-generator-in-javascript
+function splitmix32(a: number) {
+  return function () {
+    a |= 0;
+    a = (a + 0x9e3779b9) | 0;
+    let t = a ^ (a >>> 16);
+    t = Math.imul(t, 0x21f0aaad);
+    t = t ^ (t >>> 15);
+    t = Math.imul(t, 0x735a2d97);
+    return ((t = t ^ (t >>> 15)) >>> 0) / 4294967296;
+  };
+}
+
+let prng = null as any;
+
+export function seedRandom(seed: number) {
+  prng = splitmix32(seed);
+}
+
 // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/random
 export function getRandomRange(min: number, max: number) {
-  return Math.random() * (max - min) + min;
+  if (!prng) {
+    console.log("prng not seeded, generating now");
+    seedRandom(Date.now() * Math.random());
+  }
+  return prng() * (max - min) + min;
 }
 
 // https://stackoverflow.com/questions/5306680/move-an-array-element-from-one-array-position-to-another

src/reducers/Game.tsx

@@ -19,7 +19,7 @@ import {
   formatWatts,
   formatWattHours,
 } from "../helpers/Format";
-import { arrayMove } from "../helpers/Math";
+import { arrayMove, seedRandom } from "../helpers/Math";
 import { getSolarOutputFactor, getWindOutputFactor } from "../helpers/Energy";
 import { getFuelPricesPerMBTU } from "../data/FuelPrices";
 import { getWeather, getRawSolarIrradianceWM2 } from "../data/Weather";
@@ -86,6 +86,7 @@ let previousTickMs = 0;
 let previousSpeed = "PAUSED" as SpeedType;
 let previousMonth = "";
 const initialGame: GameType = {
+  seed: Date.now() * Math.random(),
   scenarioId: 0,
   location: LOCATIONS["SF"],
   difficulty: "Employee",
@@ -134,6 +135,8 @@ export const gameSlice = createSlice({
     initGame: (state, action: PayloadAction<NewGameAction>) => {
       const a = action.payload;
       state.timeline = [] as TickPresentFutureType[];
+      state.seed = Date.now() * Math.random();
+      seedRandom(state.seed);
       const scenario =
         SCENARIOS.find((s) => s.id === state.scenarioId) || SCENARIOS[0];
       state.date = getDateFromMinute(0, scenario.startingYear);