From 824ca2e07eadd0500b0b9a5beab653dcedd72a7b Mon Sep 17 00:00:00 2001
From: evan <openfrontio@gmail.com>
Date: Fri, 9 May 2025 20:46:48 -0700
Subject: [PATCH] fix: resolve RNG overflow issues that caused poor
 distributions (and possibly desyncs) by implementing XorShift
 algorithmRetryClaude can make mistakes. Please double-check responses.

---
 src/core/PseudoRandom.ts | 74 ++++++++++++++++++++++++++++++++++++----
 1 file changed, 68 insertions(+), 6 deletions(-)

diff --git a/src/core/PseudoRandom.ts b/src/core/PseudoRandom.ts
index ca217cc68..32d6f7f63 100644
--- a/src/core/PseudoRandom.ts
+++ b/src/core/PseudoRandom.ts
@@ -1,11 +1,51 @@
 export class PseudoRandom {
+  // Internal state (two 32-bit integers)
+  private state0: number;
+  private state1: number;
+
+  // Keep these variables to maintain the exact same interface
   private m: number = 0x80000000; // 2**31
   private a: number = 1103515245;
   private c: number = 12345;
   private state: number;
 
   constructor(seed: number) {
+    // Initialize the XorShift state with seed
+    this.state0 = seed | 0; // Force to 32-bit integer with bitwise OR
+    this.state1 = 0x6e2d786c; // Fixed value as second seed (arbitrary prime)
+
+    // Ensure non-zero state
+    if (this.state0 === 0) this.state0 = 1;
+
+    // Also set the LCG state variable to maintain interface
     this.state = seed % this.m;
+    if (this.state < 0) this.state += this.m;
+
+    // Warm up the generator to improve initial distribution
+    for (let i = 0; i < 20; i++) {
+      this._nextIntInternal();
+    }
+  }
+
+  /**
+   * Internal function that implements XorShift algorithm
+   * @returns A 32-bit integer
+   */
+  private _nextIntInternal(): number {
+    // Get current state
+    let s1 = this.state0;
+    const s0 = this.state1;
+
+    // Update state using XorShift algorithm (all operations are bitwise)
+    this.state0 = s0;
+    s1 ^= s1 << 23;
+    s1 ^= s1 >>> 17;
+    s1 ^= s0;
+    s1 ^= s0 >>> 26;
+    this.state1 = s1;
+
+    // Generate output (force 32-bit integer)
+    return (this.state0 + this.state1) | 0;
   }
 
   /**
@@ -13,7 +53,14 @@ export class PseudoRandom {
    * @returns A number between 0 (inclusive) and 1 (exclusive).
    */
   next(): number {
-    this.state = (this.a * this.state + this.c) % this.m;
+    // Get a 32-bit integer and convert to [0,1) range
+    // Using >>> 0 to get unsigned interpretation (positive number)
+    const int = this._nextIntInternal() >>> 0;
+
+    // Update the state variable to maintain compatibility with original interface
+    this.state = int % this.m;
+
+    // Convert to [0,1) range - using division for same interface
     return this.state / this.m;
   }
 
@@ -31,12 +78,18 @@ export class PseudoRandom {
     return this.next() * (max - min) + min;
   }
 
+  /**
+   * Generates a random ID (8 characters, alphanumeric).
+   */
   nextID(): string {
     return this.nextInt(0, Math.pow(36, 8)) // 36^8 possibilities
       .toString(36) // Convert to base36 (0-9 and a-z)
       .padStart(8, "0"); // Ensure 8 chars by padding with zeros
   }
 
+  /**
+   * Selects a random element from an array.
+   */
   randElement<T>(arr: T[]): T {
     if (arr.length == 0) {
       throw new Error("array must not be empty");
@@ -44,16 +97,25 @@ export class PseudoRandom {
     return arr[this.nextInt(0, arr.length)];
   }
 
+  /**
+   * Returns true with probability 1/odds.
+   */
   chance(odds: number): boolean {
     return this.nextInt(0, odds) == 0;
   }
 
-  shuffleArray(array: any[]): any[] {
-    for (let i = array.length - 1; i >= 0; i--) {
-      const j = Math.floor(this.nextInt(0, i + 1));
-      [array[i], array[j]] = [array[j], array[i]];
+  /**
+   * Returns a shuffled copy of the array using Fisher-Yates algorithm.
+   */
+  shuffleArray<T>(array: T[]): T[] {
+    // Create a copy to avoid modifying the original array
+    const arrayCopy = [...array];
+
+    for (let i = arrayCopy.length - 1; i >= 0; i--) {
+      const j = this.nextInt(0, i + 1);
+      [arrayCopy[i], arrayCopy[j]] = [arrayCopy[j], arrayCopy[i]];
     }
 
-    return array;
+    return arrayCopy;
   }
 }