From 15476c2f07796b34da6fb86ae8b0e529221d6d83 Mon Sep 17 00:00:00 2001
From: cs01 <cs01@users.noreply.github.com>
Date: Sun, 1 Mar 2026 19:31:19 -0800
Subject: [PATCH] add seeded prng: Math.seedRandom() and lib/random.ts lcg

---
 lib/random.ts                             | 22 ++++++++++++
 src/codegen/stdlib/math.ts                | 15 +++++++++
 tests/fixtures/math/math-seed-random.ts   | 27 +++++++++++++++
 tests/fixtures/random/random-lib-basic.ts | 41 +++++++++++++++++++++++
 4 files changed, 105 insertions(+)
 create mode 100644 lib/random.ts
 create mode 100644 tests/fixtures/math/math-seed-random.ts
 create mode 100644 tests/fixtures/random/random-lib-basic.ts

diff --git a/lib/random.ts b/lib/random.ts
new file mode 100644
index 00000000..ff4ab3aa
--- /dev/null
+++ b/lib/random.ts
@@ -0,0 +1,22 @@
+export class Random {
+  private state: number;
+
+  constructor(seed: number) {
+    this.state = seed % 4294967296.0;
+    if (this.state < 0.0) {
+      this.state = this.state + 4294967296.0;
+    }
+    if (this.state === 0.0) {
+      this.state = 1.0;
+    }
+  }
+
+  next(): number {
+    this.state = (this.state * 1664525.0 + 1013904223.0) % 4294967296.0;
+    return this.state / 4294967296.0;
+  }
+
+  nextInt(min: number, max: number): number {
+    return Math.floor(this.next() * (max - min)) + min;
+  }
+}
diff --git a/src/codegen/stdlib/math.ts b/src/codegen/stdlib/math.ts
index e672988c..721fb118 100644
--- a/src/codegen/stdlib/math.ts
+++ b/src/codegen/stdlib/math.ts
@@ -53,6 +53,7 @@ export class MathGenerator {
       "cos",
       "sign",
       "random",
+      "seedRandom",
     ];
   }
 
@@ -95,6 +96,8 @@ export class MathGenerator {
         return this.generateSign(expr, params);
       case "random":
         return this.generateRandom(expr);
+      case "seedRandom":
+        return this.generateSeedRandom(expr, params);
       default:
         return this.ctx.emitError(`Unsupported Math method: ${method}`, expr.loc);
     }
@@ -216,6 +219,18 @@ export class MathGenerator {
     return result;
   }
 
+  private generateSeedRandom(expr: MethodCallNode, params: string[]): string {
+    if (expr.args.length !== 1) {
+      return this.ctx.emitError("Math.seedRandom() requires 1 argument", expr.loc);
+    }
+    const seed = this.ctx.generateExpression(expr.args[0], params);
+    const dblSeed = this.ctx.ensureDouble(seed);
+    const i64Seed = this.ctx.nextTemp();
+    this.ctx.emit(`${i64Seed} = fptosi double ${dblSeed} to i64`);
+    this.ctx.emitCallVoid("@srand48", `i64 ${i64Seed}`);
+    return "0.0";
+  }
+
   private generateSign(expr: MethodCallNode, params: string[]): string {
     if (expr.args.length !== 1) {
       return this.ctx.emitError("Math.sign() requires 1 argument", expr.loc);
diff --git a/tests/fixtures/math/math-seed-random.ts b/tests/fixtures/math/math-seed-random.ts
new file mode 100644
index 00000000..638cf907
--- /dev/null
+++ b/tests/fixtures/math/math-seed-random.ts
@@ -0,0 +1,27 @@
+Math.seedRandom(42);
+const a1 = Math.random();
+const a2 = Math.random();
+
+Math.seedRandom(42);
+const b1 = Math.random();
+const b2 = Math.random();
+
+if (a1 !== b1 || a2 !== b2) {
+  console.log("FAIL: same seed produced different sequences");
+  process.exit(1);
+}
+
+Math.seedRandom(99);
+const c1 = Math.random();
+
+if (c1 === a1) {
+  console.log("FAIL: different seeds produced same first value");
+  process.exit(1);
+}
+
+if (a1 < 0 || a1 >= 1 || b1 < 0 || b1 >= 1) {
+  console.log("FAIL: values out of [0, 1) range");
+  process.exit(1);
+}
+
+console.log("TEST_PASSED");
diff --git a/tests/fixtures/random/random-lib-basic.ts b/tests/fixtures/random/random-lib-basic.ts
new file mode 100644
index 00000000..6dca0c20
--- /dev/null
+++ b/tests/fixtures/random/random-lib-basic.ts
@@ -0,0 +1,41 @@
+import { Random } from "../../../lib/random.js";
+
+const rng1 = new Random(12345);
+const a = rng1.next();
+const b = rng1.next();
+const c = rng1.next();
+
+if (a < 0 || a >= 1 || b < 0 || b >= 1 || c < 0 || c >= 1) {
+  console.log("FAIL: values out of [0, 1) range");
+  process.exit(1);
+}
+
+if (a === b && b === c) {
+  console.log("FAIL: all values identical");
+  process.exit(1);
+}
+
+const rng2 = new Random(12345);
+const d = rng2.next();
+const e = rng2.next();
+const f = rng2.next();
+
+if (a !== d || b !== e || c !== f) {
+  console.log("FAIL: same seed produced different sequences");
+  process.exit(1);
+}
+
+const rng3 = new Random(99999);
+const g = rng3.next();
+if (g === a) {
+  console.log("FAIL: different seeds produced same first value");
+  process.exit(1);
+}
+
+const n = rng1.nextInt(0, 10);
+if (n < 0 || n >= 10) {
+  console.log("FAIL: nextInt out of range");
+  process.exit(1);
+}
+
+console.log("TEST_PASSED");