[TS] Implement ctx.random()

crates/bindings-typescript/package.json

@@ -156,6 +156,7 @@
     "fast-text-encoding": "^1.0.0",
     "headers-polyfill": "^4.0.3",
     "prettier": "^3.3.3",
+    "pure-rand": "^7.0.1",
     "statuses": "^2.0.2",
     "url-polyfill": "^1.1.14"
   },

crates/bindings-typescript/src/lib/reducers.ts

@@ -23,6 +23,7 @@ import {
 import type { ReducerSchema } from './reducer_schema';
 import { toCamelCase, toPascalCase } from './util';
 import type { CamelCase } from './type_util';
+import type { Random } from '../server/rng';
 
 /**
  * Helper to extract the parameter types from an object type
@@ -120,6 +121,7 @@ export type ReducerCtx<SchemaDef extends UntypedSchemaDef> = Readonly<{
   connectionId: ConnectionId | null;
   db: DbView<SchemaDef>;
   senderAuth: AuthCtx;
+  random: Random;
 }>;
 
 /**

crates/bindings-typescript/src/server/rng.ts

@@ -0,0 +1,119 @@
+import type { RandomGenerator } from 'pure-rand';
+import { unsafeUniformBigIntDistribution } from 'pure-rand/distribution/UnsafeUniformBigIntDistribution';
+import { unsafeUniformIntDistribution } from 'pure-rand/distribution/UnsafeUniformIntDistribution';
+import { xoroshiro128plus } from 'pure-rand/generator/XoroShiro';
+import type { Timestamp } from '../lib/timestamp';
+
+declare global {
+  interface Math {
+    random(): never;
+  }
+}
+
+type IntArray =
+  | Int8Array
+  | Uint8Array
+  | Uint8ClampedArray
+  | Int16Array
+  | Uint16Array
+  | Int32Array
+  | Uint32Array
+  | BigInt64Array
+  | BigUint64Array;
+
+/**
+ * A collection of random-number-generating functions, seeded based on `ctx.timestamp`.
+ *
+ * ## Usage
+ *
+ * ```
+ * const floatOneToTen = ctx.random() * 10;
+ * const randomBytes = ctx.random.fill(new Uint8Array(16));
+ * const intOneToTen = ctx.random.integerInRange(0, 10);
+ * ```
+ */
+export interface Random {
+  /**
+   * Returns a random floating-point number in the range `[0.0, 1.0)`.
+   *
+   * The returned float will have 53 bits of randomness.
+   */
+  (): number;
+
+  /**
+   * Like `crypto.getRandomValues()`. Fills a `TypedArray` with random integers
+   * in a uniform distribution, mutating and returning it.
+   */
+  fill<T extends IntArray>(array: T): T;
+
+  /**
+   * Returns a random unsigned 32-bit integer in a uniform distribution in the
+   * range `[0, 2**32)`.
+   */
+  uint32(): number;
+
+  /**
+   * Returns an integer in the range `[min, max]`.
+   */
+  integerInRange(min: number, max: number): number;
+
+  /**
+   * Returns a bigint in the range `[min, max]`.
+   */
+  bigintInRange(min: bigint, max: bigint): bigint;
+}
+
+const { asUintN } = BigInt;
+
+/** Based on the function of the same name in `rand_core::SeedableRng::seed_from_u64` */
+function pcg32(state: bigint): number {
+  const MUL = 6364136223846793005n;
+  const INC = 11634580027462260723n;
+
+  state = asUintN(64, state * MUL + INC);
+  const xorshifted = Number(asUintN(32, ((state >> 18n) ^ state) >> 27n));
+  const rot = Number(asUintN(32, state >> 59n));
+  // rotate `xorshifted` right by `rot` bits
+  return (xorshifted >> rot) | (xorshifted << (32 - rot));
+}
+
+/** From the `pure-rand` README */
+function generateFloat64(rng: RandomGenerator): number {
+  const g1 = unsafeUniformIntDistribution(0, (1 << 26) - 1, rng);
+  const g2 = unsafeUniformIntDistribution(0, (1 << 27) - 1, rng);
+  const value = (g1 * Math.pow(2, 27) + g2) * Math.pow(2, -53);
+  return value;
+}
+
+export function makeRandom(seed: Timestamp): Random {
+  // Use PCG32 to turn a 64-bit seed into a 32-bit seed, as the Rust `rand` crate does.
+  const rng = xoroshiro128plus(pcg32(seed.microsSinceUnixEpoch));
+
+  const random: Random = () => generateFloat64(rng);
+
+  random.fill = array => {
+    const elem = array.at(0);
+    if (typeof elem === 'bigint') {
+      const upper = (1n << BigInt(array.BYTES_PER_ELEMENT * 8)) - 1n;
+      for (let i = 0; i < array.length; i++) {
+        array[i] = unsafeUniformBigIntDistribution(0n, upper, rng);
+      }
+    } else if (typeof elem === 'number') {
+      const upper = (1 << (array.BYTES_PER_ELEMENT * 8)) - 1;
+      for (let i = 0; i < array.length; i++) {
+        array[i] = unsafeUniformIntDistribution(0, upper, rng);
+      }
+    }
+    return array;
+  };
+
+  random.uint32 = () => rng.unsafeNext();
+
+  random.integerInRange = (min, max) =>
+    unsafeUniformIntDistribution(min, max, rng);
+
+  random.bigintInRange = (min, max) =>
+    unsafeUniformBigIntDistribution(min, max, rng);
+
+  return random;
+}

crates/bindings-typescript/src/server/runtime.ts

@@ -45,6 +45,7 @@ import type { DbView } from './db_view';
 import { SenderError, SpacetimeHostError } from './errors';
 import { Range, type Bound } from './range';
 import ViewResultHeader from '../lib/autogen/view_result_header_type';
+import { makeRandom } from './rng';
 
 const { freeze } = Object;
 
@@ -194,6 +195,7 @@ export const makeReducerCtx = (
   connectionId,
   db: getDbView(),
   senderAuth: AuthCtxImpl.fromSystemTables(connectionId, sender),
+  random: makeRandom(timestamp),
 });
 
 /**

crates/bindings-typescript/tsup.config.ts

@@ -131,7 +131,7 @@ export default defineConfig([
       ],
     },
     external: ['undici', /^spacetime:sys.*$/],
-    noExternal: ['base64-js', 'fast-text-encoding', 'statuses'],
+    noExternal: ['base64-js', 'fast-text-encoding', 'statuses', 'pure-rand'],
     outExtension,
     esbuildOptions: commonEsbuildTweaks(),
   },

crates/core/src/host/v8/builtins/delete_math_random.js

@@ -0,0 +1,11 @@
+delete Math.random;
+Object.defineProperty(Math, 'random', {
+  writable: true,
+  enumerable: false,
+  configurable: true,
+  get() {
+    throw new TypeError(
+      'Math.random is not available in SpacetimeDB modules. Use ctx.random instead.'
+    );
+  },
+});

crates/core/src/host/v8/builtins/mod.rs

@@ -16,6 +16,7 @@ pub(super) fn evalute_builtins(scope: &mut PinScope<'_, '_>) -> ExcResult<()> {
         };
     }
     eval_builtin!("text_encoding.js")?;
+    eval_builtin!("delete_math_random.js")?;
     Ok(())
 }