[autorelease] Use xoroshiro128** for random/getRandomBytes.

This provides full determinism, and also avoids the use of `crypto`
API that requires `require` on Node v18 (used in VSCode).

lib/api-browser.js

@@ -1,13 +1,3 @@
-import { setRandomBytesImpl } from './wasi-virt.js';
-
-setRandomBytesImpl(function(length) {
-    const QUOTA = 65536; // QuotaExceededError thrown when requesting more
-    const bytes = new Uint8Array(length);
-    for (var offset = 0; offset < length; offset += QUOTA)
-      crypto.getRandomValues(bytes.slice(offset, offset + QUOTA));
-    return bytes;
-});
-
 import { BaseApplication } from './api-base.js';
 
 export class Application extends BaseApplication {

lib/api-node.js

@@ -1,8 +1,3 @@
-import { randomBytes } from 'crypto';
-import { setRandomBytesImpl } from './wasi-virt.js';
-
-setRandomBytesImpl(randomBytes);
-
 import { readFile } from 'fs/promises';
 import { BaseApplication } from './api-base.js';
 

lib/wasi-virt.js

@@ -16,11 +16,30 @@ function wallClockNow() {
   return { seconds, nanoseconds };
 }
 
-let randomBytesImpl = function() { throw 'not implemented'; };
-export function setRandomBytesImpl(impl) { randomBytesImpl = impl; }
+class Xoroshiro128StarStar {
+  constructor(seed) {
+    if (BigInt(seed) === 0n) {
+      throw new Error("xoroshiro128** must be seeded with a non-zero state")
+    }
+    this.s = [BigInt(seed) & 0xffffffffffffffffn, (BigInt(seed) >> 64n) & 0xffffffffffffffffn];
+  }
 
-export function getRandomBytes(len) {
-  return randomBytesImpl(Number(len));
+  next() {
+    function trunc64(x) { return x & 0xffffffffffffffffn; }
+    function rotl(x, k) { return (x << k) | (x >> (64n - k)); }
+
+    let [s0, s1] = this.s;
+    const r = trunc64(rotl(s0 * 5n, 7n) * 9n);
+    s1 ^= s0;
+    s0 = trunc64(rotl(s0, 24n) ^ s1 ^ (s1 << 16n));
+    s1 = trunc64(rotl(s1, 37n));
+    this.s = [s0, s1];
+    return r;
+  }
+
+  getBytes(length) {
+    return Uint8Array.from({ length }, () => Number(BigInt.asUintN(8, this.next() >> 32n)));
+  }
 }
 
 class IoError extends Error {}
@@ -335,6 +354,8 @@ export class Environment {
   root = new Directory({});
 
   constructor() {
+    this.prng = new Xoroshiro128StarStar(1n);
+
     this.terminalInputStream = new InputStream();
     this.terminalOutputStream = new TerminalOutputStream();
 
@@ -350,7 +371,7 @@ export class Environment {
         now: wallClockNow
       },
       random: {
-        getRandomBytes,
+        getRandomBytes(length) { return $this.prng.getBytes(Number(length)); }
       },
       io: {
         Error: IoError,