create tidepool-utils js package, with prng impl

tidepool-utils/.gitignore

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+node_modules/

tidepool-utils/lib/prng.js

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+export class Rng {
+    genU32() { throw new Error('not implemented'); }
+
+    // precisely mirrors Rust's `rand::Rng::gen_range()` for `Range<u32>`
+    genRange(min, max) {
+        let range = max - min;
+        let zone = (range << Math.clz32(range)) - 1 >>> 0;
+        let v, lo, hi;
+        do {
+            v = this.genU32();
+            lo = Math.imul(v, range) >>> 0; // (u32) (v * range)
+        } while (lo > zone);
+        hi = BigInt(v) * BigInt(range) >> 32n;
+        return min + Number(hi);
+    }
+}
+
+export class SplitMix64 extends Rng {
+    constructor(seed) {
+        super();
+        this.seed = BigInt(seed);
+    }
+
+    clone() {
+        return new this.constructor(this.seed);
+    }
+
+    genU32() {
+        this.seed = BigInt.asUintN(64, this.seed + 0x9E3779B97F4A7C15n);
+        let z = this.seed;
+        z ^= BigInt.asUintN(64, z >> 33n);
+        z  = BigInt.asUintN(64, z * 0x62A9D9ED799705F5n);
+        z ^= BigInt.asUintN(64, z >> 28n);
+        z  = BigInt.asUintN(64, z * 0xCB24D0A5C88C35B3n);
+        return Number(z >> 32n);
+    }
+}

tidepool-utils/package-lock.json

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+{
+  "name": "tidepool-utils",
+  "version": "1.0.0",
+  "lockfileVersion": 2,
+  "requires": true,
+  "packages": {
+    "": {
+      "name": "tidepool-utils",
+      "version": "1.0.0",
+      "license": "LGPL-2.1-or-later",
+      "dependencies": {
+        "tetris-fumen": "^1.1.3"
+      }
+    },
+    "node_modules/tetris-fumen": {
+      "version": "1.1.3",
+      "resolved": "https://registry.npmjs.org/tetris-fumen/-/tetris-fumen-1.1.3.tgz",
+      "integrity": "sha512-cMGKYvkL11fH5iRk8SpIl4+5gREWk084Q8VPtkhUc0pSyCjT99+I5+SaBjlBG/WkP2U3ve30OyFfvCTUSbl2Qg=="
+    }
+  },
+  "dependencies": {
+    "tetris-fumen": {
+      "version": "1.1.3",
+      "resolved": "https://registry.npmjs.org/tetris-fumen/-/tetris-fumen-1.1.3.tgz",
+      "integrity": "sha512-cMGKYvkL11fH5iRk8SpIl4+5gREWk084Q8VPtkhUc0pSyCjT99+I5+SaBjlBG/WkP2U3ve30OyFfvCTUSbl2Qg=="
+    }
+  }
+}

tidepool-utils/package.json

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+{
+  "name": "tidepool-utils",
+  "type": "module",
+  "version": "1.0.0",
+  "description": "Utilities for working with JSON data generated by tidepool",
+  "author": "iitalics",
+  "license": "LGPL-2.1-or-later",
+  "dependencies": {
+    "tetris-fumen": "^1.1.3"
+  }
+}