crypto: add randomInt function

Author: olalonde

doc/api/crypto.md

@@ -2800,6 +2800,46 @@ threadpool request. To minimize threadpool task length variation, partition
 large `randomFill` requests when doing so as part of fulfilling a client
 request.
 
+### `crypto.randomInt([min, ]max[, callback])`
+<!-- YAML
+added:
+  - v14.8.0
+-->
+
+* `min` {integer} Start of random range. **Default**: `0`.
+* `max` {integer} End of random range (non-inclusive).
+* `callback` {Function} `function(err, n) {}`.
+
+Return a random integer `n` such that `min <= n < max`.  This
+implementation avoids [modulo
+bias](https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle#Modulo_bias).
+
+The maximum supported range value (`max - min`) is `2^48 - 1`.
+
+If the `callback` function is not provided, the random integer is generated
+synchronously.
+
+```js
+// Asynchronous
+crypto.randomInt(3, (err, n) => {
+  if (err) throw err;
+  console.log(`Random number chosen from (0, 1, 2): ${n}`);
+});
+```
+
+```js
+// Synchronous
+const n = crypto.randomInt(3);
+console.log(`Random number chosen from (0, 1, 2): ${n}`);
+```
+
+```js
+crypto.randomInt(1, 7, (err, n) => {
+  if (err) throw err;
+  console.log(`The dice rolled: ${n}`);
+});
+```
+
 ### `crypto.scrypt(password, salt, keylen[, options], callback)`
 <!-- YAML
 added: v10.5.0

lib/crypto.js

@@ -52,7 +52,8 @@ const {
 const {
   randomBytes,
   randomFill,
-  randomFillSync
+  randomFillSync,
+  randomInt
 } = require('internal/crypto/random');
 const {
   pbkdf2,
@@ -184,6 +185,7 @@ module.exports = {
   randomBytes,
   randomFill,
   randomFillSync,
+  randomInt,
   scrypt,
   scryptSync,
   sign: signOneShot,

lib/internal/crypto/random.js

@@ -3,6 +3,7 @@
 const {
   MathMin,
   NumberIsNaN,
+  NumberIsInteger
 } = primordials;
 
 const { AsyncWrap, Providers } = internalBinding('async_wrap');
@@ -119,6 +120,74 @@ function randomFill(buf, offset, size, cb) {
   _randomBytes(buf, offset, size, wrap);
 }
 
+// Largest integer we can read from a buffer.
+// e.g.: Buffer.from("ff".repeat(6), "hex").readUIntBE(0, 6);
+const RAND_MAX = 281474976710655;
+
+function randomInt(min, max, cb) {
+  // randomInt(max, cb)
+  // randomInt(max)
+  if (typeof max === 'function' || typeof max === 'undefined') {
+    cb = max;
+    max = min;
+    min = 0;
+  }
+  const isSync = typeof cb === 'undefined';
+  if (!isSync && typeof cb !== 'function') {
+    throw new ERR_INVALID_CALLBACK(cb);
+  }
+  if (!NumberIsInteger(min)) {
+    throw new ERR_INVALID_ARG_TYPE('min', 'integer', min);
+  }
+  if (!NumberIsInteger(max)) {
+    throw new ERR_INVALID_ARG_TYPE('max', 'integer', max);
+  }
+  if (!(max > min)) {
+    throw new ERR_OUT_OF_RANGE('max', `> ${min}`, max);
+  }
+
+  // First we generate a random int between [0..range)
+  const range = max - min;
+
+  if (!(range <= RAND_MAX)) {
+    throw new ERR_OUT_OF_RANGE('max - min', `<= ${RAND_MAX}`, range);
+  }
+
+  const excess = RAND_MAX % range;
+  const randLimit = RAND_MAX - excess;
+
+  if (isSync) {
+    // Sync API
+    while (true) {
+      const x = randomBytes(6).readUIntBE(0, 6);
+      // If x > (maxVal - (maxVal % range)), we will get "modulo bias"
+      if (x > randLimit) {
+        // Try again
+        continue;
+      }
+      const n = (x % range) + min;
+      return n;
+    }
+  } else {
+    // Async API
+    const pickAttempt = () => {
+      randomBytes(6, (err, bytes) => {
+        if (err) return cb(err);
+        const x = bytes.readUIntBE(0, 6);
+        // If x > (maxVal - (maxVal % range)), we will get "modulo bias"
+        if (x > randLimit) {
+          // Try again
+          return pickAttempt();
+        }
+        const n = (x % range) + min;
+        cb(null, n);
+      });
+    };
+
+    pickAttempt();
+  }
+}
+
 function handleError(ex, buf) {
   if (ex) throw ex;
   return buf;
@@ -127,5 +196,6 @@ function handleError(ex, buf) {
 module.exports = {
   randomBytes,
   randomFill,
-  randomFillSync
+  randomFillSync,
+  randomInt
 };

test/parallel/test-crypto-random.js

@@ -315,3 +315,147 @@ assert.throws(
   assert.strictEqual(desc.writable, true);
   assert.strictEqual(desc.enumerable, false);
 });
+
+
+{
+  const randomInts = [];
+  for (let i = 0; i < 100; i++) {
+    crypto.randomInt(1, 4, common.mustCall((err, n) => {
+      assert.ifError(err);
+      assert.ok(n >= 1);
+      assert.ok(n < 4);
+      randomInts.push(n);
+      if (randomInts.length === 100) {
+        assert.ok(!randomInts.includes(0));
+        assert.ok(randomInts.includes(1));
+        assert.ok(randomInts.includes(2));
+        assert.ok(randomInts.includes(3));
+        assert.ok(!randomInts.includes(4));
+      }
+    }));
+  }
+}
+{
+  const randomInts = [];
+  for (let i = 0; i < 100; i++) {
+    crypto.randomInt(-10, -7, common.mustCall((err, n) => {
+      assert.ifError(err);
+      assert.ok(n >= -10);
+      assert.ok(n < -7);
+      randomInts.push(n);
+      if (randomInts.length === 100) {
+        assert.ok(!randomInts.includes(-11));
+        assert.ok(randomInts.includes(-10));
+        assert.ok(randomInts.includes(-9));
+        assert.ok(randomInts.includes(-8));
+        assert.ok(!randomInts.includes(-7));
+      }
+    }));
+  }
+}
+{
+  const randomInts = [];
+  for (let i = 0; i < 100; i++) {
+    crypto.randomInt(3, common.mustCall((err, n) => {
+      assert.ifError(err);
+      assert.ok(n >= 0);
+      assert.ok(n < 3);
+      randomInts.push(n);
+      if (randomInts.length === 100) {
+        assert.ok(randomInts.includes(0));
+        assert.ok(randomInts.includes(1));
+        assert.ok(randomInts.includes(2));
+      }
+    }));
+  }
+}
+{
+  // Synchronous API
+  const randomInts = [];
+  for (let i = 0; i < 100; i++) {
+    const n = crypto.randomInt(3);
+    assert.ok(n >= 0);
+    assert.ok(n < 3);
+    randomInts.push(n);
+  }
+
+  assert.ok(randomInts.includes(0));
+  assert.ok(randomInts.includes(1));
+  assert.ok(randomInts.includes(2));
+}
+{
+  // Synchronous API with min
+  const randomInts = [];
+  for (let i = 0; i < 100; i++) {
+    const n = crypto.randomInt(3, 6);
+    assert.ok(n >= 3);
+    assert.ok(n < 6);
+    randomInts.push(n);
+  }
+
+  assert.ok(randomInts.includes(3));
+  assert.ok(randomInts.includes(4));
+  assert.ok(randomInts.includes(5));
+}
+{
+
+  ['10', true, NaN, null, {}, []].forEach((i) => {
+    assert.throws(() => crypto.randomInt(i, 100, common.mustNotCall()), {
+      code: 'ERR_INVALID_ARG_TYPE',
+      name: 'TypeError',
+      message: 'The "min" argument must be integer.' +
+               `${common.invalidArgTypeHelper(i)}`,
+    });
+
+    assert.throws(() => crypto.randomInt(0, i, common.mustNotCall()), {
+      code: 'ERR_INVALID_ARG_TYPE',
+      name: 'TypeError',
+      message: 'The "max" argument must be integer.' +
+               `${common.invalidArgTypeHelper(i)}`,
+    });
+
+    assert.throws(() => crypto.randomInt(i, common.mustNotCall()), {
+      code: 'ERR_INVALID_ARG_TYPE',
+      name: 'TypeError',
+      message: 'The "max" argument must be integer.' +
+               `${common.invalidArgTypeHelper(i)}`,
+    });
+  });
+
+  assert.throws(() => crypto.randomInt(0, 0, common.mustNotCall()), {
+    code: 'ERR_OUT_OF_RANGE',
+    name: 'RangeError',
+    message: 'The value of "max" is out of range. It must be > 0. Received 0'
+  });
+
+  assert.throws(() => crypto.randomInt(0, -1, common.mustNotCall()), {
+    code: 'ERR_OUT_OF_RANGE',
+    name: 'RangeError',
+    message: 'The value of "max" is out of range. It must be > 0. Received -1'
+  });
+
+  assert.throws(() => crypto.randomInt(0, common.mustNotCall()), {
+    code: 'ERR_OUT_OF_RANGE',
+    name: 'RangeError',
+    message: 'The value of "max" is out of range. It must be > 0. Received 0'
+  });
+
+  assert.throws(() => crypto.randomInt(2 ** 48, common.mustNotCall()), {
+    code: 'ERR_OUT_OF_RANGE',
+    name: 'RangeError',
+    message: 'The value of "max - min" is out of range. ' +
+             `It must be <= ${(2 ** 48) - 1}. ` +
+             'Received 281_474_976_710_656'
+  });
+
+  [1, true, NaN, null, {}, []].forEach((i) => {
+    assert.throws(
+      () => crypto.randomInt(1, 2, i),
+      {
+        code: 'ERR_INVALID_CALLBACK',
+        name: 'TypeError',
+        message: `Callback must be a function. Received ${inspect(i)}`
+      }
+    );
+  });
+}