test(mersenne): add tests for value ranges (#2470)

6 files changed, 190 insertions, 59 deletions

diff --git a/src/internal/mersenne.ts b/src/internal/mersenne.ts
index 6be5825d..2372e364 100644
--- a/src/internal/mersenne.ts
+++ b/src/internal/mersenne.ts
@@ -73,7 +73,7 @@ import type { Randomizer } from '../randomizer';
  *
  * @internal
  */
-class MersenneTwister19937 {
+export class MersenneTwister19937 {
   private readonly N = 624;
   private readonly M = 397;
   private readonly MATRIX_A = 0x9908b0df; // constant vector a
diff --git a/test/__snapshots__/mersenne.spec.ts.snap b/test/internal/__snapshots__/mersenne.spec.ts.snap
index c55b1152..c55b1152 100644
--- a/test/__snapshots__/mersenne.spec.ts.snap
+++ b/test/internal/__snapshots__/mersenne.spec.ts.snap
diff --git a/test/internal/mersenne-test-utils.ts b/test/internal/mersenne-test-utils.ts
new file mode 100644
index 00000000..ee7a43da
--- /dev/null
+++ b/test/internal/mersenne-test-utils.ts
@@ -0,0 +1,17 @@
+// Moved to a separate file to avoid importing the tests
+
+/**
+ * The maximum value that can be returned by `MersenneTwister19937.genrandReal2()`.
+ * This is the max possible value with 32 bits of precision that is less than 1.
+ */
+export const TWISTER_32CO_MAX_VALUE = 0.9999999997671694;
+/**
+ * The maximum value that can be returned by `MersenneTwister19937.genrandRes53()`.
+ * This is the max possible value with 53 bits of precision that is less than 1.
+ */
+export const TWISTER_53CO_MAX_VALUE = 0.9999999999999999;
+// Re-exported because the value might change in the future
+/**
+ * The maximum value that can be returned by `next()`.
+ */
+export const MERSENNE_MAX_VALUE = TWISTER_32CO_MAX_VALUE;
diff --git a/test/internal/mersenne.spec.ts b/test/internal/mersenne.spec.ts
new file mode 100644
index 00000000..bbee0502
--- /dev/null
+++ b/test/internal/mersenne.spec.ts
@@ -0,0 +1,136 @@
+import { beforeAll, beforeEach, describe, expect, it } from 'vitest';
+import {
+  generateMersenne32Randomizer,
+  MersenneTwister19937,
+} from '../../src/internal/mersenne';
+import type { Randomizer } from '../../src/randomizer';
+import { seededRuns } from '../support/seeded-runs';
+import { times } from '../support/times';
+import {
+  MERSENNE_MAX_VALUE,
+  TWISTER_32CO_MAX_VALUE,
+  TWISTER_53CO_MAX_VALUE,
+} from './mersenne-test-utils';
+
+const NON_SEEDED_BASED_RUN = 25;
+
+function newTwister(
+  seed: number = Math.random() * Number.MAX_SAFE_INTEGER
+): MersenneTwister19937 {
+  const twister = new MersenneTwister19937();
+  twister.initGenrand(seed);
+  return twister;
+}
+
+describe('MersenneTwister19937', () => {
+  describe('genrandInt32()', () => {
+    it('should be able to return 0', () => {
+      const twister = newTwister(257678572);
+
+      // There is no single value seed that can produce 0 in the first call
+      for (let i = 0; i < 5; i++) {
+        twister.genrandInt32();
+      }
+
+      const actual = twister.genrandInt32();
+      expect(actual).toBe(0);
+    });
+
+    it('should be able to return 2^32-1', () => {
+      const twister = newTwister(2855577693);
+      const actual = twister.genrandInt32();
+      expect(actual).toBe(2 ** 32 - 1);
+    });
+  });
+
+  describe('genrandReal2()', () => {
+    it('should be able to return 0', () => {
+      const twister = newTwister();
+      // shortcut to return minimal value
+      // the test above shows that it is possible to return 0
+      twister.genrandInt32 = () => 0;
+      const actual = twister.genrandReal2();
+      expect(actual).toBe(0);
+    });
+
+    it('should be able to return almost 1', () => {
+      const twister = newTwister();
+      // shortcut to return maximal value
+      // the test above shows that it is possible to return 2^32-1
+      twister.genrandInt32 = () => 2 ** 32 - 1;
+      const actual = twister.genrandReal2();
+      expect(actual).toBe(TWISTER_32CO_MAX_VALUE);
+    });
+  });
+
+  describe('genrandRes53()', () => {
+    it('should be able to return 0', () => {
+      const twister = newTwister();
+      // shortcut to return minimal value
+      // the test above shows that it is possible to return 0
+      twister.genrandInt32 = () => 0;
+      const actual = twister.genrandRes53();
+      expect(actual).toBe(0);
+    });
+
+    it('should be able to return almost 1', () => {
+      const twister = newTwister();
+      // shortcut to return maximal value
+      // the test above shows that it is possible to return 2^32-1
+      twister.genrandInt32 = () => 2 ** 32 - 1;
+      const actual = twister.genrandRes53();
+      expect(actual).toBe(TWISTER_53CO_MAX_VALUE);
+    });
+  });
+});
+
+describe('generateMersenne32Randomizer()', () => {
+  const randomizer: Randomizer = generateMersenne32Randomizer();
+
+  it('should return a result matching the interface', () => {
+    expect(randomizer).toBeDefined();
+    expect(randomizer).toBeTypeOf('object');
+    expect(randomizer.next).toBeTypeOf('function');
+    expect(randomizer.seed).toBeTypeOf('function');
+  });
+
+  describe.each(
+    [...seededRuns, ...seededRuns.map((v) => [v, 1, 2])].map((v) => [v])
+  )('seed: %j', (seed) => {
+    beforeEach(() => {
+      randomizer.seed(seed);
+    });
+
+    it('should return deterministic value for next()', () => {
+      const actual = randomizer.next();
+
+      expect(actual).toMatchSnapshot();
+    });
+  });
+
+  function randomSeed(): number {
+    return Math.ceil(Math.random() * 1_000_000_000);
+  }
+
+  // Create and log-back the seed for debug purposes
+  describe.each(
+    times(NON_SEEDED_BASED_RUN).flatMap(() => [
+      [randomSeed()],
+      [[randomSeed(), randomSeed()]],
+    ])
+  )('random seeded tests %j', (seed) => {
+    beforeAll(() => {
+      randomizer.seed(seed);
+    });
+
+    describe('next', () => {
+      it('should return random number from interval [0, 1)', () => {
+        const actual = randomizer.next();
+
+        expect(actual).toBeGreaterThanOrEqual(0);
+        expect(actual).toBeLessThanOrEqual(MERSENNE_MAX_VALUE);
+        expect(actual).toBeLessThan(1);
+      });
+    });
+  });
+});
diff --git a/test/mersenne.spec.ts b/test/mersenne.spec.ts
deleted file mode 100644
index 6c7d5a18..00000000
--- a/test/mersenne.spec.ts
+++ /dev/null
@@ -1,57 +0,0 @@
-import { beforeAll, beforeEach, describe, expect, it } from 'vitest';
-import { generateMersenne32Randomizer } from '../src/internal/mersenne';
-import type { Randomizer } from '../src/randomizer';
-import { seededRuns } from './support/seeded-runs';
-import { times } from './support/times';
-
-const NON_SEEDED_BASED_RUN = 25;
-
-describe('generateMersenne32Randomizer()', () => {
-  const randomizer: Randomizer = generateMersenne32Randomizer();
-
-  it('should return a result matching the interface', () => {
-    expect(randomizer).toBeDefined();
-    expect(randomizer).toBeTypeOf('object');
-    expect(randomizer.next).toBeTypeOf('function');
-    expect(randomizer.seed).toBeTypeOf('function');
-  });
-
-  describe.each(
-    [...seededRuns, ...seededRuns.map((v) => [v, 1, 2])].map((v) => [v])
-  )('seed: %j', (seed) => {
-    beforeEach(() => {
-      randomizer.seed(seed);
-    });
-
-    it('should return deterministic value for next()', () => {
-      const actual = randomizer.next();
-
-      expect(actual).toMatchSnapshot();
-    });
-  });
-
-  function randomSeed(): number {
-    return Math.ceil(Math.random() * 1_000_000_000);
-  }
-
-  // Create and log-back the seed for debug purposes
-  describe.each(
-    times(NON_SEEDED_BASED_RUN).flatMap(() => [
-      [randomSeed()],
-      [[randomSeed(), randomSeed()]],
-    ])
-  )('random seeded tests %j', (seed) => {
-    beforeAll(() => {
-      randomizer.seed(seed);
-    });
-
-    describe('next', () => {
-      it('should return random number from interval [0, 1)', () => {
-        const actual = randomizer.next();
-
-        expect(actual).toBeGreaterThanOrEqual(0);
-        expect(actual).toBeLessThan(1);
-      });
-    });
-  });
-});
diff --git a/test/modules/number.spec.ts b/test/modules/number.spec.ts
index 66018ee6..699200b2 100644
--- a/test/modules/number.spec.ts
+++ b/test/modules/number.spec.ts
@@ -1,6 +1,7 @@
 import validator from 'validator';
 import { describe, expect, it } from 'vitest';
-import { faker, FakerError } from '../../src';
+import { faker, FakerError, SimpleFaker } from '../../src';
+import { MERSENNE_MAX_VALUE } from '../internal/mersenne-test-utils';
 import { seededTests } from '../support/seeded-runs';
 
 describe('number', () => {
@@ -507,4 +508,38 @@ describe('number', () => {
       });
     });
   });
+
+  describe('value range tests', () => {
+    const customFaker = new SimpleFaker();
+    // @ts-expect-error: access private member field
+    const randomizer = customFaker._randomizer;
+    describe('int', () => {
+      it('should be able to return 0', () => {
+        randomizer.next = () => 0;
+        const actual = customFaker.number.int();
+        expect(actual).toBe(0);
+      });
+
+      // TODO @ST-DDT 2023-10-12: This requires a randomizer with 53 bits of precision
+      it.todo('should be able to return MAX_SAFE_INTEGER', () => {
+        randomizer.next = () => MERSENNE_MAX_VALUE;
+        const actual = customFaker.number.int();
+        expect(actual).toBe(Number.MAX_SAFE_INTEGER);
+      });
+    });
+
+    describe('float', () => {
+      it('should be able to return 0', () => {
+        randomizer.next = () => 0;
+        const actual = customFaker.number.float();
+        expect(actual).toBe(0);
+      });
+
+      it('should be able to return almost 1', () => {
+        randomizer.next = () => MERSENNE_MAX_VALUE;
+        const actual = customFaker.number.float();
+        expect(actual).toBe(MERSENNE_MAX_VALUE);
+      });
+    });
+  });
 });