From de21aa759a599efdafedfb31f122792c5cf851ca Mon Sep 17 00:00:00 2001
From: Lumi Pakkanen <lumi.pakkanen@gmail.com>
Date: Tue, 19 Dec 2023 08:24:16 +0200
Subject: [PATCH] Implement primality check and prime array generation

---
 src/__tests__/primes.spec.ts | 66 +++++++++++++++++++++++++++++--
 src/primes.ts                | 76 ++++++++++++++++++++++++++++++++++++
 2 files changed, 138 insertions(+), 4 deletions(-)

diff --git a/src/__tests__/primes.spec.ts b/src/__tests__/primes.spec.ts
index f0bd3a9..e5d0390 100644
--- a/src/__tests__/primes.spec.ts
+++ b/src/__tests__/primes.spec.ts
@@ -1,22 +1,80 @@
 import {describe, it, expect} from 'vitest';
-import {PRIMES} from '../primes';
+import {PRIMES, isPrime, primes} from '../primes';
 
 describe('Array of prime numbers', () => {
   it('has no gaps', () => {
     expect(PRIMES[0]).toBe(2);
     let index = 1;
     for (let n = 3; n <= 7919; n += 2) {
-      let isPrime = true;
+      let isPrime_ = true;
       for (let i = 3; i * i <= n; ++i) {
         if (n % i === 0) {
-          isPrime = false;
+          isPrime_ = false;
           break;
         }
       }
-      if (isPrime) {
+      if (isPrime_) {
         expect(PRIMES[index]).toBe(n);
         index++;
       }
     }
   });
 });
+
+const LARGER_PRIMES = [
+  7927, 7933, 7937, 7949, 7951, 7963, 7993, 8009, 8011, 8017, 8039, 8053, 8059,
+  8069, 8081, 8087, 8089, 8093,
+];
+
+describe('Primeness detector', () => {
+  it('works for small primes', () => {
+    for (let n = -1.5; n < 7920.5; n += 0.5) {
+      if (isPrime(n)) {
+        expect(PRIMES).includes(n);
+      } else {
+        expect(PRIMES).not.includes(n);
+      }
+    }
+  });
+  it.each(LARGER_PRIMES)('works for %s', (prime: number) => {
+    expect(isPrime(prime)).toBe(true);
+  });
+  it('works for larger composites', () => {
+    for (let n = 7919.25; n < 8095; n += 0.25) {
+      if (LARGER_PRIMES.includes(n)) {
+        continue;
+      }
+      expect(isPrime(n)).toBe(false);
+    }
+  });
+});
+
+describe('Lists of primes', () => {
+  it('works from implicit 2 to 7', () => {
+    expect(primes(7)).toEqual([2, 3, 5, 7]);
+  });
+
+  it('works from implicit 2 to below 16', () => {
+    expect(primes(16)).toEqual([2, 3, 5, 7, 11, 13]);
+  });
+
+  it('produces an empty array with negative bounds', () => {
+    expect(primes(-100, -50)).toHaveLength(0);
+  });
+
+  it('produces an empty array when end < start', () => {
+    expect(primes(10, 1)).toHaveLength(0);
+  });
+
+  it('supports start and end parameters', () => {
+    expect(primes(12, 31)).toEqual([13, 17, 19, 23, 29, 31]);
+  });
+
+  it('works with medium-sized values', () => {
+    expect(primes(7907, 7933)).toEqual([7907, 7919, 7927, 7933]);
+  });
+
+  it('works with larger values', () => {
+    expect(primes(7920, 8094)).toEqual(LARGER_PRIMES);
+  });
+});
diff --git a/src/primes.ts b/src/primes.ts
index 046942e..481be00 100644
--- a/src/primes.ts
+++ b/src/primes.ts
@@ -101,3 +101,79 @@ PRIME_CENTS[0] = 1200; // Ensure that octaves are exact.
  * BigInt representation of the primes.
  */
 export const BIG_INT_PRIMES = PRIMES.map(BigInt);
+
+/**
+ * Check a number for primality.
+ * @param n Number to check.
+ * @returns True if the number is prime, false otherwise.
+ */
+export function isPrime(n: number) {
+  if (!Number.isInteger(n) || n < 2) {
+    return false;
+  }
+  if (n < 7927) {
+    return PRIMES.includes(n);
+  }
+  if (n > 62837328) {
+    throw new Error('Prime check only implemented up to 62837328');
+  }
+  for (const prime of PRIMES) {
+    if (n % prime === 0) {
+      return false;
+    }
+  }
+  return true;
+}
+
+/**
+ * Generate an array of prime numbers.
+ * @param start Smallest prime number to include (or the next smallest prime).
+ * @param end Largest prime number to include (or the previous largest prime).
+ * @returns An array of primes p in ascending order such that start <= p <= end.
+ */
+export function primes(start: number, end?: number) {
+  if (end === undefined) {
+    end = start;
+    start = 2;
+  }
+  if (start > end || end < 2) {
+    return [];
+  }
+  if (start <= 2) {
+    if (end < 3) {
+      return [2];
+    }
+    start = 2;
+  } else {
+    start = Math.ceil((start + 1) * 0.5) * 2 - 1;
+    while (!isPrime(start)) {
+      start += 2;
+    }
+  }
+  end = Math.floor((end + 1) * 0.5) * 2 - 1;
+  while (!isPrime(end)) {
+    end -= 2;
+  }
+
+  const startIndex = PRIMES.indexOf(start);
+  const endIndex = PRIMES.indexOf(end);
+
+  let result: number[];
+  if (endIndex < 0) {
+    if (startIndex >= 0) {
+      result = PRIMES.slice(startIndex);
+      start = 7927;
+    } else {
+      result = [];
+    }
+  } else {
+    return PRIMES.slice(startIndex, endIndex + 1);
+  }
+
+  for (let n = start; n <= end; n += 2) {
+    if (isPrime(n)) {
+      result.push(n);
+    }
+  }
+  return result;
+}