Add Project Euler problem 187 solution 1

DIRECTORY.md

@@ -990,6 +990,8 @@
     * [Sol1](project_euler/problem_174/sol1.py)
   * Problem 180
     * [Sol1](project_euler/problem_180/sol1.py)
+  * Problem 187
+    * [Sol1](project_euler/problem_187/sol1.py)
   * Problem 188
     * [Sol1](project_euler/problem_188/sol1.py)
   * Problem 191

project_euler/problem_187/sol1.py

@@ -0,0 +1,58 @@
+"""
+Project Euler Problem 187: https://projecteuler.net/problem=187
+
+A composite is a number containing at least two prime factors.
+For example, 15 = 3 x 5; 9 = 3 x 3; 12 = 2 x 2 x 3.
+
+There are ten composites below thirty containing precisely two,
+not necessarily distinct, prime factors: 4, 6, 9, 10, 14, 15, 21, 22, 25, 26.
+
+How many composite integers, n < 10^8, have precisely two,
+not necessarily distinct, prime factors?
+"""
+
+from math import isqrt
+
+
+def calculate_prime_numbers(max_number: int) -> list[int]:
+    """
+    Returns prime numbers below max_number
+
+    >>> calculate_prime_numbers(10)
+    [2, 3, 5, 7]
+    """
+
+    is_prime = [True] * max_number
+    for i in range(2, isqrt(max_number - 1) + 1):
+        if is_prime[i]:
+            for j in range(i**2, max_number, i):
+                is_prime[j] = False
+
+    return [i for i in range(2, max_number) if is_prime[i]]
+
+
+def solution(max_number: int = 10**8) -> int:
+    """
+    Returns the number of composite integers below max_number have precisely two,
+    not necessarily distinct, prime factors
+
+    >>> solution(30)
+    10
+    """
+
+    prime_numbers = calculate_prime_numbers(max_number // 2)
+
+    semiprimes_count = 0
+    left = 0
+    right = len(prime_numbers) - 1
+    while left <= right:
+        while prime_numbers[left] * prime_numbers[right] >= max_number:
+            right -= 1
+        semiprimes_count += right - left + 1
+        left += 1
+
+    return semiprimes_count
+
+
+if __name__ == "__main__":
+    print(f"{solution() = }")