Add solution for problem 35.

README.md

@@ -1,6 +1,6 @@
 # [Project Euler](https://projecteuler.net) Solutions
 
-[![solved: 34 problems](https://img.shields.io/badge/solved-34_problems-f93.svg)](./src)
+[![solved: 35 problems](https://img.shields.io/badge/solved-35_problems-f93.svg)](./src)
 ![Python: 3.7](https://img.shields.io/badge/Python-3.7-3776ab.svg)
 [![Build Status](https://travis-ci.com/FranzDiebold/project-euler-solutions.svg?branch=master)](https://travis-ci.com/FranzDiebold/project-euler-solutions)
 [![license: MIT](https://img.shields.io/badge/license-MIT-brightgreen.svg)](./LICENSE.md)

src/p035_circular_primes.py

@@ -0,0 +1,55 @@
+"""
+Problem 35: Circular primes
+http://projecteuler.net/problem=35
+
+The number, 197, is called a circular prime because all rotations of the digits:
+197, 971, and 719, are themselves prime.
+
+There are thirteen such primes below 100: 2, 3, 5, 7, 11, 13, 17, 31, 37, 71, 73, 79, and 97.
+
+How many circular primes are there below one million?
+"""
+
+from typing import Set, Iterable
+
+from src.common.primes import is_prime
+
+
+def _get_primes_set(threshold: int) -> Set:
+    """Get all prime numbers up to a threshold `threshold` as set."""
+    return {number for number in range(2, threshold) if is_prime(number)}
+
+
+def _get_circular_numbers(number: int) -> Iterable[int]:
+    """Get all circular numbers for a given number `number`.
+
+    Example:
+    1234 -> 1234, 2341, 3412, 4123
+    """
+    number_str = str(number)
+    for idx in range(len(number_str)):
+        yield int(number_str[idx:] + number_str[:idx])
+
+
+def _get_circular_primes(threshold: int) -> Iterable[int]:
+    """Get all circular primes up to a threshold `threshold`."""
+    primes = _get_primes_set(threshold)
+    for prime in primes:
+        is_circular_prime = True
+        for circular_number in _get_circular_numbers(prime):
+            if circular_number not in primes:
+                is_circular_prime = False
+                break
+        if is_circular_prime:
+            yield prime
+
+
+def main() -> None:
+    """Main function."""
+    threshold = int(1e6)
+    print(f'There are {len(list(_get_circular_primes(threshold))):,} circular primes ' \
+          f'below {threshold:,}.')
+
+
+if __name__ == '__main__':
+    main()