Add solution to project_euler/problem_060

project_euler/problem_060/sol1.py

@@ -0,0 +1,192 @@
+"""
+Project Euler Problem 60: https://projecteuler.net/problem=60
+
+The primes 3, 7, 109, and 673, are quite remarkable. By taking any two primes and
+concatenating them in any order the result will always be prime. For example,
+taking 7 and 109, both 7109 and 1097 are prime. The sum of these four primes, 792,
+represents the lowest sum for a set of four primes with this property.
+
+Find the lowest sum for a set of five primes for which any two primes concatenate to
+produce another prime.
+
+On the algorithm:
+- Generate PRIMES_SIZE primes
+- Iterate through the primes, starting from the smallest available. On the top-level of
+the recursion, generate a list of primes that can be concatenated to the current prime.
+This list will be used for the next recursive calls.
+- Find all matches for that prime and store them in a list that will be used for the
+next recursive calls
+- Test matches for the next recursive calls, register the smallest sum found
+- Concatenation is done with arithmetic operations. String processing is generally
+slower.
+"""
+import collections.abc
+import math
+from time import time
+
+PRIMES_SIZE = 1.2e3
+COUNTER = False
+
+
+def get_order(target: int) -> int:
+    """
+    Get the order of a number n, in order to facilitate concatenation.
+    get_order(n) = x -> 10^x <= n < 10^(x+1)
+
+    >>> get_order(1)
+    0
+    >>> get_order(10)
+    1
+    >>> get_order(12345)
+    4
+    """
+
+    return int(math.log(target, 10))
+
+
+def test_concatenate(num1: int, num2: int) -> bool:
+    """
+    Test if two numbers concatenate to form a prime, for both possible arrangements.
+    Use arithmetic operations to concatenate.
+
+    >> test_concatenate(3, 7)
+    True
+    >>> test_concatenate(2, 10)
+    False
+    >>> test_concatenate(673, 109)
+    True
+    """
+
+    if not is_prime(num1 * 10 ** (get_order(num2) + 1) + num2):
+        return False
+
+    if not is_prime(num2 * 10 ** (get_order(num1) + 1) + num1):
+        return False
+
+    return True
+
+
+def is_prime(target: int) -> bool:
+    """
+    Simple prime test
+
+    >>> is_prime(2)
+    True
+    >>> is_prime(100000007)
+    True
+    >>> is_prime(10000007)
+    False
+    """
+    for divider in range(2, int(target**0.5) + 1):
+        if int(target) % divider == 0:
+            return False
+    return True
+
+
+def prime_generator() -> collections.abc.Iterator[int]:
+    """
+    Custom prime generator for primes used in this problem
+    Skip 2 and 5: no primes end with 2 or 5 except for 2 and 5, so they're not useful
+    for this problem
+
+    >>> [[next(x) for _ in range(5)] for x in [(prime_generator())]][0]
+    [3, 7, 11, 13, 17]
+    """
+
+    local_output: int = 3
+    yield local_output
+    local_output = 5
+
+    while True:
+        local_output += 2
+        if is_prime(local_output):
+            yield local_output
+
+
+def solution_helper(
+    depth: int, start_idx: int, to_test: list[int], matches: list[int]
+) -> list[int]:
+    """
+    Recursive helper function for solution(), search for more primes from
+    matches[start_idx:] that can be concatenated to all primes from to_test until
+    recursion reaches depth 0. Return the list of primes if a solution is found, False
+    otherwise.
+
+    >>> solution_helper(depth=3, start_idx=1, to_test=[3],matches=[3, 7, 100, 109, 673])
+    [3, 7, 109, 673]
+    >>> solution_helper(depth=2, start_idx=0, to_test=[7], matches=[10, 20, 30, 40, 50])
+    []
+    """
+
+    if depth == 0:
+        return to_test
+
+    for i in range(start_idx, len(matches)):
+        # Test all previous matches:
+        passes = True
+        for j in range(len(to_test)):
+            if not test_concatenate(matches[i], to_test[j]):
+                passes = False
+                break
+        if not passes:
+            continue
+        if output := solution_helper(depth - 1, i + 1, to_test + [matches[i]], matches):
+            return output
+    return []
+
+
+def solution(n_primes: int = 5) -> int:
+    """
+    This function behaves similarly to solution_helper, but it is not recursive and
+    defines some variables that are used in the recursive calls. It also defines a list
+    of matches for every prime it tests, optimizing search time.
+
+    >>> solution(n_primes=2)
+    14
+    >>> solution(n_primes=3)
+    405
+    >>> solution(n_primes=4)
+    3146
+    """
+
+    # Generate primes and start variables
+    start = time()
+    generator: collections.abc.Iterator[int] = prime_generator()
+    primes: list[int] = []
+    output: int = int(5e4)  # initialize with theoretical max value
+    for _ in range(int(PRIMES_SIZE)):
+        primes.append(next(generator))
+    if COUNTER:
+        print(f"{int(PRIMES_SIZE)} primes generated in {time() - start}s")
+
+    # Main loop
+    limit = output ** (1 / n_primes) * n_primes
+    for i in range(len(primes)):
+        # Break main loop if the current minimal number is larger than the nth root
+        # of the current output times n, with n being the amount of primes searched.
+        # The reason for this is to reduce the search space with a reasonable upper
+        # bound. Analysis with lower values for n_primes shows that this is a valid
+        # optimization.
+        if primes[i] > limit:
+            break
+
+        # Iterate larger primes, store in matches. This should optimize the nested loops
+        matches: list[int] = []
+        prime: int = primes[i]
+        for j in range(i + 1, len(primes)):
+            if test_concatenate(prime, primes[j]):
+                matches.append(primes[j])
+
+        # Match every candidate with every other candidate until 5 are found
+        if found := solution_helper(
+            depth=n_primes - 1, start_idx=+1, to_test=[prime], matches=matches
+        ):
+            output = min(output, sum(found))
+
+    if COUNTER:
+        print(f"Done ({time() - start}s)")
+    return output
+
+
+if __name__ == "__main__":
+    print(f"{solution() = }")