Project Euler 68 Solution

DIRECTORY.md

@@ -786,6 +786,8 @@
   * Problem 067
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_067/sol1.py)
     * [Sol2](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_067/sol2.py)
+  * Problem 068
+    * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_068/sol1.py)
   * Problem 069
     * [Sol1](https://github.com/TheAlgorithms/Python/blob/master/project_euler/problem_069/sol1.py)
   * Problem 070

project_euler/problem_068/sol1.py

@@ -0,0 +1,133 @@
+"""
+Project Euler Problem 68: https://projecteuler.net/problem=68
+
+Magic 5-gon ring
+
+Problem Statement:
+Consider the following "magic" 3-gon ring,
+filled with the numbers 1 to 6, and each line adding to nine.
+
+   4
+    \
+     3
+    / \
+   1 - 2 - 6
+  /
+ 5
+
+Working clockwise, and starting from the group of three
+with the numerically lowest external node (4,3,2 in this example),
+each solution can be described uniquely.
+For example, the above solution can be described by the set: 4,3,2; 6,2,1; 5,1,3.
+
+It is possible to complete the ring with four different totals: 9, 10, 11, and 12.
+There are eight solutions in total.
+Total   Solution Set
+9       4,2,3; 5,3,1; 6,1,2
+9       4,3,2; 6,2,1; 5,1,3
+10      2,3,5; 4,5,1; 6,1,3
+10      2,5,3; 6,3,1; 4,1,5
+11      1,4,6; 3,6,2; 5,2,4
+11      1,6,4; 5,4,2; 3,2,6
+12      1,5,6; 2,6,4; 3,4,5
+12      1,6,5; 3,5,4; 2,4,6
+
+By concatenating each group it is possible to form 9-digit strings;
+the maximum string for a 3-gon ring is 432621513.
+
+Using the numbers 1 to 10, and depending on arrangements,
+it is possible to form 16- and 17-digit strings.
+What is the maximum 16-digit string for a "magic" 5-gon ring?
+"""
+
+from itertools import permutations
+
+
+def solution(gon_side: int = 5) -> int:
+    """
+    Find the maximum number for a "magic" gon_side-gon ring
+
+    The gon_side parameter should be in the range [3, 5],
+    other side numbers aren't tested
+
+    >>> solution(3)
+    432621513
+    >>> solution(4)
+    426561813732
+    >>> solution()
+    6531031914842725
+    >>> solution(6)
+    Traceback (most recent call last):
+    ValueError: gon_side must be in the range [3, 5]
+    """
+    if gon_side < 3 or gon_side > 5:
+        raise ValueError("gon_side must be in the range [3, 5]")
+
+    # Since it's 16, we know 10 is on the outer ring
+    # Put the big numbers at the end so that they are never the first number
+    small_numbers = list(range(gon_side + 1, 0, -1))
+    big_numbers = list(range(gon_side + 2, gon_side * 2 + 1))
+
+    for perm in permutations(small_numbers + big_numbers):
+        numbers = generate_gon_ring(gon_side, list(perm))
+        if is_magic_gon(numbers):
+            return int("".join(str(n) for n in numbers))
+
+    raise ValueError(f"Magic {gon_side}-gon ring is impossible")
+
+
+def generate_gon_ring(gon_side: int, perm: list[int]) -> list[int]:
+    """
+    Generate a gon_side-gon ring from a permutation state
+    The permutation state is the ring, but every duplicate is removed
+
+    >>> generate_gon_ring(3, [4, 2, 3, 5, 1, 6])
+    [4, 2, 3, 5, 3, 1, 6, 1, 2]
+    >>> generate_gon_ring(5, [6, 5, 4, 3, 2, 1, 7, 8, 9, 10])
+    [6, 5, 4, 3, 4, 2, 1, 2, 7, 8, 7, 9, 10, 9, 5]
+    """
+    result = [0] * (gon_side * 3)
+    result[0:3] = perm[0:3]
+    perm.append(perm[1])
+
+    magic_number = 1 if gon_side < 5 else 2
+
+    for i in range(1, len(perm) // 3 + magic_number):
+        result[3 * i] = perm[2 * i + 1]
+        result[3 * i + 1] = result[3 * i - 1]
+        result[3 * i + 2] = perm[2 * i + 2]
+
+    return result
+
+
+def is_magic_gon(numbers: list[int]) -> bool:
+    """
+    Check if the solution set is a magic n-gon ring
+    Check that the first number is the smallest number on the outer ring
+    Take a list, and check if the sum of each 3 numbers chunk is equal to the same total
+
+    >>> is_magic_gon([4, 2, 3, 5, 3, 1, 6, 1, 2])
+    True
+    >>> is_magic_gon([4, 3, 2, 6, 2, 1, 5, 1, 3])
+    True
+    >>> is_magic_gon([2, 3, 5, 4, 5, 1, 6, 1, 3])
+    True
+    >>> is_magic_gon([1, 2, 3, 4, 5, 6, 7, 8, 9])
+    False
+    >>> is_magic_gon([1])
+    Traceback (most recent call last):
+    ValueError: a gon ring should have a length that is a multiple of 3
+    """
+    if len(numbers) % 3 != 0:
+        raise ValueError("a gon ring should have a length that is a multiple of 3")
+
+    if min(numbers[::3]) != numbers[0]:
+        return False
+
+    total = sum(numbers[:3])
+
+    return all(sum(numbers[i : i + 3]) == total for i in range(3, len(numbers), 3))
+
+
+if __name__ == "__main__":
+    print(solution())