2017: d14: ex1: add solution

2017/d14/ex1/ex1.py

@@ -0,0 +1,57 @@
+#!/usr/bin/env python
+
+import functools
+import itertools
+import operator
+import sys
+
+
+def solve(input: str) -> int:
+    def knot_hash(byte_string: str) -> str:
+        def compute_sparse_hash(lengths: list[int]) -> list[int]:
+            circle = list(range(256))
+            cur_pos, skip_size = 0, 0
+            for _ in range(64):
+                for n in lengths:
+                    # Invalid length
+                    assert n < len(circle)  # Sanity check
+                    # Reverse
+                    for i, j in zip(
+                        range(cur_pos, cur_pos + n // 2),
+                        # Avoid off-by-one by going further than necessary
+                        range(cur_pos + n - 1, cur_pos, -1),
+                    ):
+                        i %= len(circle)
+                        j %= len(circle)
+                        circle[i], circle[j] = circle[j], circle[i]
+                    # Move
+                    cur_pos += n + skip_size
+                    # Increase
+                    skip_size += 1
+            return circle
+
+        def compute_dense_hash(sparse_hash: list[int]) -> list[int]:
+            assert len(sparse_hash) == 256  # Sanity check
+            return [
+                functools.reduce(operator.xor, chunk)
+                for chunk in itertools.batched(sparse_hash, 16)
+            ]
+
+        lengths = [ord(c) for c in byte_string]
+        lengths += [17, 31, 73, 47, 23]  # Additional lengths
+        sparse_hash = compute_sparse_hash(lengths)
+        dense_hash = compute_dense_hash(sparse_hash)
+        return "".join(f"{n:02x}" for n in dense_hash)
+
+    input = input.strip()
+    hashes = [int(knot_hash(f"{input}-{i}"), 16) for i in range(128)]
+    return sum(n.bit_count() for n in hashes)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()