Day 9 solutions

adventofcode/answers.py

@@ -9,4 +9,5 @@
     6: {1: 1108800, 2: 36919753},
     7: {1: 251106089, 2: 249620106},
     8: {1: 16271, 2: 14265111103729},
+    9: {1: 1731106378, 2: 1087},
 }

adventofcode/d9.py

@@ -0,0 +1,50 @@
+import itertools
+from typing import Iterable
+
+
+def _parse_input(lines: Iterable[str]) -> Iterable[list[int]]:
+    for line in lines:
+        yield [int(x) for x in line.strip().split()]
+
+
+def construct_tree_with_diffs(seq: list[int]) -> list[list[int]]:
+    tree: list[list[int]] = [seq]
+    seq_to_check = seq
+    while True:
+        diff = [b - a for a, b in itertools.pairwise(seq_to_check)]
+        if all(x == 0 for x in diff):
+            break
+
+        tree.append(diff)
+        seq_to_check = diff
+    return tree
+
+
+def p1(input_str: str) -> int:
+    input_data = _parse_input(input_str.splitlines())
+
+    def solve(seq: list[int]) -> int:
+        tree = construct_tree_with_diffs(seq)
+
+        tree[-1].append(tree[-1][-1])
+        for i in reversed(range(len(tree) - 1)):
+            tree[i].append(tree[i + 1][-1] + tree[i][-1])
+
+        return tree[0][-1]
+
+    return sum(solve(seq) for seq in input_data)
+
+
+def p2(input_str: str) -> int:
+    input_data = _parse_input(input_str.splitlines())
+
+    def solve(seq: list[int]) -> int:
+        tree = construct_tree_with_diffs(seq)
+
+        tree[-1].insert(0, tree[-1][0])
+        for i in reversed(range(len(tree) - 1)):
+            tree[i].insert(0, tree[i][0] - tree[i + 1][0])
+
+        return tree[0][0]
+
+    return sum(solve(seq) for seq in input_data)