From 3967b92757ca95a895fa5a178b9fa77533f5d72f Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <bruno@belanyi.fr>
Date: Mon, 13 Dec 2021 14:25:44 +0100
Subject: [PATCH] 2021: d13: ex1: add solution

---
 2021/d13/ex1/ex1.py | 71 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)
 create mode 100755 2021/d13/ex1/ex1.py

diff --git a/2021/d13/ex1/ex1.py b/2021/d13/ex1/ex1.py
new file mode 100755
index 0000000..fb71b61
--- /dev/null
+++ b/2021/d13/ex1/ex1.py
@@ -0,0 +1,71 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+from copy import deepcopy
+from typing import Iterable, List, Tuple, cast
+
+Paper = List[List[bool]]
+Instruction = Tuple[str, int]
+
+
+def solve(input: List[str]) -> int:
+    def transpose(paper: Paper) -> Paper:
+        return cast(Paper, list(zip(*paper)))
+
+    def fold_along_y(paper: Paper, y: int) -> Paper:
+        assert not any(paper[y])  # Sanity check
+        assert y >= len(paper) // 2  # Ensure that we can actually fold the paper
+
+        paper = deepcopy(paper)
+        for i, j in zip(itertools.count(y, -1), range(y, len(paper))):
+            paper[i] = [(dot_i or dot_j) for dot_i, dot_j in zip(paper[i], paper[j])]
+
+        paper = paper[:y]
+        return paper
+
+    def fold_along_x(paper: Paper, x: int) -> Paper:
+        return transpose(fold_along_y(transpose(paper), x))
+
+    def do_folds(paper: Paper, instructions: Iterable[Instruction]) -> Paper:
+        for axis, n in instructions:
+            if axis == "x":
+                paper = fold_along_x(paper, n)
+            elif axis == "y":
+                paper = fold_along_y(paper, n)
+            else:
+                assert False  # Sanity check
+
+        return paper
+
+    def parse() -> Tuple[Paper, List[Instruction]]:
+        paper_part = itertools.takewhile(lambda s: s != "", input)
+        fold_part = itertools.islice(
+            itertools.dropwhile(lambda s: s != "", input), 1, None
+        )
+
+        points = {(int(x), int(y)) for x, y in map(lambda s: s.split(","), paper_part)}
+        folds = [
+            (axis[-1], int(n)) for axis, n in map(lambda s: s.split("="), fold_part)
+        ]
+
+        width, height = max(p[0] for p in points), max(p[1] for p in points)
+
+        paper = [
+            [(x, y) in points for x in range(width + 1)] for y in range(height + 1)
+        ]
+
+        return paper, folds
+
+    paper, instructions = parse()
+
+    return sum(itertools.chain.from_iterable(do_folds(paper, instructions[:1])))
+
+
+def main() -> None:
+    input = [line.strip() for line in sys.stdin.readlines()]
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()