Poetry stuff.

src/holt59/aoc/2023/day10.py

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+import os
+import sys
+from typing import Literal, cast
+
+VERBOSE = os.getenv("AOC_VERBOSE") == "True"
+
+Symbol = Literal["|", "-", "L", "J", "7", "F", ".", "S"]
+
+lines: list[list[Symbol]] = [
+    [cast(Symbol, symbol) for symbol in line] for line in sys.stdin.read().splitlines()
+]
+
+# find starting point
+si, sj = next(
+    (i, j)
+    for i in range(len(lines))
+    for j in range(len(lines[0]))
+    if lines[i][j] == "S"
+)
+
+# find one of the two outputs
+ni, nj = si, sj
+for ni, nj, chars in (
+    (si - 1, sj, "|7F"),
+    (si + 1, sj, "|LJ"),
+    (si, sj - 1, "-LF"),
+    (si, sj + 1, "-J7"),
+):
+    if lines[ni][nj] in chars:
+        break
+
+# part 1 - find the loop (re-used in part 2)
+loop = [(si, sj), (ni, nj)]
+while True:
+    pi, pj = loop[-2]
+    i, j = loop[-1]
+
+    sym = lines[i][j]
+
+    if sym == "|" and pi > i or sym in "JL" and pi == i:
+        i -= 1
+    elif sym == "|" and pi < i or sym in "7F" and pi == i:
+        i += 1
+    elif sym == "-" and pj > j or sym in "J7" and pj == j:
+        j -= 1
+    elif sym == "-" and pj < j or sym in "LF" and pj == j:
+        j += 1
+
+    if (i, j) == (si, sj):
+        break
+
+    loop.append((i, j))
+
+answer_1 = len(loop) // 2
+print(f"answer 1 is {answer_1}")
+
+# part 2
+
+# replace S by an appropriate character for the loop below
+di1, dj1 = loop[1][0] - loop[0][0], loop[1][1] - loop[0][1]
+di2, dj2 = loop[0][0] - loop[-1][0], loop[0][1] - loop[-1][1]
+mapping: dict[tuple[int, int], dict[tuple[int, int], Symbol]] = {
+    (0, 1): {(0, 1): "-", (-1, 0): "F", (1, 0): "L"},
+    (0, -1): {(0, -1): "-", (-1, 0): "7", (1, 0): "J"},
+    (1, 0): {(1, 0): "|", (0, 1): "7", (0, -1): "F"},
+    (-1, 0): {(-1, 0): "|", (0, -1): "L", (0, 1): "J"},
+}
+lines[si][sj] = mapping[di1, dj1][di2, dj2]
+
+# find the points inside the loop using an adaptation of ray casting for a discrete
+# grid (https://stackoverflow.com/a/218081/2666289)
+#
+# use a set for faster '... in loop' check
+#
+loop_s = set(loop)
+inside: set[tuple[int, int]] = set()
+for i in range(len(lines)):
+    cnt = 0
+    for j in range(len(lines[0])):
+        if (i, j) not in loop_s and cnt % 2 == 1:
+            inside.add((i, j))
+
+        if (i, j) in loop_s and lines[i][j] in "|LJ":
+            cnt += 1
+
+if VERBOSE:
+    for i in range(len(lines)):
+        for j in range(len(lines[0])):
+            if (i, j) == (si, sj):
+                print("\033[91mS\033[0m", end="")
+            elif (i, j) in loop:
+                print(lines[i][j], end="")
+            elif (i, j) in inside:
+                print("\033[92mI\033[0m", end="")
+            else:
+                print(".", end="")
+        print()
+
+answer_2 = len(inside)
+print(f"answer 2 is {answer_2}")