🎼

y2019/p10.py

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+import itertools
+import math
+import sys
+from functools import partial
+
+
+def measure(start, other):
+    dx, dy = other[0] - start[0], other[1] - start[1]
+    angle = math.atan2(-dx, dy)
+    return -math.pi if angle == math.pi else angle, math.hypot(dx, dy), other
+
+
+def seek(start, asteroids):
+    pending = sorted(measure(start, ast) for ast in asteroids)
+    while pending:
+        copy = pending.copy()
+        pending = []
+        out = {}
+        for angle, dist, ast in copy:
+            if angle in out:
+                pending.append((angle, dist, ast))
+            else:
+                out[angle] = ast
+        yield list(out.values())
+
+
+text = sys.stdin.read()
+asteroids = {
+    (X, Y)
+    for Y, line in enumerate(text.splitlines())
+    for X, cell in enumerate(line)
+    if cell == '#'
+}
+
+visibility = {ast: len(next(seek(ast, asteroids))) for ast in asteroids}
+origin = max(visibility, key=visibility.get)
+print(visibility[origin])
+
+ordering = list(itertools.chain(*seek(origin, asteroids)))
+x, y = ordering[200 - 1]
+print(100 * x + y)