Solve day 10

2019/10_asteroids.py

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+#!/usr/bin/env python3
+import sys
+from collections import defaultdict, deque
+from itertools import combinations
+from math import atan2, gcd, pi
+
+def read_asteroids(f):
+    for y, line in enumerate(f):
+        for x, c in enumerate(line):
+            if c == '#':
+                yield x, y
+
+def angle(dx, dy):
+    return pi / 2 - atan2(dx, dy)
+
+def raytrace(asteroids):
+    xmax = max(x for x, y in asteroids)
+    ymax = max(y for x, y in asteroids)
+    exists = set(asteroids)
+    visible = defaultdict(dict)
+
+    def trace(x, y, dx, dy):
+        x += dx
+        y += dy
+        while 0 <= x <= xmax and 0 <= y <= ymax:
+            if (x, y) in exists:
+                return x, y
+            x += dx
+            y += dy
+
+    for a, b in combinations(asteroids, 2):
+        ax, ay = a
+        bx, by = b
+        dx = bx - ax
+        dy = by - ay
+        div = gcd(dx, dy)
+        if trace(ax, ay, dx // div, dy // div) == b:
+            visible[a][angle(dx, dy)] = b
+            visible[b][angle(-dx, -dy)] = a
+
+    return visible
+
+def shoot_laser(visible, station):
+    targets = deque(sorted(visible[station].items()))
+    while targets:
+        angle, target = targets.popleft()
+        yield target
+        replacement = visible[target].get(angle, None)
+        if replacement is not None:
+            targets.append((angle, replacement))
+
+# part 1
+asteroids = list(read_asteroids(sys.stdin))
+visible = raytrace(asteroids)
+station = max(visible, key=lambda x: len(visible[x]))
+print(len(visible[station]))
+
+# part 2
+targets = shoot_laser(visible, station)
+for i in range(200):
+    x, y = next(targets)
+print(x * 100 + y)

2019/input/10

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