2019 day 10

adventofcode/2019/day10/day10.py

@@ -0,0 +1,114 @@
+import time
+import math
+import cmath
+from itertools import combinations
+from collections import defaultdict
+from bisect import insort
+
+
+def dist(a, b):
+    "euclidean distance between two points (as complex numbers)"
+    return abs(a - b)
+
+
+def main(inp):
+    asteroids = set()
+    for y, l in enumerate(inp):
+        for x, c in enumerate(l):
+            if c == "#":
+                asteroids.add(complex(x,y))
+    base = part1(asteroids)
+    #base = 22+28j
+
+    # construct a list of asteroids for each angle, sorted by distance from the base
+    angles = defaultdict(list)
+    for a in asteroids:
+        if a == base:
+            continue
+        angle = (math.degrees(cmath.phase(a - base)) + 90) % 360
+        elt = (abs(a - base), a)
+        insort(angles[angle], elt)
+
+    i = 0
+    for angle in sorted(angles):
+        i += 1
+        a = angles[angle].pop(0)
+        if i == 200:
+            _, a = a
+            print("Part 2: ", int(a.real*100+a.imag))
+            break
+
+
+def part1(asteroids):
+    """
+    # ray cast solution not working
+    start_time = time.perf_counter()
+    sight = defaultdict(set)
+    for a, b in combinations(asteroids, 2):
+        if has_direct_sight_ray(a, b, asteroids):
+            sight[a].add(b)
+            sight[b].add(a)
+    end_time = time.perf_counter() - start_time
+    print(f"compute lines of sight ray {end_time:.4f}")
+    m = max(sight.items(), key=lambda x: len(x[1]))
+    print(m[0], len(m[1]))
+    """
+    #start_time = time.perf_counter()
+    # construct line of sight for each asteroid
+    sight = defaultdict(set)
+    for a, b in combinations(asteroids, 2):
+        if has_direct_sight(a, b, asteroids):
+            sight[a].add(b)
+            sight[b].add(a)
+    #end_time = time.perf_counter() - start_time
+    #print(f"compute lines of sight {end_time:.4f}")
+
+    m = max(sight.items(), key=lambda x: len(x[1]))
+    #print(m[0], len(m[1]))
+    print("Part 1: ", len(m[1]))
+    return m[0]
+
+
+def has_direct_sight(a, c, asteroids):
+    """
+    Returns true if a can see c, i.e. they have no other asteroid between
+    them. An asteroid b is between a and c if:
+    dist(a,c) = dist(a,b) + dist(b,c).
+    """
+    ac = dist(a, c)
+    eps = 0.00001  # avoid floating-point shenanigans
+    for b in asteroids:  # check if any asteroid is blocking the view between a and c
+        if b == a or b == c:
+            continue
+        if abs(dist(a, b) + dist(b, c) - ac) <= eps:
+            return False
+    return True
+
+
+def has_direct_sight_ray(a, c, asteroids):
+    # FIXME not working...
+    """
+    Cast a ray from a in the direction of c and return true if there is no
+    obstacle between a and c
+    """
+    if c.real < a.real:
+        a, c = c, a
+    angle = cmath.phase(c - a)
+    t = 0
+    x = a.real
+    y = a.imag
+    while True:
+        x = a.real + t * math.cos(angle)
+        y = a.imag + t * math.sin(angle)
+        t += 1
+        if x >= c.real or y >= c.imag:
+            return True
+        if complex(int(x), int(y)) in asteroids:
+            return False
+    assert False, "Unreachable"
+
+
+if __name__ == "__main__":
+    import fileinput
+    main(fileinput.input())
+