Solve 2018 day 15

2018/d15.py

@@ -0,0 +1,117 @@
+from lib import Grid2D, add2
+from collections import deque
+
+
+DIRS = [Grid2D.N, Grid2D.W, Grid2D.E, Grid2D.S]
+
+
+def find_move(unit, grid):
+    assert grid[unit] in "GE"
+    target_fields = set()
+    enemies = grid.find("G") if grid[unit] == "E" else grid.find("E")
+    for e in enemies:
+        for dir in DIRS:
+            nb = add2(e, dir)
+            if grid[nb] == ".":
+                target_fields.add(nb)
+
+    # we are already next to an enemy
+    if unit in target_fields:
+        return None
+
+    seen = set()
+    states = deque([(unit, [])])
+    while states:
+        pos, hist = states.popleft()
+
+        if pos in seen:
+            continue
+        else:
+            seen.add(pos)
+
+        for dir in DIRS:
+            nb = add2(pos, dir)
+            if nb in target_fields:
+                if hist:
+                    return hist[0]
+                else:
+                    return nb
+            elif grid[nb] == ".":
+                nhist = hist + [nb]
+                states.append((nb, nhist))
+
+    # we did not find a path to an enemy
+    return None
+
+
+def part_2(data):
+    for elf_attack in range(3, 100):
+        g = Grid2D(data)
+        pos_to_health = {pos: 200 for pos in g.find("GE")}
+        len_elfs_orig = len(g.find("E"))
+
+        rounds = 0
+        done = False
+        while not done:
+            for unit in sorted(list(pos_to_health.keys())):
+                if not unit in pos_to_health:
+                    continue  # unit died in the meantime
+                enemy_type = "G" if g[unit] == "E" else "E"
+
+                # move stage
+                for dir in DIRS:
+                    nb = add2(unit, dir)
+                    if g[nb] == enemy_type:
+                        break  # we are already next to an enemy
+                else:
+                    npos = find_move(unit, g)
+                    if npos is not None:
+                        assert npos not in pos_to_health and g[npos] == "."
+                        pos_to_health[npos] = pos_to_health[unit]
+                        del pos_to_health[unit]
+                        g[npos] = g[unit]
+                        g[unit] = "."
+                        unit = npos
+
+                # attack stage
+                enemy_type = "G" if g[unit] == "E" else "E"
+                enemy_hit = 999
+                enemy = None
+                for dir in DIRS:
+                    nb = add2(unit, dir)
+                    if g[nb] == enemy_type and pos_to_health[nb] < enemy_hit:
+                        enemy_hit = pos_to_health[nb]
+                        enemy = nb
+
+                if enemy is not None:
+                    if g[enemy] == "G":
+                        pos_to_health[enemy] -= elf_attack
+                    else:
+                        pos_to_health[enemy] -= 3
+                    if pos_to_health[enemy] <= 0:
+                        del pos_to_health[enemy]
+                        g[enemy] = "."
+
+                if len(g.find("E")) == 0 or len(g.find("G")) == 0:
+                    done = True
+                    break
+            else:
+                rounds += 1
+
+        if elf_attack == 3:
+            print(sum(pos_to_health.values()) * rounds)
+
+        if len(g.find("E")) == len_elfs_orig:
+            print(sum(pos_to_health.values()) * (rounds))
+            break
+
+
+def main():
+    input_file = __file__.replace(".py", ".txt")
+    with open(input_file) as f:
+        data = f.read()
+    part_2(data)
+
+
+if __name__ == "__main__":
+    main()

README.md

@@ -103,7 +103,7 @@ written in Python.
 - Day 12: 180:00
 - Day 13: 73:09
 - Day 14: 20:48
-- Day 15:
+- Day 15: 185:11
 
 
 # 2022