106 lines
2.7 KiB
Python
106 lines
2.7 KiB
Python
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from heapq import heappush, heappop
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def search(start, is_goal, neighbors, heuristic=None, worst=None):
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if heuristic == None:
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def heuristic(x):
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return 0
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if not callable(is_goal):
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goal = is_goal
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def is_goal(this):
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return this == goal
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if not isinstance(start, list):
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start = [start]
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i = 0 # tiebreaker
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q = []
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for s in start:
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i += 1
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heappush(q, (heuristic(s), i, s, None))
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done = {}
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if worst:
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best_worst = min(worst(s) for s in start)
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while q:
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z, _, this, prev = heappop(q)
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if this in done:
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continue
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cost_so_far = z - heuristic(this)
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#print(this,z, cost_so_far)
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done[this] = (cost_so_far, prev)
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if is_goal(this):
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print("astar: visited", len(done), "nodes")
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print("astar: pending", len(q), "nodes")
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# reconsruct the path
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n = this
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path = []
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while n is not None:
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path.append(n)
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_, n = done[n]
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path.reverse()
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return cost_so_far, this, path
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for c, n in neighbors(this):
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c = cost_so_far + c
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if n not in done or c < done[n][0]:
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h = heuristic(n)
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if worst:
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if c+h > best_worst:
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# if the best possible cost for this node
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# is worse than the lowest worst-case cost we've seen
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# then don't even bother exploring it
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continue
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w = worst(n)
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if c+w < best_worst:
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best_worst = c+w
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i += 1
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heappush(q, (c+h, i, n, this))
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return float('inf'), None, []
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def test():
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data = [
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"aabqponm",
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"abcryxxl",
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"accszzxk",
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"acctuvwj",
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"abdefghi",
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]
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#data = [
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# "aabbaaba",
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# "abcbaaba",
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# "accabcbb",
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# "accbbbba",
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# "abbbabab",
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#]
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start = (0,0)
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goal = (5,2)
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def get(x,y):
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return ord(data[y][x])
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def neighbors(src):
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x,y = src
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here = get(x,y)
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n = []
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def push(x,y):
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if 0 <= y < len(data):
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if 0 <= x < len(data[y]):
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if get(x,y) <= here+1:
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n.append((1, (x,y)))
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push(x-1, y)
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push(x+1,y)
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push(x, y-1)
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push(x, y+1)
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return n
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def heuristic(n):
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return abs(goal[0] - n[0]) + abs(goal[1] - n[1])
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c, _, path = search(start, goal, neighbors, heuristic)
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print(*data, sep="\n")
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print(*path, sep="\n")
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assert c == 31, c
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if __name__ == '__main__':
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test()
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