Missionaries and cannibals.

Author: razvan

src/ch2/cannibals.py

@@ -0,0 +1,92 @@
+"""
+Missionaries and cannibals
+
+"""
+from __future__ import annotations
+from attrs import define, field, Factory
+from typing import List, Optional, Iterator
+from ch2.maze import bfs, to_path
+
+MAX_NUM: int = 3
+
+
+@define(frozen=True)
+class MCState:
+    missionaries: int
+    cannibals: int
+    boat: bool = True
+    _wm: int = field(init=False, default=Factory(
+        lambda self: self.missionaries, takes_self=True))
+    _wc: int = field(init=False, default=Factory(
+        lambda self: self.cannibals, takes_self=True))
+    _em: int = field(init=False, default=Factory(
+        lambda self: MAX_NUM-self.missionaries, takes_self=True))
+    _ec: int = field(init=False, default=Factory(
+        lambda self: MAX_NUM-self.cannibals, takes_self=True))
+
+    @property
+    def is_legal(self) -> bool:
+        if self._wm > 0 and self._wm < self._wc:
+            return False
+        if self._em > 0 and self._em < self._ec:
+            return False
+        return True
+
+    def goal_test(self) -> bool:
+        return self.is_legal and self._em == MAX_NUM and self._ec == MAX_NUM
+
+    def successors(self) -> List[MCState]:
+        result: List[MCState] = []
+        if self.boat:  # west
+            if self._wm > 1:
+                result.append(MCState(self._wm - 2, self._wc, not self.boat))
+            if self._wm > 0:
+                result.append(MCState(self._wm - 1, self._wc, not self.boat))
+            if self._wc > 1:
+                result.append(MCState(self._wm, self._wc - 2, not self.boat))
+            if self._wc > 0:
+                result.append(MCState(self._wm, self._wc - 1, not self.boat))
+            if (self._wm > 0) and (self._wc > 0):
+                result.append(
+                    MCState(self._wm - 1, self._wc - 1, not self.boat))
+        else:  # east
+            if self._em > 1:
+                result.append(MCState(self._wm + 2, self._wc, not self.boat))
+            if self._em > 0:
+                result.append(MCState(self._wm + 1, self._wc, not self.boat))
+            if self._ec > 1:
+                result.append(MCState(self._wm, self._wc + 2, not self.boat))
+            if self._ec > 0:
+                result.append(MCState(self._wm, self._wc + 1, not self.boat))
+            if (self._em > 0) and (self._ec > 0):
+                result.append(
+                    MCState(self._wm + 1, self._wc + 1, not self.boat))
+        return [s for s in result if s.is_legal]
+
+
+def display_solution(path: Iterator[MCState]) -> None:
+    old_state = next(path)
+    print(
+        f"West is starting with [{old_state.missionaries}] missionaries and [{old_state.cannibals}] cannibals")
+    for s in path:
+        if s.boat:
+            print("E -> W moved [{}] missionaries and [{}] cannibals (W={}m+{}c, E={}m+{}c, Boat=West)".format(
+                old_state._em - s._em, old_state._ec - s._ec, s._wm, s._wc, s._em, s._ec))
+        else:
+            print("W -> E moved [{}] missionaries and [{}] cannibals (W={}m+{}c, E={}m+{}c, Boat=East)".format(
+                old_state._wm - s._wm, old_state._wc - s._wc, s._wm, s._wc, s._em, s._ec))
+        old_state = s
+
+
+def main():
+    start = MCState(MAX_NUM, MAX_NUM, True)
+    solution = bfs(start, MCState.goal_test, MCState.successors)
+    if solution is None:
+        print("No solution found!")
+    else:
+        path = to_path(solution)
+        display_solution(path)
+
+
+if __name__ == '__main__':
+    main()