Solve problem 81 to 83 in Python and learn A* algorithm along the way.

python/e082.py

@@ -1,8 +1,50 @@
+from functools import namedtuple
+from lib_a_star import A_Star
+from e081 import get_grid
+
 
 def euler_082():
-    return 0
+    Node = namedtuple("Node", ["row", "col"])
+
+    def cost(grid):
+        n_rows = len(grid)
+        n_cols = len(grid[0])
+        start_nodes = [Node(row, 0) for row in range(n_rows)]
+        end_nodes = [Node(row, n_cols - 1) for row in range(n_rows)]
+
+        def h(node):
+            c_self = grid[node.row][node.col]
+            c_steps = n_cols - node.col
+            return c_self + c_steps
+
+        def d(current_node, next_node):
+            return grid[next_node.row][next_node.col]
+
+        def neighbors(node):
+            neighbors = []
+            # right neighbor
+            if not node.col + 1 == n_cols:
+                neighbors.append(Node(node.row, node.col + 1))
+            # down neighbor
+            if not node.row + 1 == n_rows:
+                neighbors.append(Node(node.row + 1, node.col))
+            # up neighbor
+            if not node.row == 0:
+                neighbors.append(Node(node.row - 1, node.col))
+            return neighbors
+
+        a = A_Star(start_nodes, end_nodes, h, d, neighbors)
+        return a.cost
+
+    grid = [[131, 673, 234, 103, 18],
+            [201, 96, 342, 965, 150],
+            [630, 803, 746, 422, 111],
+            [537, 699, 497, 121, 956],
+            [805, 732, 524, 37, 331]]
+    assert(cost(grid) == 994)  # Example from problem statement.
+    return cost(get_grid())
 
 
 if __name__ == "__main__":
     print("e082.py: " + str(euler_082()))
-    assert(euler_082() == 0)
+    assert(euler_082() == 260324)