// find a solution to the initial board (using the A* algorithm)
public Solver(Board initial) {
Board twin = initial.twin();
SearchNode minNode = null;
boolean isTwinRound = false;
MinPQ<SearchNode> currentPQ = null;
minPQ.insert(new SearchNode(initial));
minPQForTwin.insert(new SearchNode(twin));
while (true) {
// Searching solution in the initial board and and its twin board
// simultaneously(alternating in loops).
// It has been proven by Math theory that exactly one of the two
// will lead to the goal board. If a solution is found in the twin
// board, it immediately proves that the initial board is not solvable,
// and the search will terminate there.
// Otherwise, the initial board will reach a solution.
if (isTwinRound) {
currentPQ = minPQForTwin;
} else {
currentPQ = minPQ;
}
minNode = currentPQ.delMin();
if (minNode.getBoard().isGoal()) {
break;
} else {
for (Board neighbor : minNode.getBoard().neighbors()) {
// Insert the neighbors into the MinPQ if:
// 1. Current node contains the initial board(has no prev node)
// 2. The new neighbor is not the same as current node's previous search node.
// This is a critical optimization used to reduce unnecessary
// exploration of already visited search nodes.
if (minNode.getPrev() == null || !neighbor.equals(minNode.getPrev().getBoard())) {
currentPQ.insert(new SearchNode(neighbor, minNode));
}
}
// Flip the state of the isTwinRound flag
isTwinRound = isTwinRound ? false : true;
}
}
if (isTwinRound) {
isSolvable = false;
solution = null;
} else {
isSolvable = true;
solution = minNode;
moves = solution.getMoves();
}
}
