Real-time A* is a variation of A*.
Search continues on the basis of choosing paths that have minimum values of f(node) = g(node) + h(node). However, g(node) is the distance of the node from the current node, rather than from the root node.
Hence, the algorithm will backtrack if the cost of doing so plus the estimated cost of solving the problem from the new node is less than the estimated cost of solving the problem from the current node.
Implementing real-time A* means maintaining a hash table of previously visited states with their h(node) values.
Iterative-Deepening A* (IDA*)
By combining iterative-deepening with A*, we produce an algorithm that is optimal and complete (like A*) and that has the low memory requirements of depth-first search.
IDA* is a form of iterative-deepening search where successive iterations impose a greater limit on f(node) rather than on the depth of a node.
IDA* performs well in problems where the heuristic value f (node) has relatively few possible values.
For example, using the Manhattan distance as a heuristic in solving the eight-queens problem, the value of f (node) can only have values 1, 2, 3, or 4.
In this case, the IDA* algorithm only needs to run through a maximum of four iterations, and it has a time complexity not dissimilar from that of A*, but with a significantly improved space complexity because it is effectively running depth-first search.
In cases such as the traveling salesman problem where the value of f (node) is different for every state, the IDA* method has to expand 1 + 2 + 3 + . . . + n nodes = O(n2) where A* would expand n nodes.
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