Search Visualisation in Prolog

SWI-Prolog based tracer for visualisation search algorithms such as depth first, breadth first, best first, A*, ... search

Instructions

Install dependencies using yarn then run ./serve.sh and visit http://localhost:4000

Here's a demo of the visualisation:

• The dark circle indicates the agent, it tracks the path through the search tree.

• Light blue cells indicate those that have been visited by the algorithm.

• Dark blue cells indicate those that are on the current path being explored.

• The width/height of the grid can be adjusted

• The search algorithm can be toggled through any that are implemented on the server

• The starting position of the agent can be chosen by dragging the agent to the desired starting cell

• The goal position can be selected by clicking on a cell.

• Searches can be aborted using the reset button

Implementing new search algorithms

Search algorithms are abstractly defined by the prolog record search_strategy. Search strategies define methods for ...

• Combining the current agenda with the agenda items created for the children reachable from the current state
• Computing the cost of an agenda item given h and g predicates
• Depth bounds

The following predicates define a search strategy:

• combine_agenda(+OldAgenda:list(agenda_item), +ChildAgenda:list(agenda_item), -NewAgenda:list(agenda_item)) combines OldAgenda, the current agenda (minus the agenda item we popped off) and ChildAgenda, agenda items corresponding to the children found by children/2 defined in the search_problem.
• cost(+G:callable/3, +H:callable/3, +From:state, +CostToCurrent:integer, +To:state, -FCost:f(CostToNode:integer, HeuristicCostToGoal:integer)) evaluates the child state To reachable from From using G and H, predicates defining the cost of moving from From to To and from To to a goal. Note that G differs from the its traditional definition in the A algorithm literature, instead of computing the cost of a state from scratch it is much easier to compute the cost difference of a single move in the search true keeping a running total along a path.

Implementing new search problems

Search problems are abstractly defined by the prolog record search_problem, there are two example problems:

• grid search
• black-white counter puzzle (see the Simply Logical chapter for an explanation)

Search problems are defined by 5 predicates:

• start(-StartState:state), first argument unifies with the starting state of the search problem, e.g. the starting position of the agent in a grid search, or an initial board state in a game tree search.
• goal(+State:state) holds if State is a goal state or not.
• children(+State:state, -ChildStates:list(state)) the reachable child states from State
• h(+State, -Cost:integer), cost unifies with the h-value of State. The h-value is the heuristic estimate for how must it costs to reach the goal state from State.
• g(+State:state, -Cost:integer), cost unifies with the cost of reaching State from the start state as defined by start/1.

The predicates are wrapped up into a search_problem record using search_problem:make_search_record.

The state type is user defined--the search algorithms are orthogonal to the representation, you can choose your state representation however you see fit.

Contributing

See DEVELOPMENT.md for technical details.