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POA* / Dijkstra's algorithm simple implementation (Pascal)
 

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    <p>I'm trying to implement A* path finding algorithm (now it's Dijkstra's algorithm i.e without heuristic) using this article <a href="http://www.policyalmanac.org/games/aStarTutorial.htm" rel="nofollow noreferrer">http://www.policyalmanac.org/games/aStarTutorial.htm</a>. But I can't figure out what's wrong in my code (it finds incorrect path). </p> <p><img src="https://i.stack.imgur.com/amlAv.png" alt="enter image description here"></p> <p>instead of the empty begin ... end; it should be this step: </p> <blockquote> <p>If it is on the open list already, check to see if this path to that square is better, using G cost as the measure. A lower G cost means that this is a better path. If so, change the parent of the square to the current square, and recalculate the G and F scores of the square.</p> </blockquote> <p>but I think it is not important because there is no diagonal movement.</p> <pre><code>uses crt; const MAXX = 20; MAXY = 25; type TArr = array [0..MAXY, 0..MAXX] of integer; TCell = record x: integer; y: integer; end; TListCell = record x: integer; y: integer; G: integer; parent: TCell; end; TListArr = array [1..10000] of TListCell; TList = record arr: TListArr; len: integer; end; var i, j, minind, ind, c: integer; start, finish: TCell; current: TListCell; field: TArr; opened, closed: TList; procedure ShowField; var i, j: integer; begin textcolor(15); for i := 0 to MAXX do begin for j := 0 to MAXY do begin case field[j, i] of 99: textcolor(8); // not walkable 71: textcolor(14); // walkable 11: textcolor(10); // start 21: textcolor(12); // finish 15: textcolor(2); // path 14: textcolor(5); 16: textcolor(6); end; write(field[j, i], ' '); end; writeln; end; textcolor(15); end; procedure AddClosed(a: TListCell); begin closed.arr[closed.len + 1] := a; inc(closed.len); end; procedure AddOpened(x, y, G: integer); begin opened.arr[opened.len + 1].x := x; opened.arr[opened.len + 1].y := y; opened.arr[opened.len + 1].G := G; inc(opened.len); end; procedure DelOpened(n: integer); var i: integer; begin AddClosed(opened.arr[n]); for i := n to opened.len - 1 do opened.arr[i] := opened.arr[i + 1]; dec(opened.len); end; procedure SetParent(var a: TListCell; parx, pary: integer); begin a.parent.x := parx; a.parent.y := pary; end; function GetMin(var a: TList): integer; var i, min, mini: integer; begin min := MaxInt; mini := 0; for i := 1 to a.len do if a.arr[i].G &lt; min then begin min := a.arr[i].G; mini := i; end; GetMin := mini; end; function FindCell(a: TList; x, y: integer): integer; var i: integer; begin FindCell := 0; for i := 1 to a.len do if (a.arr[i].x = x) and (a.arr[i].y = y) then begin FindCell := i; break; end; end; procedure ProcessNeighbourCell(x, y: integer); begin if (field[current.x + x, current.y + y] &lt;&gt; 99) then // if walkable if (FindCell(closed, current.x + x, current.y + y) &lt;= 0) then // and not visited before if (FindCell(opened, current.x + x, current.y + y) &lt;= 0) then // and not added to list already begin AddOpened(current.x + x, current.y + y, current.G + 10); SetParent(opened.arr[opened.len], current.x, current.y); // field[opened.arr[opened.len].x, opened.arr[opened.len].y]:=16; end else begin end; end; begin randomize; for i := 0 to MAXX do for j := 0 to MAXY do field[j, i] := 99; for i := 1 to MAXX - 1 do for j := 1 to MAXY - 1 do if random(5) mod 5 = 0 then field[j, i] := 99 else field[j, i] := 71; // start and finish positions coordinates start.x := 5; start.y := 3; finish.x := 19; finish.y := 16; field[start.x, start.y] := 11; field[finish.x, finish.y] := 21; ShowField; writeln; opened.len := 0; closed.len := 0; AddOpened(start.x, start.y, 0); SetParent(opened.arr[opened.len], -1, -1); current.x := start.x; current.y := start.y; repeat minind := GetMin(opened); current.x := opened.arr[minind].x; current.y := opened.arr[minind].y; current.G := opened.arr[minind].G; DelOpened(minind); ProcessNeighbourCell(1, 0); // look at the cell to the right ProcessNeighbourCell(-1, 0); // look at the cell to the left ProcessNeighbourCell(0, 1); // look at the cell above ProcessNeighbourCell(0, -1); // look at the cell below if (FindCell(opened, finish.x, finish.y) &gt; 0) then break; until opened.len = 0; // count and mark path c := 0; while ((current.x &lt;&gt; start.x) or (current.y &lt;&gt; start.y)) do begin field[current.x, current.y] := 15; ind := FindCell(closed, current.x, current.y); current.x := closed.arr[ind].parent.x; current.y := closed.arr[ind].parent.y; inc(c); end; ShowField; writeln(c); readln; end. </code></pre> <p><strong>Edit</strong> Feb 1 '12: updated code, also fixed path marking (there should be or instead and), looks like it works now :)</p>
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