Project Euler #81 - Shortest Path in F#

gistfile1.txt

// Project Euler #81 - Shortest Path in F#

// Doesn't seem very functional, but my first shot at it.

// Using Dijkstra's algorithm from wikipedia.
let matrix = 
    File.ReadAllLines(@"Matrix.txt") |> Array.map( fun line -> line.Split(",".ToCharArray()) |> Array.map( Int32.Parse));  
    
    

type Coord = {
    row: int;
    col: int;
    }

type Node = {
    rc : Coord;
    distance : int;
    mutable unvisited : bool;
    mutable tentative_distance : int;
    }

let MAX_ROW = matrix.GetUpperBound(0)+1;
let MAX_COL = MAX_ROW;
let MAX_DISTANCE = Int32.MaxValue;

let array = Array2D.init MAX_ROW MAX_COL (fun i j -> { new Node with distance = matrix.[i].[j] and unvisited = true and tentative_distance = MAX_DISTANCE and rc = { row = i; col = j } } ); 

let neighborNodes (c:Node) = [
    // Right
    if c.rc.row < MAX_ROW-1 then
        yield array.[c.rc.row+1,c.rc.col];
    // Down
    if c.rc.col < MAX_COL-1 then
        yield array.[c.rc.row, c.rc.col+1];

];
    

let unvisitedNodes  (arr: Node[,]) = [
    for ii in 0..(MAX_ROW-1) do
        for jj in 0..(MAX_COL-1) do
            if( arr.[ii,jj].unvisited) then
                yield arr.[ii,jj];
];

let nodeWithSmallestTentativeDistance (arr:Node[,]) = 
    unvisitedNodes arr |> List.minBy( fun x -> x.tentative_distance);

let mutable CurrentNode = array.[0,0];
CurrentNode.tentative_distance <- CurrentNode.distance;

let DestinationNode = array.[MAX_ROW-1, MAX_COL-1];

while DestinationNode.unvisited = true do
    for n in (neighborNodes CurrentNode) do
        //printf "%A\n" n.rc
        if n.unvisited then
            let tentativeDistance = CurrentNode.tentative_distance + n.distance;
            if tentativeDistance < n.tentative_distance then
                n.tentative_distance <- tentativeDistance;
    CurrentNode.unvisited <- false;
 
    if DestinationNode.unvisited = true then
        CurrentNode <- nodeWithSmallestTentativeDistance array;

printf "Answer is: %A\n" DestinationNode.tentative_distance ;
printf "Done\n";