solution to the heathrow route planning problem from learnyousomeerlang.com

heathrow.fsx

type Move =
    | AFwd
    | BFwd
    | XCross

// list of amove, bmove, xcross moves tuples (costs)
let moveCosts = [(50, 10, 30); (5, 90, 20); (40, 2, 25); (10, 8,0)]
let bottomGraphVal = ((0, ([]: Move list)), (0, ([] : Move list)))

let calcNewSetAndMoves ((aVal, (aRoute)), (bVal, (bRoute))) (aFwd, bFwd, xCross) =
    let valToAStraightFromA = aVal + aFwd
    let valToACrossFromB = bVal + bFwd + xCross
    let newAVal = min valToAStraightFromA valToACrossFromB
    let newAMoves =
        if  valToAStraightFromA < valToACrossFromB then AFwd :: aRoute
        else XCross :: BFwd :: bRoute
            
    //-----------------------------------

    let valToBStraightFromB = bVal + bFwd
    let valToBCrossFromA = aVal + aFwd + xCross
    let newBVal = min valToBStraightFromB valToBCrossFromA
    let newBMoves =
        if  valToBStraightFromB < valToBCrossFromA then BFwd :: bRoute
        else XCross :: AFwd :: aRoute

    ((newAVal, newAMoves), (newBVal, newBMoves))

let myTestRes = calcNewSetAndMoves bottomGraphVal (50, 10, 30)

let foldAccFn = 
    fun (curState) (curEntry ) ->
    calcNewSetAndMoves curState curEntry

let main = 
    let retVal = List.fold foldAccFn bottomGraphVal moveCosts
    let ((aVal,aList),(bVal,bList)) = retVal
    (aVal, List.rev aList, bVal, List.rev bList)