2 people like it.

Find largest mass in 2D array

This is a modification of the flood fill algorithm to find the largest contiguous block of items in a 2D array. Also includes a simple flood fill finder given a canvas and the target point

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type Board<'T> = 'T[,]

type X = int

type Y = int

type Position = X * Y

type PositionList = Position list 

type ProcessedPositions = PositionList

type ContiguousPoints = PositionList

type MassFinder = ContiguousPoints * ProcessedPositions

type Earth = 
    | Land
    | Water

let board = array2D [[Land;  Land;  Land;  Land;];
                     [Water; Land;  Land;  Water;];
                     [Land;  Water; Water; Water;];
                     [Water; Land;  Land;  Water;]]

let boardInt = array2D [[0;  0;  0;  1;];
                        [1; 0;  0;  1;];
                        [0;  1; 1; 1;];
                        [1; 0;  0;  1;]]
(* 
    Helper methods to move the position around
*)
                                          
let moveRight position = 
    let (x,y) = position
    (x + 1, y)

let moveLeft position = 
    let (x,y) = position
    (x - 1, y)

let moveUp position = 
    let (x,y) = position
    (x, y + 1)

let moveDown position = 
    let (x,y) = position
    (x, y - 1)

(*
    Size helpers
*)

let xSize board = Array2D.length1 board

let ySize board = Array2D.length2 board

let offBoard position board = 
    let (x,y) = position
    x < 0 || y < 0 || x >= (xSize board) || y >= (ySize board)

(*
    Alias to push elements onto a list
*)

let markPosition position previousSpots = position::previousSpots

(*
    Determines if the position on the board equals the target
*)

let positionOnTarget position board target = 
    if offBoard position board then 
        false
    else
        let (x, y) = position
        (Array2D.get board x y) = target

(*
    Alias to find if we already processed a position
*)

let positionExists position list = 
    List.exists(fun pos -> pos = position) list

(* 
   Iterate over each element in a 2d array, passing the x and y
   coordinate and the board, to the supplied function
   which can return an item. The items are all cons together
   and the function returns a new list
*)

let forEachElement (applier:(X -> Y -> Board<'a> -> 'b)) (twoDimArray:Board<'a>) =
    let mutable items = [] 
    for x in 0..(xSize board) do
        for y in 0..(ySize board) do            
            items <- (applier x y twoDimArray)::items
    items


(*
    Looks for a specified contigoius block
    and keeps track of processed positions using a 
    reference cell of a list of positions (supplied by the caller)
*)

let findMassStartingAt (position:Position) (board:Board<'A>) (target:'A) (positionSeed:ProcessedPositions) : MassFinder = 
    let rec findMassStartingAt' position (currentMass:ContiguousPoints, processedList:ProcessedPositions) cont = 
            // if you move off the board return
        if offBoard position board then
            cont((currentMass, processedList))

        // if you already processed this position then don't do anything
        else if positionExists position processedList then
            cont((currentMass, processedList))
        else  
            
            // branch out left, up, right, and down and see what you can find
            let up = moveUp position
            let down = moveDown position
            let left = moveLeft position
            let right = moveRight position
            
            let found = positionOnTarget position board target

            match found with 
                | true ->                    
                           let massState = (position::currentMass, position::processedList)

                           findMassStartingAt' up  massState (fun foundMassUp -> 
                           findMassStartingAt' down foundMassUp (fun foundMassDown ->
                           findMassStartingAt' left foundMassDown (fun foundMassLeft ->
                           findMassStartingAt' right foundMassLeft cont))) 

                | false -> 
                    // if you didn't find anything return the masses that you 
                    // found prevoiusly
                    cont((currentMass, processedList))

    findMassStartingAt' position ([], positionSeed) id

(*
    Finds all contiguous blocks of the specified type
    and returns a list of lists (each list is the points for a specific
    block)
*)

let getContiguousBlocks board target = 
    
    // go through each board element and find masses starting at the
    // the current position
    // filter out any positions that found no masses
    let findMass x y board = findMassStartingAt (x, y) board target []

    forEachElement findMass board
        |> List.map fst
        |> List.filter (List.isEmpty >> not)
        

(*
    Returns a list of points representing a contigious block 
    of the type that the point was at. 
*)

let floodFillArea (point:Position) (canvas:Board<'T>) =
    let (x, y) = point
    let itemAtPoint = Array2D.get canvas x y
    
    findMassStartingAt point canvas itemAtPoint [] |> fst


(* 
    Test functions to run it
*)

let masses = getContiguousBlocks board Land

let largestList = List.maxBy(List.length) masses

let massAt = floodFillArea (2, 2) boardInt

let sizeOfMassAt22 = List.length massAt

System.Console.WriteLine("Largest mass is " + (List.length largestList).ToString());
System.Console.WriteLine("Mass size at (2,2) is " + sizeOfMassAt22.ToString());
type X = int

Full name: Script.X
Multiple items
val int : value:'T -> int (requires member op_Explicit)

Full name: Microsoft.FSharp.Core.Operators.int

--------------------
type int = int32

Full name: Microsoft.FSharp.Core.int

--------------------
type int<'Measure> = int

Full name: Microsoft.FSharp.Core.int<_>
type Y = int

Full name: Script.Y
type Position = X * Y

Full name: Script.Position
type PositionList = Position list

Full name: Script.PositionList
type 'T list = List<'T>

Full name: Microsoft.FSharp.Collections.list<_>
type ProcessedPositions = PositionList

Full name: Script.ProcessedPositions
type ContiguousPoints = PositionList

Full name: Script.ContiguousPoints
type MassFinder = ContiguousPoints * ProcessedPositions

Full name: Script.MassFinder
type Earth =
  | Land
  | Water

Full name: Script.Earth
union case Earth.Land: Earth
union case Earth.Water: Earth
val board : Earth [,]

Full name: Script.board
val array2D : rows:seq<#seq<'T>> -> 'T [,]

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.array2D
val boardInt : int [,]

Full name: Script.boardInt
val moveRight : int * 'a -> int * 'a

Full name: Script.moveRight
val position : int * 'a
val x : int
val y : 'a
val moveLeft : int * 'a -> int * 'a

Full name: Script.moveLeft
val moveUp : 'a * int -> 'a * int

Full name: Script.moveUp
val position : 'a * int
val x : 'a
val y : int
val moveDown : 'a * int -> 'a * int

Full name: Script.moveDown
val xSize : board:'a [,] -> int

Full name: Script.xSize
val board : 'a [,]
module Array2D

from Microsoft.FSharp.Collections
val length1 : array:'T [,] -> int

Full name: Microsoft.FSharp.Collections.Array2D.length1
val ySize : board:'a [,] -> int

Full name: Script.ySize
val length2 : array:'T [,] -> int

Full name: Microsoft.FSharp.Collections.Array2D.length2
val offBoard : int * int -> board:'a [,] -> bool

Full name: Script.offBoard
val position : int * int
val markPosition : position:'a -> previousSpots:'a list -> 'a list

Full name: Script.markPosition
val position : 'a
val previousSpots : 'a list
val positionOnTarget : int * int -> board:'a [,] -> target:'a -> bool (requires equality)

Full name: Script.positionOnTarget
val board : 'a [,] (requires equality)
val target : 'a (requires equality)
val get : array:'T [,] -> index1:int -> index2:int -> 'T

Full name: Microsoft.FSharp.Collections.Array2D.get
val positionExists : position:'a -> list:'a list -> bool (requires equality)

Full name: Script.positionExists
val position : 'a (requires equality)
Multiple items
val list : 'a list (requires equality)

--------------------
type 'T list = List<'T>

Full name: Microsoft.FSharp.Collections.list<_>
Multiple items
module List

from Microsoft.FSharp.Collections

--------------------
type List<'T> =
  | ( [] )
  | ( :: ) of Head: 'T * Tail: 'T list
  interface IEnumerable
  interface IEnumerable<'T>
  member Head : 'T
  member IsEmpty : bool
  member Item : index:int -> 'T with get
  member Length : int
  member Tail : 'T list
  static member Cons : head:'T * tail:'T list -> 'T list
  static member Empty : 'T list

Full name: Microsoft.FSharp.Collections.List<_>
val exists : predicate:('T -> bool) -> list:'T list -> bool

Full name: Microsoft.FSharp.Collections.List.exists
val pos : 'a (requires equality)
val forEachElement : applier:(X -> Y -> Board<'a> -> 'b) -> twoDimArray:Board<'a> -> 'b list

Full name: Script.forEachElement
val applier : (X -> Y -> Board<'a> -> 'b)
type Board<'T> = 'T [,]

Full name: Script.Board<_>
val twoDimArray : Board<'a>
val mutable items : 'b list
val x : int32
val y : int32
val findMassStartingAt : X * Y -> board:Board<'A> -> target:'A -> positionSeed:ProcessedPositions -> MassFinder (requires equality)

Full name: Script.findMassStartingAt
val position : Position
val board : Board<'A> (requires equality)
val target : 'A (requires equality)
val positionSeed : ProcessedPositions
val findMassStartingAt' : (int * int -> ContiguousPoints * ProcessedPositions -> (ContiguousPoints * ProcessedPositions -> 'a) -> 'a)
val currentMass : ContiguousPoints
val processedList : ProcessedPositions
val cont : (ContiguousPoints * ProcessedPositions -> 'a)
val up : int * int
val down : int * int
val left : int * int
val right : int * int
val found : bool
val massState : (int * int) list * (int * int) list
val foundMassUp : ContiguousPoints * ProcessedPositions
val foundMassDown : ContiguousPoints * ProcessedPositions
val foundMassLeft : ContiguousPoints * ProcessedPositions
val id : x:'T -> 'T

Full name: Microsoft.FSharp.Core.Operators.id
val getContiguousBlocks : board:Board<'a> -> target:'a -> ContiguousPoints list (requires equality)

Full name: Script.getContiguousBlocks
val board : Board<'a> (requires equality)
val findMass : (X -> Y -> Board<'a> -> MassFinder) (requires equality)
val x : X
val y : Y
val map : mapping:('T -> 'U) -> list:'T list -> 'U list

Full name: Microsoft.FSharp.Collections.List.map
val fst : tuple:('T1 * 'T2) -> 'T1

Full name: Microsoft.FSharp.Core.Operators.fst
val filter : predicate:('T -> bool) -> list:'T list -> 'T list

Full name: Microsoft.FSharp.Collections.List.filter
val isEmpty : list:'T list -> bool

Full name: Microsoft.FSharp.Collections.List.isEmpty
val not : value:bool -> bool

Full name: Microsoft.FSharp.Core.Operators.not
val floodFillArea : X * Y -> canvas:Board<'T> -> ContiguousPoints (requires equality)

Full name: Script.floodFillArea
val point : Position
val canvas : Board<'T> (requires equality)
val itemAtPoint : 'T (requires equality)
val masses : ContiguousPoints list

Full name: Script.masses
val largestList : Position list

Full name: Script.largestList
val maxBy : projection:('T -> 'U) -> list:'T list -> 'T (requires comparison)

Full name: Microsoft.FSharp.Collections.List.maxBy
val length : list:'T list -> int

Full name: Microsoft.FSharp.Collections.List.length
val massAt : ContiguousPoints

Full name: Script.massAt
val sizeOfMassAt22 : int

Full name: Script.sizeOfMassAt22
namespace System
type Console =
  static member BackgroundColor : ConsoleColor with get, set
  static member Beep : unit -> unit + 1 overload
  static member BufferHeight : int with get, set
  static member BufferWidth : int with get, set
  static member CapsLock : bool
  static member Clear : unit -> unit
  static member CursorLeft : int with get, set
  static member CursorSize : int with get, set
  static member CursorTop : int with get, set
  static member CursorVisible : bool with get, set
  ...

Full name: System.Console
System.Console.WriteLine() : unit
   (+0 other overloads)
System.Console.WriteLine(value: string) : unit
   (+0 other overloads)
System.Console.WriteLine(value: obj) : unit
   (+0 other overloads)
System.Console.WriteLine(value: uint64) : unit
   (+0 other overloads)
System.Console.WriteLine(value: int64) : unit
   (+0 other overloads)
System.Console.WriteLine(value: uint32) : unit
   (+0 other overloads)
System.Console.WriteLine(value: int) : unit
   (+0 other overloads)
System.Console.WriteLine(value: float32) : unit
   (+0 other overloads)
System.Console.WriteLine(value: float) : unit
   (+0 other overloads)
System.Console.WriteLine(value: decimal) : unit
   (+0 other overloads)
System.Int32.ToString() : string
System.Int32.ToString(provider: System.IFormatProvider) : string
System.Int32.ToString(format: string) : string
System.Int32.ToString(format: string, provider: System.IFormatProvider) : string

More information

Link:http://fssnip.net/ik
Posted:8 years ago
Author:devshorts
Tags: flood fill , algorithms