#### Problem

In the 20 x 20 grid below, four numbers along a diagonal line have been marked in red.

08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08

49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00

81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65

52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91

22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80

24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50

32 98 81 28 64 23 67 102638 40 67 59 54 70 66 18 38 64 70

67 26 20 68 02 62 12 20 956394 39 63 08 40 91 66 49 94 21

24 55 58 05 66 73 99 26 97 177878 96 83 14 88 34 89 63 72

21 36 23 09 75 00 76 44 20 45 351400 61 33 97 34 31 33 95

78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92

16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57

86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58

19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40

04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66

88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69

04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36

20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16

20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54

01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48

The product of these numbers is 26 x 63 x 78 x 14 = 1788696.

What is the greatest product of four adjacent numbers in any direction (up, down, left, right, or diagonally) in the 20 x 20 grid?

#### Solution

open System.IO let initArray = File.ReadAllLines(@"C:\TEMP\euler11.txt") |> Seq.map (fun l -> l.Split(' ') |> Seq.map int32 |> Seq.toArray) |> Seq.toArray let height, width = initArray.Length, initArray |> Seq.map (fun l -> l.Length) |> Seq.max let twoDArray = Array2D.init height width (fun i j -> initArray.[i].[j]) let Up (array2D:int[,]) h w n = let lowerBound = h-(n-1) let upperBound = h if lowerBound < 0 || upperBound > height-1 then [] else [lowerBound..upperBound] |> List.map (fun y -> array2D.[y, w]) let Left (array2D:int[,]) h w n = let lowerBound = w-(n-1) let upperBound = w if lowerBound < 0 || upperBound > width-1 then [] else [lowerBound..upperBound] |> List.map (fun x -> array2D.[h, x]) let LeftDiag (array2D:int[,]) h w n = let lowerWBound = w-(n-1) let upperWBound = w let lowerHBound = h-(n-1) let upperHBound = h if lowerWBound < 0 || upperWBound > width-1 || lowerHBound < 0 || upperHBound > height-1 then [] else let wCoordinates = [lowerWBound..upperWBound] let hCoordinates = [lowerHBound..upperHBound] List.map2 (fun y x -> array2D.[y, x]) hCoordinates wCoordinates let RightDiag (array2D:int[,]) h w n = let lowerWBound = w let upperWBound = w+(n-1) let lowerHBound = h-(n-1) let upperHBound = h if lowerWBound < 0 || upperWBound > width-1 || lowerHBound < 0 || upperHBound > height-1 then [] else let wCoordinates = [lowerWBound..upperWBound] let hCoordinates = [lowerHBound..upperHBound] |> List.rev List.map2 (fun y x -> array2D.[y, x]) hCoordinates wCoordinates let quartets = seq { for y in 3 .. width-1 do for x in 3 .. height-1 do yield Up twoDArray x y 4 yield Left twoDArray x y 4 yield LeftDiag twoDArray x y 4 yield RightDiag twoDArray x y 4 } let CalcProduct numbers = numbers |> Seq.fold (fun acc n -> acc * n) 1 let maxProduct = quartets |> Seq.map CalcProduct |> Seq.max

**Getting Started**

The first thing I needed to do is to get this 20×20 grid of data into F# somehow, and to do so I copied and pasted the grid into a text file *C:\TEMP\euler11.txt*. *File.ReadAllLines* will only get us as far as reading the data into a string array though, which still isn’t very useful to us. So in order to get the data into a useful format, I first had to turn this string[] into an int32[][]:

|> Seq.map (fun l -> l.Split(' ') |> Seq.map int32 |> Seq.toArray)

This this line of code takes a line of string such as:

“08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08”

and split it up using the space character, cast each of the string segments into an int32 and return the int32 values as an array.

The next step is entirely optional, in short it converts the int32[][] (that is, an array of int32 arrays) into a int32[,] (a 2-dimensional array). The main difference between a normal Array and an Array2D is how you access its member elements – Array.[x].[y] versus Array2D.[x, y].

**The Interesting Bits**

Now, to find the answer you can iterate through each element in the 2 dimensional array and for each element calculate the product of each 4 adjacent numbers centred around that element (see image on the left) and find the max product.

However, this approach introduces a large amount of duplicates. For instance, the right pattern from 28 gives [28; 66; 33; 13] which yields the same product as the left pattern from 13.

You can cut down on the number of duplicates by only processing the first 4 patterns, shown in red. For each of the patterns I have created a corresponding function which takes a 2D array, a height and width value which identifies the element in the 2D array, a number which indicates the number of adjacent numbers that make up the pattern, and returns the int32 array.

There are some edge cases of course, take the element at (0, 0) position for instance, there aren’t enough elements in any of the directions to fill up the patterns. In these cases, the functions will simply return an empty array. This also means that there is no need to consider any of the elements whose x (horizontal) and z (vertical) coordinates is less than 3.

**The Finishing Touch**

The final step involves getting a sequence of all the 4-number arrays generated by the aforementioned patterns, I have chosen to use yield and put the results into a sequence:

quartets;; val it : seq<int list> = seq [[97; 40; 73; 23]; [52; 70; 95; 23]; [8; 49; 31; 23]; [23; 55; 81; 0]; ...]

And for each array work out its product (using the *CalcProduct* function) and then find the greatest product in the array.