Phillip Trelford's Array

This week I had pleasure of meeting Vagif Abilov on Tomas Petricek and I’s Advanced F# course at Skills Matter. Vagif mentioned a Ruby implementation of Conway’s Game of Life being squeezed into 140 characters for tweeting and how that might look in F#.

Here’s the 140 characters of Ruby:

life=->g,s{(0..s*s-1).map{|i|->n{n==3||(g[i]&&n==2)||nil}

[[g[i-s-1],g[i-s],g[i-s+1],g[i-1],g[i+1],g[i+s-1],g[i+s],g[i+s+1]]

.compact.count]}}

And 127 characters of F#:

let f s g = g|>List.mapi(fun i x->match Seq.sumBy(fun o->

g.[abs((i+o)%(s*s))])[-1-s;-s;1-s;-1;1;-1+s;s;1+s]with 3->1|2->x|_->0)

Like the Ruby version, the F# implementation above takes a shortcut when computing points outside of the grid.

This is what the function looks like with added white space, variable names and comments:

let generate size grid =
    grid |> List.mapi (fun i value->
        /// Neighbours count
        let count =
            let s = size
            [-1-s;-s;1-s;
             -1;     1;
             -1+s;s; 1+s]
            |> Seq.sumBy (fun o ->
                let offset = abs((i + o) % grid.Length)
                grid.[offset]
            )
        // Evaluate life
        match value, count with 
        | _, 3 -> 1
        | n, 2 -> n
        | _, _ -> 0 
    )

The  game grid can is expressed as a list:

let grid = [
    0;0;0;0;0
    0;0;1;0;0
    0;0;1;1;0
    0;0;1;0;0
    0;0;0;0;0
    ]

Visualizing the grid on the console:

let show (size:int) (grid:int list) =
    for y=0 to (grid.Length/size)-1 do
       [0..size-1] 
       |> List.map (fun x -> grid.[y * size + x].ToString())
       |> String.concat "" 
       |> printfn "%s"

Or watch it running in your browser: http://trelford.com/Life/Life.htm

(I used Pit to compile the F# code to JavaScript)

Life.fs (1.77 kb)