F# Diamond

Diamond.fs

open System
type CharSpec = {Char:char; SndOffset: int}
type DiamondSpec = CharSpec list

let printDiamondRow charSpec = 
    match charSpec with
    | {Char = x; SndOffset = 0} -> sprintf "%c" x
    | {Char = x; SndOffset = y} -> sprintf "%c%s%c" x (String(' ', (y-1))) x

let padRow size (row:string) = 
    let padding = String(' ', (size-(row.Length))/2)
    sprintf "%s%s%s" padding row padding

let join separator (strings:string list) = String.Join(separator, strings)

let makeDiamondSpec x = 
    let letters  = ['A' .. x]
    let makeOffset rowNumber = 2*(rowNumber)

    let rec splitList list first second index =
        match list with
        | [] -> ((List.rev first) @ second),(index*2-1)
        | y::[] -> 
            let charSpec = {Char = y; SndOffset = (makeOffset index)}
            splitList [] (charSpec::first) second (index+1)
        | y::rest -> 
            let charSpec = {Char = y; SndOffset = (makeOffset index)}
            splitList rest (charSpec::first) (charSpec::second) (index+1)

    splitList letters [] [] 0

let makeDiamond c = 
    let diamondSpec,size = makeDiamondSpec 'F' 
    diamondSpec |> List.map (printDiamondRow >> (padRow size)) |> join (Environment.NewLine)

[<EntryPoint>]
let main argv = 
    let result = makeDiamond 'F'
    printfn "%s" result
    Console.ReadLine() |> ignore
    0 // return an integer exit code