Turns an integer into an array of strings that correspond to the correct wav file names to read out.

SpeechElements

let emptyIfZero f x =
    match x with
    | 0 -> []
    | _ -> f x
 
let elementsBetween0And20 = emptyIfZero (fun x -> [ string x ])
let elementsBetween21And99 x = string (x - (x % 10)) :: elementsBetween0And20 (x % 10)
let ifGreaterThan y f g x = if x > y then f x else g x
let elementsFor2DigitNumber = ifGreaterThan 20 elementsBetween21And99 elementsBetween0And20
let elementsBetween100And999 x = string (x / 100) :: "hundred" :: emptyIfZero (fun x -> "and" :: elementsFor2DigitNumber x) (x % 100)
let elementsFor3DigitNumber = ifGreaterThan 99 elementsBetween100And999 elementsFor2DigitNumber
    
let generateSeparatorText n x y =
    match n with
    | "thousand" when y > 0 && y < 100 && x % 1000 > 0 -> [ n; "and" ]
    | "thousand" when y > 0 && y < 100 -> [ "and" ]
    | _ when x % 1000 = 0 -> []
    | _ -> [ n ]

let rec generateElementsRec l x =
    match l with
    | n::ns when x > 999 -> elementsFor3DigitNumber (x % 1000) :: generateSeparatorText n (x / 1000) (x % 1000) :: generateElementsRec ns (x / 1000)
    | _ -> [ elementsFor3DigitNumber x ]
    
let generateElements x =
    match x with
    | 0 -> [ "0" ]
    | _ when x < 0 -> raise <| System.ArgumentOutOfRangeException()
    | _ ->
        generateElementsRec [ "thousand"; "million"; "billion" ] x
        |> List.rev
        |> List.collect id

First 3 revisions of this were buggy! (Try 5000076 as input)