open System;
open System.Collections.Generic;

type dna = A | C | G | T;;

let DNASTR = "ACGT"  // translates 0,1,2,3 to printable DNA symbol
let STRDNA = Dictionary<char,dna>();
STRDNA.['A'] <- A
STRDNA.['C'] <- C
STRDNA.['G'] <- G
STRDNA.['T'] <- T

// convert dna-string to list of type dna values
let stringdna (s:string) =
   let mutable dnalist = []
   let mutable i = s.Length-1  // decrement from end to preserve order
   while i>=0 do
     dnalist <- STRDNA.[ s.[i] ] :: dnalist
     i <- i-1
   dnalist
///

// What's wrong with using strings?
// let yourdna = stringdna "I AM A FARM ANIMAL";  
// this can't possibly be detected at compile time, because to the
// compiler all strings are the same, they're called strings.

//let hisdna = [A;C;G;F;R;M];  // compiler type error

let CODEDNA = [|A;C;G;T|]  // array index translates to type dna
let DNACODE x =   // convert DNA value to 2-bit binary values
  match x with
    | A -> 0
    | C -> 1
    | G -> 2
    | T -> 3;;

// convert list of DNA symbols to list of 32 bit integers, each int
// can encode 16 DNA symbols.  The length of original sequence is stored
// via the lref reference, since the length may not be a multiple of 16
let dnaint (ds:dna list) lref = 
   let mutable dsq = ds  // lref is a reference, for storing length
   lref := ds.Length;        // record length of original sequence
   while dsq.Length%16<>0 do
      dsq <- A::dsq   // add dummy A's to front so %8=0
   let mutable intlist = []
   let mutable i = (dsq.Length-1)
   let mutable k,ax = 0,0
   while i>0 do
     ax <- 0;  k <- i
     while k>= i-15 do
       ax <- ax+DNACODE(dsq.[k])
       if k>i-15 then ax <- ax*4 // shift left 2 bits
       k <- k-1
     // inner while
     intlist <- ax::intlist
     i <- i-16
   // outer while
   intlist;;

let intdna (intl:int list) len = // convert integer list to dna sequence of len
   let mutable dnalist = []
   let mutable i = intl.Length-1
   while i>=0 do 
     let mutable ai = intl.[i]
     for k in 0..15 do
        dnalist <- (CODEDNA.[ai%4]) :: dnalist
        ai <- ai/4   // right-shift 4 bits
     i <- i-1
   // while
   let dl = dnalist.Length
   dnalist <- dnalist.[(dl-len)..(dl-1)] // trim
   dnalist;;

let rec reverse ax m =
   match m with
     | [] -> ax
     | car::cdr -> reverse (car::ax) cdr

let len = ref 0
let x = dnaint [A;C;G;T;G;C;A;C;C;T;G;G;A;T;G;C] len
Console.WriteLine(string(x))
let y = reverse [] (intdna x !len)
for c in y do 
   printf "%A" c
printfn ""