(* $Id: diff.ml,v 5.1 2006-11-03 10:31:18 ddr Exp $ *)

(* Parts of Code of GNU diff (analyze.c) translated from C to OCaml
   and adjusted. Basic algorithm described by Eugene W.Myers in:
     "An O(ND) Difference Algorithm and Its Variations" *)

exception DiagReturn of int

let diag fd bd sh xv yv xoff xlim yoff ylim =
  let dmin = xoff - ylim in
  let dmax = xlim - yoff in
  let fmid = xoff - yoff in
  let bmid = xlim - ylim in
  let odd = (fmid - bmid) land 1 <> 0 in
  fd.(sh + fmid) <- xoff;
  bd.(sh + bmid) <- xlim;
  try
    let rec loop fmin fmax bmin bmax =
      let fmin =
        if fmin > dmin then (
          fd.(sh + fmin - 2) <- -1;
          fmin - 1)
        else fmin + 1
      in
      let fmax =
        if fmax < dmax then (
          fd.(sh + fmax + 2) <- -1;
          fmax + 1)
        else fmax - 1
      in
      (let rec loop d =
         if d < fmin then ()
         else
           let tlo = fd.(sh + d - 1) in
           let thi = fd.(sh + d + 1) in
           let x = if tlo >= thi then tlo + 1 else thi in
           let x =
             let rec loop xv yv xlim ylim x y =
               if x < xlim && y < ylim && xv x == yv y then
                 loop xv yv xlim ylim (x + 1) (y + 1)
               else x
             in
             loop xv yv xlim ylim x (x - d)
           in
           fd.(sh + d) <- x;
           if odd && bmin <= d && d <= bmax && bd.(sh + d) <= fd.(sh + d) then
             raise (DiagReturn d)
           else loop (d - 2)
       in
       loop fmax);
      let bmin =
        if bmin > dmin then (
          bd.(sh + bmin - 2) <- max_int;
          bmin - 1)
        else bmin + 1
      in
      let bmax =
        if bmax < dmax then (
          bd.(sh + bmax + 2) <- max_int;
          bmax + 1)
        else bmax - 1
      in
      (let rec loop d =
         if d < bmin then ()
         else
           let tlo = bd.(sh + d - 1) in
           let thi = bd.(sh + d + 1) in
           let x = if tlo < thi then tlo else thi - 1 in
           let x =
             let rec loop xv yv xoff yoff x y =
               if x > xoff && y > yoff && xv (x - 1) == yv (y - 1) then
                 loop xv yv xoff yoff (x - 1) (y - 1)
               else x
             in
             loop xv yv xoff yoff x (x - d)
           in
           bd.(sh + d) <- x;
           if (not odd) && fmin <= d && d <= fmax && bd.(sh + d) <= fd.(sh + d)
           then raise (DiagReturn d)
           else loop (d - 2)
       in
       loop bmax);
      loop fmin fmax bmin bmax
    in
    loop fmid fmid bmid bmid
  with DiagReturn i -> i

let diff_loop a ai b bi n m =
  let fd = Array.make (n + m + 3) 0 in
  let bd = Array.make (n + m + 3) 0 in
  let sh = m + 1 in
  let xvec i = a.(ai.(i)) in
  let yvec j = b.(bi.(j)) in
  let chng1 = Array.make (Array.length a) true in
  let chng2 = Array.make (Array.length b) true in
  for i = 0 to n - 1 do
    chng1.(ai.(i)) <- false
  done;
  for j = 0 to m - 1 do
    chng2.(bi.(j)) <- false
  done;
  (let rec loop xoff xlim yoff ylim =
     let xoff, yoff =
       let rec loop xoff yoff =
         if xoff < xlim && yoff < ylim && xvec xoff == yvec yoff then
           loop (xoff + 1) (yoff + 1)
         else (xoff, yoff)
       in
       loop xoff yoff
     in
     let xlim, ylim =
       let rec loop xlim ylim =
         if xlim > xoff && ylim > yoff && xvec (xlim - 1) == yvec (ylim - 1)
         then loop (xlim - 1) (ylim - 1)
         else (xlim, ylim)
       in
       loop xlim ylim
     in
     if xoff = xlim then
       for y = yoff to ylim - 1 do
         chng2.(bi.(y)) <- true
       done
     else if yoff = ylim then
       for x = xoff to xlim - 1 do
         chng1.(ai.(x)) <- true
       done
     else
       let d = diag fd bd sh xvec yvec xoff xlim yoff ylim in
       let b = bd.(sh + d) in
       loop xoff b yoff (b - d);
       loop b xlim (b - d) ylim
   in
   loop 0 n 0 m);
  (chng1, chng2)

(* [make_indexer a b] returns an array of index of items of [a] which
   are also present in [b]; this way, the main algorithm can skip items
   which, anyway, are different. This improves the speed much.
     The same time, this function updates the items of so that all
   equal items point to the same unique item. All items comparisons in
   the main algorithm can therefore be done with [==] instead of [=],
   what can improve speed much. *)
let make_indexer a b =
  let n = Array.length a in
  let htb = Hashtbl.create (10 * Array.length b) in
  Array.iteri
    (fun i e ->
      try b.(i) <- Hashtbl.find htb e with Not_found -> Hashtbl.add htb e e)
    b;
  let ai = Array.make n 0 in
  let k =
    let rec loop i k =
      if i = n then k
      else
        let k =
          try
            a.(i) <- Hashtbl.find htb a.(i);
            (* line found (since "Not_found" not raised) *)
            ai.(k) <- i;
            k + 1
          with Not_found -> k
        in
        loop (i + 1) k
    in
    loop 0 0
  in
  Array.sub ai 0 k

let f a b =
  let ai = make_indexer a b in
  let bi = make_indexer b a in
  let n = Array.length ai in
  let m = Array.length bi in
  diff_loop a ai b bi n m
