(****
 *
 * Auxillary functions for working with srings.  The exported functions are
 * IndexOf and Reverse.  See their descriptions for details.
 *
 * The non-exported functions are helpers used by the exported functions.
 * 
 *)
module Strings;

  export IndexOf, Reverse;

  function IndexOf(base_string:string, search_string:string) -> integer =
    (
	if search_string = "" or base_string = ""
 	then 0
	else IndexOf1(base_string, search_string, 1);
    )
    
    description: (*
        Return the first position of search_string within base_string.  If
        search_string appears nowhere in base_string, return 0.  Also return 0
        if base_string and/or search_string is the empty string.
    *);
  end IndexOf;

  function IndexOf1(base_string:string, search_string:string, pos:integer) ->
    integer = (
	if pos > #base_string
	then 0
	else
	    if base_string[pos .. pos + #search_string - 1] = search_string
	    then pos
	    else IndexOf1(base_string, search_string, pos+1)
    )

    description: (*
        Work doer for IndexOf.
    *);
  end IndexOf1;

  function Reverse(s:string) -> string =
    (
	Reverse1(s, "")
    )

    description: (*
	Return the characterwise reversal of the given string.
    *);
  end Reverse;

  function Reverse1(s:string, rs:string) -> string =
    (
	if s = ""
	then rs
	else Reverse1(s[1 .. #s-1], rs + s[#s])
    )

    description: (*
        Work doer for Reverse.
    *);
  end Reverse1;

end Strings;