// file:    Parse.java
// author:  Robert Keller
// purpose: Parse function object for kwic
// note:    Could also use a StringTokenizer instead of the methods here.

class Parse implements Function1
{
static private Character blank = new Character(' ');

/**
 * Parse a title String.
 * @param title the String to be parsed
 * @return a list of words in the title
 */

public Object apply(Object title)
  {
  return parse1(OpenList.explode((String)title));
  }


/**
 * parse1 returns a list of words from an original String.  The words
 * are formed from consecutive characters, as separated by blanks in the
 * String.
 * @param Chars characters of the String to be parsed
 * @return a list of words in the title
 */

// rex: parse1([]) => [];
// rex: parse1([' ' | Chars]) => parse1(Chars);
// rex: parse1([NonBlank | Chars]) => parse2([NonBlank], Chars);

OpenList parse1(OpenList Chars)
  {
  if( Chars.isEmpty() ) return OpenList.nil;

  if( Chars.first().equals(blank) ) return parse1(Chars.rest());

  return parse2(OpenList.list(Chars.first()), Chars.rest());
  }


/**
 * parse2 is an auxiliary method for parse1.  It collects non-blank characters
 * in an accumulator.  When a blank is found, or the end of the list of
 * characters is reached, it reverses the accumulator and returns a list
 * beginning with that, followed by the result of calling parse1 recursively.
 * @param Acc accumulated characters
 * @param Chars characters of the String to be parsed
 * @return a list of words in the title
 */

// rex: parse2(Acc, []) => [reverse(implode(Acc))];
// rex: parse2(Acc, [' ' | Chars]) => [reverse(implode(Acc)) | parse1(Chars)];
// rex: parse2(Acc, [Char | Chars]) => parse2([Char | Acc], Chars);

OpenList parse2(OpenList Acc, OpenList Chars)
  {
  if( Chars.isEmpty() ) return OpenList.list(Acc.reverse().implode());

  if( Chars.first().equals(blank) ) 
      return OpenList.cons(Acc.reverse().implode(), parse1(Chars.rest()));

  return parse2(OpenList.cons(Chars.first(), Acc), Chars.rest());
  }
}