#<p>
#   Read from a text file with the ability to <i>push back</i>
#   input lines.  Multiple lines may be pushed back onto the
#   file.
#</p>
#<p>
# <b>Author:</b> Steve Wampler (<i>sbw@tapestry.tucson.az.us</i>)
#</p>
#<p>
#  This file is in the <i>public domain</i>.
#</p>

package util

import lang

#<p>
#  Reads in lines from a file, but allows lines to be pushed back
#   back into the input stream for rereading.  There is no limit
#   (aside from memory constraints) on the number of lines pushed back.
#</p>
class PushBack : Object(f,        # Input file (defaults to <tt>&input</tt>)
                        pbBuffer, # pushback buffer
                        reader    # hook to real <tt>read</tt> procedure
                       )

   #<p>
   #   Read in the next line.  Reads in any pushed back lines first
   #   before reading a line from the file.
   #</p>
   method read()
      return ::pop(pbBuffer) | reader(f)
   end

   #<p>
   #   Push a line back onto the input stream.
   #</p>
   method pushback(line)  # line to push back onto input stream
      ::push(pbBuffer, line)
   end

#<p>
#   Provide an instance of a pushback file reader.
#</p>
initially (aFile) # File to read from
   /aFile := &input
   f := aFile
   pbBuffer := []
   reader := ::proc("read")
end