# Natural Language Toolkit: Chunk parsing API
#
# Copyright (C) 2001-2010 NLTK Project
# Author: Edward Loper <edloper@gradient.cis.upenn.edu>
#         Steven Bird <sb@csse.unimelb.edu.au> (minor additions)
# URL: <http://www.nltk.org/>
# For license information, see LICENSE.TXT

##//////////////////////////////////////////////////////
##  Chunk Parser Interface
##//////////////////////////////////////////////////////

from nltk.parse import ParserI
import nltk

class ChunkParserI(ParserI):
    """
    A processing interface for identifying non-overlapping groups in
    unrestricted text.  Typically, chunk parsers are used to find base
    syntactic constituants, such as base noun phrases.  Unlike
    L{ParserI}, C{ChunkParserI} guarantees that the C{parse} method
    will always generate a parse.
    """
    def parse(self, tokens):
        """
        @return: the best chunk structure for the given tokens
        and return a tree.
        
        @param tokens: The list of (word, tag) tokens to be chunked.
        @type tokens: C{list} of L{tuple}
        @rtype: L{Tree}
        """
        assert 0, "ChunkParserI is an abstract interface"

    def evaluate(self, gold):
        """
        Score the accuracy of the chunker against the gold standard.
        Remove the chunking the gold standard text, rechunk it using
        the chunker, and return a L{ChunkScore<nltk.chunk.util.ChunkScore>}
        object reflecting the performance of this chunk peraser.

        @type gold: C{list} of L{Tree}
        @param gold: The list of chunked sentences to score the chunker on.
        @rtype:  L{ChunkScore<nltk.chunk.util.ChunkScore>}
        """
        chunkscore = nltk.chunk.util.ChunkScore()
        for correct in gold:
            chunkscore.score(correct, self.parse(correct.leaves()))
        return chunkscore
        
