"""
Iterator based sre token scanner
"""
import sre_parse, sre_compile, sre_constants
from sre_constants import BRANCH, SUBPATTERN
import re

__all__ = ['Scanner', 'pattern']

FLAGS = (re.VERBOSE | re.MULTILINE | re.DOTALL)
class Scanner(object):
    def __init__(self, lexicon, flags=FLAGS):
        self.actions = [None]
        # combine phrases into a compound pattern
        s = sre_parse.Pattern()
        s.flags = flags
        p = []
        for idx, token in enumerate(lexicon):
            phrase = token.pattern
            try:
                subpattern = sre_parse.SubPattern(s,
                    [(SUBPATTERN, (idx + 1, sre_parse.parse(phrase, flags)))])
            except sre_constants.error:
                raise
            p.append(subpattern)
            self.actions.append(token)

        p = sre_parse.SubPattern(s, [(BRANCH, (None, p))])
        self.scanner = sre_compile.compile(p)


    def iterscan(self, string, idx=0, context=None):
        """
        Yield match, end_idx for each match
        """
        match = self.scanner.scanner(string, idx).match
        actions = self.actions
        lastend = idx
        end = len(string)
        while True:
            m = match()
            if m is None:
                break
            matchbegin, matchend = m.span()
            if lastend == matchend:
                break
            action = actions[m.lastindex]
            if action is not None:
                rval, next_pos = action(m, context)
                if next_pos is not None and next_pos != matchend:
                    # "fast forward" the scanner
                    matchend = next_pos
                    match = self.scanner.scanner(string, matchend).match
                yield rval, matchend
            lastend = matchend
            
def pattern(pattern, flags=FLAGS):
    def decorator(fn):
        fn.pattern = pattern
        fn.regex = re.compile(pattern, flags)
        return fn
    return decorator
