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# CSV I/O
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import csv
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import StringIO
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import strings
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import util
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delims = ',\t`'
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tsv_delim = '\t'
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escape = '\\'
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ending_placeholder = r'\n'
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def is_tsv(dialect): return dialect.delimiter == tsv_delim
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def sniff(line):
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    '''Automatically detects the dialect'''
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    dialect = csv.Sniffer().sniff(line, delims)
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    # TSVs usually don't quote fields (nor doublequote embedded quotes)
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    if is_tsv(dialect): dialect.quoting = csv.QUOTE_NONE
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    else: dialect.doublequote = True # Sniffer doesn't turn this on by default
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    return dialect
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def stream_info(stream, parse_header=False):
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    '''Automatically detects the dialect based on the header line
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    @return NamedTuple {header_line, header, dialect}'''
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    info = util.NamedTuple()
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    info.header_line = stream.readline()
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    info.header = None
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    if info.header_line != '':
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        info.dialect = sniff(info.header_line)
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        if parse_header:
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            info.header = reader_class(info.dialect)(
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                StringIO.StringIO(info.header_line), info.dialect).next()
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    else: info.dialect = None # line of '' indicates EOF = empty stream
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    return info
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class TsvReader:
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    '''Unlike csv.reader, for TSVs, interprets \ as escaping a line ending but
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    ignores it before everything else (e.g. \N for NULL). Also interprets the
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    '\n' escape sequence as a newline.'''
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    def __init__(self, stream, dialect):
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        assert is_tsv(dialect)
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        self.stream = stream
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        self.dialect = dialect
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    def __iter__(self): return self
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    def next(self):
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        record = ''
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        ending = None
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        while True:
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            line = self.stream.readline()
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            if line == '': raise StopIteration
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            raw_contents, ending = strings.extract_line_ending(line)
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            contents = strings.remove_suffix(escape, raw_contents)
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            record += contents
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            if len(contents) == len(raw_contents): break # no line continuation
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            record += ending_placeholder
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        row = csv.reader(StringIO.StringIO(record), self.dialect).next()
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        return [v.replace(ending_placeholder, '\n') for v in row]
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def reader_class(dialect):
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    if is_tsv(dialect): return TsvReader
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    else: return csv.reader
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def make_reader(stream, dialect): return reader_class(dialect)(stream, dialect)
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def reader_and_header(stream):
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    '''Automatically detects the dialect based on the header line
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    @return tuple (reader, header)'''
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    info = stream_info(stream, parse_header=True)
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    return (make_reader(stream, info.dialect), info.header)
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##### csv modifications
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# Note that these methods only work on *instances* of Dialect classes
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csv.Dialect.__eq__ = lambda self, other: self.__dict__ == other.__dict__
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csv.Dialect.__ne__ = lambda self, other: not (self == other)
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##### Row filters
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class Filter:
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    '''Wraps a reader, filtering each row'''
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    def __init__(self, filter_, reader):
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        self.reader = reader
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        self.filter = filter_
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    def __iter__(self): return self
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    def next(self): return self.filter(self.reader.next())
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std_nulls = [r'\N']
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empty_nulls = [''] + std_nulls
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class NullFilter(Filter):
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    '''Translates special string values to None'''
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    def __init__(self, reader, nulls=std_nulls):
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        map_ = dict.fromkeys(nulls, None)
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        def filter_(row): return [map_.get(v, v) for v in row]
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        Filter.__init__(self, filter_, reader)
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class StripFilter(Filter):
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    '''Strips whitespace'''
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    def __init__(self, reader):
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        def filter_(row): return [v.strip() for v in row]
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        Filter.__init__(self, filter_, reader)
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