Yield tokens and offsets.

def token_offsets(tokens):
    """Yield tokens and offsets."""
    end_offset = 0
    previous_end_row = 0
    previous_end_column = 0
    for t in tokens:
        token_type = t[0]
        token_string = t[1]
        (start_row, start_column) = t[2]
        (end_row, end_column) = t[3]

        # Account for the whitespace between tokens.
        end_offset += start_column
        if previous_end_row == start_row:
            end_offset -= previous_end_column

        # Record the start offset of the token.
        start_offset = end_offset

        # Account for the length of the token itself.
        end_offset += len(token_string)

        yield (token_type,
               token_string,
               start_offset,
               end_offset)

        previous_end_row = end_row
        previous_end_column = end_column