emitter.py

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# (c) Copyright Rosetta Commons Member Institutions.
# (c) This file is part of the Rosetta software suite and is made available under license.
# (c) The Rosetta software is developed by the contributing members of the Rosetta Commons.
# (c) For more information, see http://www.rosettacommons.org. Questions about this can be
# (c) addressed to University of Washington UW TechTransfer, email: license@u.washington.edu.

# Emitter expects events obeying the following grammar:
# stream ::= STREAM-START document* STREAM-END
# document ::= DOCUMENT-START node DOCUMENT-END
# node ::= SCALAR | sequence | mapping
# sequence ::= SEQUENCE-START node* SEQUENCE-END
# mapping ::= MAPPING-START (node node)* MAPPING-END

__all__ = ['Emitter', 'EmitterError']

from error import YAMLError
from events import *

import re

class EmitterError(YAMLError):
    pass

class ScalarAnalysis(object):
    def __init__(self, scalar, empty, multiline,
            allow_flow_plain, allow_block_plain,
            allow_single_quoted, allow_double_quoted,
            allow_block):
        self.scalar = scalar
        self.empty = empty
        self.multiline = multiline
        self.allow_flow_plain = allow_flow_plain
        self.allow_block_plain = allow_block_plain
        self.allow_single_quoted = allow_single_quoted
        self.allow_double_quoted = allow_double_quoted
        self.allow_block = allow_block

class Emitter(object):

    DEFAULT_TAG_PREFIXES = {
        u'!' : u'!',
        u'tag:yaml.org,2002:' : u'!!',
    }

    def __init__(self, stream, canonical=None, indent=None, width=None,
            allow_unicode=None, line_break=None):

        # The stream should have the methods `write` and possibly `flush`.
        self.stream = stream

        # Encoding can be overriden by STREAM-START.
        self.encoding = None

        # Emitter is a state machine with a stack of states to handle nested
        # structures.
        self.states = []
        self.state = self.expect_stream_start

        # Current event and the event queue.
        self.events = []
        self.event = None

        # The current indentation level and the stack of previous indents.
        self.indents = []
        self.indent = None

        # Flow level.
        self.flow_level = 0

        # Contexts.
        self.root_context = False
        self.sequence_context = False
        self.mapping_context = False
        self.simple_key_context = False

        # Characteristics of the last emitted character:
        #  - current position.
        #  - is it a whitespace?
        #  - is it an indention character
        #    (indentation space, '-', '?', or ':')?
        self.line = 0
        self.column = 0
        self.whitespace = True
        self.indention = True

        # Formatting details.
        self.canonical = canonical
        self.allow_unicode = allow_unicode
        self.best_indent = 2
        if indent and 1 < indent < 10:
            self.best_indent = indent
        self.best_width = 80
        if width and width > self.best_indent*2:
            self.best_width = width
        self.best_line_break = u'\n'
        if line_break in [u'\r', u'\n', u'\r\n']:
            self.best_line_break = line_break

        # Tag prefixes.
        self.tag_prefixes = None

        # Prepared anchor and tag.
        self.prepared_anchor = None
        self.prepared_tag = None

        # Scalar analysis and style.
        self.analysis = None
        self.style = None

    def emit(self, event):
        self.events.append(event)
        while not self.need_more_events():
            self.event = self.events.pop(0)
            self.state()
            self.event = None

    # In some cases, we wait for a few next events before emitting.

    def need_more_events(self):
        if not self.events:
            return True
        event = self.events[0]
        if isinstance(event, DocumentStartEvent):
            return self.need_events(1)
        elif isinstance(event, SequenceStartEvent):
            return self.need_events(2)
        elif isinstance(event, MappingStartEvent):
            return self.need_events(3)
        else:
            return False

    def need_events(self, count):
        level = 0
        for event in self.events[1:]:
            if isinstance(event, (DocumentStartEvent, CollectionStartEvent)):
                level += 1
            elif isinstance(event, (DocumentEndEvent, CollectionEndEvent)):
                level -= 1
            elif isinstance(event, StreamEndEvent):
                level = -1
            if level < 0:
                return False
        return (len(self.events) < count+1)

    def increase_indent(self, flow=False, indentless=False):
        self.indents.append(self.indent)
        if self.indent is None:
            if flow:
                self.indent = self.best_indent
            else:
                self.indent = 0
        elif not indentless:
            self.indent += self.best_indent

    # States.

    # Stream handlers.

    def expect_stream_start(self):
        if isinstance(self.event, StreamStartEvent):
            if self.event.encoding:
                self.encoding = self.event.encoding
            self.write_stream_start()
            self.state = self.expect_first_document_start
        else:
            raise EmitterError("expected StreamStartEvent, but got %s"
                    % self.event)

    def expect_nothing(self):
        raise EmitterError("expected nothing, but got %s" % self.event)

    # Document handlers.

    def expect_first_document_start(self):
        return self.expect_document_start(first=True)

    def expect_document_start(self, first=False):
        if isinstance(self.event, DocumentStartEvent):
            if self.event.version:
                version_text = self.prepare_version(self.event.version)
                self.write_version_directive(version_text)
            self.tag_prefixes = self.DEFAULT_TAG_PREFIXES.copy()
            if self.event.tags:
                handles = self.event.tags.keys()
                handles.sort()
                for handle in handles:
                    prefix = self.event.tags[handle]
                    self.tag_prefixes[prefix] = handle
                    handle_text = self.prepare_tag_handle(handle)
                    prefix_text = self.prepare_tag_prefix(prefix)
                    self.write_tag_directive(handle_text, prefix_text)
            implicit = (first and not self.event.explicit and not self.canonical
                    and not self.event.version and not self.event.tags
                    and not self.check_empty_document())
            if not implicit:
                self.write_indent()
                self.write_indicator(u'---', True)
                if self.canonical:
                    self.write_indent()
            self.state = self.expect_document_root
        elif isinstance(self.event, StreamEndEvent):
            self.write_stream_end()
            self.state = self.expect_nothing
        else:
            raise EmitterError("expected DocumentStartEvent, but got %s"
                    % self.event)

    def expect_document_end(self):
        if isinstance(self.event, DocumentEndEvent):
            self.write_indent()
            if self.event.explicit:
                self.write_indicator(u'...', True)
                self.write_indent()
            self.flush_stream()
            self.state = self.expect_document_start
        else:
            raise EmitterError("expected DocumentEndEvent, but got %s"
                    % self.event)

    def expect_document_root(self):
        self.states.append(self.expect_document_end)
        self.expect_node(root=True)

    # Node handlers.

    def expect_node(self, root=False, sequence=False, mapping=False,
            simple_key=False):
        self.root_context = root
        self.sequence_context = sequence
        self.mapping_context = mapping
        self.simple_key_context = simple_key
        if isinstance(self.event, AliasEvent):
            self.expect_alias()
        elif isinstance(self.event, (ScalarEvent, CollectionStartEvent)):
            self.process_anchor(u'&')
            self.process_tag()
            if isinstance(self.event, ScalarEvent):
                self.expect_scalar()
            elif isinstance(self.event, SequenceStartEvent):
                if self.flow_level or self.canonical or self.event.flow_style   \
                        or self.check_empty_sequence():
                    self.expect_flow_sequence()
                else:
                    self.expect_block_sequence()
            elif isinstance(self.event, MappingStartEvent):
                if self.flow_level or self.canonical or self.event.flow_style   \
                        or self.check_empty_mapping():
                    self.expect_flow_mapping()
                else:
                    self.expect_block_mapping()
        else:
            raise EmitterError("expected NodeEvent, but got %s" % self.event)

    def expect_alias(self):
        if self.event.anchor is None:
            raise EmitterError("anchor is not specified for alias")
        self.process_anchor(u'*')
        self.state = self.states.pop()

    def expect_scalar(self):
        self.increase_indent(flow=True)
        self.process_scalar()
        self.indent = self.indents.pop()
        self.state = self.states.pop()

    # Flow sequence handlers.

    def expect_flow_sequence(self):
        self.write_indicator(u'[', True, whitespace=True)
        self.flow_level += 1
        self.increase_indent(flow=True)
        self.state = self.expect_first_flow_sequence_item

    def expect_first_flow_sequence_item(self):
        if isinstance(self.event, SequenceEndEvent):
            self.indent = self.indents.pop()
            self.flow_level -= 1
            self.write_indicator(u']', False)
            self.state = self.states.pop()
        else:
            if self.canonical or self.column > self.best_width:
                self.write_indent()
            self.states.append(self.expect_flow_sequence_item)
            self.expect_node(sequence=True)

    def expect_flow_sequence_item(self):
        if isinstance(self.event, SequenceEndEvent):
            self.indent = self.indents.pop()
            self.flow_level -= 1
            if self.canonical:
                self.write_indicator(u',', False)
                self.write_indent()
            self.write_indicator(u']', False)
            self.state = self.states.pop()
        else:
            self.write_indicator(u',', False)
            if self.canonical or self.column > self.best_width:
                self.write_indent()
            self.states.append(self.expect_flow_sequence_item)
            self.expect_node(sequence=True)

    # Flow mapping handlers.

    def expect_flow_mapping(self):
        self.write_indicator(u'{', True, whitespace=True)
        self.flow_level += 1
        self.increase_indent(flow=True)
        self.state = self.expect_first_flow_mapping_key

    def expect_first_flow_mapping_key(self):
        if isinstance(self.event, MappingEndEvent):
            self.indent = self.indents.pop()
            self.flow_level -= 1
            self.write_indicator(u'}', False)
            self.state = self.states.pop()
        else:
            if self.canonical or self.column > self.best_width:
                self.write_indent()
            if not self.canonical and self.check_simple_key():
                self.states.append(self.expect_flow_mapping_simple_value)
                self.expect_node(mapping=True, simple_key=True)
            else:
                self.write_indicator(u'?', True)
                self.states.append(self.expect_flow_mapping_value)
                self.expect_node(mapping=True)

    def expect_flow_mapping_key(self):
        if isinstance(self.event, MappingEndEvent):
            self.indent = self.indents.pop()
            self.flow_level -= 1
            if self.canonical:
                self.write_indicator(u',', False)
                self.write_indent()
            self.write_indicator(u'}', False)
            self.state = self.states.pop()
        else:
            self.write_indicator(u',', False)
            if self.canonical or self.column > self.best_width:
                self.write_indent()
            if not self.canonical and self.check_simple_key():
                self.states.append(self.expect_flow_mapping_simple_value)
                self.expect_node(mapping=True, simple_key=True)
            else:
                self.write_indicator(u'?', True)
                self.states.append(self.expect_flow_mapping_value)
                self.expect_node(mapping=True)

    def expect_flow_mapping_simple_value(self):
        self.write_indicator(u':', False)
        self.states.append(self.expect_flow_mapping_key)
        self.expect_node(mapping=True)

    def expect_flow_mapping_value(self):
        if self.canonical or self.column > self.best_width:
            self.write_indent()
        self.write_indicator(u':', True)
        self.states.append(self.expect_flow_mapping_key)
        self.expect_node(mapping=True)

    # Block sequence handlers.

    def expect_block_sequence(self):
        indentless = (self.mapping_context and not self.indention)
        self.increase_indent(flow=False, indentless=indentless)
        self.state = self.expect_first_block_sequence_item

    def expect_first_block_sequence_item(self):
        return self.expect_block_sequence_item(first=True)

    def expect_block_sequence_item(self, first=False):
        if not first and isinstance(self.event, SequenceEndEvent):
            self.indent = self.indents.pop()
            self.state = self.states.pop()
        else:
            self.write_indent()
            self.write_indicator(u'-', True, indention=True)
            self.states.append(self.expect_block_sequence_item)
            self.expect_node(sequence=True)

    # Block mapping handlers.

    def expect_block_mapping(self):
        self.increase_indent(flow=False)
        self.state = self.expect_first_block_mapping_key

    def expect_first_block_mapping_key(self):
        return self.expect_block_mapping_key(first=True)

    def expect_block_mapping_key(self, first=False):
        if not first and isinstance(self.event, MappingEndEvent):
            self.indent = self.indents.pop()
            self.state = self.states.pop()
        else:
            self.write_indent()
            if self.check_simple_key():
                self.states.append(self.expect_block_mapping_simple_value)
                self.expect_node(mapping=True, simple_key=True)
            else:
                self.write_indicator(u'?', True, indention=True)
                self.states.append(self.expect_block_mapping_value)
                self.expect_node(mapping=True)

    def expect_block_mapping_simple_value(self):
        self.write_indicator(u':', False)
        self.states.append(self.expect_block_mapping_key)
        self.expect_node(mapping=True)

    def expect_block_mapping_value(self):
        self.write_indent()
        self.write_indicator(u':', True, indention=True)
        self.states.append(self.expect_block_mapping_key)
        self.expect_node(mapping=True)

    # Checkers.

    def check_empty_sequence(self):
        return (isinstance(self.event, SequenceStartEvent) and self.events
                and isinstance(self.events[0], SequenceEndEvent))

    def check_empty_mapping(self):
        return (isinstance(self.event, MappingStartEvent) and self.events
                and isinstance(self.events[0], MappingEndEvent))

    def check_empty_document(self):
        if not isinstance(self.event, DocumentStartEvent) or not self.events:
            return False
        event = self.events[0]
        return (isinstance(event, ScalarEvent) and event.anchor is None
                and event.tag is None and event.implicit and event.value == u'')

    def check_simple_key(self):
        length = 0
        if isinstance(self.event, NodeEvent) and self.event.anchor is not None:
            if self.prepared_anchor is None:
                self.prepared_anchor = self.prepare_anchor(self.event.anchor)
            length += len(self.prepared_anchor)
        if isinstance(self.event, (ScalarEvent, CollectionStartEvent))  \
                and self.event.tag is not None:
            if self.prepared_tag is None:
                self.prepared_tag = self.prepare_tag(self.event.tag)
            length += len(self.prepared_tag)
        if isinstance(self.event, ScalarEvent):
            if self.analysis is None:
                self.analysis = self.analyze_scalar(self.event.value)
            length += len(self.analysis.scalar)
        return (length < 128 and (isinstance(self.event, AliasEvent)
            or (isinstance(self.event, ScalarEvent)
                    and not self.analysis.empty and not self.analysis.multiline)
            or self.check_empty_sequence() or self.check_empty_mapping()))

    # Anchor, Tag, and Scalar processors.

    def process_anchor(self, indicator):
        if self.event.anchor is None:
            self.prepared_anchor = None
            return
        if self.prepared_anchor is None:
            self.prepared_anchor = self.prepare_anchor(self.event.anchor)
        if self.prepared_anchor:
            self.write_indicator(indicator+self.prepared_anchor, True)
        self.prepared_anchor = None

    def process_tag(self):
        tag = self.event.tag
        if isinstance(self.event, ScalarEvent):
            if self.style is None:
                self.style = self.choose_scalar_style()
            if ((not self.canonical or tag is None) and
                ((self.style == '' and self.event.implicit[0])
                        or (self.style != '' and self.event.implicit[1]))):
                self.prepared_tag = None
                return
            if self.event.implicit[0] and tag is None:
                tag = u'!'
                self.prepared_tag = None
        else:
            if (not self.canonical or tag is None) and self.event.implicit:
                self.prepared_tag = None
                return
        if tag is None:
            raise EmitterError("tag is not specified")
        if self.prepared_tag is None:
            self.prepared_tag = self.prepare_tag(tag)
        if self.prepared_tag:
            self.write_indicator(self.prepared_tag, True)
        self.prepared_tag = None

    def choose_scalar_style(self):
        if self.analysis is None:
            self.analysis = self.analyze_scalar(self.event.value)
        if self.event.style == '"' or self.canonical:
            return '"'
        if not self.event.style and self.event.implicit[0]:
            if (not (self.simple_key_context and
                    (self.analysis.empty or self.analysis.multiline))
                and (self.flow_level and self.analysis.allow_flow_plain
                    or (not self.flow_level and self.analysis.allow_block_plain))):
                return ''
        if self.event.style and self.event.style in '|>':
            if (not self.flow_level and not self.simple_key_context
                    and self.analysis.allow_block):
                return self.event.style
        if not self.event.style or self.event.style == '\'':
            if (self.analysis.allow_single_quoted and
                    not (self.simple_key_context and self.analysis.multiline)):
                return '\''
        return '"'

    def process_scalar(self):
        if self.analysis is None:
            self.analysis = self.analyze_scalar(self.event.value)
        if self.style is None:
            self.style = self.choose_scalar_style()
        split = (not self.simple_key_context)
        #if self.analysis.multiline and split    \
        #        and (not self.style or self.style in '\'\"'):
        #    self.write_indent()
        if self.style == '"':
            self.write_double_quoted(self.analysis.scalar, split)
        elif self.style == '\'':
            self.write_single_quoted(self.analysis.scalar, split)
        elif self.style == '>':
            self.write_folded(self.analysis.scalar)
        elif self.style == '|':
            self.write_literal(self.analysis.scalar)
        else:
            self.write_plain(self.analysis.scalar, split)
        self.analysis = None
        self.style = None

    # Analyzers.

    def prepare_version(self, version):
        major, minor = version
        if major != 1:
            raise EmitterError("unsupported YAML version: %d.%d" % (major, minor))
        return u'%d.%d' % (major, minor)

    def prepare_tag_handle(self, handle):
        if not handle:
            raise EmitterError("tag handle must not be empty")
        if handle[0] != u'!' or handle[-1] != u'!':
            raise EmitterError("tag handle must start and end with '!': %r"
                    % (handle.encode('utf-8')))
        for ch in handle[1:-1]:
            if not (u'0' <= ch <= u'9' or u'A' <= ch <= 'Z' or u'a' <= ch <= 'z'    \
                    or ch in u'-_'):
                raise EmitterError("invalid character %r in the tag handle: %r"
                        % (ch.encode('utf-8'), handle.encode('utf-8')))
        return handle

    def prepare_tag_prefix(self, prefix):
        if not prefix:
            raise EmitterError("tag prefix must not be empty")
        chunks = []
        start = end = 0
        if prefix[0] == u'!':
            end = 1
        while end < len(prefix):
            ch = prefix[end]
            if u'0' <= ch <= u'9' or u'A' <= ch <= 'Z' or u'a' <= ch <= 'z'  \
                    or ch in u'-;/?!:@&=+$,_.~*\'()[]':
                end += 1
            else:
                if start < end:
                    chunks.append(prefix[start:end])
                start = end = end+1
                data = ch.encode('utf-8')
                for ch in data:
                    chunks.append(u'%%%02X' % ord(ch))
        if start < end:
            chunks.append(prefix[start:end])
        return u''.join(chunks)

    def prepare_tag(self, tag):
        if not tag:
            raise EmitterError("tag must not be empty")
        if tag == u'!':
            return tag
        handle = None
        suffix = tag
        for prefix in self.tag_prefixes:
            if tag.startswith(prefix)   \
                    and (prefix == u'!' or len(prefix) < len(tag)):
                handle = self.tag_prefixes[prefix]
                suffix = tag[len(prefix):]
        chunks = []
        start = end = 0
        while end < len(suffix):
            ch = suffix[end]
            if u'0' <= ch <= u'9' or u'A' <= ch <= 'Z' or u'a' <= ch <= 'z'  \
                    or ch in u'-;/?:@&=+$,_.~*\'()[]'   \
                    or (ch == u'!' and handle != u'!'):
                end += 1
            else:
                if start < end:
                    chunks.append(suffix[start:end])
                start = end = end+1
                data = ch.encode('utf-8')
                for ch in data:
                    chunks.append(u'%%%02X' % ord(ch))
        if start < end:
            chunks.append(suffix[start:end])
        suffix_text = u''.join(chunks)
        if handle:
            return u'%s%s' % (handle, suffix_text)
        else:
            return u'!<%s>' % suffix_text

    def prepare_anchor(self, anchor):
        if not anchor:
            raise EmitterError("anchor must not be empty")
        for ch in anchor:
            if not (u'0' <= ch <= u'9' or u'A' <= ch <= 'Z' or u'a' <= ch <= 'z'    \
                    or ch in u'-_'):
                raise EmitterError("invalid character %r in the anchor: %r"
                        % (ch.encode('utf-8'), anchor.encode('utf-8')))
        return anchor

    def analyze_scalar(self, scalar):

        # Empty scalar is a special case.
        if not scalar:
            return ScalarAnalysis(scalar=scalar, empty=True, multiline=False,
                    allow_flow_plain=False, allow_block_plain=True,
                    allow_single_quoted=True, allow_double_quoted=True,
                    allow_block=False)

        # Indicators and special characters.
        block_indicators = False
        flow_indicators = False
        line_breaks = False
        special_characters = False

        # Whitespaces.
        inline_spaces = False          # non-space space+ non-space
        inline_breaks = False          # non-space break+ non-space
        leading_spaces = False         # ^ space+ (non-space | $)
        leading_breaks = False         # ^ break+ (non-space | $)
        trailing_spaces = False        # (^ | non-space) space+ $
        trailing_breaks = False        # (^ | non-space) break+ $
        inline_breaks_spaces = False   # non-space break+ space+ non-space
        mixed_breaks_spaces = False    # anything else

        # Check document indicators.
        if scalar.startswith(u'---') or scalar.startswith(u'...'):
            block_indicators = True
            flow_indicators = True

        # First character or preceded by a whitespace.
        preceeded_by_space = True

        # Last character or followed by a whitespace.
        followed_by_space = (len(scalar) == 1 or
                scalar[1] in u'\0 \t\r\n\x85\u2028\u2029')

        # The current series of whitespaces contain plain spaces.
        spaces = False

        # The current series of whitespaces contain line breaks.
        breaks = False

        # The current series of whitespaces contain a space followed by a
        # break.
        mixed = False

        # The current series of whitespaces start at the beginning of the
        # scalar.
        leading = False

        index = 0
        while index < len(scalar):
            ch = scalar[index]

            # Check for indicators.

            if index == 0:
                # Leading indicators are special characters.
                if ch in u'#,[]{}&*!|>\'\"%@`':
                    flow_indicators = True
                    block_indicators = True
                if ch in u'?:':
                    flow_indicators = True
                    if followed_by_space:
                        block_indicators = True
                if ch == u'-' and followed_by_space:
                    flow_indicators = True
                    block_indicators = True
            else:
                # Some indicators cannot appear within a scalar as well.
                if ch in u',?[]{}':
                    flow_indicators = True
                if ch == u':':
                    flow_indicators = True
                    if followed_by_space:
                        block_indicators = True
                if ch == u'#' and preceeded_by_space:
                    flow_indicators = True
                    block_indicators = True

            # Check for line breaks, special, and unicode characters.

            if ch in u'\n\x85\u2028\u2029':
                line_breaks = True
            if not (ch == u'\n' or u'\x20' <= ch <= u'\x7E'):
                if (ch == u'\x85' or u'\xA0' <= ch <= u'\uD7FF'
                        or u'\uE000' <= ch <= u'\uFFFD') and ch != u'\uFEFF':
                    unicode_characters = True
                    if not self.allow_unicode:
                        special_characters = True
                else:
                    special_characters = True

            # Spaces, line breaks, and how they are mixed. State machine.

            # Start or continue series of whitespaces.
            if ch in u' \n\x85\u2028\u2029':
                if spaces and breaks:
                    if ch != u' ':      # break+ (space+ break+)    => mixed
                        mixed = True
                elif spaces:
                    if ch != u' ':      # (space+ break+)   => mixed
                        breaks = True
                        mixed = True
                elif breaks:
                    if ch == u' ':      # break+ space+
                        spaces = True
                else:
                    leading = (index == 0)
                    if ch == u' ':      # space+
                        spaces = True
                    else:               # break+
                        breaks = True

            # Series of whitespaces ended with a non-space.
            elif spaces or breaks:
                if leading:
                    if spaces and breaks:
                        mixed_breaks_spaces = True
                    elif spaces:
                        leading_spaces = True
                    elif breaks:
                        leading_breaks = True
                else:
                    if mixed:
                        mixed_breaks_spaces = True
                    elif spaces and breaks:
                        inline_breaks_spaces = True
                    elif spaces:
                        inline_spaces = True
                    elif breaks:
                        inline_breaks = True
                spaces = breaks = mixed = leading = False

            # Series of whitespaces reach the end.
            if (spaces or breaks) and (index == len(scalar)-1):
                if spaces and breaks:
                    mixed_breaks_spaces = True
                elif spaces:
                    trailing_spaces = True
                    if leading:
                        leading_spaces = True
                elif breaks:
                    trailing_breaks = True
                    if leading:
                        leading_breaks = True
                spaces = breaks = mixed = leading = False

            # Prepare for the next character.
            index += 1
            preceeded_by_space = (ch in u'\0 \t\r\n\x85\u2028\u2029')
            followed_by_space = (index+1 >= len(scalar) or
                    scalar[index+1] in u'\0 \t\r\n\x85\u2028\u2029')

        # Let's decide what styles are allowed.
        allow_flow_plain = True
        allow_block_plain = True
        allow_single_quoted = True
        allow_double_quoted = True
        allow_block = True

        # Leading and trailing whitespace are bad for plain scalars. We also
        # do not want to mess with leading whitespaces for block scalars.
        if leading_spaces or leading_breaks or trailing_spaces:
            allow_flow_plain = allow_block_plain = allow_block = False

        # Trailing breaks are fine for block scalars, but unacceptable for
        # plain scalars.
        if trailing_breaks:
            allow_flow_plain = allow_block_plain = False

        # The combination of (space+ break+) is only acceptable for block
        # scalars.
        if inline_breaks_spaces:
            allow_flow_plain = allow_block_plain = allow_single_quoted = False

        # Mixed spaces and breaks, as well as special character are only
        # allowed for double quoted scalars.
        if mixed_breaks_spaces or special_characters:
            allow_flow_plain = allow_block_plain =  \
            allow_single_quoted = allow_block = False

        # We don't emit multiline plain scalars.
        if line_breaks:
            allow_flow_plain = allow_block_plain = False

        # Flow indicators are forbidden for flow plain scalars.
        if flow_indicators:
            allow_flow_plain = False

        # Block indicators are forbidden for block plain scalars.
        if block_indicators:
            allow_block_plain = False

        return ScalarAnalysis(scalar=scalar,
                empty=False, multiline=line_breaks,
                allow_flow_plain=allow_flow_plain,
                allow_block_plain=allow_block_plain,
                allow_single_quoted=allow_single_quoted,
                allow_double_quoted=allow_double_quoted,
                allow_block=allow_block)

    # Writers.

    def flush_stream(self):
        if hasattr(self.stream, 'flush'):
            self.stream.flush()

    def write_stream_start(self):
        # Write BOM if needed.
        if self.encoding and self.encoding.startswith('utf-16'):
            self.stream.write(u'\xFF\xFE'.encode(self.encoding))

    def write_stream_end(self):
        self.flush_stream()

    def write_indicator(self, indicator, need_whitespace,
            whitespace=False, indention=False):
        if self.whitespace or not need_whitespace:
            data = indicator
        else:
            data = u' '+indicator
        self.whitespace = whitespace
        self.indention = self.indention and indention
        self.column += len(data)
        if self.encoding:
            data = data.encode(self.encoding)
        self.stream.write(data)

    def write_indent(self):
        indent = self.indent or 0
        if not self.indention or self.column > indent   \
                or (self.column == indent and not self.whitespace):
            self.write_line_break()
        if self.column < indent:
            self.whitespace = True
            data = u' '*(indent-self.column)
            self.column = indent
            if self.encoding:
                data = data.encode(self.encoding)
            self.stream.write(data)

    def write_line_break(self, data=None):
        if data is None:
            data = self.best_line_break
        self.whitespace = True
        self.indention = True
        self.line += 1
        self.column = 0
        if self.encoding:
            data = data.encode(self.encoding)
        self.stream.write(data)

    def write_version_directive(self, version_text):
        data = u'%%YAML %s' % version_text
        if self.encoding:
            data = data.encode(self.encoding)
        self.stream.write(data)
        self.write_line_break()

    def write_tag_directive(self, handle_text, prefix_text):
        data = u'%%TAG %s %s' % (handle_text, prefix_text)
        if self.encoding:
            data = data.encode(self.encoding)
        self.stream.write(data)
        self.write_line_break()

    # Scalar streams.

    def write_single_quoted(self, text, split=True):
        self.write_indicator(u'\'', True)
        spaces = False
        breaks = False
        start = end = 0
        while end <= len(text):
            ch = None
            if end < len(text):
                ch = text[end]
            if spaces:
                if ch is None or ch != u' ':
                    if start+1 == end and self.column > self.best_width and split   \
                            and start != 0 and end != len(text):
                        self.write_indent()
                    else:
                        data = text[start:end]
                        self.column += len(data)
                        if self.encoding:
                            data = data.encode(self.encoding)
                        self.stream.write(data)
                    start = end
            elif breaks:
                if ch is None or ch not in u'\n\x85\u2028\u2029':
                    if text[start] == u'\n':
                        self.write_line_break()
                    for br in text[start:end]:
                        if br == u'\n':
                            self.write_line_break()
                        else:
                            self.write_line_break(br)
                    self.write_indent()
                    start = end
            else:
                if ch is None or ch in u' \n\x85\u2028\u2029' or ch == u'\'':
                    if start < end:
                        data = text[start:end]
                        self.column += len(data)
                        if self.encoding:
                            data = data.encode(self.encoding)
                        self.stream.write(data)
                        start = end
            if ch == u'\'':
                data = u'\'\''
                self.column += 2
                if self.encoding:
                    data = data.encode(self.encoding)
                self.stream.write(data)
                start = end + 1
            if ch is not None:
                spaces = (ch == u' ')
                breaks = (ch in u'\n\x85\u2028\u2029')
            end += 1
        self.write_indicator(u'\'', False)

    ESCAPE_REPLACEMENTS = {
        u'\0':      u'0',
        u'\x07':    u'a',
        u'\x08':    u'b',
        u'\x09':    u't',
        u'\x0A':    u'n',
        u'\x0B':    u'v',
        u'\x0C':    u'f',
        u'\x0D':    u'r',
        u'\x1B':    u'e',
        u'\"':      u'\"',
        u'\\':      u'\\',
        u'\x85':    u'N',
        u'\xA0':    u'_',
        u'\u2028':  u'L',
        u'\u2029':  u'P',
    }

    def write_double_quoted(self, text, split=True):
        self.write_indicator(u'"', True)
        start = end = 0
        while end <= len(text):
            ch = None
            if end < len(text):
                ch = text[end]
            if ch is None or ch in u'"\\\x85\u2028\u2029\uFEFF' \
                    or not (u'\x20' <= ch <= u'\x7E'
                        or (self.allow_unicode
                            and (u'\xA0' <= ch <= u'\uD7FF'
                                or u'\uE000' <= ch <= u'\uFFFD'))):
                if start < end:
                    data = text[start:end]
                    self.column += len(data)
                    if self.encoding:
                        data = data.encode(self.encoding)
                    self.stream.write(data)
                    start = end
                if ch is not None:
                    if ch in self.ESCAPE_REPLACEMENTS:
                        data = u'\\'+self.ESCAPE_REPLACEMENTS[ch]
                    elif ch <= u'\xFF':
                        data = u'\\x%02X' % ord(ch)
                    elif ch <= u'\uFFFF':
                        data = u'\\u%04X' % ord(ch)
                    else:
                        data = u'\\U%08X' % ord(ch)
                    self.column += len(data)
                    if self.encoding:
                        data = data.encode(self.encoding)
                    self.stream.write(data)
                    start = end+1
            if 0 < end < len(text)-1 and (ch == u' ' or start >= end)   \
                    and self.column+(end-start) > self.best_width and split:
                data = text[start:end]+u'\\'
                if start < end:
                    start = end
                self.column += len(data)
                if self.encoding:
                    data = data.encode(self.encoding)
                self.stream.write(data)
                self.write_indent()
                self.whitespace = False
                self.indention = False
                if text[start] == u' ':
                    data = u'\\'
                    self.column += len(data)
                    if self.encoding:
                        data = data.encode(self.encoding)
                    self.stream.write(data)
            end += 1
        self.write_indicator(u'"', False)

    def determine_chomp(self, text):
        tail = text[-2:]
        while len(tail) < 2:
            tail = u' '+tail
        if tail[-1] in u'\n\x85\u2028\u2029':
            if tail[-2] in u'\n\x85\u2028\u2029':
                return u'+'
            else:
                return u''
        else:
            return u'-'

    def write_folded(self, text):
        chomp = self.determine_chomp(text)
        self.write_indicator(u'>'+chomp, True)
        self.write_indent()
        leading_space = False
        spaces = False
        breaks = False
        start = end = 0
        while end <= len(text):
            ch = None
            if end < len(text):
                ch = text[end]
            if breaks:
                if ch is None or ch not in u'\n\x85\u2028\u2029':
                    if not leading_space and ch is not None and ch != u' '  \
                            and text[start] == u'\n':
                        self.write_line_break()
                    leading_space = (ch == u' ')
                    for br in text[start:end]:
                        if br == u'\n':
                            self.write_line_break()
                        else:
                            self.write_line_break(br)
                    if ch is not None:
                        self.write_indent()
                    start = end
            elif spaces:
                if ch != u' ':
                    if start+1 == end and self.column > self.best_width:
                        self.write_indent()
                    else:
                        data = text[start:end]
                        self.column += len(data)
                        if self.encoding:
                            data = data.encode(self.encoding)
                        self.stream.write(data)
                    start = end
            else:
                if ch is None or ch in u' \n\x85\u2028\u2029':
                    data = text[start:end]
                    if self.encoding:
                        data = data.encode(self.encoding)
                    self.stream.write(data)
                    if ch is None:
                        self.write_line_break()
                    start = end
            if ch is not None:
                breaks = (ch in u'\n\x85\u2028\u2029')
                spaces = (ch == u' ')
            end += 1

    def write_literal(self, text):
        chomp = self.determine_chomp(text)
        self.write_indicator(u'|'+chomp, True)
        self.write_indent()
        breaks = False
        start = end = 0
        while end <= len(text):
            ch = None
            if end < len(text):
                ch = text[end]
            if breaks:
                if ch is None or ch not in u'\n\x85\u2028\u2029':
                    for br in text[start:end]:
                        if br == u'\n':
                            self.write_line_break()
                        else:
                            self.write_line_break(br)
                    if ch is not None:
                        self.write_indent()
                    start = end
            else:
                if ch is None or ch in u'\n\x85\u2028\u2029':
                    data = text[start:end]
                    if self.encoding:
                        data = data.encode(self.encoding)
                    self.stream.write(data)
                    if ch is None:
                        self.write_line_break()
                    start = end
            if ch is not None:
                breaks = (ch in u'\n\x85\u2028\u2029')
            end += 1

    def write_plain(self, text, split=True):
        if not text:
            return
        if not self.whitespace:
            data = u' '
            self.column += len(data)
            if self.encoding:
                data = data.encode(self.encoding)
            self.stream.write(data)
        self.writespace = False
        self.indention = False
        spaces = False
        breaks = False
        start = end = 0
        while end <= len(text):
            ch = None
            if end < len(text):
                ch = text[end]
            if spaces:
                if ch != u' ':
                    if start+1 == end and self.column > self.best_width and split:
                        self.write_indent()
                        self.writespace = False
                        self.indention = False
                    else:
                        data = text[start:end]
                        self.column += len(data)
                        if self.encoding:
                            data = data.encode(self.encoding)
                        self.stream.write(data)
                    start = end
            elif breaks:
                if ch not in u'\n\x85\u2028\u2029':
                    if text[start] == u'\n':
                        self.write_line_break()
                    for br in text[start:end]:
                        if br == u'\n':
                            self.write_line_break()
                        else:
                            self.write_line_break(br)
                    self.write_indent()
                    self.whitespace = False
                    self.indention = False
                    start = end
            else:
                if ch is None or ch in u' \n\x85\u2028\u2029':
                    data = text[start:end]
                    self.column += len(data)
                    if self.encoding:
                        data = data.encode(self.encoding)
                    self.stream.write(data)
                    start = end
            if ch is not None:
                spaces = (ch == u' ')
                breaks = (ch in u'\n\x85\u2028\u2029')
            end += 1