scannerc.go

package yaml

import (
	"bytes"
	"fmt"
)

// Introduction
// ************
//
// The following notes assume that you are familiar with the YAML specification
// (http://yaml.org/spec/1.2/spec.html).  We mostly follow it, although in
// some cases we are less restrictive that it requires.
//
// The process of transforming a YAML stream into a sequence of events is
// divided on two steps: Scanning and Parsing.
//
// The Scanner transforms the input stream into a sequence of tokens, while the
// parser transform the sequence of tokens produced by the Scanner into a
// sequence of parsing events.
//
// The Scanner is rather clever and complicated. The Parser, on the contrary,
// is a straightforward implementation of a recursive-descendant parser (or,
// LL(1) parser, as it is usually called).
//
// Actually there are two issues of Scanning that might be called "clever", the
// rest is quite straightforward.  The issues are "block collection start" and
// "simple keys".  Both issues are explained below in details.
//
// Here the Scanning step is explained and implemented.  We start with the list
// of all the tokens produced by the Scanner together with short descriptions.
//
// Now, tokens:
//
//      STREAM-START(encoding)          # The stream start.
//      STREAM-END                      # The stream end.
//      VERSION-DIRECTIVE(major,minor)  # The '%YAML' directive.
//      TAG-DIRECTIVE(handle,prefix)    # The '%TAG' directive.
//      DOCUMENT-START                  # '---'
//      DOCUMENT-END                    # '...'
//      BLOCK-SEQUENCE-START            # Indentation increase denoting a block
//      BLOCK-MAPPING-START             # sequence or a block mapping.
//      BLOCK-END                       # Indentation decrease.
//      FLOW-SEQUENCE-START             # '['
//      FLOW-SEQUENCE-END               # ']'
//      BLOCK-SEQUENCE-START            # '{'
//      BLOCK-SEQUENCE-END              # '}'
//      BLOCK-ENTRY                     # '-'
//      FLOW-ENTRY                      # ','
//      KEY                             # '?' or nothing (simple keys).
//      VALUE                           # ':'
//      ALIAS(anchor)                   # '*anchor'
//      ANCHOR(anchor)                  # '&anchor'
//      TAG(handle,suffix)              # '!handle!suffix'
//      SCALAR(value,style)             # A scalar.
//
// The following two tokens are "virtual" tokens denoting the beginning and the
// end of the stream:
//
//      STREAM-START(encoding)
//      STREAM-END
//
// We pass the information about the input stream encoding with the
// STREAM-START token.
//
// The next two tokens are responsible for tags:
//
//      VERSION-DIRECTIVE(major,minor)
//      TAG-DIRECTIVE(handle,prefix)
//
// Example:
//
//      %YAML   1.1
//      %TAG    !   !foo
//      %TAG    !yaml!  tag:yaml.org,2002:
//      ---
//
// The correspoding sequence of tokens:
//
//      STREAM-START(utf-8)
//      VERSION-DIRECTIVE(1,1)
//      TAG-DIRECTIVE("!","!foo")
//      TAG-DIRECTIVE("!yaml","tag:yaml.org,2002:")
//      DOCUMENT-START
//      STREAM-END
//
// Note that the VERSION-DIRECTIVE and TAG-DIRECTIVE tokens occupy a whole
// line.
//
// The document start and end indicators are represented by:
//
//      DOCUMENT-START
//      DOCUMENT-END
//
// Note that if a YAML stream contains an implicit document (without '---'
// and '...' indicators), no DOCUMENT-START and DOCUMENT-END tokens will be
// produced.
//
// In the following examples, we present whole documents together with the
// produced tokens.
//
//      1. An implicit document:
//
//          'a scalar'
//
//      Tokens:
//
//          STREAM-START(utf-8)
//          SCALAR("a scalar",single-quoted)
//          STREAM-END
//
//      2. An explicit document:
//
//          ---
//          'a scalar'
//          ...
//
//      Tokens:
//
//          STREAM-START(utf-8)
//          DOCUMENT-START
//          SCALAR("a scalar",single-quoted)
//          DOCUMENT-END
//          STREAM-END
//
//      3. Several documents in a stream:
//
//          'a scalar'
//          ---
//          'another scalar'
//          ---
//          'yet another scalar'
//
//      Tokens:
//
//          STREAM-START(utf-8)
//          SCALAR("a scalar",single-quoted)
//          DOCUMENT-START
//          SCALAR("another scalar",single-quoted)
//          DOCUMENT-START
//          SCALAR("yet another scalar",single-quoted)
//          STREAM-END
//
// We have already introduced the SCALAR token above.  The following tokens are
// used to describe aliases, anchors, tag, and scalars:
//
//      ALIAS(anchor)
//      ANCHOR(anchor)
//      TAG(handle,suffix)
//      SCALAR(value,style)
//
// The following series of examples illustrate the usage of these tokens:
//
//      1. A recursive sequence:
//
//          &A [ *A ]
//
//      Tokens:
//
//          STREAM-START(utf-8)
//          ANCHOR("A")
//          FLOW-SEQUENCE-START
//          ALIAS("A")
//          FLOW-SEQUENCE-END
//          STREAM-END
//
//      2. A tagged scalar:
//
//          !!float "3.14"  # A good approximation.
//
//      Tokens:
//
//          STREAM-START(utf-8)
//          TAG("!!","float")
//          SCALAR("3.14",double-quoted)
//          STREAM-END
//
//      3. Various scalar styles:
//
//          --- # Implicit empty plain scalars do not produce tokens.
//          --- a plain scalar
//          --- 'a single-quoted scalar'
//          --- "a double-quoted scalar"
//          --- |-
//            a literal scalar
//          --- >-
//            a folded
//            scalar
//
//      Tokens:
//
//          STREAM-START(utf-8)
//          DOCUMENT-START
//          DOCUMENT-START
//          SCALAR("a plain scalar",plain)
//          DOCUMENT-START
//          SCALAR("a single-quoted scalar",single-quoted)
//          DOCUMENT-START
//          SCALAR("a double-quoted scalar",double-quoted)
//          DOCUMENT-START
//          SCALAR("a literal scalar",literal)
//          DOCUMENT-START
//          SCALAR("a folded scalar",folded)
//          STREAM-END
//
// Now it's time to review collection-related tokens. We will start with
// flow collections:
//
//      FLOW-SEQUENCE-START
//      FLOW-SEQUENCE-END
//      FLOW-MAPPING-START
//      FLOW-MAPPING-END
//      FLOW-ENTRY
//      KEY
//      VALUE
//
// The tokens FLOW-SEQUENCE-START, FLOW-SEQUENCE-END, FLOW-MAPPING-START, and
// FLOW-MAPPING-END represent the indicators '[', ']', '{', and '}'
// correspondingly.  FLOW-ENTRY represent the ',' indicator.  Finally the
// indicators '?' and ':', which are used for denoting mapping keys and values,
// are represented by the KEY and VALUE tokens.
//
// The following examples show flow collections:
//
//      1. A flow sequence:
//
//          [item 1, item 2, item 3]
//
//      Tokens:
//
//          STREAM-START(utf-8)
//          FLOW-SEQUENCE-START
//          SCALAR("item 1",plain)
//          FLOW-ENTRY
//          SCALAR("item 2",plain)
//          FLOW-ENTRY
//          SCALAR("item 3",plain)
//          FLOW-SEQUENCE-END
//          STREAM-END
//
//      2. A flow mapping:
//
//          {
//              a simple key: a value,  # Note that the KEY token is produced.
//              ? a complex key: another value,
//          }
//
//      Tokens:
//
//          STREAM-START(utf-8)
//          FLOW-MAPPING-START
//          KEY
//          SCALAR("a simple key",plain)
//          VALUE
//          SCALAR("a value",plain)
//          FLOW-ENTRY
//          KEY
//          SCALAR("a complex key",plain)
//          VALUE
//          SCALAR("another value",plain)
//          FLOW-ENTRY
//          FLOW-MAPPING-END
//          STREAM-END
//
// A simple key is a key which is not denoted by the '?' indicator.  Note that
// the Scanner still produce the KEY token whenever it encounters a simple key.
//
// For scanning block collections, the following tokens are used (note that we
// repeat KEY and VALUE here):
//
//      BLOCK-SEQUENCE-START
//      BLOCK-MAPPING-START
//      BLOCK-END
//      BLOCK-ENTRY
//      KEY
//      VALUE
//
// The tokens BLOCK-SEQUENCE-START and BLOCK-MAPPING-START denote indentation
// increase that precedes a block collection (cf. the INDENT token in Python).
// The token BLOCK-END denote indentation decrease that ends a block collection
// (cf. the DEDENT token in Python).  However YAML has some syntax pecularities
// that makes detections of these tokens more complex.
//
// The tokens BLOCK-ENTRY, KEY, and VALUE are used to represent the indicators
// '-', '?', and ':' correspondingly.
//
// The following examples show how the tokens BLOCK-SEQUENCE-START,
// BLOCK-MAPPING-START, and BLOCK-END are emitted by the Scanner:
//
//      1. Block sequences:
//
//          - item 1
//          - item 2
//          -
//            - item 3.1
//            - item 3.2
//          -
//            key 1: value 1
//            key 2: value 2
//
//      Tokens:
//
//          STREAM-START(utf-8)
//          BLOCK-SEQUENCE-START
//          BLOCK-ENTRY
//          SCALAR("item 1",plain)
//          BLOCK-ENTRY
//          SCALAR("item 2",plain)
//          BLOCK-ENTRY
//          BLOCK-SEQUENCE-START
//          BLOCK-ENTRY
//          SCALAR("item 3.1",plain)
//          BLOCK-ENTRY
//          SCALAR("item 3.2",plain)
//          BLOCK-END
//          BLOCK-ENTRY
//          BLOCK-MAPPING-START
//          KEY
//          SCALAR("key 1",plain)
//          VALUE
//          SCALAR("value 1",plain)
//          KEY
//          SCALAR("key 2",plain)
//          VALUE
//          SCALAR("value 2",plain)
//          BLOCK-END
//          BLOCK-END
//          STREAM-END
//
//      2. Block mappings:
//
//          a simple key: a value   # The KEY token is produced here.
//          ? a complex key
//          : another value
//          a mapping:
//            key 1: value 1
//            key 2: value 2
//          a sequence:
//            - item 1
//            - item 2
//
//      Tokens:
//
//          STREAM-START(utf-8)
//          BLOCK-MAPPING-START
//          KEY
//          SCALAR("a simple key",plain)
//          VALUE
//          SCALAR("a value",plain)
//          KEY
//          SCALAR("a complex key",plain)
//          VALUE
//          SCALAR("another value",plain)
//          KEY
//          SCALAR("a mapping",plain)
//          BLOCK-MAPPING-START
//          KEY
//          SCALAR("key 1",plain)
//          VALUE
//          SCALAR("value 1",plain)
//          KEY
//          SCALAR("key 2",plain)
//          VALUE
//          SCALAR("value 2",plain)
//          BLOCK-END
//          KEY
//          SCALAR("a sequence",plain)
//          VALUE
//          BLOCK-SEQUENCE-START
//          BLOCK-ENTRY
//          SCALAR("item 1",plain)
//          BLOCK-ENTRY
//          SCALAR("item 2",plain)
//          BLOCK-END
//          BLOCK-END
//          STREAM-END
//
// YAML does not always require to start a new block collection from a new
// line.  If the current line contains only '-', '?', and ':' indicators, a new
// block collection may start at the current line.  The following examples
// illustrate this case:
//
//      1. Collections in a sequence:
//
//          - - item 1
//            - item 2
//          - key 1: value 1
//            key 2: value 2
//          - ? complex key
//            : complex value
//
//      Tokens:
//
//          STREAM-START(utf-8)
//          BLOCK-SEQUENCE-START
//          BLOCK-ENTRY
//          BLOCK-SEQUENCE-START
//          BLOCK-ENTRY
//          SCALAR("item 1",plain)
//          BLOCK-ENTRY
//          SCALAR("item 2",plain)
//          BLOCK-END
//          BLOCK-ENTRY
//          BLOCK-MAPPING-START
//          KEY
//          SCALAR("key 1",plain)
//          VALUE
//          SCALAR("value 1",plain)
//          KEY
//          SCALAR("key 2",plain)
//          VALUE
//          SCALAR("value 2",plain)
//          BLOCK-END
//          BLOCK-ENTRY
//          BLOCK-MAPPING-START
//          KEY
//          SCALAR("complex key")
//          VALUE
//          SCALAR("complex value")
//          BLOCK-END
//          BLOCK-END
//          STREAM-END
//
//      2. Collections in a mapping:
//
//          ? a sequence
//          : - item 1
//            - item 2
//          ? a mapping
//          : key 1: value 1
//            key 2: value 2
//
//      Tokens:
//
//          STREAM-START(utf-8)
//          BLOCK-MAPPING-START
//          KEY
//          SCALAR("a sequence",plain)
//          VALUE
//          BLOCK-SEQUENCE-START
//          BLOCK-ENTRY
//          SCALAR("item 1",plain)
//          BLOCK-ENTRY
//          SCALAR("item 2",plain)
//          BLOCK-END
//          KEY
//          SCALAR("a mapping",plain)
//          VALUE
//          BLOCK-MAPPING-START
//          KEY
//          SCALAR("key 1",plain)
//          VALUE
//          SCALAR("value 1",plain)
//          KEY
//          SCALAR("key 2",plain)
//          VALUE
//          SCALAR("value 2",plain)
//          BLOCK-END
//          BLOCK-END
//          STREAM-END
//
// YAML also permits non-indented sequences if they are included into a block
// mapping.  In this case, the token BLOCK-SEQUENCE-START is not produced:
//
//      key:
//      - item 1    # BLOCK-SEQUENCE-START is NOT produced here.
//      - item 2
//
// Tokens:
//
//      STREAM-START(utf-8)
//      BLOCK-MAPPING-START
//      KEY
//      SCALAR("key",plain)
//      VALUE
//      BLOCK-ENTRY
//      SCALAR("item 1",plain)
//      BLOCK-ENTRY
//      SCALAR("item 2",plain)
//      BLOCK-END
//

// Ensure that the buffer contains the required number of characters.
// Return true on success, false on failure (reader error or memory error).
func cache(parser *yaml_parser_t, length int) bool {
	// [Go] This was inlined: !cache(A, B) -> unread < B && !update(A, B)
	return parser.unread >= length || yaml_parser_update_buffer(parser, length)
}

// Advance the buffer pointer.
func skip(parser *yaml_parser_t) {
	parser.mark.index++
	parser.mark.column++
	parser.unread--
	parser.buffer_pos += width(parser.buffer[parser.buffer_pos])
}

func skip_line(parser *yaml_parser_t) {
	if is_crlf(parser.buffer, parser.buffer_pos) {
		parser.mark.index += 2
		parser.mark.column = 0
		parser.mark.line++
		parser.unread -= 2
		parser.buffer_pos += 2
	} else if is_break(parser.buffer, parser.buffer_pos) {
		parser.mark.index++
		parser.mark.column = 0
		parser.mark.line++
		parser.unread--
		parser.buffer_pos += width(parser.buffer[parser.buffer_pos])
	}
}

// Copy a character to a string buffer and advance pointers.
func read(parser *yaml_parser_t, s []byte) []byte {
	w := width(parser.buffer[parser.buffer_pos])
	if w == 0 {
		panic("invalid character sequence")
	}
	if len(s) == 0 {
		s = make([]byte, 0, 32)
	}
	if w == 1 && len(s)+w <= cap(s) {
		s = s[:len(s)+1]
		s[len(s)-1] = parser.buffer[parser.buffer_pos]
		parser.buffer_pos++
	} else {
		s = append(s, parser.buffer[parser.buffer_pos:parser.buffer_pos+w]...)
		parser.buffer_pos += w
	}
	parser.mark.index++
	parser.mark.column++
	parser.unread--
	return s
}

// Copy a line break character to a string buffer and advance pointers.
func read_line(parser *yaml_parser_t, s []byte) []byte {
	buf := parser.buffer
	pos := parser.buffer_pos
	switch {
	case buf[pos] == '\r' && buf[pos+1] == '\n':
		// CR LF . LF
		s = append(s, '\n')
		parser.buffer_pos += 2
		parser.mark.index++
		parser.unread--
	case buf[pos] == '\r' || buf[pos] == '\n':
		// CR|LF . LF
		s = append(s, '\n')
		parser.buffer_pos += 1
	case buf[pos] == '\xC2' && buf[pos+1] == '\x85':
		// NEL . LF
		s = append(s, '\n')
		parser.buffer_pos += 2
	case buf[pos] == '\xE2' && buf[pos+1] == '\x80' && (buf[pos+2] == '\xA8' || buf[pos+2] == '\xA9'):
		// LS|PS . LS|PS
		s = append(s, buf[parser.buffer_pos:pos+3]...)
		parser.buffer_pos += 3
	default:
		return s
	}
	parser.mark.index++
	parser.mark.column = 0
	parser.mark.line++
	parser.unread--
	return s
}

// Get the next token.
func yaml_parser_scan(parser *yaml_parser_t, token *yaml_token_t) bool {
	// Erase the token object.
	*token = yaml_token_t{} // [Go] Is this necessary?

	// No tokens after STREAM-END or error.
	if parser.stream_end_produced || parser.error != yaml_NO_ERROR {
		return true
	}

	// Ensure that the tokens queue contains enough tokens.
	if !parser.token_available {
		if !yaml_parser_fetch_more_tokens(parser) {
			return false
		}
	}

	// Fetch the next token from the queue.
	*token = parser.tokens[parser.tokens_head]
	parser.tokens_head++
	parser.tokens_parsed++
	parser.token_available = false

	if token.typ == yaml_STREAM_END_TOKEN {
		parser.stream_end_produced = true
	}
	return true
}

// Set the scanner error and return false.
func yaml_parser_set_scanner_error(parser *yaml_parser_t, context string, context_mark yaml_mark_t, problem string) bool {
	parser.error = yaml_SCANNER_ERROR
	parser.context = context
	parser.context_mark = context_mark
	parser.problem = problem
	parser.problem_mark = parser.mark
	return false
}

func yaml_parser_set_scanner_tag_error(parser *yaml_parser_t, directive bool, context_mark yaml_mark_t, problem string) bool {
	context := "while parsing a tag"
	if directive {
		context = "while parsing a %TAG directive"
	}
	return yaml_parser_set_scanner_error(parser, context, context_mark, problem)
}

func trace(args ...interface{}) func() {
	pargs := append([]interface{}{"+++"}, args...)
	fmt.Println(pargs...)
	pargs = append([]interface{}{"---"}, args...)
	return func() { fmt.Println(pargs...) }
}

// Ensure that the tokens queue contains at least one token which can be
// returned to the Parser.
func yaml_parser_fetch_more_tokens(parser *yaml_parser_t) bool {
	// While we need more tokens to fetch, do it.
	for {
		// Check if we really need to fetch more tokens.
		need_more_tokens := false

		if parser.tokens_head == len(parser.tokens) {
			// Queue is empty.
			need_more_tokens = true
		} else {
			// Check if any potential simple key may occupy the head position.
			if !yaml_parser_stale_simple_keys(parser) {
				return false
			}

			for i := range parser.simple_keys {
				simple_key := &parser.simple_keys[i]
				if simple_key.possible && simple_key.token_number == parser.tokens_parsed {
					need_more_tokens = true
					break
				}
			}
		}

		// We are finished.
		if !need_more_tokens {
			break
		}
		// Fetch the next token.
		if !yaml_parser_fetch_next_token(parser) {
			return false
		}
	}

	parser.token_available = true
	return true
}

// The dispatcher for token fetchers.
func yaml_parser_fetch_next_token(parser *yaml_parser_t) bool {
	// Ensure that the buffer is initialized.
	if parser.unread < 1 && !yaml_parser_update_buffer(parser, 1) {
		return false
	}

	// Check if we just started scanning.  Fetch STREAM-START then.
	if !parser.stream_start_produced {
		return yaml_parser_fetch_stream_start(parser)
	}

	// Eat whitespaces and comments until we reach the next token.
	if !yaml_parser_scan_to_next_token(parser) {
		return false
	}

	// Remove obsolete potential simple keys.
	if !yaml_parser_stale_simple_keys(parser) {
		return false
	}

	// Check the indentation level against the current column.
	if !yaml_parser_unroll_indent(parser, parser.mark.column) {
		return false
	}

	// Ensure that the buffer contains at least 4 characters.  4 is the length
	// of the longest indicators ('--- ' and '... ').
	if parser.unread < 4 && !yaml_parser_update_buffer(parser, 4) {
		return false
	}

	// Is it the end of the stream?
	if is_z(parser.buffer, parser.buffer_pos) {
		return yaml_parser_fetch_stream_end(parser)
	}

	// Is it a directive?
	if parser.mark.column == 0 && parser.buffer[parser.buffer_pos] == '%' {
		return yaml_parser_fetch_directive(parser)
	}

	buf := parser.buffer
	pos := parser.buffer_pos

	// Is it the document start indicator?
	if parser.mark.column == 0 && buf[pos] == '-' && buf[pos+1] == '-' && buf[pos+2] == '-' && is_blankz(buf, pos+3) {
		return yaml_parser_fetch_document_indicator(parser, yaml_DOCUMENT_START_TOKEN)
	}

	// Is it the document end indicator?
	if parser.mark.column == 0 && buf[pos] == '.' && buf[pos+1] == '.' && buf[pos+2] == '.' && is_blankz(buf, pos+3) {
		return yaml_parser_fetch_document_indicator(parser, yaml_DOCUMENT_END_TOKEN)
	}

	// Is it the flow sequence start indicator?
	if buf[pos] == '[' {
		return yaml_parser_fetch_flow_collection_start(parser, yaml_FLOW_SEQUENCE_START_TOKEN)
	}

	// Is it the flow mapping start indicator?
	if parser.buffer[parser.buffer_pos] == '{' {
		return yaml_parser_fetch_flow_collection_start(parser, yaml_FLOW_MAPPING_START_TOKEN)
	}

	// Is it the flow sequence end indicator?
	if parser.buffer[parser.buffer_pos] == ']' {
		return yaml_parser_fetch_flow_collection_end(parser,
			yaml_FLOW_SEQUENCE_END_TOKEN)
	}

	// Is it the flow mapping end indicator?
	if parser.buffer[parser.buffer_pos] == '}' {
		return yaml_parser_fetch_flow_collection_end(parser,
			yaml_FLOW_MAPPING_END_TOKEN)
	}

	// Is it the flow entry indicator?
	if parser.buffer[parser.buffer_pos] == ',' {
		return yaml_parser_fetch_flow_entry(parser)
	}

	// Is it the block entry indicator?
	if parser.buffer[parser.buffer_pos] == '-' && is_blankz(parser.buffer, parser.buffer_pos+1) {
		return yaml_parser_fetch_block_entry(parser)
	}

	// Is it the key indicator?
	if parser.buffer[parser.buffer_pos] == '?' && (parser.flow_level > 0 || is_blankz(parser.buffer, parser.buffer_pos+1)) {
		return yaml_parser_fetch_key(parser)
	}

	// Is it the value indicator?
	if parser.buffer[parser.buffer_pos] == ':' && (parser.flow_level > 0 || is_blankz(parser.buffer, parser.buffer_pos+1)) {
		return yaml_parser_fetch_value(parser)
	}

	// Is it an alias?
	if parser.buffer[parser.buffer_pos] == '*' {
		return yaml_parser_fetch_anchor(parser, yaml_ALIAS_TOKEN)
	}

	// Is it an anchor?
	if parser.buffer[parser.buffer_pos] == '&' {
		return yaml_parser_fetch_anchor(parser, yaml_ANCHOR_TOKEN)
	}

	// Is it a tag?
	if parser.buffer[parser.buffer_pos] == '!' {
		return yaml_parser_fetch_tag(parser)
	}

	// Is it a literal scalar?
	if parser.buffer[parser.buffer_pos] == '|' && parser.flow_level == 0 {
		return yaml_parser_fetch_block_scalar(parser, true)
	}

	// Is it a folded scalar?
	if parser.buffer[parser.buffer_pos] == '>' && parser.flow_level == 0 {
		return yaml_parser_fetch_block_scalar(parser, false)
	}

	// Is it a single-quoted scalar?
	if parser.buffer[parser.buffer_pos] == '\'' {
		return yaml_parser_fetch_flow_scalar(parser, true)
	}

	// Is it a double-quoted scalar?
	if parser.buffer[parser.buffer_pos] == '"' {
		return yaml_parser_fetch_flow_scalar(parser, false)
	}

	// Is it a plain scalar?
	//
	// A plain scalar may start with any non-blank characters except
	//
	//      '-', '?', ':', ',', '[', ']', '{', '}',
	//      '#', '&', '*', '!', '|', '>', '\'', '\"',
	//      '%', '@', '`'.
	//
	// In the block context (and, for the '-' indicator, in the flow context
	// too), it may also start with the characters
	//
	//      '-', '?', ':'
	//
	// if it is followed by a non-space character.
	//
	// The last rule is more restrictive than the specification requires.
	// [Go] Make this logic more reasonable.
	//switch parser.buffer[parser.buffer_pos] {
	//case '-', '?', ':', ',', '?', '-', ',', ':', ']', '[', '}', '{', '&', '#', '!', '*', '>', '|', '"', '\'', '@', '%', '-', '`':
	//}
	if !(is_blankz(parser.buffer, parser.buffer_pos) || parser.buffer[parser.buffer_pos] == '-' ||
		parser.buffer[parser.buffer_pos] == '?' || parser.buffer[parser.buffer_pos] == ':' ||
		parser.buffer[parser.buffer_pos] == ',' || parser.buffer[parser.buffer_pos] == '[' ||
		parser.buffer[parser.buffer_pos] == ']' || parser.buffer[parser.buffer_pos] == '{' ||
		parser.buffer[parser.buffer_pos] == '}' || parser.buffer[parser.buffer_pos] == '#' ||
		parser.buffer[parser.buffer_pos] == '&' || parser.buffer[parser.buffer_pos] == '*' ||
		parser.buffer[parser.buffer_pos] == '!' || parser.buffer[parser.buffer_pos] == '|' ||
		parser.buffer[parser.buffer_pos] == '>' || parser.buffer[parser.buffer_pos] == '\'' ||
		parser.buffer[parser.buffer_pos] == '"' || parser.buffer[parser.buffer_pos] == '%' ||
		parser.buffer[parser.buffer_pos] == '@' || parser.buffer[parser.buffer_pos] == '`') ||
		(parser.buffer[parser.buffer_pos] == '-' && !is_blank(parser.buffer, parser.buffer_pos+1)) ||
		(parser.flow_level == 0 &&
			(parser.buffer[parser.buffer_pos] == '?' || parser.buffer[parser.buffer_pos] == ':') &&
			!is_blankz(parser.buffer, parser.buffer_pos+1)) {
		return yaml_parser_fetch_plain_scalar(parser)
	}

	// If we don't determine the token type so far, it is an error.
	return yaml_parser_set_scanner_error(parser,
		"while scanning for the next token", parser.mark,
		"found character that cannot start any token")
}

// Check the list of potential simple keys and remove the positions that
// cannot contain simple keys anymore.
func yaml_parser_stale_simple_keys(parser *yaml_parser_t) bool {
	// Check for a potential simple key for each flow level.
	for i := range parser.simple_keys {
		simple_key := &parser.simple_keys[i]

		// The specification requires that a simple key
		//
		//  - is limited to a single line,
		//  - is shorter than 1024 characters.
		if simple_key.possible && (simple_key.mark.line < parser.mark.line || simple_key.mark.index+1024 < parser.mark.index) {

			// Check if the potential simple key to be removed is required.
			if simple_key.required {
				return yaml_parser_set_scanner_error(parser,
					"while scanning a simple key", simple_key.mark,
					"could not find expected ':'")
			}
			simple_key.possible = false
		}
	}
	return true
}

// Check if a simple key may start at the current position and add it if
// needed.
func yaml_parser_save_simple_key(parser *yaml_parser_t) bool {
	// A simple key is required at the current position if the scanner is in
	// the block context and the current column coincides with the indentation
	// level.

	required := parser.flow_level == 0 && parser.indent == parser.mark.column

	//
	// If the current position may start a simple key, save it.
	//
	if parser.simple_key_allowed {
		simple_key := yaml_simple_key_t{
			possible:     true,
			required:     required,
			token_number: parser.tokens_parsed + (len(parser.tokens) - parser.tokens_head),
		}
		simple_key.mark = parser.mark

		if !yaml_parser_remove_simple_key(parser) {
			return false
		}
		parser.simple_keys[len(parser.simple_keys)-1] = simple_key
	}
	return true
}

// Remove a potential simple key at the current flow level.
func yaml_parser_remove_simple_key(parser *yaml_parser_t) bool {
	i := len(parser.simple_keys) - 1
	if parser.simple_keys[i].possible {
		// If the key is required, it is an error.
		if parser.simple_keys[i].required {
			return yaml_parser_set_scanner_error(parser,
				"while scanning a simple key", parser.simple_keys[i].mark,
				"could not find expected ':'")
		}
	}
	// Remove the key from the stack.
	parser.simple_keys[i].possible = false
	return true
}

// Increase the flow level and resize the simple key list if needed.
func yaml_parser_increase_flow_level(parser *yaml_parser_t) bool {
	// Reset the simple key on the next level.
	parser.simple_keys = append(parser.simple_keys, yaml_simple_key_t{})

	// Increase the flow level.
	parser.flow_level++
	return true
}

// Decrease the flow level.
func yaml_parser_decrease_flow_level(parser *yaml_parser_t) bool {
	if parser.flow_level > 0 {
		parser.flow_level--
		parser.simple_keys = parser.simple_keys[:len(parser.simple_keys)-1]
	}
	return true
}

// Push the current indentation level to the stack and set the new level
// the current column is greater than the indentation level.  In this case,
// append or insert the specified token into the token queue.
func yaml_parser_roll_indent(parser *yaml_parser_t, column, number int, typ yaml_token_type_t, mark yaml_mark_t) bool {
	// In the flow context, do nothing.
	if parser.flow_level > 0 {
		return true
	}

	if parser.indent < column {
		// Push the current indentation level to the stack and set the new
		// indentation level.
		parser.indents = append(parser.indents, parser.indent)
		parser.indent = column

		// Create a token and insert it into the queue.
		token := yaml_token_t{
			typ:        typ,
			start_mark: mark,
			end_mark:   mark,
		}
		if number > -1 {
			number -= parser.tokens_parsed
		}
		yaml_insert_token(parser, number, &token)
	}
	return true
}

// Pop indentation levels from the indents stack until the current level
// becomes less or equal to the column.  For each indentation level, append
// the BLOCK-END token.
func yaml_parser_unroll_indent(parser *yaml_parser_t, column int) bool {
	// In the flow context, do nothing.
	if parser.flow_level > 0 {
		return true
	}

	// Loop through the indentation levels in the stack.
	for parser.indent > column {
		// Create a token and append it to the queue.
		token := yaml_token_t{
			typ:        yaml_BLOCK_END_TOKEN,
			start_mark: parser.mark,
			end_mark:   parser.mark,
		}
		yaml_insert_token(parser, -1, &token)

		// Pop the indentation level.
		parser.indent = parser.indents[len(parser.indents)-1]
		parser.indents = parser.indents[:len(parser.indents)-1]
	}
	return true
}

// Initialize the scanner and produce the STREAM-START token.
func yaml_parser_fetch_stream_start(parser *yaml_parser_t) bool {

	// Set the initial indentation.
	parser.indent = -1

	// Initialize the simple key stack.
	parser.simple_keys = append(parser.simple_keys, yaml_simple_key_t{})

	// A simple key is allowed at the beginning of the stream.
	parser.simple_key_allowed = true

	// We have started.
	parser.stream_start_produced = true

	// Create the STREAM-START token and append it to the queue.
	token := yaml_token_t{
		typ:        yaml_STREAM_START_TOKEN,