import * as chars from './chars';
import {ListWrapper} from './facade/collection';
import {NumberWrapper, StringWrapper, isBlank, isPresent} from './facade/lang';
import {HtmlTagContentType, NAMED_ENTITIES, getHtmlTagDefinition} from './html_tags';
import {DEFAULT_INTERPOLATION_CONFIG, InterpolationConfig} from './interpolation_config';
import {ParseError, ParseLocation, ParseSourceFile, ParseSourceSpan} from './parse_util';

export enum HtmlTokenType {
  TAG_OPEN_START,
  TAG_OPEN_END,
  TAG_OPEN_END_VOID,
  TAG_CLOSE,
  TEXT,
  ESCAPABLE_RAW_TEXT,
  RAW_TEXT,
  COMMENT_START,
  COMMENT_END,
  CDATA_START,
  CDATA_END,
  ATTR_NAME,
  ATTR_VALUE,
  DOC_TYPE,
  EXPANSION_FORM_START,
  EXPANSION_CASE_VALUE,
  EXPANSION_CASE_EXP_START,
  EXPANSION_CASE_EXP_END,
  EXPANSION_FORM_END,
  EOF
}

export class HtmlToken {
  constructor(
      public type: HtmlTokenType, public parts: string[], public sourceSpan: ParseSourceSpan) {}
}

export class HtmlTokenError extends ParseError {
  constructor(errorMsg: string, public tokenType: HtmlTokenType, span: ParseSourceSpan) {
    super(span, errorMsg);
  }
}

export class HtmlTokenizeResult {
  constructor(public tokens: HtmlToken[], public errors: HtmlTokenError[]) {}
}

export function tokenizeHtml(
    sourceContent: string, sourceUrl: string, tokenizeExpansionForms: boolean = false,
    interpolationConfig: InterpolationConfig = DEFAULT_INTERPOLATION_CONFIG): HtmlTokenizeResult {
  return new _HtmlTokenizer(
             new ParseSourceFile(sourceContent, sourceUrl), tokenizeExpansionForms,
             interpolationConfig)
      .tokenize();
}

var CR_OR_CRLF_REGEXP = /\r\n?/g;

function unexpectedCharacterErrorMsg(charCode: number): string {
  var char = charCode === chars.$EOF ? 'EOF' : StringWrapper.fromCharCode(charCode);
  return `Unexpected character "${char}"`;
}

function unknownEntityErrorMsg(entitySrc: string): string {
  return `Unknown entity "${entitySrc}" - use the "&#<decimal>;" or  "&#x<hex>;" syntax`;
}

class ControlFlowError {
  constructor(public error: HtmlTokenError) {}
}

// See http://www.w3.org/TR/html51/syntax.html#writing
class _HtmlTokenizer {
  private _input: string;
  private _length: number;
  // Note: this is always lowercase!
  private _peek: number = -1;
  private _nextPeek: number = -1;
  private _index: number = -1;
  private _line: number = 0;
  private _column: number = -1;
  private _currentTokenStart: ParseLocation;
  private _currentTokenType: HtmlTokenType;
  private _expansionCaseStack: HtmlTokenType[] = [];

  tokens: HtmlToken[] = [];
  errors: HtmlTokenError[] = [];

  constructor(
      private file: ParseSourceFile, private tokenizeExpansionForms: boolean,
      private interpolationConfig: InterpolationConfig = DEFAULT_INTERPOLATION_CONFIG) {
    this._input = file.content;
    this._length = file.content.length;
    this._advance();
  }

  private _processCarriageReturns(content: string): string {
    // http://www.w3.org/TR/html5/syntax.html#preprocessing-the-input-stream
    // In order to keep the original position in the source, we can not
    // pre-process it.
    // Instead CRs are processed right before instantiating the tokens.
    return StringWrapper.replaceAll(content, CR_OR_CRLF_REGEXP, '\n');
  }

  tokenize(): HtmlTokenizeResult {
    while (this._peek !== chars.$EOF) {
      var start = this._getLocation();
      try {
        if (this._attemptCharCode(chars.$LT)) {
          if (this._attemptCharCode(chars.$BANG)) {
            if (this._attemptCharCode(chars.$LBRACKET)) {
              this._consumeCdata(start);
            } else if (this._attemptCharCode(chars.$MINUS)) {
              this._consumeComment(start);
            } else {
              this._consumeDocType(start);
            }
          } else if (this._attemptCharCode(chars.$SLASH)) {
            this._consumeTagClose(start);
          } else {
            this._consumeTagOpen(start);
          }
        } else if (
            isExpansionFormStart(this._input, this._index, this.interpolationConfig.start) &&
            this.tokenizeExpansionForms) {
          this._consumeExpansionFormStart();

        } else if (
            isExpansionCaseStart(this._peek) && this._isInExpansionForm() &&
            this.tokenizeExpansionForms) {
          this._consumeExpansionCaseStart();

        } else if (
            this._peek === chars.$RBRACE && this._isInExpansionCase() &&
            this.tokenizeExpansionForms) {
          this._consumeExpansionCaseEnd();

        } else if (
            this._peek === chars.$RBRACE && this._isInExpansionForm() &&
            this.tokenizeExpansionForms) {
          this._consumeExpansionFormEnd();

        } else {
          this._consumeText();
        }
      } catch (e) {
        if (e instanceof ControlFlowError) {
          this.errors.push(e.error);
        } else {
          throw e;
        }
      }
    }
    this._beginToken(HtmlTokenType.EOF);
    this._endToken([]);
    return new HtmlTokenizeResult(mergeTextTokens(this.tokens), this.errors);
  }

  private _getLocation(): ParseLocation {
    return new ParseLocation(this.file, this._index, this._line, this._column);
  }

  private _getSpan(start?: ParseLocation, end?: ParseLocation): ParseSourceSpan {
    if (isBlank(start)) {
      start = this._getLocation();
    }
    if (isBlank(end)) {
      end = this._getLocation();
    }
    return new ParseSourceSpan(start, end);
  }

  private _beginToken(type: HtmlTokenType, start: ParseLocation = null) {
    if (isBlank(start)) {
      start = this._getLocation();
    }
    this._currentTokenStart = start;
    this._currentTokenType = type;
  }

  private _endToken(parts: string[], end: ParseLocation = null): HtmlToken {
    if (isBlank(end)) {
      end = this._getLocation();
    }
    var token = new HtmlToken(
        this._currentTokenType, parts, new ParseSourceSpan(this._currentTokenStart, end));
    this.tokens.push(token);
    this._currentTokenStart = null;
    this._currentTokenType = null;
    return token;
  }

  private _createError(msg: string, span: ParseSourceSpan): ControlFlowError {
    var error = new HtmlTokenError(msg, this._currentTokenType, span);
    this._currentTokenStart = null;
    this._currentTokenType = null;
    return new ControlFlowError(error);
  }

  private _advance() {
    if (this._index >= this._length) {
      throw this._createError(unexpectedCharacterErrorMsg(chars.$EOF), this._getSpan());
    }
    if (this._peek === chars.$LF) {
      this._line++;
      this._column = 0;
    } else if (this._peek !== chars.$LF && this._peek !== chars.$CR) {
      this._column++;
    }
    this._index++;
    this._peek = this._index >= this._length ? chars.$EOF :
                                               StringWrapper.charCodeAt(this._input, this._index);
    this._nextPeek = this._index + 1 >= this._length ?
        chars.$EOF :
        StringWrapper.charCodeAt(this._input, this._index + 1);
  }

  private _attemptCharCode(charCode: number): boolean {
    if (this._peek === charCode) {
      this._advance();
      return true;
    }
    return false;
  }

  private _attemptCharCodeCaseInsensitive(charCode: number): boolean {
    if (compareCharCodeCaseInsensitive(this._peek, charCode)) {
      this._advance();
      return true;
    }
    return false;
  }

  private _requireCharCode(charCode: number) {
    var location = this._getLocation();
    if (!this._attemptCharCode(charCode)) {
      throw this._createError(
          unexpectedCharacterErrorMsg(this._peek), this._getSpan(location, location));
    }
  }

  private _attemptStr(chars: string): boolean {
    const initialPosition = this._savePosition();
    for (var i = 0; i < chars.length; i++) {
      if (!this._attemptCharCode(StringWrapper.charCodeAt(chars, i))) {
        // If attempting to parse the string fails, we want to reset the parser
        // to where it was before the attempt
        this._restorePosition(initialPosition);
        return false;
      }
    }
    return true;
  }

  private _attemptStrCaseInsensitive(chars: string): boolean {
    for (var i = 0; i < chars.length; i++) {
      if (!this._attemptCharCodeCaseInsensitive(StringWrapper.charCodeAt(chars, i))) {
        return false;
      }
    }
    return true;
  }

  private _requireStr(chars: string) {
    var location = this._getLocation();
    if (!this._attemptStr(chars)) {
      throw this._createError(unexpectedCharacterErrorMsg(this._peek), this._getSpan(location));
    }
  }

  private _attemptCharCodeUntilFn(predicate: Function) {
    while (!predicate(this._peek)) {
      this._advance();
    }
  }

  private _requireCharCodeUntilFn(predicate: Function, len: number) {
    var start = this._getLocation();
    this._attemptCharCodeUntilFn(predicate);
    if (this._index - start.offset < len) {
      throw this._createError(unexpectedCharacterErrorMsg(this._peek), this._getSpan(start, start));
    }
  }

  private _attemptUntilChar(char: number) {
    while (this._peek !== char) {
      this._advance();
    }
  }

  private _readChar(decodeEntities: boolean): string {
    if (decodeEntities && this._peek === chars.$AMPERSAND) {
      return this._decodeEntity();
    } else {
      var index = this._index;
      this._advance();
      return this._input[index];
    }
  }

  private _decodeEntity(): string {
    var start = this._getLocation();
    this._advance();
    if (this._attemptCharCode(chars.$HASH)) {
      let isHex = this._attemptCharCode(chars.$x) || this._attemptCharCode(chars.$X);
      let numberStart = this._getLocation().offset;
      this._attemptCharCodeUntilFn(isDigitEntityEnd);
      if (this._peek != chars.$SEMICOLON) {
        throw this._createError(unexpectedCharacterErrorMsg(this._peek), this._getSpan());
      }
      this._advance();
      let strNum = this._input.substring(numberStart, this._index - 1);
      try {
        let charCode = NumberWrapper.parseInt(strNum, isHex ? 16 : 10);
        return StringWrapper.fromCharCode(charCode);
      } catch (e) {
        let entity = this._input.substring(start.offset + 1, this._index - 1);
        throw this._createError(unknownEntityErrorMsg(entity), this._getSpan(start));
      }
    } else {
      let startPosition = this._savePosition();
      this._attemptCharCodeUntilFn(isNamedEntityEnd);
      if (this._peek != chars.$SEMICOLON) {
        this._restorePosition(startPosition);
        return '&';
      }
      this._advance();
      let name = this._input.substring(start.offset + 1, this._index - 1);
      let char = (NAMED_ENTITIES as any)[name];
      if (isBlank(char)) {
        throw this._createError(unknownEntityErrorMsg(name), this._getSpan(start));
      }
      return char;
    }
  }

  private _consumeRawText(
      decodeEntities: boolean, firstCharOfEnd: number, attemptEndRest: Function): HtmlToken {
    var tagCloseStart: ParseLocation;
    var textStart = this._getLocation();
    this._beginToken(
        decodeEntities ? HtmlTokenType.ESCAPABLE_RAW_TEXT : HtmlTokenType.RAW_TEXT, textStart);
    var parts: string[] = [];
    while (true) {
      tagCloseStart = this._getLocation();
      if (this._attemptCharCode(firstCharOfEnd) && attemptEndRest()) {
        break;
      }
      if (this._index > tagCloseStart.offset) {
        parts.push(this._input.substring(tagCloseStart.offset, this._index));
      }
      while (this._peek !== firstCharOfEnd) {
        parts.push(this._readChar(decodeEntities));
      }
    }
    return this._endToken([this._processCarriageReturns(parts.join(''))], tagCloseStart);
  }

  private _consumeComment(start: ParseLocation) {
    this._beginToken(HtmlTokenType.COMMENT_START, start);
    this._requireCharCode(chars.$MINUS);
    this._endToken([]);
    var textToken = this._consumeRawText(false, chars.$MINUS, () => this._attemptStr('->'));
    this._beginToken(HtmlTokenType.COMMENT_END, textToken.sourceSpan.end);
    this._endToken([]);
  }

  private _consumeCdata(start: ParseLocation) {
    this._beginToken(HtmlTokenType.CDATA_START, start);
    this._requireStr('CDATA[');
    this._endToken([]);
    var textToken = this._consumeRawText(false, chars.$RBRACKET, () => this._attemptStr(']>'));
    this._beginToken(HtmlTokenType.CDATA_END, textToken.sourceSpan.end);
    this._endToken([]);
  }

  private _consumeDocType(start: ParseLocation) {
    this._beginToken(HtmlTokenType.DOC_TYPE, start);
    this._attemptUntilChar(chars.$GT);
    this._advance();
    this._endToken([this._input.substring(start.offset + 2, this._index - 1)]);
  }

  private _consumePrefixAndName(): string[] {
    var nameOrPrefixStart = this._index;
    var prefix: string = null;
    while (this._peek !== chars.$COLON && !isPrefixEnd(this._peek)) {
      this._advance();
    }
    var nameStart: number;
    if (this._peek === chars.$COLON) {
      this._advance();
      prefix = this._input.substring(nameOrPrefixStart, this._index - 1);
      nameStart = this._index;
    } else {
      nameStart = nameOrPrefixStart;
    }
    this._requireCharCodeUntilFn(isNameEnd, this._index === nameStart ? 1 : 0);
    var name = this._input.substring(nameStart, this._index);
    return [prefix, name];
  }

  private _consumeTagOpen(start: ParseLocation) {
    let savedPos = this._savePosition();
    let lowercaseTagName: string;
    try {
      if (!isAsciiLetter(this._peek)) {
        throw this._createError(unexpectedCharacterErrorMsg(this._peek), this._getSpan());
      }
      var nameStart = this._index;
      this._consumeTagOpenStart(start);
      lowercaseTagName = this._input.substring(nameStart, this._index).toLowerCase();
      this._attemptCharCodeUntilFn(isNotWhitespace);
      while (this._peek !== chars.$SLASH && this._peek !== chars.$GT) {
        this._consumeAttributeName();
        this._attemptCharCodeUntilFn(isNotWhitespace);
        if (this._attemptCharCode(chars.$EQ)) {
          this._attemptCharCodeUntilFn(isNotWhitespace);
          this._consumeAttributeValue();
        }
        this._attemptCharCodeUntilFn(isNotWhitespace);
      }
      this._consumeTagOpenEnd();
    } catch (e) {
      if (e instanceof ControlFlowError) {
        // When the start tag is invalid, assume we want a "<"
        this._restorePosition(savedPos);
        // Back to back text tokens are merged at the end
        this._beginToken(HtmlTokenType.TEXT, start);
        this._endToken(['<']);
        return;
      }

      throw e;
    }

    var contentTokenType = getHtmlTagDefinition(lowercaseTagName).contentType;
    if (contentTokenType === HtmlTagContentType.RAW_TEXT) {
      this._consumeRawTextWithTagClose(lowercaseTagName, false);
    } else if (contentTokenType === HtmlTagContentType.ESCAPABLE_RAW_TEXT) {
      this._consumeRawTextWithTagClose(lowercaseTagName, true);
    }
  }

  private _consumeRawTextWithTagClose(lowercaseTagName: string, decodeEntities: boolean) {
    var textToken = this._consumeRawText(decodeEntities, chars.$LT, () => {
      if (!this._attemptCharCode(chars.$SLASH)) return false;
      this._attemptCharCodeUntilFn(isNotWhitespace);
      if (!this._attemptStrCaseInsensitive(lowercaseTagName)) return false;
      this._attemptCharCodeUntilFn(isNotWhitespace);
      if (!this._attemptCharCode(chars.$GT)) return false;
      return true;
    });
    this._beginToken(HtmlTokenType.TAG_CLOSE, textToken.sourceSpan.end);
    this._endToken([null, lowercaseTagName]);
  }

  private _consumeTagOpenStart(start: ParseLocation) {
    this._beginToken(HtmlTokenType.TAG_OPEN_START, start);
    var parts = this._consumePrefixAndName();
    this._endToken(parts);
  }

  private _consumeAttributeName() {
    this._beginToken(HtmlTokenType.ATTR_NAME);
    var prefixAndName = this._consumePrefixAndName();
    this._endToken(prefixAndName);
  }

  private _consumeAttributeValue() {
    this._beginToken(HtmlTokenType.ATTR_VALUE);
    var value: string;
    if (this._peek === chars.$SQ || this._peek === chars.$DQ) {
      var quoteChar = this._peek;
      this._advance();
      var parts: string[] = [];
      while (this._peek !== quoteChar) {
        parts.push(this._readChar(true));
      }
      value = parts.join('');
      this._advance();
    } else {
      var valueStart = this._index;
      this._requireCharCodeUntilFn(isNameEnd, 1);
      value = this._input.substring(valueStart, this._index);
    }
    this._endToken([this._processCarriageReturns(value)]);
  }

  private _consumeTagOpenEnd() {
    var tokenType = this._attemptCharCode(chars.$SLASH) ? HtmlTokenType.TAG_OPEN_END_VOID :
                                                          HtmlTokenType.TAG_OPEN_END;
    this._beginToken(tokenType);
    this._requireCharCode(chars.$GT);
    this._endToken([]);
  }

  private _consumeTagClose(start: ParseLocation) {
    this._beginToken(HtmlTokenType.TAG_CLOSE, start);
    this._attemptCharCodeUntilFn(isNotWhitespace);
    let prefixAndName = this._consumePrefixAndName();
    this._attemptCharCodeUntilFn(isNotWhitespace);
    this._requireCharCode(chars.$GT);
    this._endToken(prefixAndName);
  }

  private _consumeExpansionFormStart() {
    this._beginToken(HtmlTokenType.EXPANSION_FORM_START, this._getLocation());
    this._requireCharCode(chars.$LBRACE);
    this._endToken([]);

    this._beginToken(HtmlTokenType.RAW_TEXT, this._getLocation());
    let condition = this._readUntil(chars.$COMMA);
    this._endToken([condition], this._getLocation());
    this._requireCharCode(chars.$COMMA);
    this._attemptCharCodeUntilFn(isNotWhitespace);

    this._beginToken(HtmlTokenType.RAW_TEXT, this._getLocation());
    let type = this._readUntil(chars.$COMMA);
    this._endToken([type], this._getLocation());
    this._requireCharCode(chars.$COMMA);
    this._attemptCharCodeUntilFn(isNotWhitespace);

    this._expansionCaseStack.push(HtmlTokenType.EXPANSION_FORM_START);
  }

  private _consumeExpansionCaseStart() {
    this._beginToken(HtmlTokenType.EXPANSION_CASE_VALUE, this._getLocation());
    let value = this._readUntil(chars.$LBRACE).trim();
    this._endToken([value], this._getLocation());
    this._attemptCharCodeUntilFn(isNotWhitespace);

    this._beginToken(HtmlTokenType.EXPANSION_CASE_EXP_START, this._getLocation());
    this._requireCharCode(chars.$LBRACE);
    this._endToken([], this._getLocation());
    this._attemptCharCodeUntilFn(isNotWhitespace);

    this._expansionCaseStack.push(HtmlTokenType.EXPANSION_CASE_EXP_START);
  }

  private _consumeExpansionCaseEnd() {
    this._beginToken(HtmlTokenType.EXPANSION_CASE_EXP_END, this._getLocation());
    this._requireCharCode(chars.$RBRACE);
    this._endToken([], this._getLocation());
    this._attemptCharCodeUntilFn(isNotWhitespace);

    this._expansionCaseStack.pop();
  }

  private _consumeExpansionFormEnd() {
    this._beginToken(HtmlTokenType.EXPANSION_FORM_END, this._getLocation());
    this._requireCharCode(chars.$RBRACE);
    this._endToken([]);

    this._expansionCaseStack.pop();
  }

  private _consumeText() {
    var start = this._getLocation();
    this._beginToken(HtmlTokenType.TEXT, start);

    var parts: string[] = [];
    let interpolation = false;

    do {
      const savedPos = this._savePosition();
      // _attemptStr advances the position when it is true.
      // To push interpolation symbols, we have to reset it.
      if (this._attemptStr(this.interpolationConfig.start)) {
        this._restorePosition(savedPos);
        for (let i = 0; i < this.interpolationConfig.start.length; i++) {
          parts.push(this._readChar(true));
        }
        interpolation = true;
      } else if (this._attemptStr(this.interpolationConfig.end) && interpolation) {
        this._restorePosition(savedPos);
        for (let i = 0; i < this.interpolationConfig.end.length; i++) {
          parts.push(this._readChar(true));
        }
        interpolation = false;
      } else {
        this._restorePosition(savedPos);
        parts.push(this._readChar(true));
      }
    } while (!this._isTextEnd(interpolation));

    this._endToken([this._processCarriageReturns(parts.join(''))]);
  }

  private _isTextEnd(interpolation: boolean): boolean {
    if (this._peek === chars.$LT || this._peek === chars.$EOF) return true;
    if (this.tokenizeExpansionForms) {
      const savedPos = this._savePosition();
      if (isExpansionFormStart(this._input, this._index, this.interpolationConfig.start))
        return true;
      this._restorePosition(savedPos);
      if (this._peek === chars.$RBRACE && !interpolation &&
          (this._isInExpansionCase() || this._isInExpansionForm()))
        return true;
    }
    return false;
  }

  private _savePosition(): number[] {
    return [this._peek, this._index, this._column, this._line, this.tokens.length];
  }

  private _readUntil(char: number): string {
    let start = this._index;
    this._attemptUntilChar(char);
    return this._input.substring(start, this._index);
  }

  private _restorePosition(position: number[]): void {
    this._peek = position[0];
    this._index = position[1];
    this._column = position[2];
    this._line = position[3];
    let nbTokens = position[4];
    if (nbTokens < this.tokens.length) {
      // remove any extra tokens
      this.tokens = ListWrapper.slice(this.tokens, 0, nbTokens);
    }
  }

  private _isInExpansionCase(): boolean {
    return this._expansionCaseStack.length > 0 &&
        this._expansionCaseStack[this._expansionCaseStack.length - 1] ===
        HtmlTokenType.EXPANSION_CASE_EXP_START;
  }

  private _isInExpansionForm(): boolean {
    return this._expansionCaseStack.length > 0 &&
        this._expansionCaseStack[this._expansionCaseStack.length - 1] ===
        HtmlTokenType.EXPANSION_FORM_START;
  }
}

function isNotWhitespace(code: number): boolean {
  return !isWhitespace(code) || code === chars.$EOF;
}

function isWhitespace(code: number): boolean {
  return (code >= chars.$TAB && code <= chars.$SPACE) || (code === chars.$NBSP);
}

function isNameEnd(code: number): boolean {
  return isWhitespace(code) || code === chars.$GT || code === chars.$SLASH || code === chars.$SQ ||
      code === chars.$DQ || code === chars.$EQ;
}

function isPrefixEnd(code: number): boolean {
  return (code < chars.$a || chars.$z < code) && (code < chars.$A || chars.$Z < code) &&
      (code < chars.$0 || code > chars.$9);
}

function isDigitEntityEnd(code: number): boolean {
  return code == chars.$SEMICOLON || code == chars.$EOF || !isAsciiHexDigit(code);
}

function isNamedEntityEnd(code: number): boolean {
  return code == chars.$SEMICOLON || code == chars.$EOF || !isAsciiLetter(code);
}

function isExpansionFormStart(input: string, offset: number, interpolationStart: string): boolean {
  const substr = input.substring(offset);
  return StringWrapper.charCodeAt(substr, 0) === chars.$LBRACE &&
      StringWrapper.charCodeAt(substr, 1) !== chars.$LBRACE &&
      !substr.startsWith(interpolationStart);
}

function isExpansionCaseStart(peek: number): boolean {
  return peek === chars.$EQ || isAsciiLetter(peek);
}

function isAsciiLetter(code: number): boolean {
  return code >= chars.$a && code <= chars.$z || code >= chars.$A && code <= chars.$Z;
}

function isAsciiHexDigit(code: number): boolean {
  return code >= chars.$a && code <= chars.$f || code >= chars.$A && code <= chars.$F ||
      code >= chars.$0 && code <= chars.$9;
}

function compareCharCodeCaseInsensitive(code1: number, code2: number): boolean {
  return toUpperCaseCharCode(code1) == toUpperCaseCharCode(code2);
}

function toUpperCaseCharCode(code: number): number {
  return code >= chars.$a && code <= chars.$z ? code - chars.$a + chars.$A : code;
}

function mergeTextTokens(srcTokens: HtmlToken[]): HtmlToken[] {
  let dstTokens: HtmlToken[] = [];
  let lastDstToken: HtmlToken;
  for (let i = 0; i < srcTokens.length; i++) {
    let token = srcTokens[i];
    if (isPresent(lastDstToken) && lastDstToken.type == HtmlTokenType.TEXT &&
        token.type == HtmlTokenType.TEXT) {
      lastDstToken.parts[0] += token.parts[0];
      lastDstToken.sourceSpan.end = token.sourceSpan.end;
    } else {
      lastDstToken = token;
      dstTokens.push(lastDstToken);
    }
  }

  return dstTokens;
}
