preprocessor.js

'use strict';

const unicode = require('../common/unicode');
const ERR = require('../common/error-codes');

//Aliases
const $ = unicode.CODE_POINTS;

//Const
const DEFAULT_BUFFER_WATERLINE = 1 << 16;

//Preprocessor
//NOTE: HTML input preprocessing
//(see: http://www.whatwg.org/specs/web-apps/current-work/multipage/parsing.html#preprocessing-the-input-stream)
class Preprocessor {
    constructor() {
        this.html = null;

        this.pos = -1;
        this.lastGapPos = -1;
        this.lastCharPos = -1;

        this.gapStack = [];

        this.skipNextNewLine = false;

        this.lastChunkWritten = false;
        this.endOfChunkHit = false;
        this.bufferWaterline = DEFAULT_BUFFER_WATERLINE;
    }

    _err() {
        // NOTE: err reporting is noop by default. Enabled by mixin.
    }

    _addGap() {
        this.gapStack.push(this.lastGapPos);
        this.lastGapPos = this.pos;
    }

    _processSurrogate(cp) {
        //NOTE: try to peek a surrogate pair
        if (this.pos !== this.lastCharPos) {
            const nextCp = this.html.charCodeAt(this.pos + 1);

            if (unicode.isSurrogatePair(nextCp)) {
                //NOTE: we have a surrogate pair. Peek pair character and recalculate code point.
                this.pos++;

                //NOTE: add gap that should be avoided during retreat
                this._addGap();

                return unicode.getSurrogatePairCodePoint(cp, nextCp);
            }
        }

        //NOTE: we are at the end of a chunk, therefore we can't infer surrogate pair yet.
        else if (!this.lastChunkWritten) {
            this.endOfChunkHit = true;
            return $.EOF;
        }

        //NOTE: isolated surrogate
        this._err(ERR.surrogateInInputStream);

        return cp;
    }

    dropParsedChunk() {
        if (this.pos > this.bufferWaterline) {
            this.lastCharPos -= this.pos;
            this.html = this.html.substring(this.pos);
            this.pos = 0;
            this.lastGapPos = -1;
            this.gapStack = [];
        }
    }

    write(chunk, isLastChunk) {
        if (this.html) {
            this.html += chunk;
        } else {
            this.html = chunk;
        }

        this.lastCharPos = this.html.length - 1;
        this.endOfChunkHit = false;
        this.lastChunkWritten = isLastChunk;
    }

    insertHtmlAtCurrentPos(chunk) {
        this.html = this.html.substring(0, this.pos + 1) + chunk + this.html.substring(this.pos + 1, this.html.length);

        this.lastCharPos = this.html.length - 1;
        this.endOfChunkHit = false;
    }

    advance() {
        this.pos++;

        if (this.pos > this.lastCharPos) {
            this.endOfChunkHit = !this.lastChunkWritten;
            return $.EOF;
        }

        let cp = this.html.charCodeAt(this.pos);

        //NOTE: any U+000A LINE FEED (LF) characters that immediately follow a U+000D CARRIAGE RETURN (CR) character
        //must be ignored.
        if (this.skipNextNewLine && cp === $.LINE_FEED) {
            this.skipNextNewLine = false;
            this._addGap();
            return this.advance();
        }

        //NOTE: all U+000D CARRIAGE RETURN (CR) characters must be converted to U+000A LINE FEED (LF) characters
        if (cp === $.CARRIAGE_RETURN) {
            this.skipNextNewLine = true;
            return $.LINE_FEED;
        }

        this.skipNextNewLine = false;

        if (unicode.isSurrogate(cp)) {
            cp = this._processSurrogate(cp);
        }

        //OPTIMIZATION: first check if code point is in the common allowed
        //range (ASCII alphanumeric, whitespaces, big chunk of BMP)
        //before going into detailed performance cost validation.
        const isCommonValidRange =
            (cp > 0x1f && cp < 0x7f) || cp === $.LINE_FEED || cp === $.CARRIAGE_RETURN || (cp > 0x9f && cp < 0xfdd0);

        if (!isCommonValidRange) {
            this._checkForProblematicCharacters(cp);
        }

        return cp;
    }

    _checkForProblematicCharacters(cp) {
        if (unicode.isControlCodePoint(cp)) {
            this._err(ERR.controlCharacterInInputStream);
        } else if (unicode.isUndefinedCodePoint(cp)) {
            this._err(ERR.noncharacterInInputStream);
        }
    }

    retreat() {
        if (this.pos === this.lastGapPos) {
            this.lastGapPos = this.gapStack.pop();
            this.pos--;
        }

        this.pos--;
    }
}

module.exports = Preprocessor;