# All files filter-parse.js

98.96% Statements 95/96
97.14% Branches 34/35
100% Functions 8/8
98.9% Lines 90/91

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     "use strict";

    let interlaceUtils = require("./interlace");
    let paethPredictor = require("./paeth-predictor");

    function getByteWidth(width, bpp, depth) {
      let byteWidth = width * bpp;
      if (depth !== 8) {
        byteWidth = Math.ceil(byteWidth / (8 / depth));
      }
      return byteWidth;
    }

    let Filter = (module.exports = function (bitmapInfo, dependencies) {
      let width = bitmapInfo.width;
      let height = bitmapInfo.height;
      let interlace = bitmapInfo.interlace;
      let bpp = bitmapInfo.bpp;
      let depth = bitmapInfo.depth;

      this.read = dependencies.read;
      this.write = dependencies.write;
      this.complete = dependencies.complete;

      this._imageIndex = 0;
      this._images = [];
      if (interlace) {
        let passes = interlaceUtils.getImagePasses(width, height);
        for (let i = 0; i < passes.length; i++) {
          this._images.push({
            byteWidth: getByteWidth(passes[i].width, bpp, depth),
            height: passes[i].height,
            lineIndex: 0,
          });
        }
      } else {
        this._images.push({
          byteWidth: getByteWidth(width, bpp, depth),
          height: height,
          lineIndex: 0,
        });
      }

      // when filtering the line we look at the pixel to the left
      // the spec also says it is done on a byte level regardless of the number of pixels
      // so if the depth is byte compatible (8 or 16) we subtract the bpp in order to compare back
      // a pixel rather than just a different byte part. However if we are sub byte, we ignore.
      if (depth === 8) {
        this._xComparison = bpp;
      } else if (depth === 16) {
        this._xComparison = bpp * 2;
      } else {
        this._xComparison = 1;
      }
    });

    Filter.prototype.start = function () {
      this.read(
        this._images[this._imageIndex].byteWidth + 1,
        this._reverseFilterLine.bind(this)
      );
    };

    Filter.prototype._unFilterType1 = function (
      rawData,
      unfilteredLine,
      byteWidth
    ) {
      let xComparison = this._xComparison;
      let xBiggerThan = xComparison - 1;

      for (let x = 0; x < byteWidth; x++) {
        let rawByte = rawData[1 + x];
        let f1Left = x > xBiggerThan ? unfilteredLine[x - xComparison] : 0;
        unfilteredLine[x] = rawByte + f1Left;
      }
    };

    Filter.prototype._unFilterType2 = function (
      rawData,
      unfilteredLine,
      byteWidth
    ) {
      let lastLine = this._lastLine;

      for (let x = 0; x < byteWidth; x++) {
        let rawByte = rawData[1 + x];
        let f2Up = lastLine ? lastLine[x] : 0;
        unfilteredLine[x] = rawByte + f2Up;
      }
    };

    Filter.prototype._unFilterType3 = function (
      rawData,
      unfilteredLine,
      byteWidth
    ) {
      let xComparison = this._xComparison;
      let xBiggerThan = xComparison - 1;
      let lastLine = this._lastLine;

      for (let x = 0; x < byteWidth; x++) {
        let rawByte = rawData[1 + x];
        let f3Up = lastLine ? lastLine[x] : 0;
        let f3Left = x > xBiggerThan ? unfilteredLine[x - xComparison] : 0;
        let f3Add = Math.floor((f3Left + f3Up) / 2);
        unfilteredLine[x] = rawByte + f3Add;
      }
    };

    Filter.prototype._unFilterType4 = function (
      rawData,
      unfilteredLine,
      byteWidth
    ) {
      let xComparison = this._xComparison;
      let xBiggerThan = xComparison - 1;
      let lastLine = this._lastLine;

      for (let x = 0; x < byteWidth; x++) {
        let rawByte = rawData[1 + x];
        let f4Up = lastLine ? lastLine[x] : 0;
        let f4Left = x > xBiggerThan ? unfilteredLine[x - xComparison] : 0;
        let f4UpLeft = x > xBiggerThan && lastLine ? lastLine[x - xComparison] : 0;
        let f4Add = paethPredictor(f4Left, f4Up, f4UpLeft);
        unfilteredLine[x] = rawByte + f4Add;
      }
    };

    Filter.prototype._reverseFilterLine = function (rawData) {
      let filter = rawData[0];
      let unfilteredLine;
      let currentImage = this._images[this._imageIndex];
      let byteWidth = currentImage.byteWidth;

      if (filter === 0) {
        unfilteredLine = rawData.slice(1, byteWidth + 1);
      } else {
        unfilteredLine = Buffer.alloc(byteWidth);

        switch (filter) {
          case 1:
            this._unFilterType1(rawData, unfilteredLine, byteWidth);
            break;
          case 2:
            this._unFilterType2(rawData, unfilteredLine, byteWidth);
            break;
          case 3:
            this._unFilterType3(rawData, unfilteredLine, byteWidth);
            break;
          case 4:
            this._unFilterType4(rawData, unfilteredLine, byteWidth);
            break;
          default:
            throw new Error("Unrecognised filter type - " + filter);
        }
      }

      this.write(unfilteredLine);

      currentImage.lineIndex++;
      if (currentImage.lineIndex >= currentImage.height) {
        this._lastLine = null;
        this._imageIndex++;
        currentImage = this._images[this._imageIndex];
      } else {
        this._lastLine = unfilteredLine;
      }

      if (currentImage) {
        // read, using the byte width that may be from the new current image
        this.read(currentImage.byteWidth + 1, this._reverseFilterLine.bind(this));
      } else {
        this._lastLine = null;
        this.complete();
      }
    };