let Canvas2Image = (function() {

  // check if support sth.
  let $support = (() => {
    let canvas = document.createElement('canvas'),
      ctx = canvas.getContext('2d');

    return {
      canvas: !!ctx,
      imageData: !!ctx.getImageData,
      dataURL: !!canvas.toDataURL,
      btoa: !!window.btoa
    };
  })();

  let downloadMime = 'image/octet-stream';

  function scaleCanvas(canvas, width, height) {
    let w = canvas.width,
      h = canvas.height;

    if (width === void 0) {
      width = w;
    }
    if (height === void 0) {
      height = h;
    }

    let retCanvas = document.createElement('canvas');
    let retCtx = retCanvas.getContext('2d');

    retCanvas.width = width;
    retCanvas.height = height;
    retCtx.drawImage(canvas, 0, 0, w, h, 0, 0, width, height);
    return retCanvas;
  }

  function getDataURL(canvas, type, width, height) {
    canvas = scaleCanvas(canvas, width, height);
    return canvas.toDataURL(type);
  }

  function saveFile(strData) {
    return strData;
  }

  function genImage(strData) {
    let img = document.createElement('img');

    img.src = strData;
    return img;
  }
  function fixType(type) {
    type = type.toLowerCase().replace(/jpg/i, 'jpeg');
    let r = type.match(/png|jpeg|bmp|gif/)[0];

    return 'image/' + r;
  }
  function encodeData(data) {
    if (!window.btoa) {
      throw 'btoa undefined';
    }
    let str = '';

    if (typeof data === 'string') {
      str = data;
    } else {
      for (let i = 0; i < data.length; i++) {
        str += String.fromCharCode(data[i]);
      }
    }

    return btoa(str);
  }
  function getImageData(canvas) {
    let w = canvas.width,
      h = canvas.height;

    return canvas.getContext('2d').getImageData(0, 0, w, h);
  }
  function makeURI(strData, type) {
    return 'data:' + type + ';base64,' + strData;
  }


  /**
	 * create bitmap image
	 * 按照规则生成图片响应头和响应体
	 */
  let genBitmapImage = function(oData) {

    //
    // BITMAPFILEHEADER: http://msdn.microsoft.com/en-us/library/windows/desktop/dd183374(v=vs.85).aspx
    // BITMAPINFOHEADER: http://msdn.microsoft.com/en-us/library/dd183376.aspx
    //

    let biWidth = oData.width;
    let biHeight = oData.height;
    let biSizeImage = biWidth * biHeight * 3;
    let bfSize = biSizeImage + 54; // total header size = 54 bytes

    //
    //  typedef struct tagBITMAPFILEHEADER {
    //  	WORD bfType;
    //  	DWORD bfSize;
    //  	WORD bfReserved1;
    //  	WORD bfReserved2;
    //  	DWORD bfOffBits;
    //  } BITMAPFILEHEADER;
    //
    let BITMAPFILEHEADER = [
      // WORD bfType -- The file type signature; must be "BM"
      0x42, 0x4D,

      // DWORD bfSize -- The size, in bytes, of the bitmap file
      bfSize & 0xff, bfSize >> 8 & 0xff, bfSize >> 16 & 0xff, bfSize >> 24 & 0xff,

      // WORD bfReserved1 -- Reserved; must be zero
      0, 0,

      // WORD bfReserved2 -- Reserved; must be zero
      0, 0,

      // DWORD bfOffBits -- The offset, in bytes, from the beginning of the BITMAPFILEHEADER structure to the bitmap bits.
      54, 0, 0, 0
    ];

    //
    //  typedef struct tagBITMAPINFOHEADER {
    //  	DWORD biSize;
    //  	LONG  biWidth;
    //  	LONG  biHeight;
    //  	WORD  biPlanes;
    //  	WORD  biBitCount;
    //  	DWORD biCompression;
    //  	DWORD biSizeImage;
    //  	LONG  biXPelsPerMeter;
    //  	LONG  biYPelsPerMeter;
    //  	DWORD biClrUsed;
    //  	DWORD biClrImportant;
    //  } BITMAPINFOHEADER, *PBITMAPINFOHEADER;
    //
    let BITMAPINFOHEADER = [
      // DWORD biSize -- The number of bytes required by the structure
      40, 0, 0, 0,

      // LONG biWidth -- The width of the bitmap, in pixels
      biWidth & 0xff, biWidth >> 8 & 0xff, biWidth >> 16 & 0xff, biWidth >> 24 & 0xff,

      // LONG biHeight -- The height of the bitmap, in pixels
      biHeight & 0xff, biHeight >> 8 & 0xff, biHeight >> 16 & 0xff, biHeight >> 24 & 0xff,

      // WORD biPlanes -- The number of planes for the target device. This value must be set to 1
      1, 0,

      // WORD biBitCount -- The number of bits-per-pixel, 24 bits-per-pixel -- the bitmap
      // has a maximum of 2^24 colors (16777216, Truecolor)
      24, 0,

      // DWORD biCompression -- The type of compression, BI_RGB (code 0) -- uncompressed
      0, 0, 0, 0,

      // DWORD biSizeImage -- The size, in bytes, of the image. This may be set to zero for BI_RGB bitmaps
      biSizeImage & 0xff, biSizeImage >> 8 & 0xff, biSizeImage >> 16 & 0xff, biSizeImage >> 24 & 0xff,

      // LONG biXPelsPerMeter, unused
      0, 0, 0, 0,

      // LONG biYPelsPerMeter, unused
      0, 0, 0, 0,

      // DWORD biClrUsed, the number of color indexes of palette, unused
      0, 0, 0, 0,

      // DWORD biClrImportant, unused
      0, 0, 0, 0
    ];

    let iPadding = (4 - ((biWidth * 3) % 4)) % 4;

    let aImgData = oData.data;

    let strPixelData = '';
    let biWidth4 = biWidth << 2;
    let y = biHeight;
    let fromCharCode = String.fromCharCode;

    do {
      let iOffsetY = biWidth4 * (y - 1);
      let strPixelRow = '';

      for (let x = 0; x < biWidth; x++) {
        let iOffsetX = x << 2;

        strPixelRow += fromCharCode(aImgData[iOffsetY + iOffsetX + 2]) +
                    fromCharCode(aImgData[iOffsetY + iOffsetX + 1]) +
                    fromCharCode(aImgData[iOffsetY + iOffsetX]);
      }

      for (let c = 0; c < iPadding; c++) {
        strPixelRow += String.fromCharCode(0);
      }

      strPixelData += strPixelRow;
    } while (--y);

    let strEncoded = encodeData(BITMAPFILEHEADER.concat(BITMAPINFOHEADER)) + encodeData(strPixelData);

    return strEncoded;
  };

    /**
	 * saveAsImage
	 * @param canvasElement
	 * @param {String} image type
	 * @param {Number} [optional] png width
	 * @param {Number} [optional] png height
	 */
  let saveAsImage = function(canvas, width, height, type) {
    if ($support.canvas && $support.dataURL) {
      if (typeof canvas === 'string') {
        canvas = document.getElementById(canvas);
      }
      if (type === void 0) {
        type = 'png';
      }
      type = fixType(type);
      if (/bmp/.test(type)) {
        let data = getImageData(scaleCanvas(canvas, width, height));
        let strData = genBitmapImage(data);

        return saveFile(makeURI(strData, downloadMime));
      } else {
        let strData = getDataURL(canvas, type, width, height);

        return saveFile(strData.replace(type, downloadMime));
      }
    }
  };

  let convertToImage = function(canvas, width, height, type) {
    if ($support.canvas && $support.dataURL) {
      if (typeof canvas === 'string') {
        canvas = document.getElementById(canvas);
      }

      if (typeof type === 'undefined') {
        type = 'png';
      }

      type = fixType(type);

      if (/bmp/.test(type)) {
        let data = getImageData(scaleCanvas(canvas, width, height));
        let strData = genBitmapImage(data);

        return genImage(makeURI(strData, 'image/bmp'));
      } else {
        let strData = getDataURL(canvas, type, width, height);

        return genImage(strData);
      }
    }
  };



  return {
    saveAsImage: saveAsImage,
    saveAsPNG: function(canvas, width, height) {
      return saveAsImage(canvas, width, height, 'png');
    },
    saveAsJPEG: function(canvas, width, height) {
      return saveAsImage(canvas, width, height, 'jpeg');
    },
    saveAsGIF: function(canvas, width, height) {
      return saveAsImage(canvas, width, height, 'gif');
    },
    saveAsBMP: function(canvas, width, height) {
      return saveAsImage(canvas, width, height, 'bmp');
    },

    convertToImage: convertToImage,
    convertToPNG: function(canvas, width, height) {
      return convertToImage(canvas, width, height, 'png');
    },
    convertToJPEG: function(canvas, width, height) {
      return convertToImage(canvas, width, height, 'jpeg');
    },
    convertToGIF: function(canvas, width, height) {
      return convertToImage(canvas, width, height, 'gif');
    },
    convertToBMP: function(canvas, width, height) {
      return convertToImage(canvas, width, height, 'bmp');
    }
  };

}());

const capture = async(dom, width, height) => {
  const html2canvas = await import(/* webpackChunkName: "component.html2canvas" */ 'html2canvas');
  const canvasObj = await html2canvas.default(dom);

  return Canvas2Image.saveAsPNG(canvasObj, width, height);
};

export default {
  capture
};