'use strict';

var constants = require('./constants');

module.exports = function(dataIn, width, height, options) {
  var outHasAlpha = [constants.COLORTYPE_COLOR_ALPHA, constants.COLORTYPE_ALPHA].indexOf(options.colorType) !== -1;
  if (options.colorType === options.inputColorType) {
    var bigEndian = (function() {
      var buffer = new ArrayBuffer(2);
      new DataView(buffer).setInt16(0, 256, true /* littleEndian */);
      // Int16Array uses the platform's endianness.
      return new Int16Array(buffer)[0] !== 256;
    })();
    // If no need to convert to grayscale and alpha is present/absent in both, take a fast route
     if (options.bitDepth === 8 || (options.bitDepth === 16 && bigEndian)){
         return dataIn;
    }
  }

  // map to a UInt16 array if data is 16bit, fix endianness below
  var data = options.bitDepth !== 16 ? dataIn : new Uint16Array(dataIn.buffer);

  var maxValue = 255;
  var inBpp = constants.COLORTYPE_TO_BPP_MAP[options.inputColorType];
  var outBpp = constants.COLORTYPE_TO_BPP_MAP[options.colorType];
  if (options.bitDepth === 16) {
    maxValue = 65535;
    outBpp *= 2;
  }
  var outData = new Buffer(width * height * outBpp);

  var inIndex = 0;
  var outIndex = 0;

  var bgColor = options.bgColor || {};
  if (bgColor.red === undefined) {
    bgColor.red = maxValue;
  }
  if (bgColor.green === undefined) {
    bgColor.green = maxValue;
  }
  if (bgColor.blue === undefined) {
    bgColor.blue = maxValue;
  }

  function getRGBA(data, inIndex) {
    var red, green, blue, alpha = maxValue;
    switch (options.inputColorType) {
      case constants.COLORTYPE_COLOR_ALPHA:
        alpha = data[inIndex + 3];
        red = data[inIndex];
        green = data[inIndex+1];
        blue = data[inIndex+2];
        break;
      case constants.COLORTYPE_COLOR:
        red = data[inIndex];
        green = data[inIndex+1];
        blue = data[inIndex+2];
        break;
      case constants.COLORTYPE_ALPHA:
        alpha = data[inIndex + 1];
        red = data[inIndex];
        green = red;
        blue = red;
        break;
      case constants.COLORTYPE_GRAYSCALE:
        red = data[inIndex];
        green = red;
        blue = red;
        break;
      default:
        throw new Error('input color type:' + options.inputColorType + ' is not supported at present');
    }

    if (options.inputHasAlpha) {
      if (!outHasAlpha) {
        alpha /= maxValue;
        red = Math.min(Math.max(Math.round((1 - alpha) * bgColor.red + alpha * red), 0), maxValue);
        green = Math.min(Math.max(Math.round((1 - alpha) * bgColor.green + alpha * green), 0), maxValue);
        blue = Math.min(Math.max(Math.round((1 - alpha) * bgColor.blue + alpha * blue), 0), maxValue);
      }
    }
    return {red, green, blue, alpha};
  }

  for (var y = 0; y < height; y++) {
    for (var x = 0; x < width; x++) {
      var rgba = getRGBA(data, inIndex);

      switch (options.colorType) {
        case constants.COLORTYPE_COLOR_ALPHA:
        case constants.COLORTYPE_COLOR:
          if (options.bitDepth === 8) {
            outData[outIndex] = rgba.red;
            outData[outIndex + 1] = rgba.green;
            outData[outIndex + 2] = rgba.blue;
            if (outHasAlpha) {
              outData[outIndex + 3] = rgba.alpha;
            }
          } else {
            outData.writeUInt16BE(rgba.red, outIndex);
            outData.writeUInt16BE(rgba.green, outIndex + 2);
            outData.writeUInt16BE(rgba.blue, outIndex + 4);
            if (outHasAlpha) {
              outData.writeUInt16BE(rgba.alpha, outIndex + 6);
            }
          }
          break;
        case constants.COLORTYPE_ALPHA:
        case constants.COLORTYPE_GRAYSCALE:
          // Convert to grayscale and alpha
          var grayscale = (rgba.red + rgba.green + rgba.blue) / 3;
          if (options.bitDepth === 8) {
            outData[outIndex] = grayscale;
            if (outHasAlpha) {
              outData[outIndex + 1] = rgba.alpha;
            }
          } else {
            outData.writeUInt16BE(grayscale, outIndex);
            if (outHasAlpha) {
              outData.writeUInt16BE(rgba.alpha, outIndex + 2);
            }
          }
          break;
      }

      inIndex += inBpp;
      outIndex += outBpp;
    }
  }

  return outData;
};
