// Split original image into multiple 1024x1024 chunks to reduce memory usage
// (images have to be unpacked into typed arrays for resizing) and allow
// parallel processing of multiple tiles at a time.
//

'use strict';

/*
 * pixelFloor and pixelCeil are modified versions of Math.floor and Math.ceil
 * functions which take into account floating point arithmetic errors.
 * Those errors can cause undesired increments/decrements of sizes and offsets:
 * Math.ceil(36 / (36 / 500)) = 501
 * pixelCeil(36 / (36 / 500)) = 500
 */

var PIXEL_EPSILON = 1e-5;

function pixelFloor(x) {
  var nearest = Math.round(x);

  if (Math.abs(x - nearest) < PIXEL_EPSILON) { return nearest; }
  return Math.floor(x);
}

function pixelCeil(x) {
  var nearest = Math.round(x);

  if (Math.abs(x - nearest) < PIXEL_EPSILON) { return nearest; }
  return Math.ceil(x);
}


module.exports = function createRegions(options) {
  var scaleX = options.toWidth / options.width;
  var scaleY = options.toHeight / options.height;

  var innerTileWidth = pixelFloor(options.srcTileSize * scaleX) - 2 * options.destTileBorder;
  var innerTileHeight = pixelFloor(options.srcTileSize * scaleY) - 2 * options.destTileBorder;

  // prevent infinite loop, this should never happen
  if (innerTileWidth < 1 || innerTileHeight < 1) {
    throw new Error('Internal error in pica: target tile width/height is too small.');
  }

  var x, y;
  var innerX, innerY, toTileWidth, toTileHeight;
  var tiles = [];
  var tile;

  // we go top-to-down instead of left-to-right to make image displayed from top to
  // doesn in the browser
  for (innerY = 0; innerY < options.toHeight; innerY += innerTileHeight) {
    for (innerX = 0; innerX < options.toWidth; innerX += innerTileWidth) {
      x = innerX - options.destTileBorder;
      if (x < 0) { x = 0; }
      toTileWidth = innerX + innerTileWidth + options.destTileBorder - x;
      if (x + toTileWidth >= options.toWidth) {
        toTileWidth = options.toWidth - x;
      }

      y = innerY - options.destTileBorder;
      if (y < 0) { y = 0; }
      toTileHeight = innerY + innerTileHeight + options.destTileBorder - y;
      if (y + toTileHeight >= options.toHeight) {
        toTileHeight = options.toHeight - y;
      }

      tile = {
        toX: x,
        toY: y,
        toWidth: toTileWidth,
        toHeight: toTileHeight,

        toInnerX: innerX,
        toInnerY: innerY,
        toInnerWidth: innerTileWidth,
        toInnerHeight: innerTileHeight,

        offsetX: x / scaleX - pixelFloor(x / scaleX),
        offsetY: y / scaleY - pixelFloor(y / scaleY),
        scaleX: scaleX,
        scaleY: scaleY,

        x: pixelFloor(x / scaleX),
        y: pixelFloor(y / scaleY),
        width: pixelCeil(toTileWidth / scaleX),
        height: pixelCeil(toTileHeight / scaleY)
      };

      tiles.push(tile);
    }
  }

  return tiles;
};