Matrix find function intended for use with pixel positions

gistfile1.js

//searches the matrix for x and y pixel position using binary search
function find(x, y){
  var rowData = binarySearch(mx, 0, mx.length - 1, 'y', this.ySize, y);
  var colData = binarySearch(rowData.x, 0, rowData.x.length - 1, 'x', this.xSize, x);

  return colData; 
}

function binarySearch(arr, start, stop, bottomBoundKey, size, val){
  var midpoint, data, midBottomBound, midTopBound;

  if(stop < start){
    return null;
  }else{
    midpoint = midpoint(start, stop);
    data = arr[midpoint];
    midBottomBound = data[bottomBoundKey];
    midTopBound = midBottomBound + size;

    if(val < midBottomBound){
      //search bottom half
      return binarySearch(arr, start, midpoint, bottomBoundKey, size, val);
    }else if(val > midTopBound){
      //search top half
      return binarySearch(arr, midpoint, stop, bottomBoundKey, size, val);
    }else{
      //found
      return data;
    }
  }
}

function midpoint(start, stop){
  return Math.floor((start + stop) / 2);
}