var blit = (function(){
  var blit = {}
  blit.and = blit.atop = function(A, B, x, y){
    x = x || 0 ; y = y || 0
    B.forEach(function(lex, u, v){
      var cell = A.getCell(u+x, v+y)
      if (cell && lex.opacity > 0) {
        cell.assign(lex)
      }
    })
  }
  blit.or = blit.under = function(A, B, x, y){
    x = x || 0 ; y = y || 0
    B.forEach(function(lex, u, v){
      var cell = A.getCell(u+x, v+y)
      if (cell && cell.opacity == 0) {
        cell.assign(lex)
      }
    })
  }
  // copy the region of A beginning at x,y into B
  blit.copy_from = function(A, B, x, y){
    x = x || 0 ; y = y || 0
    B.forEach(function(lex, u, v){
      var cell = A.getCell(u+x, v+y)
      if (cell) {
        lex.assign(cell)
      }
    })
  }
  blit.copy_to = function(A, B, x, y){
    x = x || 0 ; y = y || 0
    B.forEach(function(lex, u, v){
      var cell = A.getCell(u+x, v+y)
      if (cell) {
        cell.assign(lex)
      }
    })
  }
  blit.invert = function(A, B, x, y){
    x = x || 0 ; y = y || 0
    B.forEach(function(lex, u, v){
      var cell = A.getCell(u+x, v+y)
      if (cell && lex.opacity > 0) {
        cell.fg = get_inverse(cell.fg)
        cell.bg = get_inverse(cell.bg)
      }
    })
  }
  blit.circle = function(A, lex){
    var hw = brush.w/2|0, hh = brush.h/2|0
    A.forEach(function(lex,x,y) {
      var len = Math.sqrt(Math.pow(x-hw,2)+Math.pow(y-hh,2))
      if (len > Math.abs(hw)) {
        lex.clear()
      }
    })
  }
  blit.cross = function(A, lex){
		A.forEach(function(lex,x,y) {
			if ((x+y)%2) {
				lex.clear()
			}
		})
  }

  return blit
})()