Bitmasks

See bitflags.

Bitflags

Many functions, for example, arsd.simpledisplay.ScreenPainter.drawText and arsd.terminal.Terminal.color, take a uint typed argument that is supposed to be made from a combination of enum flags defined elsewhere in the file. These are often called "bit flags".

To create one of these arguments, you use D's bitwise or operator to combine various options. Color.red | Bright will combine the values of Color.red and Bright to make a new argument that arsd.terminal.Terminal.color can comprehend. TextAlignment.Center | TextAlignment.VerticalCenter makes a combined argument for drawText's alignment parameter.

The enum values will have values that go up multiplying by two. If you see values like 1, 2, 4, 8 in an enum's members, there's a good chance it is meant to be combined with the | operator when passed to a function.

The inverse is called a "bit mask" because various bits are "masked" - imagine just seeing someone's eyes through a mask but not their nose - out by the function to deconstruct the combined result back into its individual pieces for processing. D's & operator, bitwise and, is used inside the functions to undo the result of | on the outside. You can do this too if a function returns a combined result like this. arsd.simpledisplay.MouseEvent.modifierState is an example of a struct member made out of individual bits. If you check if(event.modifierState & ModifierState.leftButtonDown) {}, you can check for the individual items.

if(event.modifierState & (ModifierState.leftButtonDown | ModifierState.rightButtonDown)) {
	// this will be true if either the left OR right buttons are down
}