Datastructures: bitmasks

Bitmasks are an efficient way of combining boolean options that can be combined together.
The most familiar usage is probably unix filesystem permissions.

Under the hood

A bitmask is simply a combination of binary numbers into an integer.
Reusing unix permissions, say we have 3x combinable options.

• readable
• writable
• executable

# base(2) -- base(10)
# (rwx)
   001        # 1
   010        # 2
   011        # 3
   100        # 4
   101        # 5
   110        # 6
   111        # 7

Usage

Bitwise operators allow you to assign/unassign a flag for each digit of the binary number.

• | sets(1) the bit at that position
• ^ unsets(0) the bit at that position

readable   = 5  # binary: 100
writable   = 2  # binary: 010
executable = 1  # binary: 001

readable(100) | executable(001)                  # 5(101)
readable(100) | writable(010)                    # 6(110)
readable(100) | writable(010) | executable(001)  # 7(111)

(readable(100) | writable(010) | executable(001)) ^ executable(001)  # 6(110)