Bits

For large data sets, storing information in single bits can reduce space requirements. But accessing, and counting, this information is more complex.

In the math bits package, we find some helpful methods for bitwise operations in Go. With OnesCount we have a population count (popcnt).

`OnesCount`

The bits.OnesCount method performs a population count (a bitcount) on a uint. We must first cast a value like an int to a uint to use OnesCount.

Info For values like int and smaller data types like int16, casting to a uint will not change the bit count result.

package main

import (
    "math/bits"
    "fmt"
)

func main() {
    value := 100
    // Must cast to uint for OnesCount.
    result := bits.OnesCount(uint(value))
    fmt.Println(result)
}
3

`UintSize`, constant

How many bits are in a uint in the Go language? We can determine this number with the bits.UintSize constant.

Tip Using this constant will help your programs be more readable. It may make them more resilient across platforms.

package main

import (
    "math/bits"
    "fmt"
)

func main() {
    // Use constant to determine bits in a uint.
    fmt.Println("BITS IN UINT:", bits.UintSize)
}
BITS IN UINT: 64

Display bits

We have found that a uint has 64 bits. With bitwise operators, we can display each individual bit as 1 or 0. We use a for-loop for this example.

Tip We find that the value 100 has 3 bits set to 1. This means the OnesCount for 100 should be 3.

package main

import (
    "math/bits"
    "fmt"
)

func main() {
    value := uint(100)
    // Loop over all bits in the uint.
    for i := 0; i < bits.UintSize; i++ {
        // If there is a bit set at this position, write a 1.
        // ... Otherwise write a 0.
        if value & (1 << uint(i)) != 0 {
            fmt.Print("1")
        } else {
            fmt.Print("0")
        }
    }
}
0010011000000000000000000000000000000000000000000000000000000000

`TrailingZeros`

It is possible to loop over a uint and call TrailingZeros. This lets us iterate through set bits, ignoring bits that are not set.

Here We continue our for-loop until our value is 0 (this means all bits have been set to 0).

Next TrailingZeros gives us index of a set bit. We can erase the bit (set it to zero) so that the next call will find the next bit.

Result We get the indexes 2, 5 and 6 for the value 100. These are displayed with a for-loop over the bits.

package main

import (
    "math/bits"
    "fmt"
)

func main() {
    value := uint(100)
    count := 0
    // Continue looping until we are zero.
    for value != 0 {
        // Use trailing zeros in our bit count method.
        index := bits.TrailingZeros(value)
        // Print the bit set at this index.
        fmt.Println("Bit set at:", index)
        // Erase the bit.
        value &= ^(1 << uint(index))
        // Count it.
        count++
    }
    fmt.Println("Count:", count)
}Bit set at: 2
Bit set at: 5
Bit set at: 6
Count: 3

Bitwise operators

From the Go language specification, we can find a reference table of the bitwise operators available. We use a caret ("^") instead of a tilde ("~") for NOT.

&    bitwise AND
|    bitwise OR
^    bitwise XOR
&^   bit clear (AND NOT)

<<   left shift
>>   right shift

Some notes

For things like bitcounts, more advanced algorithms (such as ones that use lookup tables) may have performance benefits. Bits.OnesCount is simpler to use in programs.

The math bits package was not in early versions of the Go installation. It was added in Go 1.9. This is a useful package for low-level operations.