How to implement BitSet with Go?

Question

I didn't find a BitSet package in Go, so I tried to implement it. I'd like to use a array of uint64 to store the bits.

I need the number of bits to allocate the uint64 array. With Java, I can define a constructor that takes an integer. While Go doesn't provide constructor, how can I properly initialize the BitSet 'object' when user call new()?

Answer 1

If you use a []uint64 slice to store your data, then the zero slice could function as the empty BitSet. In fact appending to a nil slice allocates a new array for you, although the Language Specification doesn't appear to guarantee that. With that kind of setup, new(BitSet) would be immediately usable. Example:

bitset.go:

package bitset

const size = 64

type bits uint64

// BitSet is a set of bits that can be set, cleared and queried.
type BitSet []bits

// Set ensures that the given bit is set in the BitSet.
func (s *BitSet) Set(i uint) {
    if len(*s) < int(i/size+1) {
        r := make([]bits, i/size+1)
        copy(r, *s)
        *s = r
    }
    (*s)[i/size] |= 1 << (i % size)
}

// Clear ensures that the given bit is cleared (not set) in the BitSet.
func (s *BitSet) Clear(i uint) {
    if len(*s) >= int(i/size+1) {
        (*s)[i/size] &^= 1 << (i % size)
    }
}

// IsSet returns true if the given bit is set, false if it is cleared.
func (s *BitSet) IsSet(i uint) bool {
    return (*s)[i/size]&(1<<(i%size)) != 0
}

bitset_test.go:

package bitset

import "fmt"

func ExampleBitSet() {
    s := new(BitSet)
    s.Set(13)
    s.Set(45)
    s.Clear(13)
    fmt.Printf("s.IsSet(13) = %t; s.IsSet(45) = %t; s.IsSet(30) = %t\n",
               s.IsSet(13), s.IsSet(45), s.IsSet(30))
    // Output: s.IsSet(13) = false; s.IsSet(45) = true; s.IsSet(30) = false
}

Answer 2

Declare bitSet as a private struct:

type bitSet struct {
  len int
  array []uint64
}

Expose the interface BitSet:

type BitSet interface {
  Has(pos int) bool
  Add(pos int) bool
  Len() int
}

Also expose a function NewBitSet:

func NewBitSet(len int) BitSet {
  return &bitSet{len, make(uint64, (len+7) / 8) }
}

This is a Go way for encapsulation: share an interface, not the implementation.

Answer 3

The short answer is, you can't properly initialize the BitSet object when a client calls new ().

The best thing you can do is make it so that your BitSet 's zero value is valid. This is what types like list.List , sync.Mutex , and big.Int do. This way you know that it's impossible for a client to get an invalid value.

The next best thing you can do is create a constuctor-like function (named NewBitSet in this case) and expect clients to call it.

Answer 4

Go's standard big.Int can be used as a bit set:

package main

import (
    "fmt"
    "math/big"
)

func main() {
    var bits big.Int
    for i := 1000; i < 2000; i++ {
        bits.SetBit(&bits, i, 1)
    }
    for i := 0; i < 10000; i++ {
        if bits.Bit(i) != 0 {
            fmt.Println(i)
        }
    }
}

https://play.golang.org/p/xbIK-boouqC