How to implement Bit array in Golang (code example)

The following column Golang Tutorial will introduce to you the implementation method of Bit array in Golang. I hope it will be helpful to friends in need!

Go languageCommon methods for implementing Bit arrays

Collections in Go language generally use map[T]bool In this form, T represents the element type. Collections are represented using map types, which is very flexible, but we can represent it in a better form. For example, in the field of data flow analysis, the set element is usually a non-negative integer, the set will contain many elements, and the set will often perform union and intersection operations. In this case, the bit array will perform more ideally than the map.

A bit array is usually represented by an unsigned number or a slice called a "word". Each bit of each element represents a value in the set. When the i-th bit of a set is set, we say that the set contains element i. The following program shows a simple bit array type and implements three functions to operate on this bit array:

package main
import (
	"bytes"
	"fmt"
)
// An IntSet is a set of small non-negative integers.
// Its zero value represents the empty set.
type IntSet struct {
	words []uint
}
const (
	bitNum = (32 << (^uint(0) >> 63)) //根据平台自动判断决定是32还是64
)
// Has reports whether the set contains the non-negative value x.
func (s *IntSet) Has(x int) bool {
	word, bit := x/bitNum, uint(x%bitNum)
	return word < len(s.words) && s.words[word]&(1<<bit) != 0
}
// Add adds the non-negative value x to the set.
func (s *IntSet) Add(x int) {
	word, bit := x/bitNum, uint(x%bitNum)
	for word >= len(s.words) {
		s.words = append(s.words, 0)
	}
	s.words[word] |= 1 << bit
}
//A与B的交集，合并A与B
// UnionWith sets s to the union of s and t.
func (s *IntSet) UnionWith(t *IntSet) {
	for i, tword := range t.words {
		if i < len(s.words) {
			s.words[i] |= tword
		} else {
			s.words = append(s.words, tword)
		}
	}
}

Because each word has 64 binary bits, in order to locate x bit, we use the quotient of x/64 as the subscript of the word, and use the value obtained by xd as the position of the bit in the word.

For example, for the number 1, add it to the bit array:

func (s *IntSet) Add(x int) {
	word, bit := x/bitNum, uint(x%bitNum) //0, 1 := 1/64, uint(1%64)
	for word >= len(s.words) { // 条件不满足
		s.words = append(s.words, 0)
	}
	s.words[word] |= 1 << bit // s.words[0] |= 1 << 1
}
// 把1存入后，words数组变为了[]uint64{2}

Similarly, if we add 66 to the bit array:

func (s *IntSet) Add(x int) {
	word, bit := x/bitNum, uint(x%bitNum) //1, 2 := 66/64, uint(66%64)
	for word >= len(s.words) { // 条件满足
		s.words = append(s.words, 0) // 此时s.words = []uint64{2, 0}
	}
	s.words[word] |= 1 << bit // s.words[1] |= 1 << 2
}
// 继续把66存入后，words数组变为了[]uint64{2, 4}

So, for words, each There are 64 values ​​that can be stored in each element. Every time the value exceeds 64, a carry is performed, that is, an element is added. (Note that 0 also occupies one bit, so 64 needs to be carried, and the first element can store 0-63).

So, for an element in words, when you want to convert it to a specific value: first get its position i, use 64 * i as the number of carries (similar to carrying every 10 digits), and then This element is converted into a binary number. Counting from right to left, the number of digits is 1, which means that the corresponding value is present. The number of digits x 64 *i is the value we store.

Correspondingly, there can be the following String() function

// String returns the set as a string of the form "{1 2 3}".
func (s *IntSet) String() string {
	var buf bytes.Buffer
	buf.WriteByte('{')
	for i, word := range s.words {
		if word == 0 {
			continue
		}
		for j := 0; j < bitNum; j++ {
			if word&(1<<uint(j)) != 0 {
				if buf.Len() > len("{") {
					buf.WriteByte(&#39; &#39;)
				}
				fmt.Fprintf(&buf, "%d", bitNum*i+j)
			}
		}
	}
	buf.WriteByte(&#39;}&#39;)
	return buf.String()
}

For example, after storing 1 and 66 previously, the conversion process is:

// []uint64{2 4}
// 对于2: 1 << 1 = 2; 所以 x = 0 * 64 + 1 
// 对于4: 1 << 2 = 4; 所以 x = 1 * 64 + 2
// 所以转换为String为{1 66}

Implement other bit arrays Method function

func (s *IntSet) Len() int {
	var len int
	for _, word := range s.words {
		for j := 0; j < bitNum; j++ {
			if word&(1<<uint(j)) != 0 {
				len++
			}
		}
	}
	return len
}
func (s *IntSet) Remove(x int) {
	word, bit := x/bitNum, uint(x%bitNum)
	if s.Has(x) {
		s.words[word] ^= 1 << bit
	}
}
func (s *IntSet) Clear() {
	s.words = append([]uint{})
}
func (s *IntSet) Copy() *IntSet {
	intSet := &IntSet{
		words: []uint{},
	}
	for _, value := range s.words {
		intSet.words = append(intSet.words, value)
	}
	return intSet
}
func (s *IntSet) AddAll(args ...int) {
	for _, x := range args {
		s.Add(x)
	}
}
//A与B的并集，A与B中均出现
func (s *IntSet) IntersectWith(t *IntSet) {
	for i, tword := range t.words {
		if i >= len(s.words) {
			continue
		}
		s.words[i] &= tword
	}
}
//A与B的差集，元素出现在A未出现在B
func (s *IntSet) DifferenceWith(t *IntSet) {
	t1 := t.Copy() //为了不改变传参t，拷贝一份
	t1.IntersectWith(s)
	for i, tword := range t1.words {
		if i < len(s.words) {
			s.words[i] ^= tword
		}
	}
}
//A与B的并差集，元素出现在A没有出现在B，或出现在B没有出现在A
func (s *IntSet) SymmetricDifference(t *IntSet) {
	for i, tword := range t.words {
		if i < len(s.words) {
			s.words[i] ^= tword
		} else {
			s.words = append(s.words, tword)
		}
	}
}
//获取比特数组中的所有元素的slice集合
func (s *IntSet) Elems() []int {
	var elems []int
	for i, word := range s.words {
		for j := 0; j < bitNum; j++ {
			if word&(1<<uint(j)) != 0 {
				elems = append(elems, bitNum*i+j)
			}
		}
	}
	return elems
}

At this point, the common method functions of bit arrays have been implemented, and you can now use it.

func main() {
	var x, y IntSet
	x.Add(1)
	x.Add(144)
	x.Add(9)
	fmt.Println("x:", x.String()) // "{1 9 144}"
	y.Add(9)
	y.Add(42)
	fmt.Println("y:", y.String()) // "{9 42}"
	x.UnionWith(&y)
	fmt.Println("x unionWith y:", x.String())         // "{1 9 42 144}"
	fmt.Println("x has 9,123:", x.Has(9), x.Has(123)) // "true false"
	fmt.Println("x len:", x.Len())                    //4
	fmt.Println("y len:", y.Len())                    //2
	x.Remove(42)
	fmt.Println("x after Remove 42:", x.String()) //{1 9 144}
	z := x.Copy()
	fmt.Println("z copy from x:", z.String()) //{1 9 144}
	x.Clear()
	fmt.Println("clear x:", x.String()) //{}
	x.AddAll(1, 2, 9)
	fmt.Println("x addAll 1,2,9:", x.String()) //{1 2 9}
	x.IntersectWith(&y)
	fmt.Println("x intersectWith y:", x.String()) //{9}
	x.AddAll(1, 2)
	fmt.Println("x addAll 1,2:", x.String()) //{1 2 9}
	x.DifferenceWith(&y)
	fmt.Println("x differenceWith y:", x.String()) //{1 2}
	x.AddAll(9, 144)
	fmt.Println("x addAll 9,144:", x.String()) //{1 2 9 144}
	x.SymmetricDifference(&y)
	fmt.Println("x symmetricDifference y:", x.String()) //{1 2 42 144}
	for _, value := range x.Elems() {
		fmt.Print(value, " ") //1 2 42 144
	}
}

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