This is the copy of original crypto/sha256 package. The only difference is that digest struct in this package is exported. It allows save/restore hash state without marshalling/unmarshalling

Hash state copy example

package main

import (
    "encoding/hex"
    "fmt"
    "github.com/maxhero/sha256"
)

func main() {
    fixedBytes := make([]byte, 80)

    hashState1 := *sha256.New().(*sha256.Digest) //value of hash state, not a pointer
    hashState1.Write(fixedBytes) //write first 80 bytes as they are the same for both messages

    hashState2 := hashState1 //save hash state

    hashState1.Write([]byte{1, 2, 3, 4}) //write last 4 bytes of first message
    hashState2.Write([]byte{5, 6, 7, 8}) //write last 4 bytes of second message

    log.Println(hex.EncodeToString(hashState1.Sum(nil)))
    log.Println(hex.EncodeToString(hashState2.Sum(nil)))
}

Output is

429eb588aa975a12e4dee3db42bd26f406d5931e9f71886b875defe218a6d119
019e19457ba8f51801274b7a487b74c2283e63d527b448ab1bd039beeb8f3f5e 

We still can use this package as original one

fixedBytes := make([]byte, 80)

hashState1 := sha256.New() //pointer on hash state 
hashState1.Write(fixedBytes)

hashState2 := hashState1 //pointer copy

hashState1.Write([]byte{1, 2, 3, 4})
hashState2.Write([]byte{5, 6, 7, 8})

fmt.Println(hex.EncodeToString(hashState1.Sum(nil)))
fmt.Println(hex.EncodeToString(hashState2.Sum(nil)))    

Output is

8a8143bb66f9e38fdecd45c6b8ad686845c0202e05d5e8bbb5daddcaafc73d3b
8a8143bb66f9e38fdecd45c6b8ad686845c0202e05d5e8bbb5daddcaafc73d3b

As you can see, in this case output hashes are the same. This is because previous code equals to

fixedBytes := make([]byte, 80)
    
hashState := sha256.New() 
hashState.Write(fixedBytes)        
hashState.Write([]byte{1, 2, 3, 4, 5, 6, 7, 8})        	
    
fmt.Println(hex.EncodeToString(hashState.Sum(nil)))    

Output is

8a8143bb66f9e38fdecd45c6b8ad686845c0202e05d5e8bbb5daddcaafc73d3b