Confused about why hashsum, encode, print is different then write, hashum, encode, print in Go?

Sorry about the title. I couldn't come up with a better way of phrasing my question and will change it happily if somebody else can.

Hasher is defined with

hasher := md5.New()

Anyway, I am curious why this:

fmt.Println(hex.EncodeToString(hasher.Sum([]byte(input))))

gives me 6869d41d8cd98f00b204e9800998ecf8427e, while this:

hasher.Write([]byte(input))
fmt.Println(hex.EncodeToString(hasher.Sum(nil))

gives me 49f68a5c8493ec2c0bf489821c21fc3b
and this:

fmt.Printf("%x\n", md5.Sum([]byte(input)))

gives me 49f68a5c8493ec2c0bf489821c21fc3b.

Generally hasher.Sum() does not hash the passed slice. The passed slice is used as the destination: it appends the current hash to it and does not change the underlying hash state. hasher.Write() obviously includes the passed slice in the hash calculation. The 2 examples are fundamentally different, the different results are nothing but expected.

Always read the documentation. hash.Hash.Sum():

> // Sum appends the current hash to b and returns the resulting slice.
> // It does not change the underlying hash state.
> Sum(b []byte) []byte

So when you first call hasher.Sum(), whatever you pass to it, it doesn't matter in terms of the result hash. If you haven't written anything into hasher previously, you'll see the initial hash.

When you next call hasher.Write([]byte(input)), you'll write the bytes of input into the hasher, so when you call hasher.Sum(nil) next, you'll see the calculated hash of input. Since you pass nil, a new slice will be allocated to accommodate the result.

When you again call hasher.Write([]byte(input)), as previously written: this won't change the hash state, the passed slice is not used as input, but only as the destination for "returning" the result, the current hash value. So you will get the same hash value as you got from the previous hasher.Sum(nil) call. Obviously, if the passed slice does not have enough capacity to store the result, a new one will be allocated / used.

See this complete, runnable example which reproduces your output:

input := "hi"
hasher := md5.New()
fmt.Println(hex.EncodeToString(hasher.Sum([]byte(input))))

hasher.Write([]byte(input))
fmt.Println(hex.EncodeToString(hasher.Sum(nil)))

fmt.Printf("%x\n", md5.Sum([]byte(input)))

Try it on the Go Playground.