hash/maphash: first call of the Reset method doesn't reset internal state

[ALTree]

The new hash/maphash package has a Reset method documented as:

Reset discards all bytes added to h. (The seed remains the same.)

Moreover, the Sum64 doc says:

Sum64 returns h's current 64-bit value, which depends on h's seed and the sequence of bytes added to h since the last call to Reset

So it appears it would be reasonable to expect this to print two times the same value:

package main

import (
	"fmt"
	"hash/maphash"
)

func main() {
	h := new(maphash.Hash)

	h.WriteString("hello")
	fmt.Printf("%#x\n", h.Sum64())

	h.Reset()

	h.WriteString("hello")
	fmt.Printf("%#x\n", h.Sum64())
}

But it doesn't:

$ gotip run hash.go
0xdf1aabcc36e56272
0x5a04b54a5a2174d6

This is because even if the seed remains the same and "all the previously added bytes are discarded", the internal state of the Hash is maintained, even after the h.Reset() call.

Is this the desired behaviour? If it is, the documentation should probably be amended to make it clear what to expect after a call to the Reset method.

cc @alandonovan @randall77

[vovapi]

There's seems to be a bug in a Hash because if you reset and hash again you'll get the same result as the second one:

package main

import (
    "fmt"
    "hash/maphash"
)

func main() {
    h := new(maphash.Hash)

    h.WriteString("hello")
    fmt.Printf("%#x\n", h.Sum64()) // 0x48e0352e23362dd2

    h.Reset()

    h.WriteString("hello")
    fmt.Printf("%#x\n", h.Sum64()) // 0xf9395b015acc3ffb

    h.Reset()

    h.WriteString("hello")
    fmt.Printf("%#x\n", h.Sum64()) // 0xf9395b015acc3ffb
}

The first hash sum seems to be ignoring all the data that was put through it and returning sum as it has an empty state:

package main

import (
    "fmt"
    "hash/maphash"
)

func main() {
    h1 := new(maphash.Hash)

    h1.WriteString("hello")
    sum1 := h1.Sum64() // first call on initSeed

    h1.Reset()

    fmt.Println(sum1 == h1.Sum64()) // true

    h2 := new(maphash.Hash)
    h2.SetSeed(h1.Seed())
    sum2 := h2.Sum64()

    fmt.Println(sum1 == sum2) // true
}

I think the issue is that the first Sum64 calls initSeed, which initializes the random seed using SetSeed. SetSeed discards previously written data.

[gopherbot]

Change https://golang.org/cl/220259 mentions this issue: hash/maphash: don't discard data on random seed init