// Copyright 2017 The Netstack Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package hash contains utility functions for hashing.
package hash

import (
	"crypto/rand"
	"encoding/binary"
	"fmt"

	"github.com/google/netstack/tcpip/header"
)

var hashIV = RandN32(1)[0]

// RandN32 generates a slice of n cryptographic random 32-bit numbers.
func RandN32(n int) []uint32 {
	b := make([]byte, 4*n)
	if _, err := rand.Read(b); err != nil {
		panic(fmt.Sprintf("unable to get random numbers: %v", err))
	}
	r := make([]uint32, n)
	for i := range r {
		r[i] = binary.LittleEndian.Uint32(b[4*i : (4*i + 4)])
	}
	return r
}

// Hash3Words calculates the Jenkins hash of 3 32-bit words. This is adapted
// from linux.
func Hash3Words(a, b, c, initval uint32) uint32 {
	const iv = 0xdeadbeef + (3 << 2)
	initval += iv

	a += initval
	b += initval
	c += initval

	c ^= b
	c -= rol32(b, 14)
	a ^= c
	a -= rol32(c, 11)
	b ^= a
	b -= rol32(a, 25)
	c ^= b
	c -= rol32(b, 16)
	a ^= c
	a -= rol32(c, 4)
	b ^= a
	b -= rol32(a, 14)
	c ^= b
	c -= rol32(b, 24)

	return c
}

// IPv4FragmentHash computes the hash of the IPv4 fragment as suggested in RFC 791.
func IPv4FragmentHash(h header.IPv4) uint32 {
	x := uint32(h.ID())<<16 | uint32(h.Protocol())
	t := h.SourceAddress()
	y := uint32(t[0]) | uint32(t[1])<<8 | uint32(t[2])<<16 | uint32(t[3])<<24
	t = h.DestinationAddress()
	z := uint32(t[0]) | uint32(t[1])<<8 | uint32(t[2])<<16 | uint32(t[3])<<24
	return Hash3Words(x, y, z, hashIV)
}

// IPv6FragmentHash computes the hash of the ipv6 fragment.
// Unlike IPv4, the protocol is not used to compute the hash.
// RFC 2640 (sec 4.5) is not very sharp on this aspect.
// As a reference, also Linux ignores the protocol to compute
// the hash (inet6_hash_frag).
func IPv6FragmentHash(h header.IPv6, f header.IPv6Fragment) uint32 {
	t := h.SourceAddress()
	y := uint32(t[0]) | uint32(t[1])<<8 | uint32(t[2])<<16 | uint32(t[3])<<24
	t = h.DestinationAddress()
	z := uint32(t[0]) | uint32(t[1])<<8 | uint32(t[2])<<16 | uint32(t[3])<<24
	return Hash3Words(f.ID(), y, z, hashIV)
}

func rol32(v, shift uint32) uint32 {
	return (v << shift) | (v >> ((-shift) & 31))
}