// Copyright 2018 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package mru

import (
	"bytes"
	"hash"

	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/crypto"
	"github.com/ethereum/go-ethereum/swarm/storage"
)

// SignedResourceUpdate represents a resource update with all the necessary information to prove ownership of the resource
type SignedResourceUpdate struct {
	resourceUpdate // actual content that will be put on the chunk, less signature
	signature      *Signature
	updateAddr     storage.Address // resulting chunk address for the update (not serialized, for internal use)
	binaryData     []byte          // resulting serialized data (not serialized, for efficiency/internal use)
}

// Verify checks that signatures are valid and that the signer owns the resource to be updated
func (r *SignedResourceUpdate) Verify() (err error) {
	if len(r.data) == 0 {
		return NewError(ErrInvalidValue, "Update does not contain data")
	}
	if r.signature == nil {
		return NewError(ErrInvalidSignature, "Missing signature field")
	}

	digest, err := r.GetDigest()
	if err != nil {
		return err
	}

	// get the address of the signer (which also checks that it's a valid signature)
	ownerAddr, err := getOwner(digest, *r.signature)
	if err != nil {
		return err
	}

	if !bytes.Equal(r.updateAddr, r.UpdateAddr()) {
		return NewError(ErrInvalidSignature, "Signature address does not match with ownerAddr")
	}

	// Check if who signed the resource update really owns the resource
	if !verifyOwner(ownerAddr, r.metaHash, r.rootAddr) {
		return NewErrorf(ErrUnauthorized, "signature is valid but signer does not own the resource: %v", err)
	}

	return nil
}

// Sign executes the signature to validate the resource
func (r *SignedResourceUpdate) Sign(signer Signer) error {

	r.binaryData = nil           //invalidate serialized data
	digest, err := r.GetDigest() // computes digest and serializes into .binaryData
	if err != nil {
		return err
	}

	signature, err := signer.Sign(digest)
	if err != nil {
		return err
	}

	// Although the Signer interface returns the public address of the signer,
	// recover it from the signature to see if they match
	ownerAddress, err := getOwner(digest, signature)
	if err != nil {
		return NewError(ErrInvalidSignature, "Error verifying signature")
	}

	if ownerAddress != signer.Address() { // sanity check to make sure the Signer is declaring the same address used to sign!
		return NewError(ErrInvalidSignature, "Signer address does not match ownerAddr")
	}

	r.signature = &signature
	r.updateAddr = r.UpdateAddr()
	return nil
}

// create an update chunk.
func (r *SignedResourceUpdate) toChunk() (storage.Chunk, error) {

	// Check that the update is signed and serialized
	// For efficiency, data is serialized during signature and cached in
	// the binaryData field when computing the signature digest in .getDigest()
	if r.signature == nil || r.binaryData == nil {
		return nil, NewError(ErrInvalidSignature, "newUpdateChunk called without a valid signature or payload data. Call .Sign() first.")
	}

	resourceUpdateLength := r.resourceUpdate.binaryLength()
	// signature is the last item in the chunk data
	copy(r.binaryData[resourceUpdateLength:], r.signature[:])

	chunk := storage.NewChunk(r.updateAddr, r.binaryData)
	return chunk, nil
}

// fromChunk populates this structure from chunk data. It does not verify the signature is valid.
func (r *SignedResourceUpdate) fromChunk(updateAddr storage.Address, chunkdata []byte) error {
	// for update chunk layout see SignedResourceUpdate definition

	//deserialize the resource update portion
	if err := r.resourceUpdate.binaryGet(chunkdata); err != nil {
		return err
	}

	// Extract the signature
	var signature *Signature
	cursor := r.resourceUpdate.binaryLength()
	sigdata := chunkdata[cursor : cursor+signatureLength]
	if len(sigdata) > 0 {
		signature = &Signature{}
		copy(signature[:], sigdata)
	}

	r.signature = signature
	r.updateAddr = updateAddr
	r.binaryData = chunkdata

	return nil

}

// GetDigest creates the resource update digest used in signatures (formerly known as keyDataHash)
// the serialized payload is cached in .binaryData
func (r *SignedResourceUpdate) GetDigest() (result common.Hash, err error) {
	hasher := hashPool.Get().(hash.Hash)
	defer hashPool.Put(hasher)
	hasher.Reset()
	dataLength := r.resourceUpdate.binaryLength()
	if r.binaryData == nil {
		r.binaryData = make([]byte, dataLength+signatureLength)
		if err := r.resourceUpdate.binaryPut(r.binaryData[:dataLength]); err != nil {
			return result, err
		}
	}
	hasher.Write(r.binaryData[:dataLength]) //everything except the signature.

	return common.BytesToHash(hasher.Sum(nil)), nil
}

// getOwner extracts the address of the resource update signer
func getOwner(digest common.Hash, signature Signature) (common.Address, error) {
	pub, err := crypto.SigToPub(digest.Bytes(), signature[:])
	if err != nil {
		return common.Address{}, err
	}
	return crypto.PubkeyToAddress(*pub), nil
}

// verifyResourceOwnerhsip checks that the signer of the update actually owns the resource
// H(ownerAddr, metaHash) is computed. If it matches the rootAddr the update chunk is claiming
// to update, it is proven that signer of the resource update owns the resource.
// See metadataHash in metadata.go for a more detailed explanation
func verifyOwner(ownerAddr common.Address, metaHash []byte, rootAddr storage.Address) bool {
	hasher := hashPool.Get().(hash.Hash)
	defer hashPool.Put(hasher)
	hasher.Reset()
	hasher.Write(metaHash)
	hasher.Write(ownerAddr.Bytes())
	rootAddr2 := hasher.Sum(nil)
	return bytes.Equal(rootAddr2, rootAddr)
}