// contracts/Messages.sol
// SPDX-License-Identifier: Apache 2

pragma solidity ^0.8.0;
pragma experimental ABIEncoderV2;

import "./ReceiverGetters.sol";
import "./ReceiverStructs.sol";
import "../libraries/external/BytesLib.sol";


contract ReceiverMessages is ReceiverGetters {
    using BytesLib for bytes;

    /// @dev parseAndVerifyVM serves to parse an encodedVM and wholy validate it for consumption
    function parseAndVerifyVM(bytes calldata encodedVM) public view returns (ReceiverStructs.VM memory vm, bool valid, string memory reason) {
        vm = parseVM(encodedVM);
        (valid, reason) = verifyVM(vm);
    }

   /**
    * @dev `verifyVM` serves to validate an arbitrary vm against a valid Guardian set
    *  - it aims to make sure the VM is for a known guardianSet
    *  - it aims to ensure the guardianSet is not expired
    *  - it aims to ensure the VM has reached quorum
    *  - it aims to verify the signatures provided against the guardianSet
    */
    function verifyVM(ReceiverStructs.VM memory vm) public view returns (bool valid, string memory reason) {
        /// @dev Obtain the current guardianSet for the guardianSetIndex provided
        ReceiverStructs.GuardianSet memory guardianSet = getGuardianSet(vm.guardianSetIndex);

       /**
        * @dev Checks whether the guardianSet has zero keys
        * WARNING: This keys check is critical to ensure the guardianSet has keys present AND to ensure 
        * that guardianSet key size doesn't fall to zero and negatively impact quorum assessment.  If guardianSet
        * key length is 0 and vm.signatures length is 0, this could compromise the integrity of both vm and 
        * signature verification.
        */
        if(guardianSet.keys.length == 0){
            return (false, "invalid guardian set");
        }

        /// @dev Checks if VM guardian set index matches the current index (unless the current set is expired).
        if(vm.guardianSetIndex != getCurrentGuardianSetIndex() && guardianSet.expirationTime < block.timestamp){
            return (false, "guardian set has expired");
        }

       /**
        * @dev We're using a fixed point number transformation with 1 decimal to deal with rounding.
        *   WARNING: This quorum check is critical to assessing whether we have enough Guardian signatures to validate a VM
        *   if making any changes to this, obtain additional peer review. If guardianSet key length is 0 and 
        *   vm.signatures length is 0, this could compromise the integrity of both vm and signature verification.
        */
        if(((guardianSet.keys.length * 10 / 3) * 2) / 10 + 1 > vm.signatures.length){
            return (false, "no quorum");
        }

        /// @dev Verify the proposed vm.signatures against the guardianSet
        (bool signaturesValid, string memory invalidReason) = verifySignatures(vm.hash, vm.signatures, guardianSet);
        if(!signaturesValid){
            return (false, invalidReason);
        }

        /// If we are here, we've validated the VM is a valid multi-sig that matches the guardianSet.
        return (true, "");
    }

    /**
     * @dev verifySignatures serves to validate arbitrary sigatures against an arbitrary guardianSet
     *  - it intentionally does not solve for expectations within guardianSet (you should use verifyVM if you need these protections)
     *  - it intentioanlly does not solve for quorum (you should use verifyVM if you need these protections)
     *  - it intentionally returns true when signatures is an empty set (you should use verifyVM if you need these protections)
     */
    function verifySignatures(bytes32 hash, ReceiverStructs.Signature[] memory signatures, ReceiverStructs.GuardianSet memory guardianSet) public pure returns (bool valid, string memory reason) {
        uint8 lastIndex = 0;
        for (uint i = 0; i < signatures.length; i++) {
            ReceiverStructs.Signature memory sig = signatures[i];

            /// Ensure that provided signature indices are ascending only
            require(i == 0 || sig.guardianIndex > lastIndex, "signature indices must be ascending");
            lastIndex = sig.guardianIndex;

            /// Check to see if the signer of the signature does not match a specific Guardian key at the provided index
            if(ecrecover(hash, sig.v, sig.r, sig.s) != guardianSet.keys[sig.guardianIndex]){
                return (false, "VM signature invalid");
            }
        }

        /// If we are here, we've validated that the provided signatures are valid for the provided guardianSet
        return (true, "");
    }

    /**
     * @dev parseVM serves to parse an encodedVM into a vm struct
     *  - it intentionally performs no validation functions, it simply parses raw into a struct
     */
    function parseVM(bytes memory encodedVM) public pure virtual returns (ReceiverStructs.VM memory vm) {
        uint index = 0;

        vm.version = encodedVM.toUint8(index);
        index += 1;
        require(vm.version == 1, "VM version incompatible");

        vm.guardianSetIndex = encodedVM.toUint32(index);
        index += 4;

        // Parse Signatures
        uint256 signersLen = encodedVM.toUint8(index);
        index += 1;
        vm.signatures = new ReceiverStructs.Signature[](signersLen);
        for (uint i = 0; i < signersLen; i++) {
            vm.signatures[i].guardianIndex = encodedVM.toUint8(index);
            index += 1;

            vm.signatures[i].r = encodedVM.toBytes32(index);
            index += 32;
            vm.signatures[i].s = encodedVM.toBytes32(index);
            index += 32;
            vm.signatures[i].v = encodedVM.toUint8(index) + 27;
            index += 1;
        }

        // Hash the body
        bytes memory body = encodedVM.slice(index, encodedVM.length - index);
        vm.hash = keccak256(abi.encodePacked(keccak256(body)));

        // Parse the body
        vm.timestamp = encodedVM.toUint32(index);
        index += 4;

        vm.nonce = encodedVM.toUint32(index);
        index += 4;

        vm.emitterChainId = encodedVM.toUint16(index);
        index += 2;

        vm.emitterAddress = encodedVM.toBytes32(index);
        index += 32;

        vm.sequence = encodedVM.toUint64(index);
        index += 8;

        vm.consistencyLevel = encodedVM.toUint8(index);
        index += 1;

        vm.payload = encodedVM.slice(index, encodedVM.length - index);
    }
}