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// Copyright 2014 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 core

import (
    "math/big"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/core/types"
    "github.com/ethereum/go-ethereum/core/vm"
)

// BlockFetcher retrieves headers by their hash
type HeaderFetcher interface {
    // GetHeader returns the hash corresponding to their hash
    GetHeader(common.Hash, uint64) *types.Header
}

// NewEVMContext creates a new context for use in the EVM.
func NewEVMContext(msg Message, header *types.Header, chain HeaderFetcher) vm.Context {
    return vm.Context{
        CanTransfer: CanTransfer,
        Transfer:    Transfer,
        GetHash:     GetHashFn(header, chain),

        Origin:      msg.From(),
        Coinbase:    header.Coinbase,
        BlockNumber: new(big.Int).Set(header.Number),
        Time:        new(big.Int).Set(header.Time),
        Difficulty:  new(big.Int).Set(header.Difficulty),
        GasLimit:    new(big.Int).Set(header.GasLimit),
        GasPrice:    new(big.Int).Set(msg.GasPrice()),
    }
}

// GetHashFn returns a GetHashFunc which retrieves header hashes by number
func GetHashFn(ref *types.Header, chain HeaderFetcher) func(n uint64) common.Hash {
    return func(n uint64) common.Hash {
        for header := chain.GetHeader(ref.ParentHash, ref.Number.Uint64()-1); header != nil; header = chain.GetHeader(header.ParentHash, header.Number.Uint64()-1) {
            if header.Number.Uint64() == n {
                return header.Hash()
            }
        }

        return common.Hash{}
    }
}

// CanTransfer checks wether there are enough funds in the address' account to make a transfer.
// This does not take the necessary gas in to account to make the transfer valid.
func CanTransfer(db vm.StateDB, addr common.Address, amount *big.Int) bool {
    return db.GetBalance(addr).Cmp(amount) >= 0
}

// Transfer subtracts amount from sender and adds amount to recipient using the given Db
func Transfer(db vm.StateDB, sender, recipient common.Address, amount *big.Int) {
    db.SubBalance(sender, amount)
    db.AddBalance(recipient, amount)
}