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// Copyright 2016 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 gasprice

import (
    "math/big"
    "sort"
    "sync"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/internal/ethapi"
    "github.com/ethereum/go-ethereum/rpc"
    "golang.org/x/net/context"
)

const (
    LpoAvgCount     = 5
    LpoMinCount     = 3
    LpoMaxBlocks    = 20
    LpoSelect       = 50
    LpoDefaultPrice = 20000000000
)

// LightPriceOracle recommends gas prices based on the content of recent
// blocks. Suitable for both light and full clients.
type LightPriceOracle struct {
    backend   ethapi.Backend
    lastHead  common.Hash
    lastPrice *big.Int
    cacheLock sync.RWMutex
    fetchLock sync.Mutex
}

// NewLightPriceOracle returns a new oracle.
func NewLightPriceOracle(backend ethapi.Backend) *LightPriceOracle {
    return &LightPriceOracle{
        backend:   backend,
        lastPrice: big.NewInt(LpoDefaultPrice),
    }
}

// SuggestPrice returns the recommended gas price.
func (self *LightPriceOracle) SuggestPrice(ctx context.Context) (*big.Int, error) {
    self.cacheLock.RLock()
    lastHead := self.lastHead
    lastPrice := self.lastPrice
    self.cacheLock.RUnlock()

    head, _ := self.backend.HeaderByNumber(ctx, rpc.LatestBlockNumber)
    headHash := head.Hash()
    if headHash == lastHead {
        return lastPrice, nil
    }

    self.fetchLock.Lock()
    defer self.fetchLock.Unlock()

    // try checking the cache again, maybe the last fetch fetched what we need
    self.cacheLock.RLock()
    lastHead = self.lastHead
    lastPrice = self.lastPrice
    self.cacheLock.RUnlock()
    if headHash == lastHead {
        return lastPrice, nil
    }

    blockNum := head.Number.Uint64()
    chn := make(chan lpResult, LpoMaxBlocks)
    sent := 0
    exp := 0
    var lps bigIntArray
    for sent < LpoAvgCount && blockNum > 0 {
        go self.getLowestPrice(ctx, blockNum, chn)
        sent++
        exp++
        blockNum--
    }
    maxEmpty := LpoAvgCount - LpoMinCount
    for exp > 0 {
        res := <-chn
        if res.err != nil {
            return nil, res.err
        }
        exp--
        if res.price != nil {
            lps = append(lps, res.price)
        } else {
            if maxEmpty > 0 {
                maxEmpty--
            } else {
                if blockNum > 0 && sent < LpoMaxBlocks {
                    go self.getLowestPrice(ctx, blockNum, chn)
                    sent++
                    exp++
                    blockNum--
                }
            }
        }
    }
    price := lastPrice
    if len(lps) > 0 {
        sort.Sort(lps)
        price = lps[(len(lps)-1)*LpoSelect/100]
    }

    self.cacheLock.Lock()
    self.lastHead = headHash
    self.lastPrice = price
    self.cacheLock.Unlock()
    return price, nil
}

type lpResult struct {
    price *big.Int
    err   error
}

// getLowestPrice calculates the lowest transaction gas price in a given block
// and sends it to the result channel. If the block is empty, price is nil.
func (self *LightPriceOracle) getLowestPrice(ctx context.Context, blockNum uint64, chn chan lpResult) {
    block, err := self.backend.BlockByNumber(ctx, rpc.BlockNumber(blockNum))
    if block == nil {
        chn <- lpResult{nil, err}
        return
    }
    txs := block.Transactions()
    if len(txs) == 0 {
        chn <- lpResult{nil, nil}
        return
    }
    // find smallest gasPrice
    minPrice := txs[0].GasPrice()
    for i := 1; i < len(txs); i++ {
        price := txs[i].GasPrice()
        if price.Cmp(minPrice) < 0 {
            minPrice = price
        }
    }
    chn <- lpResult{minPrice, nil}
}

type bigIntArray []*big.Int

func (s bigIntArray) Len() int           { return len(s) }
func (s bigIntArray) Less(i, j int) bool { return s[i].Cmp(s[j]) < 0 }
func (s bigIntArray) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }