// Copyright (c) 2015-2017 The btcsuite developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.

package blockchain

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

	"github.com/btcsuite/btcd/chaincfg"
	"github.com/btcsuite/btcd/chaincfg/chainhash"
	"github.com/btcsuite/btcd/database"
	"github.com/btcsuite/btcd/wire"
)

// blockNode represents a block within the block chain and is primarily used to
// aid in selecting the best chain to be the main chain.  The main chain is
// stored into the block database.
type blockNode struct {
	// NOTE: Additions, deletions, or modifications to the order of the
	// definitions in this struct should not be changed without considering
	// how it affects alignment on 64-bit platforms.  The current order is
	// specifically crafted to result in minimal padding.  There will be
	// hundreds of thousands of these in memory, so a few extra bytes of
	// padding adds up.

	// parent is the parent block for this node.
	parent *blockNode

	// hash is the double sha 256 of the block.
	hash chainhash.Hash

	// workSum is the total amount of work in the chain up to and including
	// this node.
	workSum *big.Int

	// height is the position in the block chain.
	height int32

	// Some fields from block headers to aid in best chain selection and
	// reconstructing headers from memory.  These must be treated as
	// immutable and are intentionally ordered to avoid padding on 64-bit
	// platforms.
	version    int32
	bits       uint32
	nonce      uint32
	timestamp  int64
	merkleRoot chainhash.Hash
}

// initBlockNode initializes a block node from the given header and height.  The
// node is completely disconnected from the chain and the workSum value is just
// the work for the passed block.  The work sum must be updated accordingly when
// the node is inserted into a chain.
//
// This function is NOT safe for concurrent access.  It must only be called when
// initially creating a node.
func initBlockNode(node *blockNode, blockHeader *wire.BlockHeader, height int32) {
	*node = blockNode{
		hash:       blockHeader.BlockHash(),
		workSum:    CalcWork(blockHeader.Bits),
		height:     height,
		version:    blockHeader.Version,
		bits:       blockHeader.Bits,
		nonce:      blockHeader.Nonce,
		timestamp:  blockHeader.Timestamp.Unix(),
		merkleRoot: blockHeader.MerkleRoot,
	}
}

// newBlockNode returns a new block node for the given block header.  It is
// completely disconnected from the chain and the workSum value is just the work
// for the passed block.  The work sum must be updated accordingly when the node
// is inserted into a chain.
func newBlockNode(blockHeader *wire.BlockHeader, height int32) *blockNode {
	var node blockNode
	initBlockNode(&node, blockHeader, height)
	return &node
}

// Header constructs a block header from the node and returns it.
//
// This function is safe for concurrent access.
func (node *blockNode) Header() wire.BlockHeader {
	// No lock is needed because all accessed fields are immutable.
	prevHash := zeroHash
	if node.parent != nil {
		prevHash = &node.parent.hash
	}
	return wire.BlockHeader{
		Version:    node.version,
		PrevBlock:  *prevHash,
		MerkleRoot: node.merkleRoot,
		Timestamp:  time.Unix(node.timestamp, 0),
		Bits:       node.bits,
		Nonce:      node.nonce,
	}
}

// Ancestor returns the ancestor block node at the provided height by following
// the chain backwards from this node.  The returned block will be nil when a
// height is requested that is after the height of the passed node or is less
// than zero.
//
// This function is safe for concurrent access.
func (node *blockNode) Ancestor(height int32) *blockNode {
	if height < 0 || height > node.height {
		return nil
	}

	n := node
	for ; n != nil && n.height != height; n = n.parent {
		// Intentionally left blank
	}

	return n
}

// RelativeAncestor returns the ancestor block node a relative 'distance' blocks
// before this node.  This is equivalent to calling Ancestor with the node's
// height minus provided distance.
//
// This function is safe for concurrent access.
func (node *blockNode) RelativeAncestor(distance int32) *blockNode {
	return node.Ancestor(node.height - distance)
}

// CalcPastMedianTime calculates the median time of the previous few blocks
// prior to, and including, the block node.
//
// This function is safe for concurrent access.
func (node *blockNode) CalcPastMedianTime() time.Time {
	// Create a slice of the previous few block timestamps used to calculate
	// the median per the number defined by the constant medianTimeBlocks.
	timestamps := make([]int64, medianTimeBlocks)
	numNodes := 0
	iterNode := node
	for i := 0; i < medianTimeBlocks && iterNode != nil; i++ {
		timestamps[i] = iterNode.timestamp
		numNodes++

		iterNode = iterNode.parent
	}

	// Prune the slice to the actual number of available timestamps which
	// will be fewer than desired near the beginning of the block chain
	// and sort them.
	timestamps = timestamps[:numNodes]
	sort.Sort(timeSorter(timestamps))

	// NOTE: The consensus rules incorrectly calculate the median for even
	// numbers of blocks.  A true median averages the middle two elements
	// for a set with an even number of elements in it.   Since the constant
	// for the previous number of blocks to be used is odd, this is only an
	// issue for a few blocks near the beginning of the chain.  I suspect
	// this is an optimization even though the result is slightly wrong for
	// a few of the first blocks since after the first few blocks, there
	// will always be an odd number of blocks in the set per the constant.
	//
	// This code follows suit to ensure the same rules are used, however, be
	// aware that should the medianTimeBlocks constant ever be changed to an
	// even number, this code will be wrong.
	medianTimestamp := timestamps[numNodes/2]
	return time.Unix(medianTimestamp, 0)
}

// blockIndex provides facilities for keeping track of an in-memory index of the
// block chain.  Although the name block chain suggests a single chain of
// blocks, it is actually a tree-shaped structure where any node can have
// multiple children.  However, there can only be one active branch which does
// indeed form a chain from the tip all the way back to the genesis block.
type blockIndex struct {
	// The following fields are set when the instance is created and can't
	// be changed afterwards, so there is no need to protect them with a
	// separate mutex.
	db          database.DB
	chainParams *chaincfg.Params

	sync.RWMutex
	index map[chainhash.Hash]*blockNode
}

// newBlockIndex returns a new empty instance of a block index.  The index will
// be dynamically populated as block nodes are loaded from the database and
// manually added.
func newBlockIndex(db database.DB, chainParams *chaincfg.Params) *blockIndex {
	return &blockIndex{
		db:          db,
		chainParams: chainParams,
		index:       make(map[chainhash.Hash]*blockNode),
	}
}

// HaveBlock returns whether or not the block index contains the provided hash.
//
// This function is safe for concurrent access.
func (bi *blockIndex) HaveBlock(hash *chainhash.Hash) bool {
	bi.RLock()
	_, hasBlock := bi.index[*hash]
	bi.RUnlock()
	return hasBlock
}

// LookupNode returns the block node identified by the provided hash.  It will
// return nil if there is no entry for the hash.
//
// This function is safe for concurrent access.
func (bi *blockIndex) LookupNode(hash *chainhash.Hash) *blockNode {
	bi.RLock()
	node := bi.index[*hash]
	bi.RUnlock()
	return node
}

// AddNode adds the provided node to the block index.  Duplicate entries are not
// checked so it is up to caller to avoid adding them.
//
// This function is safe for concurrent access.
func (bi *blockIndex) AddNode(node *blockNode) {
	bi.Lock()
	bi.index[node.hash] = node
	bi.Unlock()
}

// RemoveNode removes the provided node from the block index.  There is no check
// whether another node in the index depends on this one, so it is up to caller
// to avoid that situation.
//
// This function is safe for concurrent access.
func (bi *blockIndex) RemoveNode(node *blockNode) {
	bi.Lock()
	delete(bi.index, node.hash)
	bi.Unlock()
}