// 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 blockdag

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

	"github.com/daglabs/btcd/dagconfig"
	"github.com/daglabs/btcd/dagconfig/daghash"
	"github.com/daglabs/btcd/database"
	"github.com/daglabs/btcd/wire"
)

// blockStatus is a bit field representing the validation state of the block.
type blockStatus byte

const (
	// statusDataStored indicates that the block's payload is stored on disk.
	statusDataStored blockStatus = 1 << iota

	// statusValid indicates that the block has been fully validated.
	statusValid

	// statusValidateFailed indicates that the block has failed validation.
	statusValidateFailed

	// statusInvalidAncestor indicates that one of the block's ancestors has
	// has failed validation, thus the block is also invalid.
	statusInvalidAncestor

	// statusNone indicates that the block has no validation state flags set.
	//
	// NOTE: This must be defined last in order to avoid influencing iota.
	statusNone blockStatus = 0
)

// KnownValid returns whether the block is known to be valid. This will return
// false for a valid block that has not been fully validated yet.
func (status blockStatus) KnownValid() bool {
	return status&statusValid != 0
}

// KnownInvalid returns whether the block is known to be invalid. This may be
// because the block itself failed validation or any of its ancestors is
// invalid. This will return false for invalid blocks that have not been proven
// invalid yet.
func (status blockStatus) KnownInvalid() bool {
	return status&(statusValidateFailed|statusInvalidAncestor) != 0
}

// blockNode represents a block within the block DAG. The DAG 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.

	// parents is the parent blocks for this node.
	parents blockSet

	// selectedParent is the selected parent for this node.
	// The selected parent is the parent that if chosen will maximize the blue score of this block
	selectedParent *blockNode

	// children are all the blocks that refer to this block as a parent
	children blockSet

	// blues are all blue blocks in this block's worldview that are in its selected parent anticone
	blues []*blockNode

	// blueScore is the count of all the blue blocks in this block's past
	blueScore uint64

	// diff is the UTXO representation of the block
	// A block's UTXO is reconstituted by applying diffWith on every block in the chain of diffChildren
	// from the virtual block down to the block. See diffChild
	diff *UTXODiff

	// diffChild is the child that diff will be built from. See diff
	diffChild *blockNode

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

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

	// height is the position in the block DAG.
	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      uint64
	timestamp  int64
	merkleRoot daghash.Hash

	// status is a bitfield representing the validation state of the block. The
	// status field, unlike the other fields, may be written to and so should
	// only be accessed using the concurrent-safe NodeStatus method on
	// blockIndex once the node has been added to the global index.
	status blockStatus
}

// initBlockNode initializes a block node from the given header and parent nodes,
// calculating the height and workSum from the respective fields on the first parent.
// 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, parents blockSet, phantomK uint32) {
	*node = blockNode{
		parents:   parents,
		children:  make(blockSet),
		workSum:   big.NewInt(0),
		timestamp: time.Now().Unix(),
	}

	if blockHeader != nil {
		node.hash = blockHeader.BlockHash()
		node.workSum = CalcWork(blockHeader.Bits)
		node.version = blockHeader.Version
		node.bits = blockHeader.Bits
		node.nonce = blockHeader.Nonce
		node.timestamp = blockHeader.Timestamp.Unix()
		node.merkleRoot = blockHeader.MerkleRoot
	}

	if len(parents) > 0 {
		node.blues, node.selectedParent, node.blueScore = phantom(node, phantomK)
		node.height = calculateNodeHeight(node)
		node.workSum = node.workSum.Add(node.selectedParent.workSum, node.workSum)
	}
}

func calculateNodeHeight(node *blockNode) int32 {
	return node.parents.maxHeight() + 1
}

// newBlockNode returns a new block node for the given block header and parent
// nodes, calculating the height and workSum from the respective fields on the
// parent. This function is NOT safe for concurrent access.
func newBlockNode(blockHeader *wire.BlockHeader, parents blockSet, phantomK uint32) *blockNode {
	var node blockNode
	initBlockNode(&node, blockHeader, parents, phantomK)
	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.
	return &wire.BlockHeader{
		Version:      node.version,
		ParentHashes: node.ParentHashes(),
		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.selectedParent {
		// 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.
	// If there aren't enough blocks yet - pad remaining with genesis block's timestamp.
	timestamps := make([]int64, medianTimeBlocks)
	iterNode := node
	for i := 0; i < medianTimeBlocks; i++ {
		timestamps[i] = iterNode.timestamp

		if !iterNode.isGenesis() {
			iterNode = iterNode.selectedParent
		}
	}

	sort.Sort(timeSorter(timestamps))

	// Note: This works when medianTimeBlockCount is an odd number.
	// If it is to be changed to an even number - must take avarage of two middle values
	// Since medianTimeBlockCount is a constant, we can skip the odd/even check
	medianTimestamp := timestamps[medianTimeBlocks/2]
	return time.Unix(medianTimestamp, 0)
}

func (node *blockNode) ParentHashes() []daghash.Hash {
	return node.parents.hashes()
}

// isGenesis returns if the current block is the genesis block
func (node *blockNode) isGenesis() bool {
	return len(node.parents) == 0
}

// String returns a string that contains the block hash and height.
func (node blockNode) String() string {
	return fmt.Sprintf("%s (%d)", node.hash, node.height)
}

// 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
	dagParams *dagconfig.Params

	sync.RWMutex
	index map[daghash.Hash]*blockNode
	dirty map[*blockNode]struct{}
}

// 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, dagParams *dagconfig.Params) *blockIndex {
	return &blockIndex{
		db:        db,
		dagParams: dagParams,
		index:     make(map[daghash.Hash]*blockNode),
		dirty:     make(map[*blockNode]struct{}),
	}
}

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

// AddNode adds the provided node to the block index and marks it as dirty.
// 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.addNode(node)
	bi.dirty[node] = struct{}{}
	bi.Unlock()
}

// addNode adds the provided node to the block index, but does not mark it as
// dirty. This can be used while initializing the block index.
//
// This function is NOT safe for concurrent access.
func (bi *blockIndex) addNode(node *blockNode) {
	bi.index[node.hash] = node
}

// NodeStatus provides concurrent-safe access to the status field of a node.
//
// This function is safe for concurrent access.
func (bi *blockIndex) NodeStatus(node *blockNode) blockStatus {
	bi.RLock()
	status := node.status
	bi.RUnlock()
	return status
}

// SetStatusFlags flips the provided status flags on the block node to on,
// regardless of whether they were on or off previously. This does not unset any
// flags currently on.
//
// This function is safe for concurrent access.
func (bi *blockIndex) SetStatusFlags(node *blockNode, flags blockStatus) {
	bi.Lock()
	node.status |= flags
	bi.dirty[node] = struct{}{}
	bi.Unlock()
}

// UnsetStatusFlags flips the provided status flags on the block node to off,
// regardless of whether they were on or off previously.
//
// This function is safe for concurrent access.
func (bi *blockIndex) UnsetStatusFlags(node *blockNode, flags blockStatus) {
	bi.Lock()
	node.status &^= flags
	bi.dirty[node] = struct{}{}
	bi.Unlock()
}

// flushToDB writes all dirty block nodes to the database. If all writes
// succeed, this clears the dirty set.
func (bi *blockIndex) flushToDB() error {
	bi.Lock()
	if len(bi.dirty) == 0 {
		bi.Unlock()
		return nil
	}

	err := bi.db.Update(func(dbTx database.Tx) error {
		for node := range bi.dirty {
			err := dbStoreBlockNode(dbTx, node)
			if err != nil {
				return err
			}
		}
		return nil
	})

	// If write was successful, clear the dirty set.
	if err == nil {
		bi.dirty = make(map[*blockNode]struct{})
	}

	bi.Unlock()
	return err
}