// Copyright (c) 2013-2014 Conformal Systems LLC.
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.

package btcchain

import (
	"fmt"
	"github.com/conformal/btcutil"
	"github.com/conformal/btcwire"
)

// RuleError identifies a rule violation.  It is used to indicate that
// processing of a block or transaction failed due to one of the many validation
// rules.  The caller can use type assertions to determine if a failure was
// specifically due to a rule violation.
type RuleError string

// Error satisfies the error interface to print human-readable errors.
func (e RuleError) Error() string {
	return string(e)
}

// blockExists determines whether a block with the given hash exists either in
// the main chain or any side chains.
func (b *BlockChain) blockExists(hash *btcwire.ShaHash) bool {
	// Check memory chain first (could be main chain or side chain blocks).
	if _, ok := b.index[*hash]; ok {
		return true
	}

	// Check in database (rest of main chain not in memory).
	return b.db.ExistsSha(hash)
}

// processOrphans determines if there are any orphans which depend on the passed
// block hash (they are no longer orphans if true) and potentially accepts them.
// It repeats the process for the newly accepted blocks (to detect further
// orphans which may no longer be orphans) until there are no more.
func (b *BlockChain) processOrphans(hash *btcwire.ShaHash) error {
	// Start with processing at least the passed hash.  Leave a little room
	// for additional orphan blocks that need to be processed without
	// needing to grow the array in the common case.
	processHashes := make([]*btcwire.ShaHash, 0, 10)
	processHashes = append(processHashes, hash)
	for len(processHashes) > 0 {
		// Pop the first hash to process from the slice.
		processHash := processHashes[0]
		processHashes[0] = nil // Prevent GC leak.
		processHashes = processHashes[1:]

		// Look up all orphans that are parented by the block we just
		// accepted.  This will typically only be one, but it could
		// be multiple if multiple blocks are mined and broadcast
		// around the same time.  The one with the most proof of work
		// will eventually win out.  An indexing for loop is
		// intentionally used over a range here as range does not
		// reevaluate the slice on each iteration nor does it adjust the
		// index for the modified slice.
		for i := 0; i < len(b.prevOrphans[*processHash]); i++ {
			orphan := b.prevOrphans[*processHash][i]
			if orphan == nil {
				log.Warnf("Found a nil entry at index %d in the "+
					"orphan dependency list for block %v", i,
					processHash)
				continue
			}

			// Remove the orphan from the orphan pool.
			// It's safe to ignore the error on Sha since the hash
			// is already cached.
			orphanHash, _ := orphan.block.Sha()
			b.removeOrphanBlock(orphan)
			i--

			// Potentially accept the block into the block chain.
			err := b.maybeAcceptBlock(orphan.block, false)
			if err != nil {
				return err
			}

			// Add this block to the list of blocks to process so
			// any orphan blocks that depend on this block are
			// handled too.
			processHashes = append(processHashes, orphanHash)
		}
	}
	return nil
}

// ProcessBlock is the main workhorse for handling insertion of new blocks into
// the block chain.  It includes functionality such as rejecting duplicate
// blocks, ensuring blocks follow all rules, orphan handling, and insertion into
// the block chain along with best chain selection and reorganization.
func (b *BlockChain) ProcessBlock(block *btcutil.Block, fastAdd bool) error {
	blockHash, err := block.Sha()
	if err != nil {
		return err
	}
	log.Tracef("Processing block %v", blockHash)

	// The block must not already exist in the main chain or side chains.
	if b.blockExists(blockHash) {
		str := fmt.Sprintf("already have block %v", blockHash)
		return RuleError(str)
	}

	// The block must not already exist as an orphan.
	if _, exists := b.orphans[*blockHash]; exists {
		str := fmt.Sprintf("already have block (orphan) %v", blockHash)
		return RuleError(str)
	}

	// Perform preliminary sanity checks on the block and its transactions.
	err = CheckBlockSanity(block, b.netParams.PowLimit)
	if err != nil {
		return err
	}

	// Find the previous checkpoint and perform some additional checks based
	// on the checkpoint.  This provides a few nice properties such as
	// preventing old side chain blocks before the last checkpoint,
	// rejecting easy to mine, but otherwise bogus, blocks that could be
	// used to eat memory, and ensuring expected (versus claimed) proof of
	// work requirements since the previous checkpoint are met.
	blockHeader := &block.MsgBlock().Header
	checkpointBlock, err := b.findPreviousCheckpoint()
	if err != nil {
		return err
	}
	if checkpointBlock != nil {
		// Ensure the block timestamp is after the checkpoint timestamp.
		checkpointHeader := &checkpointBlock.MsgBlock().Header
		checkpointTime := checkpointHeader.Timestamp
		if blockHeader.Timestamp.Before(checkpointTime) {
			str := fmt.Sprintf("block %v has timestamp %v before "+
				"last checkpoint timestamp %v", blockHash,
				blockHeader.Timestamp, checkpointTime)
			return RuleError(str)
		}
		if !fastAdd {
			// Even though the checks prior to now have already ensured the
			// proof of work exceeds the claimed amount, the claimed amount
			// is a field in the block header which could be forged.  This
			// check ensures the proof of work is at least the minimum
			// expected based on elapsed time since the last checkpoint and
			// maximum adjustment allowed by the retarget rules.
			duration := blockHeader.Timestamp.Sub(checkpointTime)
			requiredTarget := CompactToBig(b.calcEasiestDifficulty(
				checkpointHeader.Bits, duration))
			currentTarget := CompactToBig(blockHeader.Bits)
			if currentTarget.Cmp(requiredTarget) > 0 {
				str := fmt.Sprintf("block target difficulty of %064x "+
					"is too low when compared to the previous "+
					"checkpoint", currentTarget)
				return RuleError(str)
			}
		}
	}

	// Handle orphan blocks.
	prevHash := &blockHeader.PrevBlock
	if !prevHash.IsEqual(zeroHash) && !b.blockExists(prevHash) {
		// Add the orphan block to the orphan pool.
		log.Infof("Adding orphan block %v with parent %v", blockHash,
			prevHash)
		b.addOrphanBlock(block)

		// Notify the caller so it can request missing blocks.
		b.sendNotification(NTOrphanBlock, blockHash)
		return nil
	}

	// The block has passed all context independent checks and appears sane
	// enough to potentially accept it into the block chain.
	err = b.maybeAcceptBlock(block, fastAdd)
	if err != nil {
		return err
	}

	// Accept any orphan blocks that depend on this block (they are no
	// longer orphans) and repeat for those accepted blocks until there are
	// no more.
	err = b.processOrphans(blockHash)
	if err != nil {
		return err
	}

	log.Debugf("Accepted block %v", blockHash)
	return nil
}