// 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 implements bitcoin block handling and chain selection rules.

The bitcoin block handling and chain selection rules are an integral, and quite
likely the most important, part of bitcoin.  Unfortunately, at the time of
this writing, these rules are also largely undocumented and had to be
ascertained from the bitcoind source code.  At its core, bitcoin is a
distributed consensus of which blocks are valid and which ones will comprise the
main block chain (public ledger) that ultimately determines accepted
transactions, so it is extremely important that fully validating nodes agree on
all rules.

At a high level, this package provides support for inserting new blocks into
the block chain according to the aforementioned rules.  It includes
functionality such as rejecting duplicate blocks, ensuring blocks and
transactions follow all rules, orphan handling, and best chain selection along
with reorganization.

Since this package does not deal with other bitcoin specifics such as network
communication or wallets, it provides a notification system which gives the
caller a high level of flexibility in how they want to react to certain events
such as orphan blocks which need their parents requested and newly connected
main chain blocks which might result in wallet updates.

Bitcoin Chain Processing Overview

Before a block is allowed into the block chain, it must go through an intensive
series of validation rules.  The following list serves as a general outline of
those rules to provide some intuition into what is going on under the hood, but
is by no means exhaustive:

 - Reject duplicate blocks
 - Perform a series of sanity checks on the block and its transactions such as
   verifying proof of work, timestamps, number and character of transactions,
   transaction amounts, script complexity, and merkle root calculations
 - Compare the block against predetermined checkpoints for expected timestamps
   and difficulty based on elapsed time since the checkpoint
 - Save the most recent orphan blocks for a limited time in case their parent
   blocks become available
 - Stop processing if the block is an orphan as the rest of the processing
   depends on the block's position within the block chain
 - Perform a series of more thorough checks that depend on the block's position
   within the block chain such as verifying block difficulties adhere to
   difficulty retarget rules, timestamps are after the median of the last
   several blocks, all transactions are finalized, checkpoint blocks match, and
   block versions are in line with the previous blocks
 - Determine how the block fits into the chain and perform different actions
   accordingly in order to ensure any side chains which have higher difficulty
   than the main chain become the new main chain
 - When a block is being connected to the main chain (either through
   reorganization of a side chain to the main chain or just extending the
   main chain), perform further checks on the block's transactions such as
   verifying transaction duplicates, script complexity for the combination of
   connected scripts, coinbase maturity, double spends, and connected
   transaction values
 - Run the transaction scripts to verify the spender is allowed to spend the
   coins
 - Insert the block into the block database

Block Processing Example

The following example program demonstrates processing a block.  This example
intentionally causes an error by attempting to process a duplicate block.

	package main

	import (
		"fmt"
		"github.com/conformal/btcchain"
		"github.com/conformal/btcdb"
		_ "github.com/conformal/btcdb/ldb"
		"github.com/conformal/btcnet"
		"github.com/conformal/btcutil"
		"os"
	)

	func main() {
		// Create a new database to store the accepted blocks into.  Typically
		// this would be opening an existing database, but we create a new db
		// here so this is a complete working example.  Also, typically the
		// calls to os.Remove would not be used either, but again, we want
		// a complete working example here, so we make sure to remove the
		// database.
		dbName := "exampledb"
		_ = os.RemoveAll(dbName)
		db, err := btcdb.CreateDB("leveldb", dbName)
		if err != nil {
			fmt.Printf("Failed to create database: %v\n", err)
			return
		}
		defer os.RemoveAll(dbName) // Ignore error.
		defer db.Close()

		// Insert the main network genesis block.  This is part of the initial
		// database setup.  Like above, this typically would not be needed when
		// opening an existing database.
		genesisBlock := btcutil.NewBlock(btcnet.MainNetParams.GenesisBlock)
		_, err = db.InsertBlock(genesisBlock)
		if err != nil {
			fmt.Printf("Failed to insert genesis block: %v\n", err)
			return
		}

		// Create a new BlockChain instance using the underlying database for
		// the main bitcoin network and ignore notifications.
		chain := btcchain.New(db, btcnet.MainNetParams, nil)

		// Process a block.  For this example, we are going to intentionally
		// cause an error by trying to process the genesis block which already
		// exists.
		err = chain.ProcessBlock(genesisBlock, false)
		if err != nil {
			fmt.Printf("Failed to process block: %v\n", err)
			return
		}
	}

Errors

Errors returned by this package are either the raw errors provided by underlying
calls or of type btcchain.RuleError.  This allows the caller to differentiate
between unexpected errors, such as database errors, versus errors due to rule
violations through type assertions.

Bitcoin Improvement Proposals

This package includes spec changes outlined by the following BIPs:

		BIP0016 (https://en.bitcoin.it/wiki/BIP_0016)
		BIP0030 (https://en.bitcoin.it/wiki/BIP_0030)
		BIP0034 (https://en.bitcoin.it/wiki/BIP_0034)
*/
package btcchain