rust-bitcoin-unsafe-fast/src/blockdata/blockchain.rs

// Rust Bitcoin Library
// Written in 2014 by
//   Andrew Poelstra <apoelstra@wpsoftware.net>
//
// To the extent possible under law, the author(s) have dedicated all
// copyright and related and neighboring rights to this software to
// the public domain worldwide. This software is distributed without
// any warranty.
//
// You should have received a copy of the CC0 Public Domain Dedication
// along with this software.
// If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
//

//! # Bitcoin Blockchain
//!
//! This module provides the structures and functions to maintain the
//! blockchain.
//!
//! Note to developers: do not expose any ref-counted pointers in the public
//! API of this module. Internally we do unsafe mutations of them and we need
//! to make sure we are holding the only references.
//!

use std::num::Zero;
use std::kinds::marker;

use blockdata::block::{Block, BlockHeader};
use blockdata::transaction::Transaction;
use blockdata::constants::{DIFFCHANGE_INTERVAL, DIFFCHANGE_TIMESPAN,
                           TARGET_BLOCK_SPACING, max_target, genesis_block};
use network::constants::{Network, BitcoinTestnet};
use network::encodable::{ConsensusDecodable, ConsensusEncodable};
use network::serialize::{BitcoinHash, SimpleDecoder, SimpleEncoder};
use util::BitArray;
use util::error::{BitcoinResult, BlockNotFound, DuplicateHash, PrevHashNotFound};
use util::uint::Uint256;
use util::hash::Sha256dHash;
use util::patricia_tree::PatriciaTree;

type BlockTree = PatriciaTree<Uint256, Box<BlockchainNode>>;
type NodePtr = *const BlockchainNode;

/// A link in the blockchain
pub struct BlockchainNode {
  /// The actual block
  pub block: Block,
  /// Total work from genesis to this point
  pub total_work: Uint256,
  /// Expected value of `block.header.bits` for this block; only changes every
  /// `blockdata::constants::DIFFCHANGE_INTERVAL;` blocks
  pub required_difficulty: Uint256,
  /// Height above genesis
  pub height: u32,
  /// Whether the transaction data is stored
  pub has_txdata: bool,
  /// Pointer to block's parent
  prev: NodePtr,
  /// Pointer to block's child
  next: NodePtr
}

impl BlockchainNode {
  /// Is the node on the main chain?
  fn is_on_main_chain(&self, chain: &Blockchain) -> bool {
    if self.block.header == unsafe { (*chain.best_tip).block.header } {
      return true;
    }
    unsafe {
      let mut scan = self.next;
      while scan.is_not_null() {
        if (*scan).block.header == (*chain.best_tip).block.header {
          return true;
        }
        scan = (*scan).next;
      }
    }
    return false;
  }
}

impl<S:SimpleEncoder<E>, E> ConsensusEncodable<S, E> for BlockchainNode {
  #[inline]
  fn consensus_encode(&self, s: &mut S) -> Result<(), E> {
    try!(self.block.consensus_encode(s));
    try!(self.total_work.consensus_encode(s));
    try!(self.required_difficulty.consensus_encode(s));
    try!(self.height.consensus_encode(s));
    try!(self.has_txdata.consensus_encode(s));
    // Don't serialize the prev or next pointers
    Ok(())
  }
}

impl<D:SimpleDecoder<E>, E> ConsensusDecodable<D, E> for BlockchainNode {
  #[inline]
  fn consensus_decode(d: &mut D) -> Result<BlockchainNode, E> {
    Ok(BlockchainNode {
      block: try!(ConsensusDecodable::consensus_decode(d)),
      total_work: try!(ConsensusDecodable::consensus_decode(d)),
      required_difficulty: try!(ConsensusDecodable::consensus_decode(d)),
      height: try!(ConsensusDecodable::consensus_decode(d)),
      has_txdata: try!(ConsensusDecodable::consensus_decode(d)),
      prev: RawPtr::null(),
      next: RawPtr::null()
    })
  }
}

impl BitcoinHash for BlockchainNode {
  fn bitcoin_hash(&self) -> Sha256dHash {
    self.block.header.bitcoin_hash()
  }
}

/// The blockchain
pub struct Blockchain {
  network: Network,
  tree: BlockTree,
  best_tip: NodePtr,
  best_hash: Sha256dHash,
  genesis_hash: Sha256dHash
}

impl<S:SimpleEncoder<E>, E> ConsensusEncodable<S, E> for Blockchain {
  #[inline]
  fn consensus_encode(&self, s: &mut S) -> Result<(), E> {
    try!(self.network.consensus_encode(s));
    try!(self.tree.consensus_encode(s));
    try!(self.best_hash.consensus_encode(s));
    try!(self.genesis_hash.consensus_encode(s));
    Ok(())
  }
}

impl<D:SimpleDecoder<E>, E> ConsensusDecodable<D, E> for Blockchain {
  fn consensus_decode(d: &mut D) -> Result<Blockchain, E> {
    let network: Network = try!(ConsensusDecodable::consensus_decode(d));
    let mut tree: BlockTree = try!(ConsensusDecodable::consensus_decode(d));
    let best_hash: Sha256dHash = try!(ConsensusDecodable::consensus_decode(d));
    let genesis_hash: Sha256dHash = try!(ConsensusDecodable::consensus_decode(d));

    // Lookup best tip
    let best = match tree.lookup(&best_hash.into_uint256(), 256) {
      Some(node) => &**node as NodePtr,
      None => {
        return Err(d.error(format!("best tip {:x} not in tree", best_hash).as_slice()));
      }
    };
    // Lookup genesis
    if tree.lookup(&genesis_hash.into_uint256(), 256).is_none() {
      return Err(d.error(format!("genesis {:x} not in tree", genesis_hash).as_slice()));
    }
    // Reconnect all prev pointers
    let raw_tree = &tree as *const _;
    for node in tree.mut_iter() {
      let hash = node.block.header.prev_blockhash.into_uint256();
      let prevptr =
        match unsafe { (*raw_tree).lookup(&hash, 256) } {
          Some(node) => &**node as NodePtr,
          None => RawPtr::null() 
        };
      node.prev = prevptr;
    }
    // Reconnect next pointers on the main chain
    unsafe {
      let mut scan = best;
      while (*scan).prev.is_not_null() {
        let prev = (*scan).prev as *mut BlockchainNode;
        (*prev).next = scan;
        scan = prev as NodePtr;
      }

      // Check that "genesis" is the genesis
      if (*scan).bitcoin_hash() != genesis_hash {
        return Err(d.error(format!("no path from tip {:x} to genesis {:x}",
                                   best_hash, genesis_hash).as_slice()));
      }
    }

    // Return the chain
    Ok(Blockchain {
      network: network,
      tree: tree,
      best_tip: best,
      best_hash: best_hash,
      genesis_hash: genesis_hash
    })
  }
}

// TODO: this should maybe be public, in which case it needs to be tagged
// with a ContravariantLifetime marker tying it to the tree's lifetime.
struct LocatorHashIter {
  index: NodePtr,
  count: uint,
  skip: uint
}

impl LocatorHashIter {
  fn new(init: NodePtr) -> LocatorHashIter {
    LocatorHashIter { index: init, count: 0, skip: 1 }
  }
}

impl Iterator<Sha256dHash> for LocatorHashIter {
  fn next(&mut self) -> Option<Sha256dHash> {
    if self.index.is_null() {
      return None;
    }
    let ret = Some(unsafe { (*self.index).bitcoin_hash() });

    // Rewind once (if we are at the genesis, this will set self.index to None)
    self.index = unsafe { (*self.index).prev };
    // If we are not at the genesis, rewind `self.skip` times, or until we are.
    if self.index.is_not_null() {
      for _ in range(1, self.skip) {
        unsafe {
          if (*self.index).prev.is_null() {
            break;
          }
          self.index = (*self.index).prev;
        }
      }
    }

    self.count += 1;
    if self.count > 10 {
      self.skip *= 2;
    }
    ret
  }
}

/// An iterator over blocks in blockheight order
pub struct BlockIter<'tree> {
  index: NodePtr,
  // Note: we don't actually touch the blockchain. But we need
  // to keep it borrowed to prevent it being mutated, since some
  // mutable blockchain methods call .mut_borrow() on the block
  // links, which would blow up if the iterator did a regular
  // borrow at the same time.
  marker: marker::ContravariantLifetime<'tree>
}

/// An iterator over blocks in reverse blockheight order. Note that this
/// is essentially the same as if we'd implemented `DoubleEndedIterator`
/// on `BlockIter` --- but we can't do that since if `BlockIter` is started
/// off the main chain, it will not reach the best tip, so the iterator
/// and its `.rev()` would be iterators over different chains! To avoid
/// this suprising behaviour we simply use separate iterators.
pub struct RevBlockIter<'tree> {
  index: NodePtr,
  // See comment in BlockIter for why we need this
  marker: marker::ContravariantLifetime<'tree>
}

/// An iterator over blocks in reverse blockheight order, which yielding only
/// stale blocks (ending at the point where it would've returned a block on
/// the main chain). It does this by checking if the `next` pointer of the
/// next-to-by-yielded block matches the currently-yielded block. If not, scan
/// forward from next-to-be-yielded block. If we hit the best tip, set the
/// next-to-by-yielded block to None instead.
///
/// So to handle reorgs, you create a `RevStaleBlockIter` starting from the last
/// known block, and play it until it runs out, rewinding every block except for
/// the last one. Since the UtxoSet `rewind` function sets its `last_hash()` to
/// the prevblockhash of the rewinded block (which will be on the main chain at
/// the end of the iteration), you can then sync it up same as if you were doing
/// a plain old fast-forward.
pub struct RevStaleBlockIter<'tree> {
  index: NodePtr,
  chain: &'tree Blockchain
}

impl<'tree> Iterator<&'tree BlockchainNode> for BlockIter<'tree> {
  fn next(&mut self) -> Option<&'tree BlockchainNode> {
    if self.index.is_null() {
      return None;
    }
    unsafe {
      let ret = Some(&*self.index);
      self.index = (*self.index).next;
      ret
    }
  }
}

impl<'tree> Iterator<&'tree BlockchainNode> for RevBlockIter<'tree> {
  fn next(&mut self) -> Option<&'tree BlockchainNode> {
    if self.index.is_null() {
      return None;
    }
    unsafe {
      let ret = Some(&*self.index);
      self.index = (*self.index).prev;
      ret
    }
  }
}

impl<'tree> Iterator<&'tree Block> for RevStaleBlockIter<'tree> {
  fn next(&mut self) -> Option<&'tree Block> { 
    if self.index.is_null() {
      return None;
    }

    unsafe {
      let ret = Some(&(*self.index).block);
      let next_index = (*self.index).prev;
      // Check if the next block is going to be on the main chain
      if next_index.is_not_null() &&
         (*next_index).next != self.index &&
         (&*next_index).is_on_main_chain(self.chain) {
        self.index = RawPtr::null();
      } else {
        self.index = next_index;
      }
      ret
    }
  }
}

/// This function emulates the GetCompact(SetCompact(n)) in the satoshi code,
/// which drops the precision to something that can be encoded precisely in
/// the nBits block header field. Savour the perversity. This is in Bitcoin
/// consensus code. What. Gaah!
fn satoshi_the_precision(n: &Uint256) -> Uint256 {
  // Shift by B bits right then left to turn the low bits to zero
  let bits = 8 * ((n.bits() + 7) / 8 - 3);
  let mut ret = n >> bits;
  // Oh, did I say B was that fucked up formula? I meant sometimes also + 8.
  if ret.bit(23) {
    ret = (ret >> 8) << 8;
  }
  ret << bits
}

impl Blockchain {
  /// Constructs a new blockchain
  pub fn new(network: Network) -> Blockchain {
    let genesis = genesis_block(network);
    let genhash = genesis.header.bitcoin_hash();
    let new_node = box BlockchainNode {
      total_work: Zero::zero(),
      required_difficulty: genesis.header.target(),
      block: genesis,
      height: 0,
      has_txdata: true,
      prev: RawPtr::null(),
      next: RawPtr::null()
    };
    let raw_ptr = &*new_node as NodePtr;
    Blockchain {
      network: network,
      tree: {
        let mut pat = PatriciaTree::new();
        pat.insert(&genhash.into_uint256(), 256, new_node);
        pat
      },
      best_hash: genhash,
      genesis_hash: genhash,
      best_tip: raw_ptr
    }
  }

  fn replace_txdata(&mut self, hash: &Uint256, txdata: Vec<Transaction>, has_txdata: bool) -> BitcoinResult<()> {
    match self.tree.lookup_mut(hash, 256) {
      Some(existing_block) => {
        unsafe {
          // existing_block is an Rc. Rust will not let us mutate it under
          // any circumstances, since if it were to be reallocated, then
          // all other references to it would be destroyed. However, we
          // just need a mutable pointer to the txdata vector; by calling
          // Vec::clone_from() rather than assigning, we can be assured that
          // no reallocation can occur, since clone_from() takes an &mut self,
          // which it does not own and therefore cannot move.
          //
          // To be clear: there will undoubtedly be some reallocation within
          // the Vec itself. We don't care about this. What we care about is
          // that the Vec (and more pointedly, its containing struct) does not
          // move, since this would invalidate the Rc that we are snookering.
          use std::mem::{forget, transmute};
          let mutable_vec: &mut Vec<Transaction> = transmute(&existing_block.block.txdata);
          mutable_vec.clone_from(&txdata);
          // If mutable_vec went out of scope unhindered, it would deallocate
          // the Vec it points to, since Rust assumes that a mutable vector
          // is a unique reference (and this one is definitely not).
          forget(mutable_vec);
          // Do the same thing with the txdata flac
          let mutable_bool: &mut bool = transmute(&existing_block.has_txdata);
          *mutable_bool = has_txdata;
          forget(mutable_bool);
        }
        Ok(())
      },
      None => Err(BlockNotFound)
    }
  }

  /// Looks up a block in the chain and returns the BlockchainNode containing it
  pub fn get_block<'a>(&'a self, hash: Sha256dHash) -> Option<&'a BlockchainNode> {
    self.tree.lookup(&hash.into_uint256(), 256).map(|node| &**node)
  }

  /// Locates a block in the chain and overwrites its txdata
  pub fn add_txdata(&mut self, block: Block) -> BitcoinResult<()> {
    self.replace_txdata(&block.header.bitcoin_hash().into_uint256(), block.txdata, true)
  }

  /// Locates a block in the chain and removes its txdata
  pub fn remove_txdata(&mut self, hash: Sha256dHash) -> BitcoinResult<()> {
    self.replace_txdata(&hash.into_uint256(), vec![], false)
  }

  /// Adds a block header to the chain
  pub fn add_header(&mut self, header: BlockHeader) -> BitcoinResult<()> {
    self.real_add_block(Block { header: header, txdata: vec![] }, false)
  }

  /// Adds a block to the chain
  pub fn add_block(&mut self, block: Block) -> BitcoinResult<()> {
    self.real_add_block(block, true)
  }

  fn real_add_block(&mut self, block: Block, has_txdata: bool) -> BitcoinResult<()> {
    // get_prev optimizes the common case where we are extending the best tip
    #[inline]
    fn get_prev<'a>(chain: &'a Blockchain, hash: Sha256dHash) -> Option<NodePtr> {
      if hash == chain.best_hash { 
        Some(chain.best_tip)
      } else {
        chain.tree.lookup(&hash.into_uint256(), 256).map(|boxptr| &**boxptr as NodePtr)
      }
    }
    // Check for multiple inserts (bitcoind from c9a09183 to 3c85d2ec doesn't
    // handle locator hashes properly and may return blocks multiple times,
    // and this may also happen in case of a reorg.
    if self.tree.lookup(&block.header.bitcoin_hash().into_uint256(), 256).is_some() {
      return Err(DuplicateHash);
    }
    // Construct node, if possible
    let new_block = match get_prev(self, block.header.prev_blockhash) {
      Some(prev) => {
        let difficulty =
          // Compute required difficulty if this is a diffchange block
          if (unsafe { (*prev).height } + 1) % DIFFCHANGE_INTERVAL == 0 {
            let timespan = unsafe {
              // Scan back DIFFCHANGE_INTERVAL blocks
              let mut scan = prev;
              for _ in range(0, DIFFCHANGE_INTERVAL - 1) {
                scan = (*scan).prev;
              }
              // Get clamped timespan between first and last blocks
              match (*prev).block.header.time - (*scan).block.header.time {
                n if n < DIFFCHANGE_TIMESPAN / 4 => DIFFCHANGE_TIMESPAN / 4,
                n if n > DIFFCHANGE_TIMESPAN * 4 => DIFFCHANGE_TIMESPAN * 4,
                n => n
              }
            };
            // Compute new target
            let mut target = unsafe { (*prev).block.header.target() };
            target = target.mul_u32(timespan);
            target = target / FromPrimitive::from_u64(DIFFCHANGE_TIMESPAN as u64).unwrap();
            // Clamp below MAX_TARGET (difficulty 1)
            let max = max_target(self.network);
            if target > max { target = max };
            // Compactify (make expressible in the 8+24 nBits float format
            satoshi_the_precision(&target)
          // On non-diffchange blocks, Testnet has a rule that any 20-minute-long
          // block intervals result the difficulty
          } else if self.network == BitcoinTestnet &&
                    block.header.time > unsafe { (*prev).block.header.time } + 2*TARGET_BLOCK_SPACING {
            max_target(self.network)
          // On the other hand, if we are in Testnet and the block interval is less
          // than 20 minutes, we need to scan backward to find a block for which the
          // previous rule did not apply, to find the "real" difficulty.
          } else if self.network == BitcoinTestnet {
            // Scan back DIFFCHANGE_INTERVAL blocks
            unsafe {
              let mut scan = prev;
              while (*scan).height % DIFFCHANGE_INTERVAL != 0 &&
                    (*scan).required_difficulty == max_target(self.network) {
                scan = (*scan).prev;
              }
              (*scan).required_difficulty
            }
          // Otherwise just use the last block's difficulty
          } else {
            unsafe { (*prev).required_difficulty }
          };
        // Create node
        let ret = box BlockchainNode {
          total_work: block.header.work().add(unsafe { &(*prev).total_work }),
          block: block,
          required_difficulty: difficulty,
          height: unsafe { (*prev).height + 1 },
          has_txdata: has_txdata,
          prev: prev,
          next: RawPtr::null()
        };
        unsafe {
          let prev = prev as *mut BlockchainNode;
          (*prev).next = &*ret as NodePtr;
        }
        ret
      },
      None => {
        return Err(PrevHashNotFound);
      }
    };

    // spv validate the block
    try!(new_block.block.header.spv_validate(&new_block.required_difficulty));

    // Insert the new block
    let raw_ptr = &*new_block as NodePtr;
    self.tree.insert(&new_block.block.header.bitcoin_hash().into_uint256(), 256, new_block);
    // Replace the best tip if necessary
    if unsafe { (*raw_ptr).total_work > (*self.best_tip).total_work } {
      self.set_best_tip(raw_ptr);
    }
    Ok(())
  }

  /// Sets the best tip (not public)
  fn set_best_tip(&mut self, tip: NodePtr) {
    // Fix next links
    unsafe {
      let mut scan = self.best_tip;
      // Scan backward
      while (*scan).prev.is_not_null() {
        // If we hit the old best, there is no need to reorg.
        if scan == self.best_tip { break; }
        // Otherwise set the next-ptr and carry on
        let prev = (*scan).prev as *mut BlockchainNode;
        (*prev).next = scan;
        scan = (*scan).prev;
      }
    }
    // Set best
    self.best_hash = unsafe { (*tip).bitcoin_hash() };
    self.best_tip = tip;
  }

  /// Returns the genesis block's blockhash
  pub fn genesis_hash(&self) -> Sha256dHash {
    self.genesis_hash
  }

  /// Returns the best tip
  pub fn best_tip<'a>(&'a self) -> &'a Block {
    unsafe { &(*self.best_tip).block }
  }

  /// Returns the best tip's blockhash
  pub fn best_tip_hash(&self) -> Sha256dHash {
    self.best_hash
  }

  /// Returns an array of locator hashes used in `getheaders` messages
  pub fn locator_hashes(&self) -> Vec<Sha256dHash> {
    LocatorHashIter::new(self.best_tip).collect()
  }

  /// An iterator over all blocks in the chain starting from `start_hash`
  pub fn iter<'a>(&'a self, start_hash: Sha256dHash) -> BlockIter<'a> {
    let start = match self.tree.lookup(&start_hash.into_uint256(), 256) {
        Some(boxptr) => &**boxptr as NodePtr,
        None => RawPtr::null()
      };
    BlockIter {
      index: start,
      marker: marker::ContravariantLifetime::<'a>
    }
  }

  /// An iterator over all blocks in reverse order to the genesis, starting with `start_hash`
  pub fn rev_iter<'a>(&'a self, start_hash: Sha256dHash) -> RevBlockIter<'a> {
    let start = match self.tree.lookup(&start_hash.into_uint256(), 256) {
        Some(boxptr) => &**boxptr as NodePtr,
        None => RawPtr::null()
      };
    RevBlockIter {
      index: start,
      marker: marker::ContravariantLifetime::<'a>
    }
  }

  /// An iterator over all blocks -not- in the best chain, in reverse order, starting from `start_hash`
  pub fn rev_stale_iter<'a>(&'a self, start_hash: Sha256dHash) -> RevStaleBlockIter<'a> {
    let start = match self.tree.lookup(&start_hash.into_uint256(), 256) {
        Some(boxptr) => {
          // If we are already on the main chain, we have a dead iterator
          if boxptr.is_on_main_chain(self) {
            RawPtr::null()
          } else {
            &**boxptr as NodePtr
          }
        }
        None => RawPtr::null()
      };
    RevStaleBlockIter { 
      index: start,
      chain: self
    }
  }
}

#[cfg(test)]
mod tests {
  use std::io::IoResult;

  use blockdata::blockchain::Blockchain;
  use blockdata::constants::genesis_block;
  use network::constants::Bitcoin;
  use network::serialize::{BitcoinHash, deserialize, serialize};

  #[test]
  fn blockchain_serialize_test() {
    let empty_chain = Blockchain::new(Bitcoin);
    assert_eq!(empty_chain.best_tip().header.bitcoin_hash(),
               genesis_block(Bitcoin).header.bitcoin_hash());

    let serial = serialize(&empty_chain);
    let deserial: IoResult<Blockchain> = deserialize(serial.unwrap());

    assert!(deserial.is_ok());
    let read_chain = deserial.unwrap();
    assert_eq!(read_chain.best_tip().header.bitcoin_hash(),
               genesis_block(Bitcoin).header.bitcoin_hash());
  }
}