//! EIP-2124 implementation based on <https://eips.ethereum.org/EIPS/eip-2124>.

#![deny(missing_docs)]
#![warn(clippy::all, clippy::pedantic, clippy::nursery)]
#![allow(clippy::too_many_lines)]

use crc::crc32;
use maplit::btreemap;
use primitive_types::H256;
use rlp::{DecoderError, Rlp, RlpStream};
use rlp_derive::{RlpDecodable, RlpEncodable};
use std::collections::{BTreeMap, BTreeSet};

/// Block number.
pub type BlockNumber = u64;

/// `CRC32` hash of all previous forks starting from genesis block.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct ForkHash(pub u32);

impl rlp::Encodable for ForkHash {
    fn rlp_append(&self, s: &mut RlpStream) {
        s.encoder().encode_value(&self.0.to_be_bytes());
    }
}

impl rlp::Decodable for ForkHash {
    fn decode(rlp: &Rlp) -> Result<Self, DecoderError> {
        rlp.decoder().decode_value(|b| {
            if b.len() != 4 {
                return Err(DecoderError::RlpInvalidLength);
            }

            let mut blob = [0; 4];
            blob.copy_from_slice(&b[..]);

            Ok(Self(u32::from_be_bytes(blob)))
        })
    }
}

impl From<H256> for ForkHash {
    fn from(genesis: H256) -> Self {
        Self(crc32::checksum_ieee(&genesis[..]))
    }
}

impl std::ops::AddAssign<BlockNumber> for ForkHash {
    fn add_assign(&mut self, block: BlockNumber) {
        let blob = block.to_be_bytes();
        self.0 = crc32::update(self.0, &crc32::IEEE_TABLE, &blob)
    }
}

impl std::ops::Add<BlockNumber> for ForkHash {
    type Output = Self;
    fn add(mut self, block: BlockNumber) -> Self {
        self += block;
        self
    }
}

/// A fork identifier as defined by EIP-2124.
/// Serves as the chain compatibility identifier.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, RlpEncodable, RlpDecodable)]
pub struct ForkId {
    /// CRC32 checksum of the all fork blocks from genesis.
    pub hash: ForkHash,
    /// Next upcoming fork block number, 0 if not yet known.
    pub next: BlockNumber,
}

/// Reason for rejecting provided `ForkId`.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum RejectReason {
    /// Remote node is outdated and needs a software update.
    RemoteStale,
    /// Local node is on an incompatible chain or needs a software update.
    LocalIncompatibleOrStale,
}

/// Filter that describes the state of blockchain and can be used to check incoming `ForkId`s for compatibility.
#[derive(Clone, Debug, PartialEq)]
pub struct ForkFilter {
    forks: BTreeMap<BlockNumber, ForkHash>,

    head: BlockNumber,

    cache: Cache,
}

#[derive(Clone, Debug, PartialEq)]
struct Cache {
    // An epoch is a period between forks.
    // When we progress from one fork to the next one we move to the next epoch.
    epoch_start: BlockNumber,
    epoch_end: Option<BlockNumber>,
    past: Vec<(BlockNumber, ForkHash)>,
    future: Vec<ForkHash>,
    fork_id: ForkId,
}

impl Cache {
    fn compute_cache(forks: &BTreeMap<BlockNumber, ForkHash>, head: BlockNumber) -> Self {
        let mut past = Vec::with_capacity(forks.len());
        let mut future = Vec::with_capacity(forks.len());

        let mut epoch_start = 0;
        let mut epoch_end = None;
        for (block, hash) in forks {
            if *block <= head {
                epoch_start = *block;
                past.push((*block, *hash));
            } else {
                if epoch_end.is_none() {
                    epoch_end = Some(*block);
                }
                future.push(*hash);
            }
        }

        let fork_id = ForkId {
            hash: past
                .last()
                .expect("there is always at least one - genesis - fork hash; qed")
                .1,
            next: epoch_end.unwrap_or(0),
        };

        Self {
            epoch_start,
            epoch_end,
            past,
            future,
            fork_id,
        }
    }
}

impl ForkFilter {
    /// Create the filter from provided head, genesis block hash, past forks and expected future forks.
    pub fn new<F>(head: BlockNumber, genesis: H256, forks: F) -> Self
    where
        F: IntoIterator<Item = BlockNumber>,
    {
        let genesis_fork_hash = ForkHash::from(genesis);
        let mut forks = forks.into_iter().collect::<BTreeSet<_>>();
        forks.remove(&0);
        let forks = forks
            .into_iter()
            .fold(
                (btreemap! { 0 => genesis_fork_hash }, genesis_fork_hash),
                |(mut acc, base_hash), block| {
                    let fork_hash = base_hash + block;
                    acc.insert(block, fork_hash);
                    (acc, fork_hash)
                },
            )
            .0;

        let cache = Cache::compute_cache(&forks, head);

        Self { forks, head, cache }
    }

    fn set_head_priv(&mut self, head: BlockNumber) -> bool {
        #[allow(clippy::option_if_let_else)]
        let recompute_cache = {
            if head < self.cache.epoch_start {
                true
            } else if let Some(epoch_end) = self.cache.epoch_end {
                head >= epoch_end
            } else {
                false
            }
        };

        if recompute_cache {
            self.cache = Cache::compute_cache(&self.forks, head);
        }
        self.head = head;

        recompute_cache
    }

    /// Set the current head
    pub fn set_head(&mut self, head: BlockNumber) {
        self.set_head_priv(head);
    }

    /// Return current fork id
    #[must_use]
    pub const fn current(&self) -> ForkId {
        self.cache.fork_id
    }

    /// Check whether the provided `ForkId` is compatible based on the validation rules in `EIP-2124`.
    ///
    /// # Errors
    /// Returns a `RejectReason` if the `ForkId` is not compatible.
    pub fn is_compatible(&self, fork_id: ForkId) -> Result<(), RejectReason> {
        // 1) If local and remote FORK_HASH matches...
        if self.current().hash == fork_id.hash {
            if fork_id.next == 0 {
                // 1b) No remotely announced fork, connect.
                return Ok(());
            }

            //... compare local head to FORK_NEXT.
            if self.head >= fork_id.next {
                // 1a) A remotely announced but remotely not passed block is already passed locally, disconnect,
                // since the chains are incompatible.
                return Err(RejectReason::LocalIncompatibleOrStale);
            } else {
                // 1b) Remotely announced fork not yet passed locally, connect.
                return Ok(());
            }
        }

        // 2) If the remote FORK_HASH is a subset of the local past forks...
        let mut it = self.cache.past.iter();
        while let Some((_, hash)) = it.next() {
            if *hash == fork_id.hash {
                // ...and the remote FORK_NEXT matches with the locally following fork block number, connect.
                if let Some((actual_fork_block, _)) = it.next() {
                    if *actual_fork_block == fork_id.next {
                        return Ok(());
                    } else {
                        return Err(RejectReason::RemoteStale);
                    }
                }

                break;
            }
        }

        // 3) If the remote FORK_HASH is a superset of the local past forks and can be completed with locally known future forks, connect.
        for future_fork_hash in &self.cache.future {
            if *future_fork_hash == fork_id.hash {
                return Ok(());
            }
        }

        // 4) Reject in all other cases.
        Err(RejectReason::LocalIncompatibleOrStale)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use hex_literal::hex;

    const GENESIS_HASH: H256 = H256(hex!(
        "d4e56740f876aef8c010b86a40d5f56745a118d0906a34e69aec8c0db1cb8fa3"
    ));

    // EIP test vectors.

    #[test]
    fn forkhash() {
        let mut fork_hash = ForkHash::from(GENESIS_HASH);
        assert_eq!(fork_hash.0, 0xfc64_ec04);

        fork_hash += 1_150_000;
        assert_eq!(fork_hash.0, 0x97c2_c34c);

        fork_hash += 1_920_000;
        assert_eq!(fork_hash.0, 0x91d1_f948);
    }

    #[test]
    fn compatibility_check() {
        let mut filter = ForkFilter::new(
            0,
            GENESIS_HASH,
            vec![
                1_150_000, 1_920_000, 2_463_000, 2_675_000, 4_370_000, 7_280_000,
            ],
        );

        // Local is mainnet Petersburg, remote announces the same. No future fork is announced.
        filter.set_head(7_987_396);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0x668d_b0af),
                next: 0
            }),
            Ok(())
        );

        // Local is mainnet Petersburg, remote announces the same. Remote also announces a next fork
        // at block 0xffffffff, but that is uncertain.
        filter.set_head(7_987_396);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0x668d_b0af),
                next: BlockNumber::max_value()
            }),
            Ok(())
        );

        // Local is mainnet currently in Byzantium only (so it's aware of Petersburg),remote announces
        // also Byzantium, but it's not yet aware of Petersburg (e.g. non updated node before the fork).
        // In this case we don't know if Petersburg passed yet or not.
        filter.set_head(7_279_999);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0xa00b_c324),
                next: 0
            }),
            Ok(())
        );

        // Local is mainnet currently in Byzantium only (so it's aware of Petersburg), remote announces
        // also Byzantium, and it's also aware of Petersburg (e.g. updated node before the fork). We
        // don't know if Petersburg passed yet (will pass) or not.
        filter.set_head(7_279_999);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0xa00b_c324),
                next: 7_280_000
            }),
            Ok(())
        );

        // Local is mainnet currently in Byzantium only (so it's aware of Petersburg), remote announces
        // also Byzantium, and it's also aware of some random fork (e.g. misconfigured Petersburg). As
        // neither forks passed at neither nodes, they may mismatch, but we still connect for now.
        filter.set_head(7_279_999);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0xa00b_c324),
                next: BlockNumber::max_value()
            }),
            Ok(())
        );

        // Local is mainnet Petersburg, remote announces Byzantium + knowledge about Petersburg. Remote is simply out of sync, accept.
        filter.set_head(7_987_396);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0xa00b_c324),
                next: 7_280_000
            }),
            Ok(())
        );

        // Local is mainnet Petersburg, remote announces Spurious + knowledge about Byzantium. Remote
        // is definitely out of sync. It may or may not need the Petersburg update, we don't know yet.
        filter.set_head(7_987_396);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0x3edd_5b10),
                next: 4_370_000
            }),
            Ok(())
        );

        // Local is mainnet Byzantium, remote announces Petersburg. Local is out of sync, accept.
        filter.set_head(7_279_999);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0x668d_b0af),
                next: 0
            }),
            Ok(())
        );

        // Local is mainnet Spurious, remote announces Byzantium, but is not aware of Petersburg. Local
        // out of sync. Local also knows about a future fork, but that is uncertain yet.
        filter.set_head(4_369_999);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0xa00b_c324),
                next: 0
            }),
            Ok(())
        );

        // Local is mainnet Petersburg. remote announces Byzantium but is not aware of further forks.
        // Remote needs software update.
        filter.set_head(7_987_396);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0xa00b_c324),
                next: 0
            }),
            Err(RejectReason::RemoteStale)
        );

        // Local is mainnet Petersburg, and isn't aware of more forks. Remote announces Petersburg +
        // 0xffffffff. Local needs software update, reject.
        filter.set_head(7_987_396);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0x5cdd_c0e1),
                next: 0
            }),
            Err(RejectReason::LocalIncompatibleOrStale)
        );

        // Local is mainnet Byzantium, and is aware of Petersburg. Remote announces Petersburg +
        // 0xffffffff. Local needs software update, reject.
        filter.set_head(7_279_999);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0x5cdd_c0e1),
                next: 0
            }),
            Err(RejectReason::LocalIncompatibleOrStale)
        );

        // Local is mainnet Petersburg, remote is Rinkeby Petersburg.
        filter.set_head(7_987_396);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0xafec_6b27),
                next: 0
            }),
            Err(RejectReason::LocalIncompatibleOrStale)
        );

        // Local is mainnet Petersburg, far in the future. Remote announces Gopherium (non existing fork)
        // at some future block 88888888, for itself, but past block for local. Local is incompatible.
        //
        // This case detects non-upgraded nodes with majority hash power (typical Ropsten mess).
        filter.set_head(88_888_888);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0x668d_b0af),
                next: 88_888_888
            }),
            Err(RejectReason::LocalIncompatibleOrStale)
        );

        // Local is mainnet Byzantium. Remote is also in Byzantium, but announces Gopherium (non existing
        // fork) at block 7279999, before Petersburg. Local is incompatible.
        filter.set_head(7_279_999);
        assert_eq!(
            filter.is_compatible(ForkId {
                hash: ForkHash(0xa00b_c324),
                next: 7_279_999
            }),
            Err(RejectReason::LocalIncompatibleOrStale)
        );
    }

    #[test]
    fn forkid_serialization() {
        assert_eq!(
            rlp::encode(&ForkId {
                hash: ForkHash(0),
                next: 0
            }),
            hex!("c6840000000080")
        );
        assert_eq!(
            rlp::encode(&ForkId {
                hash: ForkHash(0xdead_beef),
                next: 0xBADD_CAFE
            }),
            hex!("ca84deadbeef84baddcafe")
        );
        assert_eq!(
            rlp::encode(&ForkId {
                hash: ForkHash(u32::max_value()),
                next: u64::max_value()
            }),
            hex!("ce84ffffffff88ffffffffffffffff")
        );

        assert_eq!(
            rlp::decode::<ForkId>(&hex!("c6840000000080")).unwrap(),
            ForkId {
                hash: ForkHash(0),
                next: 0
            }
        );
        assert_eq!(
            rlp::decode::<ForkId>(&hex!("ca84deadbeef84baddcafe")).unwrap(),
            ForkId {
                hash: ForkHash(0xdead_beef),
                next: 0xBADD_CAFE
            }
        );
        assert_eq!(
            rlp::decode::<ForkId>(&hex!("ce84ffffffff88ffffffffffffffff")).unwrap(),
            ForkId {
                hash: ForkHash(u32::max_value()),
                next: u64::max_value()
            }
        );
    }

    #[test]
    fn compute_cache() {
        let b1 = 1_150_000;
        let b2 = 1_920_000;

        let h0 = ForkId {
            hash: ForkHash(0xfc64_ec04),
            next: b1,
        };
        let h1 = ForkId {
            hash: ForkHash(0x97c2_c34c),
            next: b2,
        };
        let h2 = ForkId {
            hash: ForkHash(0x91d1_f948),
            next: 0,
        };

        let mut fork_filter = ForkFilter::new(0, GENESIS_HASH, vec![b1, b2]);

        assert!(!fork_filter.set_head_priv(0));
        assert_eq!(fork_filter.current(), h0);

        assert!(!fork_filter.set_head_priv(1));
        assert_eq!(fork_filter.current(), h0);

        assert!(fork_filter.set_head_priv(b1 + 1));
        assert_eq!(fork_filter.current(), h1);

        assert!(!fork_filter.set_head_priv(b1));
        assert_eq!(fork_filter.current(), h1);

        assert!(fork_filter.set_head_priv(b1 - 1));
        assert_eq!(fork_filter.current(), h0);

        assert!(fork_filter.set_head_priv(b1));
        assert_eq!(fork_filter.current(), h1);

        assert!(!fork_filter.set_head_priv(b2 - 1));
        assert_eq!(fork_filter.current(), h1);

        assert!(fork_filter.set_head_priv(b2));
        assert_eq!(fork_filter.current(), h2);
    }
}