//! Checkpoint lists for checkpoint-based block verification
//!
//! Each checkpoint consists of a coinbase height and block header hash.
//!
//! Checkpoints can be used to verify their ancestors, by chaining backwards
//! to another checkpoint, via each block's parent block hash.

#[cfg(test)]
mod tests;

use crate::BoxError;

use std::{
    collections::{BTreeMap, HashSet},
    ops::RangeBounds,
    str::FromStr,
};

use zebra_chain::block;
use zebra_chain::parameters::Network;

/// The hard-coded checkpoints for mainnet, generated using the
/// `zebra-checkpoints` tool.
///
/// To regenerate the latest checkpoints, use the following commands:
/// ```sh
/// LAST_CHECKPOINT=$(tail -1 main-checkpoints.txt | cut -d' ' -f1)
/// echo "$LAST_CHECKPOINT"
/// zebra-checkpoints --cli /path/to/zcash-cli --last-checkpoint "$LAST_CHECKPOINT" >> main-checkpoints.txt &
/// tail -f main-checkpoints.txt
/// ```
///
/// See the checkpoints [./README.md] for more details.
const MAINNET_CHECKPOINTS: &str = include_str!("main-checkpoints.txt");

/// The hard-coded checkpoints for testnet, generated using the
/// `zebra-checkpoints` tool.
///
/// To use testnet, use the testnet checkpoints file, and run
/// `zebra-checkpoints [other args] -- -testnet`.
///
/// See [`MAINNET_CHECKPOINTS`] for detailed `zebra-checkpoints` usage
/// information.
const TESTNET_CHECKPOINTS: &str = include_str!("test-checkpoints.txt");

/// Network methods related to checkpoints
pub trait ParameterCheckpoint {
    /// Returns the hash for the genesis block in `network`.
    fn genesis_hash(&self) -> zebra_chain::block::Hash;
    /// Returns the hard-coded checkpoint list for `network`.
    fn checkpoint_list(&self) -> CheckpointList;
}

impl ParameterCheckpoint for Network {
    fn genesis_hash(&self) -> zebra_chain::block::Hash {
        match self {
            // zcash-cli getblockhash 0
            Network::Mainnet => "00040fe8ec8471911baa1db1266ea15dd06b4a8a5c453883c000b031973dce08"
                .parse()
                .expect("hard-coded hash parses"),
            // See `zebra_chain::parameters::network::testnet` for more details.
            Network::Testnet(params) => params.genesis_hash(),
        }
    }

    fn checkpoint_list(&self) -> CheckpointList {
        let (checkpoints_for_network, should_fallback_to_genesis_hash_as_checkpoint) = match self {
            Network::Mainnet => (MAINNET_CHECKPOINTS, false),
            Network::Testnet(params) if params.is_default_testnet() => (TESTNET_CHECKPOINTS, false),
            Network::Testnet(_params) => (TESTNET_CHECKPOINTS, true),
        };

        // Check that the list starts with the correct genesis block and parses checkpoint list.
        let first_checkpoint_height = checkpoints_for_network
            .lines()
            .next()
            .map(checkpoint_height_and_hash);

        match first_checkpoint_height {
            // parse calls CheckpointList::from_list
            Some(Ok((block::Height(0), hash))) if hash == self.genesis_hash() => {
                checkpoints_for_network
                    .parse()
                    .expect("hard-coded checkpoint list parses and validates")
            }
            _ if should_fallback_to_genesis_hash_as_checkpoint => {
                CheckpointList::from_list([(block::Height(0), self.genesis_hash())])
                    .expect("hard-coded checkpoint list parses and validates")
            }
            Some(Ok((block::Height(0), _))) => {
                panic!("the genesis checkpoint does not match the {self} genesis hash")
            }
            Some(Ok(_)) => panic!("checkpoints must start at the genesis block height 0"),
            Some(Err(err)) => panic!("{err}"),
            None => panic!(
                "there must be at least one checkpoint on default networks, for the genesis block"
            ),
        }
    }
}

/// Parses a checkpoint to a [`block::Height`] and [`block::Hash`].
fn checkpoint_height_and_hash(checkpoint: &str) -> Result<(block::Height, block::Hash), BoxError> {
    let fields = checkpoint.split(' ').collect::<Vec<_>>();
    if let [height, hash] = fields[..] {
        Ok((height.parse()?, hash.parse()?))
    } else {
        Err(format!("Invalid checkpoint format: expected 2 space-separated fields but found {}: '{checkpoint}'", fields.len()).into())
    }
}

/// A list of block height and hash checkpoints.
///
/// Checkpoints should be chosen to avoid forks or chain reorganizations,
/// which only happen in the last few hundred blocks in the chain.
/// (zcashd allows chain reorganizations up to 99 blocks, and prunes
/// orphaned side-chains after 288 blocks.)
///
/// This is actually a bijective map, but since it is read-only, we use a
/// BTreeMap, and do the value uniqueness check on initialisation.
#[derive(Clone, Debug, Eq, Hash, PartialEq)]
pub struct CheckpointList(BTreeMap<block::Height, block::Hash>);

impl FromStr for CheckpointList {
    type Err = BoxError;

    /// Parse a string into a CheckpointList.
    ///
    /// Each line has one checkpoint, consisting of a `block::Height` and
    /// `block::Hash`, separated by a single space.
    ///
    /// Assumes that the provided genesis checkpoint is correct.
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let mut checkpoint_list: Vec<(block::Height, block::Hash)> = Vec::new();

        for checkpoint in s.lines() {
            checkpoint_list.push(checkpoint_height_and_hash(checkpoint)?);
        }

        CheckpointList::from_list(checkpoint_list)
    }
}

impl CheckpointList {
    /// Create a new checkpoint list for `network` from `checkpoint_list`.
    ///
    /// Assumes that the provided genesis checkpoint is correct.
    ///
    /// Checkpoint heights and checkpoint hashes must be unique.
    /// There must be a checkpoint for a genesis block at block::Height 0.
    /// (All other checkpoints are optional.)
    pub(crate) fn from_list(
        list: impl IntoIterator<Item = (block::Height, block::Hash)>,
    ) -> Result<Self, BoxError> {
        // BTreeMap silently ignores duplicates, so we count the checkpoints
        // before adding them to the map
        let original_checkpoints: Vec<(block::Height, block::Hash)> = list.into_iter().collect();
        let original_len = original_checkpoints.len();

        let checkpoints: BTreeMap<block::Height, block::Hash> =
            original_checkpoints.into_iter().collect();

        // Check that the list starts with _some_ genesis block
        match checkpoints.iter().next() {
            Some((block::Height(0), _hash)) => {}
            Some(_) => Err("checkpoints must start at the genesis block height 0")?,
            None => Err("there must be at least one checkpoint, for the genesis block")?,
        };

        // This check rejects duplicate heights, whether they have the same or
        // different hashes
        if checkpoints.len() != original_len {
            Err("checkpoint heights must be unique")?;
        }

        let block_hashes: HashSet<&block::Hash> = checkpoints.values().collect();
        if block_hashes.len() != original_len {
            Err("checkpoint hashes must be unique")?;
        }

        // Make sure all the hashes are valid. In Bitcoin, [0; 32] is the null
        // hash. It is also used as the parent hash of genesis blocks.
        if block_hashes.contains(&block::Hash([0; 32])) {
            Err("checkpoint list contains invalid checkpoint hash: found null hash")?;
        }

        let checkpoints = CheckpointList(checkpoints);
        if checkpoints.max_height() > block::Height::MAX {
            Err("checkpoint list contains invalid checkpoint: checkpoint height is greater than the maximum block height")?;
        }

        Ok(checkpoints)
    }

    /// Return true if there is a checkpoint at `height`.
    ///
    /// See `BTreeMap::contains_key()` for details.
    pub fn contains(&self, height: block::Height) -> bool {
        self.0.contains_key(&height)
    }

    /// Returns the hash corresponding to the checkpoint at `height`,
    /// or None if there is no checkpoint at that height.
    ///
    /// See `BTreeMap::get()` for details.
    pub fn hash(&self, height: block::Height) -> Option<block::Hash> {
        self.0.get(&height).cloned()
    }

    /// Return the block height of the highest checkpoint in the checkpoint list.
    ///
    /// If there is only a single checkpoint, then the maximum height will be
    /// zero. (The genesis block.)
    pub fn max_height(&self) -> block::Height {
        self.max_height_in_range(..)
            .expect("checkpoint lists must have at least one checkpoint")
    }

    /// Return the block height of the lowest checkpoint in a sub-range.
    pub fn min_height_in_range<R>(&self, range: R) -> Option<block::Height>
    where
        R: RangeBounds<block::Height>,
    {
        self.0.range(range).map(|(height, _)| *height).next()
    }

    /// Return the block height of the highest checkpoint in a sub-range.
    pub fn max_height_in_range<R>(&self, range: R) -> Option<block::Height>
    where
        R: RangeBounds<block::Height>,
    {
        self.0.range(range).map(|(height, _)| *height).next_back()
    }

    /// Returns an iterator over all the checkpoints, in increasing height order.
    pub fn iter(&self) -> impl Iterator<Item = (&block::Height, &block::Hash)> {
        self.0.iter()
    }

    /// Returns the checkpoint at `height`, as a zero-based index.
    /// If `height` is not a checkpoint height, returns the checkpoint immediately before that height.
    pub fn prev_checkpoint_index(&self, height: block::Height) -> usize {
        self.0
            .keys()
            .rposition(|&key| key <= height)
            .expect("checkpoints must start at the genesis block height 0")
    }

    /// Returns the number of checkpoints in the list.
    //
    // Checkpoint lists are never empty by construction.
    #[allow(clippy::len_without_is_empty)]
    pub fn len(&self) -> usize {
        self.0.len()
    }
}