//! Equihash Solution and related items.

use std::{fmt, io};

use serde_big_array::BigArray;

use crate::{
    block::Header,
    serialization::{
        zcash_serialize_bytes, SerializationError, ZcashDeserialize, ZcashDeserializeInto,
        ZcashSerialize,
    },
};

#[cfg(feature = "internal-miner")]
use crate::serialization::AtLeastOne;

/// The error type for Equihash validation.
#[non_exhaustive]
#[derive(Debug, thiserror::Error)]
#[error("invalid equihash solution for BlockHeader")]
pub struct Error(#[from] equihash::Error);

/// The error type for Equihash solving.
#[derive(Copy, Clone, Debug, Eq, PartialEq, thiserror::Error)]
#[error("solver was cancelled")]
pub struct SolverCancelled;

/// The size of an Equihash solution in bytes (always 1344).
pub(crate) const SOLUTION_SIZE: usize = 1344;

/// The size of an Equihash solution in bytes on Regtest (always 36).
pub(crate) const REGTEST_SOLUTION_SIZE: usize = 36;

/// Equihash Solution in compressed format.
///
/// A wrapper around `[u8; n]` where `n` is the solution size because
/// Rust doesn't implement common traits like `Debug`, `Clone`, etc.
/// for collections like arrays beyond lengths 0 to 32.
///
/// The size of an Equihash solution in bytes is always 1344 on Mainnet and Testnet, and
/// is always 36 on Regtest so the length of this type is fixed.
#[derive(Deserialize, Serialize)]
// It's okay to use the extra space on Regtest
#[allow(clippy::large_enum_variant)]
pub enum Solution {
    /// Equihash solution on Mainnet or Testnet
    Common(#[serde(with = "BigArray")] [u8; SOLUTION_SIZE]),
    /// Equihash solution on Regtest
    Regtest(#[serde(with = "BigArray")] [u8; REGTEST_SOLUTION_SIZE]),
}

impl Solution {
    /// The length of the portion of the header used as input when verifying
    /// equihash solutions, in bytes.
    ///
    /// Excludes the 32-byte nonce, which is passed as a separate argument
    /// to the verification function.
    pub const INPUT_LENGTH: usize = 4 + 32 * 3 + 4 * 2;

    /// Returns the inner value of the [`Solution`] as a byte slice.
    fn value(&self) -> &[u8] {
        match self {
            Solution::Common(solution) => solution.as_slice(),
            Solution::Regtest(solution) => solution.as_slice(),
        }
    }

    /// Returns `Ok(())` if `EquihashSolution` is valid for `header`
    #[allow(clippy::unwrap_in_result)]
    pub fn check(&self, header: &Header) -> Result<(), Error> {
        let nonce = &header.nonce;
        let (solution, n, k) = match self {
            Solution::Common(solution) => (solution.as_slice(), 200, 9),
            Solution::Regtest(solution) => (solution.as_slice(), 48, 5),
        };

        let mut input = Vec::new();
        header
            .zcash_serialize(&mut input)
            .expect("serialization into a vec can't fail");

        // The part of the header before the nonce and solution.
        // This data is kept constant during solver runs, so the verifier API takes it separately.
        let input = &input[0..Solution::INPUT_LENGTH];

        equihash::is_valid_solution(n, k, input, nonce.as_ref(), solution)?;

        Ok(())
    }

    /// Returns a [`Solution`] containing the bytes from `solution`.
    /// Returns an error if `solution` is the wrong length.
    pub fn from_bytes(solution: &[u8]) -> Result<Self, SerializationError> {
        match solution.len() {
            // Won't panic, because we just checked the length.
            SOLUTION_SIZE => {
                let mut bytes = [0; SOLUTION_SIZE];
                bytes.copy_from_slice(solution);
                Ok(Self::Common(bytes))
            }
            REGTEST_SOLUTION_SIZE => {
                let mut bytes = [0; REGTEST_SOLUTION_SIZE];
                bytes.copy_from_slice(solution);
                Ok(Self::Regtest(bytes))
            }
            _unexpected_len => Err(SerializationError::Parse(
                "incorrect equihash solution size",
            )),
        }
    }

    /// Returns a [`Solution`] of `[0; SOLUTION_SIZE]` to be used in block proposals.
    #[cfg(feature = "getblocktemplate-rpcs")]
    pub fn for_proposal() -> Self {
        // TODO: Accept network as an argument, and if it's Regtest, return the shorter null solution.
        Self::Common([0; SOLUTION_SIZE])
    }

    /// Mines and returns one or more [`Solution`]s based on a template `header`.
    /// The returned header contains a valid `nonce` and `solution`.
    ///
    /// If `cancel_fn()` returns an error, returns early with `Err(SolverCancelled)`.
    ///
    /// The `nonce` in the header template is taken as the starting nonce. If you are running multiple
    /// solvers at the same time, start them with different nonces.
    /// The `solution` in the header template is ignored.
    ///
    /// This method is CPU and memory-intensive. It uses 144 MB of RAM and one CPU core while running.
    /// It can run for minutes or hours if the network difficulty is high.
    #[cfg(feature = "internal-miner")]
    #[allow(clippy::unwrap_in_result)]
    pub fn solve<F>(
        mut _header: Header,
        mut _cancel_fn: F,
    ) -> Result<AtLeastOne<Header>, SolverCancelled>
    where
        F: FnMut() -> Result<(), SolverCancelled>,
    {
        // TODO: Function code was removed as part of https://github.com/ZcashFoundation/zebra/issues/8180
        // Find the removed code at https://github.com/ZcashFoundation/zebra/blob/v1.5.1/zebra-chain/src/work/equihash.rs#L115-L166
        // Restore the code when conditions are met. https://github.com/ZcashFoundation/zebra/issues/8183

        Err(SolverCancelled)
    }

    // TODO: Some methods were removed as part of https://github.com/ZcashFoundation/zebra/issues/8180
    // Find the removed code at https://github.com/ZcashFoundation/zebra/blob/v1.5.1/zebra-chain/src/work/equihash.rs#L171-L196
    // Restore the code when conditions are met. https://github.com/ZcashFoundation/zebra/issues/8183
}

impl PartialEq<Solution> for Solution {
    fn eq(&self, other: &Solution) -> bool {
        self.value() == other.value()
    }
}

impl fmt::Debug for Solution {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_tuple("EquihashSolution")
            .field(&hex::encode(self.value()))
            .finish()
    }
}

// These impls all only exist because of array length restrictions.

impl Copy for Solution {}

impl Clone for Solution {
    fn clone(&self) -> Self {
        *self
    }
}

impl Eq for Solution {}

#[cfg(any(test, feature = "proptest-impl"))]
impl Default for Solution {
    fn default() -> Self {
        Self::Common([0; SOLUTION_SIZE])
    }
}

impl ZcashSerialize for Solution {
    fn zcash_serialize<W: io::Write>(&self, writer: W) -> Result<(), io::Error> {
        zcash_serialize_bytes(&self.value().to_vec(), writer)
    }
}

impl ZcashDeserialize for Solution {
    fn zcash_deserialize<R: io::Read>(mut reader: R) -> Result<Self, SerializationError> {
        let solution: Vec<u8> = (&mut reader).zcash_deserialize_into()?;
        Self::from_bytes(&solution)
    }
}