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// Copyright 2015-2020 Parity Technologies (UK) Ltd.
// This file is part of OpenEthereum.

// OpenEthereum is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// OpenEthereum is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with OpenEthereum.  If not, see <http://www.gnu.org/licenses/>.

use keccak::{keccak_256, H256};
use shared;

use std::cell::Cell;

#[derive(Default)]
pub struct SeedHashCompute {
    prev_epoch: Cell<u64>,
    prev_seedhash: Cell<H256>,
}

impl SeedHashCompute {
    #[inline]
    fn reset_cache(&self) {
        self.prev_epoch.set(0);
        self.prev_seedhash.set([0u8; 32]);
    }

    #[inline]
    pub fn hash_block_number(&self, block_number: u64) -> H256 {
        self.hash_epoch(shared::epoch(block_number))
    }

    #[inline]
    pub fn hash_epoch(&self, epoch: u64) -> H256 {
        if epoch < self.prev_epoch.get() {
            // can't build on previous hash if requesting an older block
            self.reset_cache();
        }
        if epoch > self.prev_epoch.get() {
            let seed_hash = SeedHashCompute::resume_compute_seedhash(
                self.prev_seedhash.get(),
                self.prev_epoch.get(),
                epoch,
            );
            self.prev_seedhash.set(seed_hash);
            self.prev_epoch.set(epoch);
        }
        self.prev_seedhash.get()
    }

    #[inline]
    pub fn resume_compute_seedhash(mut hash: H256, start_epoch: u64, end_epoch: u64) -> H256 {
        for _ in start_epoch..end_epoch {
            keccak_256::inplace(&mut hash);
        }
        hash
    }
}

#[cfg(test)]
mod tests {
    use super::SeedHashCompute;

    #[test]
    fn test_seed_compute_once() {
        let seed_compute = SeedHashCompute::default();
        let hash = [
            241, 175, 44, 134, 39, 121, 245, 239, 228, 236, 43, 160, 195, 152, 46, 7, 199, 5, 253,
            147, 241, 206, 98, 43, 3, 104, 17, 40, 192, 79, 106, 162,
        ];
        assert_eq!(seed_compute.hash_block_number(486382), hash);
    }

    #[test]
    fn test_seed_compute_zero() {
        let seed_compute = SeedHashCompute::default();
        assert_eq!(seed_compute.hash_block_number(0), [0u8; 32]);
    }

    #[test]
    fn test_seed_compute_after_older() {
        let seed_compute = SeedHashCompute::default();
        // calculating an older value first shouldn't affect the result
        let _ = seed_compute.hash_block_number(50000);
        let hash = [
            241, 175, 44, 134, 39, 121, 245, 239, 228, 236, 43, 160, 195, 152, 46, 7, 199, 5, 253,
            147, 241, 206, 98, 43, 3, 104, 17, 40, 192, 79, 106, 162,
        ];
        assert_eq!(seed_compute.hash_block_number(486382), hash);
    }

    #[test]
    fn test_seed_compute_after_newer() {
        let seed_compute = SeedHashCompute::default();
        // calculating an newer value first shouldn't affect the result
        let _ = seed_compute.hash_block_number(972764);
        let hash = [
            241, 175, 44, 134, 39, 121, 245, 239, 228, 236, 43, 160, 195, 152, 46, 7, 199, 5, 253,
            147, 241, 206, 98, 43, 3, 104, 17, 40, 192, 79, 106, 162,
        ];
        assert_eq!(seed_compute.hash_block_number(486382), hash);
    }
}