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    // Copyright 2015-2017 Parity Technologies (UK) Ltd.
    // This file is part of Parity.

    // Parity 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.

    // Parity 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 Parity.  If not, see <http://www.gnu.org/licenses/>.

    //! Ethash implementation
    //! See https://github.com/ethereum/wiki/wiki/Ethash

    // TODO: fix endianess for big endian

    use primal::is_prime;
    use std::cell::Cell;
    use std::mem;
    use std::ptr;
    use sha3;
    use std::slice;
    use std::path::PathBuf;
    use std::io::{self, Read, Write};
    use std::fs::{self, File};

    use parking_lot::Mutex;

    pub const ETHASH_EPOCH_LENGTH: u64 = 30000;
    pub const ETHASH_CACHE_ROUNDS: usize = 3;
    pub const ETHASH_MIX_BYTES: usize = 128;
    pub const ETHASH_ACCESSES: usize = 64;
    pub const ETHASH_DATASET_PARENTS: u32 = 256;

    const DATASET_BYTES_INIT: u64 = 1 << 30;
    const DATASET_BYTES_GROWTH: u64 = 1 << 23;
    const CACHE_BYTES_INIT: u64 = 1 << 24;
    const CACHE_BYTES_GROWTH: u64 = 1 << 17;
    const NODE_WORDS: usize = 64 / 4;
    const NODE_BYTES: usize = 64;
    const MIX_WORDS: usize = ETHASH_MIX_BYTES / 4;
    const MIX_NODES: usize = MIX_WORDS / NODE_WORDS;
    const FNV_PRIME: u32 = 0x01000193;

    /// Computation result
    pub struct ProofOfWork {
    	/// Difficulty boundary
    	pub value: H256,
    	/// Mix
    	pub mix_hash: H256,
    }

    struct Node {
    	bytes: [u8; NODE_BYTES],
    }

    impl Default for Node {
    	fn default() -> Self {
    		Node { bytes: [0u8; NODE_BYTES] }
    	}
    }

    impl Clone for Node {
    	fn clone(&self) -> Self {
    		Node { bytes: *&self.bytes }
    	}
    }

    impl Node {
    	#[inline]
    	fn as_words(&self) -> &[u32; NODE_WORDS] {
    		unsafe { mem::transmute(&self.bytes) }
    	}

    	#[inline]
    	fn as_words_mut(&mut self) -> &mut [u32; NODE_WORDS] {
    		unsafe { mem::transmute(&mut self.bytes) }
    	}
    }

    pub type H256 = [u8; 32];

    pub struct Light {
    	block_number: u64,
    	cache: Vec<Node>,
    	seed_compute: Mutex<SeedHashCompute>,
    }

    /// Light cache structure
    impl Light {
    	/// Create a new light cache for a given block number
    	pub fn new(block_number: u64) -> Light {
    		light_new(block_number)
    	}

    	/// Calculate the light boundary data
    	/// `header_hash` - The header hash to pack into the mix
    	/// `nonce` - The nonce to pack into the mix
    	pub fn compute(&self, header_hash: &H256, nonce: u64) -> ProofOfWork {
    		light_compute(self, header_hash, nonce)
    	}

    	pub fn file_path(seed_hash: H256) -> PathBuf {
    		let mut home = ::std::env::home_dir().unwrap();
    		home.push(".ethash");
    		home.push("light");
    		home.push(to_hex(&seed_hash));
    		home
    	}

    	pub fn from_file(block_number: u64) -> io::Result<Light> {
    		let seed_compute = SeedHashCompute::new();
    		let path = Light::file_path(seed_compute.get_seedhash(block_number));
    		let mut file = File::open(path)?;

    		let cache_size = get_cache_size(block_number);
    		if file.metadata()?.len() != cache_size as u64 {
    			return Err(io::Error::new(io::ErrorKind::Other, "Cache file size mismatch"));
    		}
    		let num_nodes = cache_size / NODE_BYTES;
    		let mut nodes: Vec<Node> = Vec::new();
    		nodes.resize(num_nodes, unsafe { mem::uninitialized() });
    		let buf = unsafe { slice::from_raw_parts_mut(nodes.as_mut_ptr() as *mut u8, cache_size) };
    		file.read_exact(buf)?;
    		Ok(Light {
    			cache: nodes,
    			block_number: block_number,
    			seed_compute: Mutex::new(seed_compute),
    		})
    	}

    	pub fn to_file(&self) -> io::Result<PathBuf> {
    		let seed_compute = self.seed_compute.lock();
    		let path = Light::file_path(seed_compute.get_seedhash(self.block_number));

    		if self.block_number >= ETHASH_EPOCH_LENGTH * 2 {
    			let deprecated = Light::file_path(
    				seed_compute.get_seedhash(self.block_number - ETHASH_EPOCH_LENGTH * 2));

    			if deprecated.exists() {
    				debug!(target: "ethash", "removing: {:?}", &deprecated);
    				fs::remove_file(deprecated)?;
    			}
    		}

    		fs::create_dir_all(path.parent().unwrap())?;
    		let mut file = File::create(&path)?;

    		let cache_size = self.cache.len() * NODE_BYTES;
    		let buf = unsafe { slice::from_raw_parts(self.cache.as_ptr() as *const u8, cache_size) };
    		file.write(buf)?;
    		Ok(path)
    	}
    }

    pub struct SeedHashCompute {
    	prev_epoch: Cell<u64>,
    	prev_seedhash: Cell<H256>,
    }

    impl SeedHashCompute {
    	#[inline]
    	pub fn new() -> SeedHashCompute {
    		SeedHashCompute {
    			prev_epoch: Cell::new(0),
    			prev_seedhash: Cell::new([0u8; 32]),
    		}
    	}

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

    	#[inline]
    	pub fn get_seedhash(&self, block_number: u64) -> H256 {
    		let epoch = block_number / ETHASH_EPOCH_LENGTH;
    		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 {
    			unsafe { sha3::sha3_256(hash[..].as_mut_ptr(), 32, hash[..].as_ptr(), 32) };
    		}
    		hash
    	}
    }

    pub fn slow_get_seedhash(block_number: u64) -> H256 {
    	SeedHashCompute::resume_compute_seedhash([0u8; 32], 0, block_number / ETHASH_EPOCH_LENGTH)
    }

    #[inline]
    fn fnv_hash(x: u32, y: u32) -> u32 {
    	return x.wrapping_mul(FNV_PRIME) ^ y;
    }

    #[inline]
    fn sha3_512(input: &[u8], output: &mut [u8]) {
    	unsafe { sha3::sha3_512(output.as_mut_ptr(), output.len(), input.as_ptr(), input.len()) };
    }

    #[inline]
    fn get_cache_size(block_number: u64) -> usize {
    	let mut sz: u64 = CACHE_BYTES_INIT + CACHE_BYTES_GROWTH * (block_number / ETHASH_EPOCH_LENGTH);
    	sz = sz - NODE_BYTES as u64;
    	while !is_prime(sz / NODE_BYTES as u64) {
    		sz = sz - 2 * NODE_BYTES as u64;
    	}
    	sz as usize
    }

    #[inline]
    fn get_data_size(block_number: u64) -> usize {
    	let mut sz: u64 = DATASET_BYTES_INIT + DATASET_BYTES_GROWTH * (block_number / ETHASH_EPOCH_LENGTH);
    	sz = sz - ETHASH_MIX_BYTES as u64;
    	while !is_prime(sz / ETHASH_MIX_BYTES as u64) {
    		sz = sz - 2 * ETHASH_MIX_BYTES as u64;
    	}
    	sz as usize
    }

    /// Difficulty quick check for POW preverification
    ///
    /// `header_hash`      The hash of the header
    /// `nonce`            The block's nonce
    /// `mix_hash`         The mix digest hash
    /// Boundary recovered from mix hash
    pub fn quick_get_difficulty(header_hash: &H256, nonce: u64, mix_hash: &H256) -> H256 {
    	let mut buf = [0u8; 64 + 32];
    	unsafe { ptr::copy_nonoverlapping(header_hash.as_ptr(), buf.as_mut_ptr(), 32) };
    	unsafe { ptr::copy_nonoverlapping(mem::transmute(&nonce), buf[32..].as_mut_ptr(), 8) };

    	unsafe { sha3::sha3_512(buf.as_mut_ptr(), 64, buf.as_ptr(), 40) };
    	unsafe { ptr::copy_nonoverlapping(mix_hash.as_ptr(), buf[64..].as_mut_ptr(), 32) };

    	let mut hash = [0u8; 32];
    	unsafe { sha3::sha3_256(hash.as_mut_ptr(), hash.len(), buf.as_ptr(), buf.len()) };
    	hash.as_mut_ptr();
    	hash
    }

    /// Calculate the light client data
    /// `light` - The light client handler
    /// `header_hash` - The header hash to pack into the mix
    /// `nonce` - The nonce to pack into the mix
    pub fn light_compute(light: &Light, header_hash: &H256, nonce: u64) -> ProofOfWork {
    	let full_size = get_data_size(light.block_number);
    	hash_compute(light, full_size, header_hash, nonce)
    }

    fn hash_compute(light: &Light, full_size: usize, header_hash: &H256, nonce: u64) -> ProofOfWork {
    	if full_size % MIX_WORDS != 0 {
    		panic!("Unaligned full size");
    	}
    	// pack hash and nonce together into first 40 bytes of s_mix
    	let mut s_mix: [Node; MIX_NODES + 1] = [Node::default(), Node::default(), Node::default()];
    	unsafe { ptr::copy_nonoverlapping(header_hash.as_ptr(), s_mix.get_unchecked_mut(0).bytes.as_mut_ptr(), 32) };
    	unsafe { ptr::copy_nonoverlapping(mem::transmute(&nonce), s_mix.get_unchecked_mut(0).bytes[32..].as_mut_ptr(), 8) };

    	// compute sha3-512 hash and replicate across mix
    	unsafe {
    		sha3::sha3_512(s_mix.get_unchecked_mut(0).bytes.as_mut_ptr(), NODE_BYTES, s_mix.get_unchecked(0).bytes.as_ptr(), 40);
    		let (f_mix, mut mix) = s_mix.split_at_mut(1);
    		for w in 0..MIX_WORDS {
    			*mix.get_unchecked_mut(0).as_words_mut().get_unchecked_mut(w) = *f_mix.get_unchecked(0).as_words().get_unchecked(w % NODE_WORDS);
    		}

    		let page_size = 4 * MIX_WORDS;
    		let num_full_pages = (full_size / page_size) as u32;
    		let cache: &[Node] = &light.cache;  // deref once for better performance

    		for i in 0..(ETHASH_ACCESSES as u32) {
    			let index = fnv_hash(f_mix.get_unchecked(0).as_words().get_unchecked(0) ^ i, *mix.get_unchecked(0).as_words().get_unchecked((i as usize) % MIX_WORDS)) % num_full_pages;
    			for n in 0..MIX_NODES {
    				let tmp_node = calculate_dag_item(index * MIX_NODES as u32 + n as u32, cache);
    				for w in 0..NODE_WORDS {
    					*mix.get_unchecked_mut(n).as_words_mut().get_unchecked_mut(w) = fnv_hash(*mix.get_unchecked(n).as_words().get_unchecked(w), *tmp_node.as_words().get_unchecked(w));
    				}
    			}
    		}

    		// compress mix
    		for i in 0..(MIX_WORDS / 4) {
    			let w = i * 4;
    			let mut reduction = *mix.get_unchecked(0).as_words().get_unchecked(w + 0);
    			reduction = reduction.wrapping_mul(FNV_PRIME) ^ *mix.get_unchecked(0).as_words().get_unchecked(w + 1);
    			reduction = reduction.wrapping_mul(FNV_PRIME) ^ *mix.get_unchecked(0).as_words().get_unchecked(w + 2);
    			reduction = reduction.wrapping_mul(FNV_PRIME) ^ *mix.get_unchecked(0).as_words().get_unchecked(w + 3);
    			*mix.get_unchecked_mut(0).as_words_mut().get_unchecked_mut(i) = reduction;
    		}

    		let mut mix_hash = [0u8; 32];
    		let mut buf = [0u8; 32 + 64];
    		ptr::copy_nonoverlapping(f_mix.get_unchecked_mut(0).bytes.as_ptr(), buf.as_mut_ptr(), 64);
    		ptr::copy_nonoverlapping(mix.get_unchecked_mut(0).bytes.as_ptr(), buf[64..].as_mut_ptr(), 32);
    		ptr::copy_nonoverlapping(mix.get_unchecked_mut(0).bytes.as_ptr(), mix_hash.as_mut_ptr(), 32);
    		let mut value: H256 = [0u8; 32];
    		sha3::sha3_256(value.as_mut_ptr(), value.len(), buf.as_ptr(), buf.len());
    		ProofOfWork {
    			mix_hash: mix_hash,
    			value: value,
    		}
    	}
    }

    fn calculate_dag_item(node_index: u32, cache: &[Node]) -> Node {
    	unsafe {
    		let num_parent_nodes = cache.len();
    		let init = cache.get_unchecked(node_index as usize % num_parent_nodes);
    		let mut ret = init.clone();
    		*ret.as_words_mut().get_unchecked_mut(0) ^= node_index;
    		sha3::sha3_512(ret.bytes.as_mut_ptr(), ret.bytes.len(), ret.bytes.as_ptr(), ret.bytes.len());

    		for i in 0..ETHASH_DATASET_PARENTS {
    			let parent_index = fnv_hash(node_index ^ i, *ret.as_words().get_unchecked(i as usize % NODE_WORDS)) % num_parent_nodes as u32;
    			let parent = cache.get_unchecked(parent_index as usize);
    			for w in 0..NODE_WORDS {
    				*ret.as_words_mut().get_unchecked_mut(w) = fnv_hash(*ret.as_words().get_unchecked(w), *parent.as_words().get_unchecked(w));
    			}
    		}
    		sha3::sha3_512(ret.bytes.as_mut_ptr(), ret.bytes.len(), ret.bytes.as_ptr(), ret.bytes.len());
    		ret
    	}
    }

    fn light_new(block_number: u64) -> Light {

    	let seed_compute = SeedHashCompute::new();
    	let seedhash = seed_compute.get_seedhash(block_number);
    	let cache_size = get_cache_size(block_number);

    	if cache_size % NODE_BYTES != 0 {
    		panic!("Unaligned cache size");
    	}
    	let num_nodes = cache_size / NODE_BYTES;

    	let mut nodes = Vec::with_capacity(num_nodes);
    	nodes.resize(num_nodes, Node::default());
    	unsafe {
    		sha3_512(&seedhash[0..32], &mut nodes.get_unchecked_mut(0).bytes);
    		for i in 1..num_nodes {
    			sha3::sha3_512(nodes.get_unchecked_mut(i).bytes.as_mut_ptr(), NODE_BYTES, nodes.get_unchecked(i - 1).bytes.as_ptr(), NODE_BYTES);
    		}

    		for _ in 0..ETHASH_CACHE_ROUNDS {
    			for i in 0..num_nodes {
    				let idx = *nodes.get_unchecked_mut(i).as_words().get_unchecked(0) as usize % num_nodes;
    				let mut data = nodes.get_unchecked((num_nodes - 1 + i) % num_nodes).clone();
    				for w in 0..NODE_WORDS {
    					*data.as_words_mut().get_unchecked_mut(w) ^= *nodes.get_unchecked(idx).as_words().get_unchecked(w);
    				}
    				sha3_512(&data.bytes, &mut nodes.get_unchecked_mut(i).bytes);
    			}
    		}
    	}

    	Light {
    		cache: nodes,
    		block_number: block_number,
    		seed_compute: Mutex::new(seed_compute),
    	}
    }

    static CHARS: &'static [u8] = b"0123456789abcdef";
    fn to_hex(bytes: &[u8]) -> String {
    	let mut v = Vec::with_capacity(bytes.len() * 2);
    	for &byte in bytes.iter() {
    		v.push(CHARS[(byte >> 4) as usize]);
    		v.push(CHARS[(byte & 0xf) as usize]);
    	}

    	unsafe { String::from_utf8_unchecked(v) }
    }

    #[test]
    fn test_get_cache_size() {
    	// https://github.com/ethereum/wiki/wiki/Ethash/ef6b93f9596746a088ea95d01ca2778be43ae68f#data-sizes
    	assert_eq!(16776896usize, get_cache_size(0));
    	assert_eq!(16776896usize, get_cache_size(1));
    	assert_eq!(16776896usize, get_cache_size(ETHASH_EPOCH_LENGTH - 1));
    	assert_eq!(16907456usize, get_cache_size(ETHASH_EPOCH_LENGTH));
    	assert_eq!(16907456usize, get_cache_size(ETHASH_EPOCH_LENGTH + 1));
    	assert_eq!(284950208usize, get_cache_size(2046 * ETHASH_EPOCH_LENGTH));
    	assert_eq!(285081536usize, get_cache_size(2047 * ETHASH_EPOCH_LENGTH));
    	assert_eq!(285081536usize, get_cache_size(2048 * ETHASH_EPOCH_LENGTH - 1));
    }

    #[test]
    fn test_get_data_size() {
    	// https://github.com/ethereum/wiki/wiki/Ethash/ef6b93f9596746a088ea95d01ca2778be43ae68f#data-sizes
    	assert_eq!(1073739904usize, get_data_size(0));
    	assert_eq!(1073739904usize, get_data_size(1));
    	assert_eq!(1073739904usize, get_data_size(ETHASH_EPOCH_LENGTH - 1));
    	assert_eq!(1082130304usize, get_data_size(ETHASH_EPOCH_LENGTH));
    	assert_eq!(1082130304usize, get_data_size(ETHASH_EPOCH_LENGTH + 1));
    	assert_eq!(18236833408usize, get_data_size(2046 * ETHASH_EPOCH_LENGTH));
    	assert_eq!(18245220736usize, get_data_size(2047 * ETHASH_EPOCH_LENGTH));
    }

    #[test]
    fn test_difficulty_test() {
    	let hash = [0xf5, 0x7e, 0x6f, 0x3a, 0xcf, 0xc0, 0xdd, 0x4b, 0x5b, 0xf2, 0xbe, 0xe4, 0x0a, 0xb3, 0x35, 0x8a, 0xa6, 0x87, 0x73, 0xa8, 0xd0, 0x9f, 0x5e, 0x59, 0x5e, 0xab, 0x55, 0x94, 0x05, 0x52, 0x7d, 0x72];
    	let mix_hash = [0x1f, 0xff, 0x04, 0xce, 0xc9, 0x41, 0x73, 0xfd, 0x59, 0x1e, 0x3d, 0x89, 0x60, 0xce, 0x6b, 0xdf, 0x8b, 0x19, 0x71, 0x04, 0x8c, 0x71, 0xff, 0x93, 0x7b, 0xb2, 0xd3, 0x2a, 0x64, 0x31, 0xab, 0x6d];
    	let nonce = 0xd7b3ac70a301a249;
    	let boundary_good = [0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x3e, 0x9b, 0x6c, 0x69, 0xbc, 0x2c, 0xe2, 0xa2, 0x4a, 0x8e, 0x95, 0x69, 0xef, 0xc7, 0xd7, 0x1b, 0x33, 0x35, 0xdf, 0x36, 0x8c, 0x9a, 0xe9, 0x7e, 0x53, 0x84];
    	assert_eq!(quick_get_difficulty(&hash, nonce, &mix_hash)[..], boundary_good[..]);
    	let boundary_bad = [0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x3a, 0x9b, 0x6c, 0x69, 0xbc, 0x2c, 0xe2, 0xa2, 0x4a, 0x8e, 0x95, 0x69, 0xef, 0xc7, 0xd7, 0x1b, 0x33, 0x35, 0xdf, 0x36, 0x8c, 0x9a, 0xe9, 0x7e, 0x53, 0x84];
    	assert!(quick_get_difficulty(&hash, nonce, &mix_hash)[..] != boundary_bad[..]);
    }

    #[test]
    fn test_light_compute() {
    	let hash = [0xf5, 0x7e, 0x6f, 0x3a, 0xcf, 0xc0, 0xdd, 0x4b, 0x5b, 0xf2, 0xbe, 0xe4, 0x0a, 0xb3, 0x35, 0x8a, 0xa6, 0x87, 0x73, 0xa8, 0xd0, 0x9f, 0x5e, 0x59, 0x5e, 0xab, 0x55, 0x94, 0x05, 0x52, 0x7d, 0x72];
    	let mix_hash = [0x1f, 0xff, 0x04, 0xce, 0xc9, 0x41, 0x73, 0xfd, 0x59, 0x1e, 0x3d, 0x89, 0x60, 0xce, 0x6b, 0xdf, 0x8b, 0x19, 0x71, 0x04, 0x8c, 0x71, 0xff, 0x93, 0x7b, 0xb2, 0xd3, 0x2a, 0x64, 0x31, 0xab, 0x6d];
    	let boundary = [0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x3e, 0x9b, 0x6c, 0x69, 0xbc, 0x2c, 0xe2, 0xa2, 0x4a, 0x8e, 0x95, 0x69, 0xef, 0xc7, 0xd7, 0x1b, 0x33, 0x35, 0xdf, 0x36, 0x8c, 0x9a, 0xe9, 0x7e, 0x53, 0x84];
    	let nonce = 0xd7b3ac70a301a249;
    	// difficulty = 0x085657254bd9u64;
    	let light = Light::new(486382);
    	let result = light_compute(&light, &hash, nonce);
    	assert_eq!(result.mix_hash[..], mix_hash[..]);
    	assert_eq!(result.value[..], boundary[..]);
    }

    #[test]
    fn test_seed_compute_once() {
    	let seed_compute = SeedHashCompute::new();
    	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.get_seedhash(486382), hash);
    }

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

    #[test]
    fn test_seed_compute_after_older() {
    	let seed_compute = SeedHashCompute::new();
    	// calculating an older value first shouldn't affect the result
    	let _ = seed_compute.get_seedhash(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.get_seedhash(486382), hash);
    }

    #[test]
    fn test_seed_compute_after_newer() {
    	let seed_compute = SeedHashCompute::new();
    	// calculating an newer value first shouldn't affect the result
    	let _ = seed_compute.get_seedhash(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.get_seedhash(486382), hash);
    }

    #[test]
    fn test_drop_old_data() {
    	let first = Light::new(0).to_file().unwrap();

    	let second = Light::new(ETHASH_EPOCH_LENGTH).to_file().unwrap();
    	assert!(fs::metadata(&first).is_ok());

    	let _ = Light::new(ETHASH_EPOCH_LENGTH * 2).to_file();
    	assert!(fs::metadata(&first).is_err());
    	assert!(fs::metadata(&second).is_ok());

    	let _ = Light::new(ETHASH_EPOCH_LENGTH * 3).to_file();
    	assert!(fs::metadata(&second).is_err());
    }

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