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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/>.

    use util::migration::SimpleMigration;

    /// This migration reduces the sizes of keys and moves `ExtrasIndex` byte from back to the front.
    pub struct ToV6;

    impl ToV6 {
    	fn migrate_old_key(&self, old_key: Vec<u8>, index: u8, len: usize) -> Vec<u8> {
    		let mut result = vec![];
    		result.reserve(len);
    		unsafe {
    			result.set_len(len);
    		}
    		result[0] = index;
    		let old_key_start = 33 - len;
    		result[1..].clone_from_slice(&old_key[old_key_start..32]);
    		result
    	}
    }

    impl SimpleMigration for ToV6 {

    	fn columns(&self) -> Option<u32> { None }

    	fn version(&self) -> u32 { 6 }

    	fn simple_migrate(&mut self, key: Vec<u8>, value: Vec<u8>) -> Option<(Vec<u8>, Vec<u8>)> {

    		//// at this version all extras keys are 33 bytes long.
    		if key.len() == 33 {
    			// block details key changes:
    			// - index is moved to the front
    			if key[32] == 0 {
    				return Some((self.migrate_old_key(key, 0, 33), value));
    			}

    			// block hash key changes:
    			// - key is shorter 33 -> 5 bytes
    			// - index is moved to the front
    			if key[32] == 1 {
    				return Some((self.migrate_old_key(key, 1, 5), value));
    			}

    			// transaction addresses changes:
    			// - index is moved to the front
    			if key[32] == 2 {
    				return Some((self.migrate_old_key(key, 2, 33), value));
    			}

    			// block log blooms are removed
    			if key[32] == 3 {
    				return None;
    			}

    			// blocks blooms key changes:
    			// - key is shorter 33 -> 6 bytes
    			// - index is moved to the front
    			// - index is changed 4 -> 3
    			if key[32] == 4 {
    				// i have no idea why it was reversed
    				let reverse = key.into_iter().rev().collect::<Vec<_>>();
    				let mut result = [0u8; 6];
    				// new extras index is 3
    				result[0] = 3;
    				// 9th (+ prefix) byte was the level. Now it's second.
    				result[1] = reverse[9];
    				result[2] = reverse[4];
    				result[3] = reverse[3];
    				result[4] = reverse[2];
    				result[5] = reverse[1];

    				return Some((result.to_vec(), value));
    			}

    			// blocks receipts key changes:
    			// - index is moved to the front
    			// - index is changed 5 -> 4
    			if key[32] == 5 {
    				return Some((self.migrate_old_key(key, 4, 33), value));
    			}
    		}

    		Some((key, value))
    	}
    }

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