// Copyright 2019-2020 Parity Technologies (UK) Ltd.
// This file is part of Substrate.

// Substrate 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.

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

//! Functionality for reading and storing children hashes from db.

use codec::{Encode, Decode};
use sp_blockchain;
use std::hash::Hash;
use sp_database::{Database, Transaction};
use crate::DbHash;

/// Returns the hashes of the children blocks of the block with `parent_hash`.
pub fn read_children<
	K: Eq + Hash + Clone + Encode + Decode,
	V: Eq + Hash + Clone + Encode + Decode,
>(db: &dyn Database<DbHash>, column: u32, prefix: &[u8], parent_hash: K) -> sp_blockchain::Result<Vec<V>> {
	let mut buf = prefix.to_vec();
	parent_hash.using_encoded(|s| buf.extend(s));

	let raw_val_opt = db.get(column, &buf[..]);

	let raw_val = match raw_val_opt {
		Some(val) => val,
		None => return Ok(Vec::new()),
	};

	let children: Vec<V> = match Decode::decode(&mut &raw_val[..]) {
		Ok(children) => children,
		Err(_) => return Err(sp_blockchain::Error::Backend("Error decoding children".into())),
	};

	Ok(children)
}

/// Insert the key-value pair (`parent_hash`, `children_hashes`) in the transaction.
/// Any existing value is overwritten upon write.
pub fn write_children<
	K: Eq + Hash + Clone + Encode + Decode,
	V: Eq + Hash + Clone + Encode + Decode,
>(
	tx: &mut Transaction<DbHash>,
	column: u32,
	prefix: &[u8],
	parent_hash: K,
	children_hashes: V,
) {
	let mut key = prefix.to_vec();
	parent_hash.using_encoded(|s| key.extend(s));
	tx.set_from_vec(column, &key[..], children_hashes.encode());
}

/// Prepare transaction to remove the children of `parent_hash`.
pub fn remove_children<
	K: Eq + Hash + Clone + Encode + Decode,
>(
	tx: &mut Transaction<DbHash>,
	column: u32,
	prefix: &[u8],
	parent_hash: K,
) {
	let mut key = prefix.to_vec();
	parent_hash.using_encoded(|s| key.extend(s));
	tx.remove(column, &key);
}


#[cfg(test)]
mod tests {
	use super::*;
	use std::sync::Arc;

	#[test]
	fn children_write_read_remove() {
		const PREFIX: &[u8] = b"children";
		let db = Arc::new(sp_database::MemDb::default());

		let mut tx = Transaction::new();

		let mut children1 = Vec::new();
		children1.push(1_3);
		children1.push(1_5);
		write_children(&mut tx, 0, PREFIX, 1_1, children1);

		let mut children2 = Vec::new();
		children2.push(1_4);
		children2.push(1_6);
		write_children(&mut tx, 0, PREFIX, 1_2, children2);

		db.commit(tx.clone());

		let r1: Vec<u32> = read_children(&*db, 0, PREFIX, 1_1).expect("(1) Getting r1 failed");
		let r2: Vec<u32> = read_children(&*db, 0, PREFIX, 1_2).expect("(1) Getting r2 failed");

		assert_eq!(r1, vec![1_3, 1_5]);
		assert_eq!(r2, vec![1_4, 1_6]);

		remove_children(&mut tx, 0, PREFIX, 1_2);
		db.commit(tx);

		let r1: Vec<u32> = read_children(&*db, 0, PREFIX, 1_1).expect("(2) Getting r1 failed");
		let r2: Vec<u32> = read_children(&*db, 0, PREFIX, 1_2).expect("(2) Getting r2 failed");

		assert_eq!(r1, vec![1_3, 1_5]);
		assert_eq!(r2.len(), 0);
	}
}
