tp-trie 2.1.2

Patricia trie stuff using a tetsy-scale-codec node format
Documentation 
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// This file is part of Tetcore.

// Copyright (C) 2015-2021 Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: Apache-2.0

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// 	http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! `TrieStream` implementation for Tetcore's trie format.

use tetsy_hash_db::Hasher;
use tetsy_trie_root;
use codec::Encode;
use tetcore_std::vec::Vec;
use crate::trie_constants;
use crate::node_header::{NodeKind, size_and_prefix_iterator};
use crate::node_codec::Bitmap;

const BRANCH_NODE_NO_VALUE: u8 = 254;
const BRANCH_NODE_WITH_VALUE: u8 = 255;

#[derive(Default, Clone)]
/// Codec-flavored TrieStream.
pub struct TrieStream {
	buffer: Vec<u8>,
}

impl TrieStream {
	// useful for debugging but not used otherwise
	pub fn as_raw(&self) -> &[u8] { &self.buffer }
}

fn branch_node_bit_mask(has_children: impl Iterator<Item = bool>) -> (u8, u8) {
	let mut bitmap: u16 = 0;
	let mut cursor: u16 = 1;
	for v in has_children {
		if v { bitmap |= cursor }
		cursor <<= 1;
	}
	((bitmap % 256 ) as u8, (bitmap / 256 ) as u8)
}


/// Create a leaf/branch node, encoding a number of nibbles.
fn fuse_nibbles_node<'a>(nibbles: &'a [u8], kind: NodeKind) -> impl Iterator<Item = u8> + 'a {
	let size = tetcore_std::cmp::min(trie_constants::NIBBLE_SIZE_BOUND, nibbles.len());

	let iter_start = match kind {
		NodeKind::Leaf => size_and_prefix_iterator(size, trie_constants::LEAF_PREFIX_MASK),
		NodeKind::BranchNoValue => size_and_prefix_iterator(size, trie_constants::BRANCH_WITHOUT_MASK),
		NodeKind::BranchWithValue => size_and_prefix_iterator(size, trie_constants::BRANCH_WITH_MASK),
	};
	iter_start
		.chain(if nibbles.len() % 2 == 1 { Some(nibbles[0]) } else { None })
		.chain(nibbles[nibbles.len() % 2..].chunks(2).map(|ch| ch[0] << 4 | ch[1]))
}


impl tetsy_trie_root::TrieStream for TrieStream {

	fn new() -> Self {
		TrieStream {
			buffer: Vec::new()
		}
	}

	fn append_empty_data(&mut self) {
		self.buffer.push(trie_constants::EMPTY_TRIE);
	}

	fn append_leaf(&mut self, key: &[u8], value: &[u8]) {
		self.buffer.extend(fuse_nibbles_node(key, NodeKind::Leaf));
		value.encode_to(&mut self.buffer);
	}

	fn begin_branch(
		&mut self,
		maybe_partial: Option<&[u8]>,
		maybe_value: Option<&[u8]>,
		has_children: impl Iterator<Item = bool>,
	) {
		if let Some(partial) = maybe_partial {
			if maybe_value.is_some() {
				self.buffer.extend(fuse_nibbles_node(partial, NodeKind::BranchWithValue));
			} else {
				self.buffer.extend(fuse_nibbles_node(partial, NodeKind::BranchNoValue));
			}
			let bm = branch_node_bit_mask(has_children);
			self.buffer.extend([bm.0,bm.1].iter());
		} else {
			debug_assert!(false, "trie stream codec only for no extension trie");
			self.buffer.extend(&branch_node(maybe_value.is_some(), has_children));
		}
		if let Some(value) = maybe_value {
			value.encode_to(&mut self.buffer);
		}
	}

	fn append_extension(&mut self, _key: &[u8]) {
		debug_assert!(false, "trie stream codec only for no extension trie");
	}

	fn append_substream<H: Hasher>(&mut self, other: Self) {
		let data = other.out();
		match data.len() {
			0..=31 => data.encode_to(&mut self.buffer),
			_ => H::hash(&data).as_ref().encode_to(&mut self.buffer),
		}
	}

	fn out(self) -> Vec<u8> { self.buffer }
}

fn branch_node(has_value: bool, has_children: impl Iterator<Item = bool>) -> [u8; 3] {
	let mut result = [0, 0, 0];
	branch_node_buffered(has_value, has_children, &mut result[..]);
	result
}

fn branch_node_buffered<I>(has_value: bool, has_children: I, output: &mut[u8])
	where
		I: Iterator<Item = bool>,
{
	let first = if has_value {
		BRANCH_NODE_WITH_VALUE
	} else {
		BRANCH_NODE_NO_VALUE
	};
	output[0] = first;
	Bitmap::encode(has_children, &mut output[1..]);
}