content 0.1.0

Trait for content-adressable types
Documentation 
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// Copyright 2015, 2016 Ethcore (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 std::path::PathBuf;
use std::fs::{self, File};
use std::io::{Result, Read, Write};
use std::fmt::Write as FmtWrite;
use rand::{thread_rng, Rng};

use backend::Backend;
use hash::{Hash32, NewHash, WriteThroughHasher};

fn pathbuf_from_hash(root: &PathBuf, hash: &Hash32) -> Result<PathBuf> {
	let a = format!("{:02x}", hash.0[0]);
	let b = format!("{:02x}", hash.0[1]);
	let mut rest = String::new();
	for i in 2..32 {
		write!(&mut rest, "{:02x}", hash.0[i])
			.expect("in-memory write to succeed");
	};
	let mut pathbuf = root.clone();

	pathbuf.push(a);
	pathbuf.push(b);
	try!(fs::create_dir_all(pathbuf.to_str().unwrap()));
	pathbuf.push(rest);
	Ok(pathbuf)
}

fn temporary_path(root: &PathBuf) -> PathBuf {
	let mut rand = root.clone();
	let s: String = thread_rng().gen_ascii_chars().take(10).collect();
	rand.push(s);
	rand
}

impl Backend for PathBuf {
	fn store(
		&mut self,
		source: &Fn(&mut Write, &mut Backend) -> Result<()>,
		newhash: &NewHash,
	) -> Result<Hash32> {
		let tmp_path = temporary_path(self);
		let mut file = try!(File::create(&tmp_path));
		let hash;
		{
			let mut wth = WriteThroughHasher::new(&mut file, newhash);
			try!(source(&mut wth, self));
			hash = wth.finalize();
		}

		let pathbuf = try!(pathbuf_from_hash(self, &hash));
		try!(fs::rename(tmp_path, pathbuf));
		Ok(hash)
	}
	fn request(
		&self,
		hash: &Hash32,
		read: &Fn(&mut Read) -> Result<()>,
	) -> Result<()> {
		let pathbuf = try!(pathbuf_from_hash(self, &hash));
		let mut file = try!(File::open(pathbuf));
		read(&mut file)
	}
}

#[cfg(test)]
mod tests {
	use test_common;

	#[test]
	fn disk_write_get() {
		let mut store = test_common::diskstore();
		let thing = Some(Box::new(8u8));
		let hash = store.put(&thing).unwrap();
		let retrieved = store.get(&hash).unwrap();
		assert_eq!(thing, retrieved);
	}
}