tfs 0.1.3

// Copyright (C) 2016 Cloudlabs, Inc
//
// This program 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.
//
// This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.

use std::sync::RwLock;

pub trait RwLockExt<T> {
    /// An efficient combinator for conditonally applying a write action.
    ///
    /// Acquires a read lock and applies the predicate, if it succeeds
    /// acquire a write lock and run the predicate again (since the data
    /// may have changed since releasing the read lock) and if it still
    /// succeeds run the provided action.
    ///
    /// Returns Some(the value returned by action) if the action is run,
    /// None otherwise. If the action is not run, then the predicate must
    /// have been false at least once.
    fn if_then<P, A, R>(&self, predicate: P, action: A) -> Option<R>
    where P: Fn(&T) -> bool, A: FnOnce(&mut T) -> R;
}

impl<T> RwLockExt<T> for RwLock<T> {
    fn if_then<P, A, R>(&self, predicate: P, action: A) -> Option<R>
    where P: Fn(&T) -> bool, A: FnOnce(&mut T) -> R {
        if predicate(&self.read().unwrap()) {
            let mut writer = self.write().unwrap();
            if predicate(&writer) {
                return Some(action(&mut writer))
            }
        };

        None
    }
}

pub mod test {
    use {ContentId};

    /// Generate a random block of data of the given size.
    pub fn gen_random_block(block_size: usize) -> (ContentId, Vec<u8>) {
        let data = (0..block_size).map(|_| ::rand::random::<u8>()).collect::<Vec<_>>();
        let id = ContentId::hash(&data);

        (id, data)
    }
}