/*
    Copyright (c) 2005-2020 Intel Corporation

    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.
*/

#ifndef _TBB_task_stream_H
#define _TBB_task_stream_H

#include "tbb/tbb_stddef.h"
#include <deque>
#include <climits>
#include "tbb/atomic.h" // for __TBB_Atomic*
#include "tbb/spin_mutex.h"
#include "tbb/tbb_allocator.h"
#include "scheduler_common.h"
#include "tbb_misc.h" // for FastRandom

namespace tbb {
namespace internal {

//! Essentially, this is just a pair of a queue and a mutex to protect the queue.
/** The reason std::pair is not used is that the code would look less clean
    if field names were replaced with 'first' and 'second'. **/
template< typename T, typename mutex_t >
struct queue_and_mutex {
    typedef std::deque< T, tbb_allocator<T> > queue_base_t;

    queue_base_t my_queue;
    mutex_t      my_mutex;

    queue_and_mutex () : my_queue(), my_mutex() {}
    ~queue_and_mutex () {}
};

typedef uintptr_t population_t;
const population_t one = 1;

inline void set_one_bit( population_t& dest, int pos ) {
    __TBB_ASSERT( pos>=0, NULL );
    __TBB_ASSERT( pos<int(sizeof(population_t)*CHAR_BIT), NULL );
    __TBB_AtomicOR( &dest, one<<pos );
}

inline void clear_one_bit( population_t& dest, int pos ) {
    __TBB_ASSERT( pos>=0, NULL );
    __TBB_ASSERT( pos<int(sizeof(population_t)*CHAR_BIT), NULL );
    __TBB_AtomicAND( &dest, ~(one<<pos) );
}

inline bool is_bit_set( population_t val, int pos ) {
    __TBB_ASSERT( pos>=0, NULL );
    __TBB_ASSERT( pos<int(sizeof(population_t)*CHAR_BIT), NULL );
    return (val & (one<<pos)) != 0;
}

//! The container for "fairness-oriented" aka "enqueued" tasks.
template<int Levels>
class task_stream : no_copy {
    typedef queue_and_mutex <task*, spin_mutex> lane_t;
    population_t population[Levels];
    padded<lane_t>* lanes[Levels];
    unsigned N;

public:
    task_stream() : N() {
        for(int level = 0; level < Levels; level++) {
            population[level] = 0;
            lanes[level] = NULL;
        }
    }

    void initialize( unsigned n_lanes ) {
        const unsigned max_lanes = sizeof(population_t) * CHAR_BIT;

        N = n_lanes>=max_lanes ? max_lanes : n_lanes>2 ? 1<<(__TBB_Log2(n_lanes-1)+1) : 2;
        __TBB_ASSERT( N==max_lanes || N>=n_lanes && ((N-1)&N)==0, "number of lanes miscalculated");
        __TBB_ASSERT( N <= sizeof(population_t) * CHAR_BIT, NULL );
        for(int level = 0; level < Levels; level++) {
            lanes[level] = new padded<lane_t>[N];
            __TBB_ASSERT( !population[level], NULL );
        }
    }

    ~task_stream() {
        for(int level = 0; level < Levels; level++)
            if (lanes[level]) delete[] lanes[level];
    }

    //! Push a task into a lane.
    void push( task* source, int level, FastRandom& random ) {
        // Lane selection is random. Each thread should keep a separate seed value.
        unsigned idx;
        for( ; ; ) {
            idx = random.get() & (N-1);
            spin_mutex::scoped_lock lock;
            if( lock.try_acquire(lanes[level][idx].my_mutex) ) {
                lanes[level][idx].my_queue.push_back(source);
                set_one_bit( population[level], idx ); //TODO: avoid atomic op if the bit is already set
                break;
            }
        }
    }

    //! Try finding and popping a task.
    task* pop( int level, unsigned& last_used_lane ) {
        task* result = NULL;
        // Lane selection is round-robin. Each thread should keep its last used lane.
        unsigned idx = (last_used_lane+1)&(N-1);
        for( ; population[level]; idx=(idx+1)&(N-1) ) {
            if( is_bit_set( population[level], idx ) ) {
                lane_t& lane = lanes[level][idx];
                spin_mutex::scoped_lock lock;
                if( lock.try_acquire(lane.my_mutex) && !lane.my_queue.empty() ) {
                    result = lane.my_queue.front();
                    lane.my_queue.pop_front();
                    if( lane.my_queue.empty() )
                        clear_one_bit( population[level], idx );
                    break;
                }
            }
        }
        last_used_lane = idx;
        return result;
    }

    //! Checks existence of a task.
    bool empty(int level) {
        return !population[level];
    }

    //! Destroys all remaining tasks in every lane. Returns the number of destroyed tasks.
    /** Tasks are not executed, because it would potentially create more tasks at a late stage.
        The scheduler is really expected to execute all tasks before task_stream destruction. */
    intptr_t drain() {
        intptr_t result = 0;
        for(int level = 0; level < Levels; level++)
            for(unsigned i=0; i<N; ++i) {
                lane_t& lane = lanes[level][i];
                spin_mutex::scoped_lock lock(lane.my_mutex);
                for(lane_t::queue_base_t::iterator it=lane.my_queue.begin();
                    it!=lane.my_queue.end(); ++it, ++result)
                {
                    __TBB_ASSERT( is_bit_set( population[level], i ), NULL );
                    task* t = *it;
                    tbb::task::destroy(*t);
                }
                lane.my_queue.clear();
                clear_one_bit( population[level], i );
            }
        return result;
    }
}; // task_stream

} // namespace internal
} // namespace tbb

#endif /* _TBB_task_stream_H */
