#
template<typename P>
tf::ScalablePipeline class

class to create a scalable pipeline object

Template parameters
P | type of the iterator to a range of pipes

A scalable pipeline is a composable graph object for users to create a pipeline scheduling framework using a module task in a taskflow. Unlike tf::Pipeline that instantiates all pipes upon the construction time, tf::ScalablePipeline allows variable assignments of pipes using range iterators. Users can also reset a scalable pipeline to a different range of pipes between runs. The following code creates a scalable pipeline of four parallel lines to schedule tokens through three serial pipes in a custom storage, then resetting the pipeline to a new range of five serial pipes:

    tf::Taskflow taskflow("pipeline");
    tf::Executor executor;

    const size_t num_lines = 4;

    // create data storage
    std::array<int, num_lines> buffer;

    // define the pipe callable
    auto pipe_callable = [&buffer] (tf::Pipeflow& pf) mutable {
      switch(pf.pipe()) {
        // first stage generates only 5 scheduling tokens and saves the
        // token number into the buffer.
        case 0: {
          if(pf.token() == 5) {
            pf.stop();
          }
          else {
            printf("stage 1: input token = %zu\n", pf.token());
            buffer[pf.line()] = pf.token();
          }
          return;
        }
        break;

        // other stages propagate the previous result to this pipe and
        // increment it by one
        default: {
          printf(
            "stage %zu: input buffer[%zu] = %d\n", pf.pipe(), pf.line(), buffer[pf.line()]
          );
          buffer[pf.line()] = buffer[pf.line()] + 1;
        }
        break;
      }
    };

    // create a vector of three pipes
    std::vector< tf::Pipe<std::function<void(tf::Pipeflow&)>> > pipes;

    for(size_t i=0; i<3; i++) {
      pipes.emplace_back(tf::PipeType::SERIAL, pipe_callable);
    }

    // create a pipeline of four parallel lines based on the given vector of pipes
    tf::ScalablePipeline pl(num_lines, pipes.begin(), pipes.end());

    // build the pipeline graph using composition
    tf::Task init = taskflow.emplace([](){ std::cout << "ready\n"; })
                            .name("starting pipeline");
    tf::Task task = taskflow.composed_of(pl)
                            .name("pipeline");
    tf::Task stop = taskflow.emplace([](){ std::cout << "stopped\n"; })
                            .name("pipeline stopped");

    // create task dependency
    init.precede(task);
    task.precede(stop);

    // dump the pipeline graph structure (with composition)
    taskflow.dump(std::cout);

    // run the pipeline
    executor.run(taskflow).wait();

    // reset the pipeline to a new range of five pipes and starts from
    // the initial state (i.e., token counts from zero)
    for(size_t i=0; i<2; i++) {
      pipes.emplace_back(tf::PipeType::SERIAL, pipe_callable);
    }
    pl.reset(pipes.begin(), pipes.end());

    executor.run(taskflow).wait();

The above example creates a pipeline graph that schedules five tokens over four parallel lines in a circular fashion, first going through three serial pipes and then five serial pipes:

    # initial construction of three serial pipes
    o -> o -> o
    |    |    |
    v    v    v
    o -> o -> o
    |    |    |
    v    v    v
    o -> o -> o
    |    |    |
    v    v    v
    o -> o -> o

    # resetting to a new range of five serial pipes
    o -> o -> o -> o -> o
    |    |    |    |    |
    v    v    v    v    v
    o -> o -> o -> o -> o
    |    |    |    |    |
    v    v    v    v    v
    o -> o -> o -> o -> o
    |    |    |    |    |
    v    v    v    v    v
    o -> o -> o -> o -> o

Each pipe has the same type of `tf::Pipe<std::function<void(tf::Pipeflow&)>>` and is kept in a vector that is amenable to change. We construct the scalable pipeline using two range iterators pointing to the beginning and the end of the vector. At each pipe stage, the program propagates the result to the next pipe by adding one to the result stored in a custom data storage, `buffer`. The pipeline scheduler will generate five scheduling tokens and then stop.

A scalable pipeline is move-only.

## Public types

using pipe_t = typename std::iterator_traits<P>::value_type
  pipe type

## Constructors, destructors, conversion operators

ScalablePipeline() defaulted
  default constructor
ScalablePipeline(size_t num_lines)
  constructs an empty scalable pipeline object
ScalablePipeline(size_t num_lines, P first, P last)
  constructs a scalable pipeline object
ScalablePipeline(const ScalablePipeline&) deleted
  disabled copy constructor
ScalablePipeline(ScalablePipeline&& rhs)
  move constructor

## Public functions

auto operator=(const ScalablePipeline&) -> ScalablePipeline& deleted
  disabled copy assignment operator
auto operator=(ScalablePipeline&& rhs) -> ScalablePipeline&
  move constructor
auto num_lines() const -> size_t noexcept
  queries the number of parallel lines
auto num_pipes() const -> size_t noexcept
  queries the number of pipes
void reset()
  resets the pipeline
void reset(P first, P last)
  resets the pipeline with a new range of pipes
void reset(size_t num_lines, P first, P last)
  resets the pipeline to a new line number and a new range of pipes
auto num_tokens() const -> size_t noexcept
  queries the number of generated tokens in the pipeline
auto graph() -> Graph&
  obtains the graph object associated with the pipeline construct

## Function documentation

###
template<typename P>
tf::ScalablePipeline<P>::ScalablePipeline(size_t num_lines)

constructs an empty scalable pipeline object

Parameters
num_lines | the number of parallel lines

An empty scalable pipeline does not have any pipes. The pipeline needs to be reset to a valid range of pipes before running.

###
template<typename P>
tf::ScalablePipeline<P>::ScalablePipeline(size_t num_lines, P first, P last)

constructs a scalable pipeline object

Parameters
num_lines | the number of parallel lines
first | iterator to the beginning of the range
last | iterator to the end of the range

Constructs a pipeline from the given range of pipes specified in `[first, last)` using `num_lines` parallel lines. The first pipe must define a serial direction (tf::PipeType::SERIAL) or an exception will be thrown.

Internally, the scalable pipeline copies the iterators from the specified range. Those pipe callables pointed to by these iterators must remain valid during the execution of the pipeline.

###
template<typename P>
tf::ScalablePipeline<P>::ScalablePipeline(ScalablePipeline&& rhs)

move constructor

Constructs a pipeline from the given `rhs` using move semantics (i.e. the data in `rhs` is moved into this pipeline). After the move, `rhs` is in a state as if it is just constructed. The behavior is undefined if `rhs` is running during the move.

###
template<typename P>
ScalablePipeline& tf::ScalablePipeline<P>::operator=(ScalablePipeline&& rhs)

move constructor

Replaces the contents with those of `rhs` using move semantics (i.e. the data in `rhs` is moved into this pipeline). After the move, `rhs` is in a state as if it is just constructed. The behavior is undefined if `rhs` is running during the move.

###
template<typename P>
size_t tf::ScalablePipeline<P>::num_lines() const noexcept

queries the number of parallel lines

The function returns the number of parallel lines given by the user upon the construction of the pipeline. The number of lines represents the maximum parallelism this pipeline can achieve.

###
template<typename P>
size_t tf::ScalablePipeline<P>::num_pipes() const noexcept

queries the number of pipes

The Function returns the number of pipes given by the user upon the construction of the pipeline.

###
template<typename P>
void tf::ScalablePipeline<P>::reset()

resets the pipeline

Resets the pipeline to the initial state. After resetting a pipeline, its token identifier will start from zero.

###
template<typename P>
void tf::ScalablePipeline<P>::reset(P first, P last)

resets the pipeline with a new range of pipes

Parameters
first | iterator to the beginning of the range
last | iterator to the end of the range

The member function assigns the pipeline to a new range of pipes specified in `[first, last)` and resets the pipeline to the initial state. After resetting a pipeline, its token identifier will start from zero.

Internally, the scalable pipeline copies the iterators from the specified range. Those pipe callables pointed to by these iterators must remain valid during the execution of the pipeline.

###
template<typename P>
void tf::ScalablePipeline<P>::reset(size_t num_lines, P first, P last)

resets the pipeline to a new line number and a new range of pipes

Parameters
num_lines | number of parallel lines
first | iterator to the beginning of the range
last | iterator to the end of the range

The member function resets the pipeline to a new number of parallel lines and a new range of pipes specified in `[first, last)`, as if the pipeline is just constructed. After resetting a pipeline, its token identifier will start from zero.

Internally, the scalable pipeline copies the iterators from the specified range. Those pipe callables pointed to by these iterators must remain valid during the execution of the pipeline.

###
template<typename P>
size_t tf::ScalablePipeline<P>::num_tokens() const noexcept

queries the number of generated tokens in the pipeline

The number represents the total scheduling tokens that has been generated by the pipeline so far.

###
template<typename P>
Graph& tf::ScalablePipeline<P>::graph()

obtains the graph object associated with the pipeline construct

This method is primarily used as an opaque data structure for creating a module task of the this pipeline.

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