Stream iteration pattern

The stream iteration pattern allows loops in data stream processing. An operation is applied to a data item until a predicate is satisfied. When the predicate is met, the result is sent to the output stream.

The interface to the stream iteration pattern is provided by function grppi::repeat_until(). As all functions in GrPPI, this function takes as its first argument an execution policy.

grppi::repeat_until(exec, other_arguments...);

Stream iteration variants

There is a single variant:

• Stand-alone stream iteration: Applies iteration to a stream of values produced by a generator and invokes a consumer for each value that satisfies the iteration predicate.

Note: Although there is a single variant of stream iteration, the transformation applied in the iteration loop may be some other composable pattern (farm or pipeline).

Key elements in stream iteration

The key elements in a stream iteration are a Transformer used to transform data items, and a Predicate that defines when the iteration should finish.

The Transformer may be any C++ callable entity that takes a data item and applies a transformation to it. Thus, a Transformer op is any operation that, given an input value x of type Tmakes valid the following:

U res{transformer(x)};

The Predicate may be any C++ callable entity that takes a data item and returns a value that is contextually convertible to bool. Thus, a predicate pred is any operation, that given a value x of type T, makes the following valid:

do { /*...*/ } while (!predicate(item));

A stand-alone stream iteration also has a Generator. The generator may be any C++ callable entity that produces values in a way that allows to signal an indication of end-of stream. For this purpose, the stream iteration requires that the generator produces values of any type that is compatible with a subset of an optional interface:

auto r = generator(); // r is a value generated by a generator.
if (r) {       // r is convertible to bool
  auto y = *r; // r can be dereferenced.
}

Additionally, a filter may have a Consumer. The consumer may be any C++ callable entity that takes values of the result type of the Transformer.

Details on stream iteration variants

Stand-alone stream iteration

A stand alone stream iteration has four elements:

• A Generator of values.
• A Transformer of values.
• A Predicate for terminating the iteration.
• A Consumer of generated values.

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  Example: For every natural number x, print the first value x*2^n that is greater than 1024.

  grppi::repeat_until(ex
    [i=0,max=100]() mutable -> optional<int> {
      if (i<max) return i++;
      else return {};
    },
    [](int x) { return 2*x; },
    [](int x) { return x>1024; },
    [](int x) { cout << x << endl; }
  );

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