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. This streaming pattern can only be used inside another pattern and consequently does not take an execution policy itself, but uses the execution policy of its enclosing pattern.

The interface to the stream iteration pattern is provided by function grppi::repeat_until().

grppi::pipeline(ex,
  stage1,
  grppi::repeat_until(arguments...),
  stage2,
  stage3,
  ...);

Stream iteration variants

There is a single variant:

Composable stream iteration: Defines a stream iteration that can be used as a building block by another pattern (e.g. 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));

Details on stream iteration variants

Stand-alone stream iteration

A composable stream iteration has two elements:

A Transformer of values.
A Predicate for terminating the iteration.

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

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

For composing complex patterns, the repeat_until() function may be used to create an object that may be used later in the composition.

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

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