functional_generators.php

<?php

function all_gens_valid($gens) {
  foreach ($gens as $gen) {
    if (!$gen->valid()) {
      return false;
    }
  }
  return true;
}

function gen_map($f /*, $gens... */) {
  $gens = func_get_args();
  array_shift($gens);

  while (all_gens_valid($gens)) {
    $vals = [];
    foreach ($gens as $gen) {
      $vals[] = $gen->current();
      $gen->next();
    }
    yield call_user_func_array($f, $vals);
  }
}

function gen_reduce($gen, $f, $initial=null) {
  $memo = $initial;
  foreach($gen as $item) {
    $memo = $f($memo, $item);
  }
  return $memo;
}

function gen_filter($gen, $f=null) {
  if (!$f) {
    $f = function($item) { return $item !== null; };
  }
  foreach ($gen as $k => $item) {
    if ($f($item)) {
      yield $k => $item;
    }
  }
}

function gen_combine($keyGen, $valGen) {
  while ($keyGen->valid() && $valGen->valid()) {
    yield $keyGen->current() => $valGen->current();
    $keyGen->next();
    $valGen->next();
  }
}

////////////////////////////////////////////////////////////////////////////////

function xrange($start, $limit, $step = 1) {
  if ($start < $limit) {
    if ($step <= 0) {
      throw new LogicException('Step must be +ve');
    }

    for ($i = $start; $i <= $limit; $i += $step) {
      yield $i;
    }
  } else {
    if ($step >= 0) {
      throw new LogicException('Step must be -ve');
    }

    for ($i = $start; $i >= $limit; $i += $step) {
      yield $i;
    }
  }
}

function gen_to_array($gen) {
  $a = [];
  while($gen->valid()) {
    $a[$gen->key()] = $gen->current();
    $gen->next();
  }
  return $a;
}

function array_to_gen($array) {
  foreach ($array as $k => $v) {
    yield $k => $v;
  }
}

////////////////////////////////////////////////////////////////////////////////

assert(
  gen_to_array(gen_map(function($x) { return $x + 1; }, xrange(0, 1)))
  ==
  array_map(function($x) { return $x + 1; }, range(0, 1))
);

assert(
  gen_to_array(gen_map(
    function($x, $y) { return $x + $y; },
    xrange(0, 2),
    xrange(0, 2)
  ))
  ==
  array_map(
    function($x, $y) { return $x + $y; },
    range(0, 2),
    range(0, 2)
  )
);

assert(
  gen_reduce(
    xrange(0, 10),
    function($sum, $i) { return $sum + $i; },
    0
  )
  ==
  array_reduce(
    range(0, 10),
    function($sum, $i) { return $sum + $i; },
    0
  )
);

assert(
  gen_to_array(gen_filter(
    xrange(0, 10),
    function($x) { return ($x % 2 == 0); }
  ))
  ==
  array_filter(
    range(0, 10),
    function($x) { return ($x % 2 == 0); }
  )
);

assert(
  gen_to_array(gen_filter(
    [1, null, 2, null, 3, null]
  ))
  ==
  array_filter(
    [1, null, 2, null, 3, null]
  )
);

assert(
  gen_to_array(gen_combine(
    array_to_gen([2, 4, 6]),
    array_to_gen([3, 5, 7])
  ))
  ==
  array_combine(
    [2, 4, 6],
    [3, 5, 7]
  )
);