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    rb_define_method(rb_cLazy, "slice_when", lazy_super, -1);

struct slicewhen_arg {

    VALUE pred;

    VALUE prev_elt;

    VALUE prev_elts;

    VALUE yielder;

};

static VALUE

slicewhen_ii(RB_BLOCK_CALL_FUNC_ARGLIST(i, _memo))

{

#define UPDATE_MEMO ((void)(memo = MEMO_FOR(struct slicewhen_arg, _memo)))

    struct slicewhen_arg *memo;

    int split_p;

    UPDATE_MEMO;

    ENUM_WANT_SVALUE();

    if (memo->prev_elt == Qundef) {

        /* The first element */

        memo->prev_elt = i;

        memo->prev_elts = rb_ary_new3(1, i);

    }

    else {

        split_p = RTEST(rb_funcall(memo->pred, id_call, 2, memo->prev_elt, i));

        UPDATE_MEMO;

        if (split_p) {

            rb_funcall(memo->yielder, id_lshift, 1, memo->prev_elts);

            UPDATE_MEMO;

            memo->prev_elts = rb_ary_new3(1, i);

        }

        else {

            rb_ary_push(memo->prev_elts, i);

        }

        memo->prev_elt = i;

    }

    return Qnil;

#undef UPDATE_MEMO

}

static VALUE

slicewhen_i(RB_BLOCK_CALL_FUNC_ARGLIST(yielder, enumerator))

{

    VALUE enumerable;

    VALUE arg;

    struct slicewhen_arg *memo = NEW_MEMO_FOR(struct slicewhen_arg, arg);

    enumerable = rb_ivar_get(enumerator, rb_intern("slicewhen_enum"));

    memo->pred = rb_attr_get(enumerator, rb_intern("slicewhen_pred"));

    memo->prev_elt = Qundef;

    memo->prev_elts = Qnil;

    memo->yielder = yielder;

    rb_block_call(enumerable, id_each, 0, 0, slicewhen_ii, arg);

    memo = MEMO_FOR(struct slicewhen_arg, arg);

    if (!NIL_P(memo->prev_elts))

        rb_funcall(memo->yielder, id_lshift, 1, memo->prev_elts);

    return Qnil;

}

/*

 *  call-seq:

 *     enum.slice_when {|elt_before, elt_after| bool } -> an_enumerator

 *

 *  Creates an enumerator for each chunked elements.

 *  The beginnings of chunks are defined by the block.

 *

 *  This method split each chunk using adjacent elements,

 *  _elt_before_ and _elt_after_,

 *  in the receiver enumerator.

 *  This method split chunks between _elt_before_ and _elt_after_ where

 *  the block returns true.

 *

 *  The block is called the length of the receiver enumerator minus one.

 *

 *  The result enumerator yields the chunked elements as an array.

 *  So +each+ method can be called as follows:

 *

 *    enum.slice_when { |elt_before, elt_after| bool }.each { |ary| ... }

 *

 *  Other methods of the Enumerator class and Enumerable module,

 *  such as +map+, etc., are also usable.

 *

 *  For example, one-by-one increasing subsequence can be chunked as follows:

 *

 *    a = [1,2,4,9,10,11,12,15,16,19,20,21]

 *    b = a.slice_when {|i, j| i+1 != j }

 *    p b.to_a #=> [[1, 2], [4], [9, 10, 11, 12], [15, 16], [19, 20, 21]]

 *    c = b.map {|a| a.length < 3 ? a : "#{a.first}-#{a.last}" }

 *    p c #=> [[1, 2], [4], "9-12", [15, 16], "19-21"]

 *    d = c.join(",")

 *    p d #=> "1,2,4,9-12,15,16,19-21"

 *

 *  Increasing (non-decreasing) subsequence can be chunked as follows:

 *

 *    a = [0, 9, 2, 2, 3, 2, 7, 5, 9, 5]

 *    p a.slice_when {|i, j| i > j }.to_a

 *    #=> [[0, 9], [2, 2, 3], [2, 7], [5, 9], [5]]

 *

 *  Adjacent evens and odds can be chunked as follows:

 *  (Enumerable#chunk is another way to do it.)

 *

 *    a = [7, 5, 9, 2, 0, 7, 9, 4, 2, 0]

 *    p a.slice_when {|i, j| i.even? != j.even? }.to_a

 *    #=> [[7, 5, 9], [2, 0], [7, 9], [4, 2, 0]]

 *

 *  Paragraphs (non-empty lines with trailing empty lines) can be chunked as follows:

 *  (See Enumerable#chunk to ignore empty lines.)

 *

 *    lines = ["foo\n", "bar\n", "\n", "baz\n", "qux\n"]

 *    p lines.slice_when {|l1, l2| /\A\s*\z/ =~ l1 && /\S/ =~ l2 }.to_a

 *    #=> [["foo\n", "bar\n", "\n"], ["baz\n", "qux\n"]]

 *

 */

static VALUE

enum_slice_when(VALUE enumerable)

{

    VALUE enumerator;

    VALUE pred;

    pred = rb_block_proc();

    enumerator = rb_obj_alloc(rb_cEnumerator);

    rb_ivar_set(enumerator, rb_intern("slicewhen_enum"), enumerable);

    rb_ivar_set(enumerator, rb_intern("slicewhen_pred"), pred);

    rb_block_call(enumerator, idInitialize, 0, 0, slicewhen_i, enumerator);

    return enumerator;

}

    rb_define_method(rb_mEnumerable, "slice_when", enum_slice_when, 0);

  def test_slice_when_0

    e = [].slice_when {|a, b| flunk "should not be called" }

    assert_equal([], e.to_a)

  end

  def test_slice_when_1

    e = [1].slice_when {|a, b| flunk "should not be called" }

    assert_equal([[1]], e.to_a)

  end

  def test_slice_when_2

    e = [1,2].slice_when {|a,b|

      assert_equal(1, a)

      assert_equal(2, b)

      true

    }

    assert_equal([[1], [2]], e.to_a)

    e = [1,2].slice_when {|a,b|

      assert_equal(1, a)

      assert_equal(2, b)

      false

    }

    assert_equal([[1, 2]], e.to_a)

  end

  def test_slice_when_3

    block_invocations = [

      lambda {|a, b|

        assert_equal(1, a)

        assert_equal(2, b)

        true

      },

      lambda {|a, b|

        assert_equal(2, a)

        assert_equal(3, b)

        false

      }

    ]

    e = [1,2,3].slice_when {|a,b|

      block_invocations.shift.call(a, b)

    }

    assert_equal([[1], [2, 3]], e.to_a)

    assert_equal([], block_invocations)

  end

  def test_slice_when_noblock

    assert_raise(ArgumentError) { [].slice_when }

  end

  def test_slice_when_contiguously_increasing_integers

    e = [1,4,9,10,11,12,15,16,19,20,21].slice_when {|i, j| i+1 != j }

    assert_equal([[1], [4], [9,10,11,12], [15,16], [19,20,21]], e.to_a)

  end

    assert_equal Enumerator::Lazy, [].lazy.slice_when{}.class, bug7507