https://git.ruby-lang.org/ruby.git/commit/?id=1d4bd229b8

From 1d4bd229b898671328c2a942b04f08065c640c28 Mon Sep 17 00:00:00 2001
From: Akinori MUSHA <knu@i...>
Date: Wed, 5 Jun 2019 20:39:21 +0900
Subject: Implement Enumerator::Lazy#eager [Feature #15901]


diff --git a/NEWS b/NEWS
index 7100394..1350943 100644
--- a/NEWS
+++ b/NEWS
@@ -102,6 +102,9 @@ Enumerator:: https://github.com/ruby/ruby/blob/trunk/NEWS#L102
 
   New method::
 
+    * Added Enumerator::Lazy#eager that generates a non-lazy enumerator
+      from a lazy enumerator.  [Feature #15901]
+
     * Added Enumerator::Yielder#to_proc so that a Yielder object
       can be directly passed to another method as a block
       argument.  [Feature #15618]
diff --git a/enumerator.c b/enumerator.c
index 62534ee..7a0a17b 100644
--- a/enumerator.c
+++ b/enumerator.c
@@ -1894,6 +1894,26 @@ lazy_to_enum(int argc, VALUE *argv, VALUE self) https://github.com/ruby/ruby/blob/trunk/enumerator.c#L1894
 }
 
 static VALUE
+lazy_eager_size(VALUE self, VALUE args, VALUE eobj)
+{
+    return enum_size(self);
+}
+
+/*
+ * call-seq:
+ *   lzy.eager -> enum
+ *
+ * Returns a non-lazy Enumerator converted from the lazy enumerator.
+ */
+
+static VALUE
+lazy_eager(VALUE self)
+{
+    return enumerator_init(enumerator_allocate(rb_cEnumerator),
+                           self, sym_each, 0, 0, lazy_eager_size, Qnil);
+}
+
+static VALUE
 lazyenum_yield(VALUE proc_entry, struct MEMO *result)
 {
     struct proc_entry *entry = proc_entry_ptr(proc_entry);
@@ -3741,6 +3761,7 @@ InitVM_Enumerator(void) https://github.com/ruby/ruby/blob/trunk/enumerator.c#L3761
     rb_define_method(rb_cLazy, "initialize", lazy_initialize, -1);
     rb_define_method(rb_cLazy, "to_enum", lazy_to_enum, -1);
     rb_define_method(rb_cLazy, "enum_for", lazy_to_enum, -1);
+    rb_define_method(rb_cLazy, "eager", lazy_eager, 0);
     rb_define_method(rb_cLazy, "map", lazy_map, 0);
     rb_define_method(rb_cLazy, "collect", lazy_map, 0);
     rb_define_method(rb_cLazy, "flat_map", lazy_flat_map, 0);
diff --git a/test/ruby/test_lazy_enumerator.rb b/test/ruby/test_lazy_enumerator.rb
index 72fe1a4..caa9a9c 100644
--- a/test/ruby/test_lazy_enumerator.rb
+++ b/test/ruby/test_lazy_enumerator.rb
@@ -452,6 +452,14 @@ class TestLazyEnumerator < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/ruby/test_lazy_enumerator.rb#L452
 EOS
   end
 
+  def test_lazy_eager
+    lazy = [1, 2, 3].lazy.map { |x| x * 2 }
+    enum = lazy.eager
+    assert_equal Enumerator, enum.class
+    assert_equal 3, enum.size
+    assert_equal [1, 2, 3], enum.map { |x| x / 2 }
+  end
+
   def test_lazy_to_enum
     lazy = [1, 2, 3].lazy
     def lazy.foo(*args)
-- 
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