[GR-19220] Reverse Array#shuffle implementation to match output of CRuby (#2985)

PullRequest: truffleruby/3757

Author: eregon

CHANGELOG.md

@@ -8,7 +8,8 @@ Bug fixes:
 
 Compatibility:
 
-- Fix `Hash#shift` when Hash is empty but has initial default value or initial default proc (@itarato).
+* Fix `Hash#shift` when Hash is empty but has initial default value or initial default proc (@itarato).
+* Make `Array#shuffle` produce the same results as CRuby (@rwstauner).
 
 Performance:
 

spec/ruby/core/array/shuffle_spec.rb

@@ -93,4 +93,14 @@
     -> { ArraySpecs.frozen_array.shuffle! }.should raise_error(FrozenError)
     -> { ArraySpecs.empty_frozen_array.shuffle! }.should raise_error(FrozenError)
   end
+
+  it "matches CRuby with random:" do
+    %w[a b c].shuffle(random: Random.new(1)).should == %w[a c b]
+    (0..10).to_a.shuffle(random: Random.new(10)).should == [2, 6, 8, 5, 7, 10, 3, 1, 0, 4, 9]
+  end
+
+  it "matches CRuby with srand" do
+    srand(123)
+    %w[a b c d e f g h i j k].shuffle.should == %w[a e f h i j d b g k c]
+  end
 end

src/main/ruby/truffleruby/core/array.rb

@@ -1240,9 +1240,11 @@ def shuffle!(options = undefined)
       random_generator = options[:random] if options[:random].respond_to?(:rand)
     end
 
-    size.times do |i|
-      r = i + random_generator.rand(size - i).to_int
-      raise RangeError, "random number too big #{r - i}" if r < 0 || r >= size
+    # Start at the end and work toward the beginning for compatibility with CRuby.
+    (size - 1).downto(0) do |i|
+      r = random_generator.rand(i + 1).to_int
+      raise RangeError, "random number too small #{r}" if r < 0
+      raise RangeError, "random number too big #{r}" if r > i
       swap(i, r)
     end
     self