From 6147b695870ce82ee3ad5305ce095b63889b8d9d Mon Sep 17 00:00:00 2001
From: Kevin Newton <kddnewton@gmail.com>
Date: Fri, 14 Nov 2025 11:31:23 -0500
Subject: [PATCH] Array#rfind

Implement Array#rfind, which is the same as find except from the
other side of the Array. Also implemented Array#find (as opposed to
the generic one on Enumerable because it is significantly faster
and to keep the implementations together.

[Feature #21678]
---
 array.c                 | 95 +++++++++++++++++++++++++++++++++++++++++
 test/ruby/test_array.rb | 17 ++++++++
 2 files changed, 112 insertions(+)

diff --git a/array.c b/array.c
index a6aeeeeca1..1352b5c046 100644
--- a/array.c
+++ b/array.c
@@ -2088,6 +2088,99 @@ rb_ary_fetch(int argc, VALUE *argv, VALUE ary)
     return RARRAY_AREF(ary, idx);
 }
 
+/*
+ * call-seq:
+ *   find(if_none_proc = nil) {|element| ... } -> object or nil
+ *   find(if_none_proc = nil) -> enumerator
+ *
+ * Returns the first element for which the block returns a truthy value.
+ *
+ * With a block given, calls the block with successive elements of the array;
+ * returns the first element for which the block returns a truthy value:
+ *
+ *   (0..9).find {|element| element > 2}                # => 3
+ *
+ * If no such element is found, calls +if_none_proc+ and returns its return value.
+ *
+ *   (0..9).find(proc {false}) {|element| element > 12} # => false
+ *   {foo: 0, bar: 1, baz: 2}.find {|key, value| key.start_with?('b') }            # => [:bar, 1]
+ *   {foo: 0, bar: 1, baz: 2}.find(proc {[]}) {|key, value| key.start_with?('c') } # => []
+ *
+ * With no block given, returns an Enumerator.
+ *
+ */
+
+static VALUE
+rb_ary_find(int argc, VALUE *argv, VALUE ary)
+{
+    VALUE if_none;
+    long idx;
+
+    RETURN_ENUMERATOR(ary, argc, argv);
+    if_none = rb_check_arity(argc, 0, 1) ? argv[0] : Qnil;
+
+    for (idx = 0; idx < RARRAY_LEN(ary); idx++) {
+        VALUE elem = RARRAY_AREF(ary, idx);
+        if (RTEST(rb_yield(elem))) {
+            return elem;
+        }
+    }
+
+    if (!NIL_P(if_none)) {
+        return rb_funcallv(if_none, idCall, 0, 0);
+    }
+    return Qnil;
+}
+
+/*
+ * call-seq:
+ *   rfind(if_none_proc = nil) {|element| ... } -> object or nil
+ *   rfind(if_none_proc = nil) -> enumerator
+ *
+ * Returns the last element for which the block returns a truthy value.
+ *
+ * With a block given, calls the block with successive elements of the array in
+ * reverse order; returns the last element for which the block returns a truthy
+ * value:
+ *
+ *   (0..9).rfind {|element| element < 5}                # => 4
+ *
+ * If no such element is found, calls +if_none_proc+ and returns its return value.
+ *
+ *   (0..9).rfind(proc {false}) {|element| element < -2} # => false
+ *   {foo: 0, bar: 1, baz: 2}.rfind {|key, value| key.start_with?('b') }            # => [:baz, 2]
+ *   {foo: 0, bar: 1, baz: 2}.rfind(proc {[]}) {|key, value| key.start_with?('c') } # => []
+ *
+ * With no block given, returns an Enumerator.
+ *
+ */
+
+static VALUE
+rb_ary_rfind(int argc, VALUE *argv, VALUE ary)
+{
+    VALUE if_none;
+    long len, idx;
+
+    RETURN_ENUMERATOR(ary, argc, argv);
+    if_none = rb_check_arity(argc, 0, 1) ? argv[0] : Qnil;
+
+    idx = RARRAY_LEN(ary);
+    while (idx--) {
+        VALUE elem = RARRAY_AREF(ary, idx);
+        if (RTEST(rb_yield(elem))) {
+            return elem;
+        }
+
+        len = RARRAY_LEN(ary);
+        idx = (idx >= len) ? len : idx;
+    }
+
+    if (!NIL_P(if_none)) {
+        return rb_funcallv(if_none, idCall, 0, 0);
+    }
+    return Qnil;
+}
+
 /*
  *  call-seq:
  *    find_index(object) -> integer or nil
@@ -8816,6 +8909,8 @@ Init_Array(void)
     rb_define_method(rb_cArray, "length", rb_ary_length, 0);
     rb_define_method(rb_cArray, "size", rb_ary_length, 0);
     rb_define_method(rb_cArray, "empty?", rb_ary_empty_p, 0);
+    rb_define_method(rb_cArray, "find", rb_ary_find, -1);
+    rb_define_method(rb_cArray, "rfind", rb_ary_rfind, -1);
     rb_define_method(rb_cArray, "find_index", rb_ary_index, -1);
     rb_define_method(rb_cArray, "index", rb_ary_index, -1);
     rb_define_method(rb_cArray, "rindex", rb_ary_rindex, -1);
diff --git a/test/ruby/test_array.rb b/test/ruby/test_array.rb
index a3ac0a6a0b..d93b86e795 100644
--- a/test/ruby/test_array.rb
+++ b/test/ruby/test_array.rb
@@ -3584,6 +3584,23 @@ class TestArray < Test::Unit::TestCase
     assert_equal((1..67).to_a.reverse, var_0)
   end
 
+  def test_find
+    ary = [1, 2, 3, 4, 5]
+    assert_equal(2, ary.find {|x| x % 2 == 0 })
+    assert_equal(nil, ary.find {|x| false })
+    assert_equal(:foo, ary.find(proc { :foo }) {|x| false })
+  end
+
+  def test_rfind
+    ary = [1, 2, 3, 4, 5]
+    assert_equal(4, ary.rfind {|x| x % 2 == 0 })
+    assert_equal(1, ary.rfind {|x| x < 2 })
+    assert_equal(5, ary.rfind {|x| x > 4 })
+    assert_equal(nil, ary.rfind {|x| false })
+    assert_equal(:foo, ary.rfind(proc { :foo }) {|x| false })
+    assert_equal(nil, ary.rfind {|x| ary.clear; false })
+  end
+
   private
   def need_continuation
     unless respond_to?(:callcc, true)