From eb053e7446607f5e70215bf508499ef6bab3aa4a Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@ruby-lang.org>
Date: Thu, 2 Apr 2026 15:37:37 +0900
Subject: [Feature #21979] Allow negative offset in unpack

---
 pack.c                                |  5 +++--
 spec/ruby/core/string/unpack1_spec.rb | 18 ++++++++++++++++--
 spec/ruby/core/string/unpack_spec.rb  | 18 ++++++++++++++++--
 test/ruby/test_pack.rb                | 14 ++++++++------
 4 files changed, 43 insertions(+), 12 deletions(-)

diff --git a/pack.c b/pack.c
index 94bb510e0d..24221bc3d6 100644
--- a/pack.c
+++ b/pack.c
@@ -1024,9 +1024,10 @@ pack_unpack_internal(VALUE str, VALUE fmt, enum unpack_mode mode, long offset)
     StringValue(fmt);
     rb_must_asciicompat(fmt);
 
-    if (offset < 0) rb_raise(rb_eArgError, "offset can't be negative");
     len = RSTRING_LEN(str);
-    if (offset > len) rb_raise(rb_eArgError, "offset outside of string");
+    if (offset < 0 ? (offset += len) < 0 : offset > len) {
+        rb_raise(rb_eArgError, "offset outside of string");
+    }
 
     s = RSTRING_PTR(str);
     send = s + len;
diff --git a/spec/ruby/core/string/unpack1_spec.rb b/spec/ruby/core/string/unpack1_spec.rb
index 3b0fa90900..ee10042eb8 100644
--- a/spec/ruby/core/string/unpack1_spec.rb
+++ b/spec/ruby/core/string/unpack1_spec.rb
@@ -20,8 +20,22 @@ describe "String#unpack1" do
     "؈".unpack1("C", offset: 1).should == 136
   end
 
-  it "raises an ArgumentError when the offset is negative" do
-    -> { "a".unpack1("C", offset: -1) }.should.raise(ArgumentError, "offset can't be negative")
+  describe "when the offset is negative" do
+    ruby_version_is "4.1" do
+      it "starts unpacking from the end" do
+        "abc".unpack1("C", offset: -2).should == 98
+      end
+
+      it "raises an ArgumentError if it is less than -length" do
+        -> { "a".unpack1("C", offset: -2) }.should.raise(ArgumentError, "offset outside of string")
+      end
+    end
+
+    ruby_version_is ""..."4.1" do
+      it "raises an ArgumentError" do
+        -> { "a".unpack1("C", offset: -1) }.should.raise(ArgumentError, "offset can't be negative")
+      end
+    end
   end
 
   it "returns nil if the offset is at the end of the string" do
diff --git a/spec/ruby/core/string/unpack_spec.rb b/spec/ruby/core/string/unpack_spec.rb
index 28dbbc14c4..eb4710ce14 100644
--- a/spec/ruby/core/string/unpack_spec.rb
+++ b/spec/ruby/core/string/unpack_spec.rb
@@ -18,8 +18,22 @@ describe "String#unpack" do
     "؈".unpack("CC", offset: 1).should == [136, nil]
   end
 
-  it "raises an ArgumentError when the offset is negative" do
-    -> { "a".unpack("C", offset: -1) }.should.raise(ArgumentError, "offset can't be negative")
+  describe "when the offset is negative" do
+    ruby_version_is "4.1" do
+      it "starts unpacking from the end" do
+        "abc".unpack("CC", offset: -2).should == [98, 99]
+      end
+
+      it "raises an ArgumentError if it is less than -length" do
+        -> { "a".unpack("C", offset: -2) }.should.raise(ArgumentError, "offset outside of string")
+      end
+    end
+
+    ruby_version_is ""..."4.1" do
+      it "raises an ArgumentError" do
+        -> { "a".unpack("C", offset: -1) }.should.raise(ArgumentError, "offset can't be negative")
+      end
+    end
   end
 
   it "returns nil if the offset is at the end of the string" do
diff --git a/test/ruby/test_pack.rb b/test/ruby/test_pack.rb
index 3020e02761..6e5f0fe7ff 100644
--- a/test/ruby/test_pack.rb
+++ b/test/ruby/test_pack.rb
@@ -913,27 +913,29 @@ EXPECTED
 
   def test_unpack1_offset
     assert_equal 65, "ZA".unpack1("C", offset: 1)
+    assert_equal 65, "ZA".unpack1("C", offset: -1)
     assert_equal "01000001", "YZA".unpack1("B*", offset: 2)
     assert_nil "abc".unpack1("C", offset: 3)
-    assert_raise_with_message(ArgumentError, /offset can't be negative/) {
-      "a".unpack1("C", offset: -1)
-    }
     assert_raise_with_message(ArgumentError, /offset outside of string/) {
       "a".unpack1("C", offset: 2)
     }
+    assert_raise_with_message(ArgumentError, /offset outside of string/) {
+      "a".unpack1("C", offset: -2)
+    }
     assert_nil "a".unpack1("C", offset: 1)
   end
 
   def test_unpack_offset
     assert_equal [65], "ZA".unpack("C", offset: 1)
+    assert_equal [65], "ZA".unpack("C", offset: -1)
     assert_equal ["01000001"], "YZA".unpack("B*", offset: 2)
     assert_equal [nil, nil, nil], "abc".unpack("CCC", offset: 3)
-    assert_raise_with_message(ArgumentError, /offset can't be negative/) {
-      "a".unpack("C", offset: -1)
-    }
     assert_raise_with_message(ArgumentError, /offset outside of string/) {
       "a".unpack("C", offset: 2)
     }
+    assert_raise_with_message(ArgumentError, /offset outside of string/) {
+      "a".unpack("C", offset: -2)
+    }
     assert_equal [nil], "a".unpack("C", offset: 1)
   end
 
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