1 files changed, 42 insertions, 11 deletions
diff --git a/spec/ruby/core/string/unpack1_spec.rb b/spec/ruby/core/string/unpack1_spec.rb
index f59bd92d6a..ee10042eb8 100644
--- a/spec/ruby/core/string/unpack1_spec.rb
+++ b/spec/ruby/core/string/unpack1_spec.rb
@@ -8,23 +8,54 @@ describe "String#unpack1" do
     "A".unpack1("B*").should == "01000001"
   end
 
-  ruby_version_is "3.1" do
-    it "starts unpacking from the given offset" do
-      "ZZABCD".unpack1('x3C', offset: 2).should == "ABCD".unpack('x3C')[0]
-      "ZZZZaG9nZWZ1Z2E=".unpack1("m", offset: 4).should == "hogefuga"
-      "ZA".unpack1("B*", offset: 1).should == "01000001"
+  it "starts unpacking from the given offset" do
+    "ZZABCD".unpack1('x3C', offset: 2).should == "ABCD".unpack('x3C')[0]
+    "ZZZZaG9nZWZ1Z2E=".unpack1("m", offset: 4).should == "hogefuga"
+    "ZA".unpack1("B*", offset: 1).should == "01000001"
+  end
+
+  it "traits offset as a bytes offset" do
+    "؈".unpack("CC").should == [216, 136]
+    "؈".unpack1("C").should == 216
+    "؈".unpack1("C", offset: 1).should == 136
+  end
+
+  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
 
-    it "raises an ArgumentError when the offset is negative" do
-      -> { "a".unpack1("C", offset: -1) }.should raise_error(ArgumentError)
+    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
+    "a".unpack1("C", offset: 1).should == nil
+  end
+
+  it "raises an ArgumentError when the offset is larger than the string bytesize" do
+    -> { "a".unpack1("C", offset: 2) }.should.raise(ArgumentError, "offset outside of string")
+  end
+
+  context "with format 'm0'" do
+    # unpack1("m0") takes a special code path that calls Pack.unpackBase46Strict instead of Pack.unpack_m,
+    # which is why we repeat the tests for unpack("m0") here.
 
-    it "returns nil if the offset is at the end of the string" do
-      "a".unpack1("C", offset: 1).should == nil
+    it "decodes base64" do
+      "dGVzdA==".unpack1("m0").should == "test"
     end
 
-    it "raises an ArgumentError when the offset is larget than the string" do
-      -> { "a".unpack1("C", offset: 2) }.should raise_error(ArgumentError)
+    it "raises an ArgumentError for an invalid base64 character" do
+      -> { "dGV%zdA==".unpack1("m0") }.should.raise(ArgumentError)
     end
   end
 end