Make String#casecmp and casecmp? spec-compliant

spec/core/string/casecmp_spec.rb

@@ -30,9 +30,7 @@
   end
 
   it "returns nil if incompatible encodings" do
-    NATFIXME 'returns nil if incompatible encodings', exception: SpecFailedException do
-      "あれ".casecmp("れ".encode(Encoding::EUC_JP)).should be_nil
-    end
+    "あれ".casecmp("れ".encode(Encoding::EUC_JP)).should be_nil
   end
 
   describe "in UTF-8 mode" do
@@ -119,6 +117,11 @@
       "B".casecmp(a).should == 1
     end
   end
+
+  it "returns 0 for empty strings in different encodings" do
+    ''.b.casecmp('').should == 0
+    ''.b.casecmp(''.encode("UTF-32LE")).should == 0
+  end
 end
 
 describe 'String#casecmp? independent of case' do
@@ -140,9 +143,7 @@
   end
 
   it "returns nil if incompatible encodings" do
-    NATFIXME 'returns nil if incompatible encodings', exception: SpecFailedException do
-      "あれ".casecmp?("れ".encode(Encoding::EUC_JP)).should be_nil
-    end
+    "あれ".casecmp?("れ".encode(Encoding::EUC_JP)).should be_nil
   end
 
   describe 'for UNICODE characters' do
@@ -189,12 +190,15 @@
   end
 
   it "case folds" do
-    NATFIXME 'Pending unicode casemap support', exception: SpecFailedException do
-      "ß".casecmp?("ss").should be_true
-    end
+    "ß".casecmp?("ss").should be_true
   end
 
   it "returns nil if other can't be converted to a string" do
     "abc".casecmp?(mock('abc')).should be_nil
   end
+
+  it "returns true for empty strings in different encodings" do
+    ''.b.should.casecmp?('')
+    ''.b.should.casecmp?(''.encode("UTF-32LE"))
+  end
 end

src/string_object.cpp

@@ -749,6 +749,9 @@ Value StringObject::cmp(Env *env, Value other) {
         return NilObject::the();
     }
 
+    if (m_string.is_empty() && other_str->m_string.is_empty())
+        return Value::integer(0);
+
     auto comparison = m_string.cmp(other_str->m_string);
 
     if (comparison == 0 && !(is_ascii_only() && other->as_string()->is_ascii_only())) {
@@ -2798,19 +2801,38 @@ Value StringObject::casecmp(Env *env, Value other) {
     other = StringObject::try_convert(env, other);
     if (other->is_nil())
         return NilObject::the();
+
+    auto other_str = other->as_string_or_raise(env);
+
+    if (is_empty() && other_str->is_empty())
+        return Value::integer(0);
+
+    if (!negotiate_compatible_encoding(other_str))
+        return NilObject::the();
+
     auto str1 = this->downcase(env, "ascii"_s, nullptr);
-    auto str2 = other->as_string()->downcase(env, "ascii"_s, nullptr);
+    auto str2 = other_str->downcase(env, "ascii"_s, nullptr);
     return str1->cmp(env, Value(str2));
 }
 
 Value StringObject::is_casecmp(Env *env, Value other) {
     other = StringObject::try_convert(env, other);
     if (other->is_nil())
         return NilObject::the();
-    auto str1 = this->downcase(env, nullptr, nullptr);
-    auto str2 = other->as_string()->downcase(env, nullptr, nullptr);
+
+    auto other_str = other->as_string_or_raise(env);
+
+    if (is_empty() && other_str->is_empty())
+        return Value::integer(0);
+
+    if (!negotiate_compatible_encoding(other_str))
+        return NilObject::the();
+
+    auto str1 = this->downcase(env, "fold"_s, nullptr);
+    auto str2 = other_str->downcase(env, "fold"_s, nullptr);
     if (str1->string() == str2->string())
         return TrueObject::the();
+
     return FalseObject::the();
 }