Move Time#xmlschema in core and optimize it

[Feature #20707]

Converting Time into RFC3339 / ISO8601 representation is an significant
hotspot for applications that serialize data in JSON, XML or other formats.

By moving it into core we can optimize it much further than what `strftime` will
allow.

```
compare-ruby: ruby 3.4.0dev (2024-08-29T13:11:40Z master 6b08a50a62) +YJIT [arm64-darwin23]
built-ruby: ruby 3.4.0dev (2024-08-30T13:17:32Z native-xmlschema 34041ff71f) +YJIT [arm64-darwin23]
warming up......

|                        |compare-ruby|built-ruby|
|:-----------------------|-----------:|---------:|
|time.xmlschema          |      1.087M|    5.190M|
|                        |           -|     4.78x|
|utc_time.xmlschema      |      1.464M|    6.848M|
|                        |           -|     4.68x|
|time.xmlschema(6)       |    859.960k|    4.646M|
|                        |           -|     5.40x|
|utc_time.xmlschema(6)   |      1.080M|    5.917M|
|                        |           -|     5.48x|
|time.xmlschema(9)       |    893.909k|    4.668M|
|                        |           -|     5.22x|
|utc_time.xmlschema(9)   |      1.056M|    5.707M|
|                        |           -|     5.40x|
```

9 files changed, 207 insertions, 3 deletions

diff --git a/benchmark/time_xmlschema.yml b/benchmark/time_xmlschema.yml
new file mode 100644
index 0000000000..f746d9c6c8
--- /dev/null
+++ b/benchmark/time_xmlschema.yml
@@ -0,0 +1,23 @@
+prelude: |
+  # frozen_string_literal
+  unless Time.method_defined?(:xmlschema)
+    class Time
+      def xmlschema(fraction_digits=0)
+        fraction_digits = fraction_digits.to_i
+        s = strftime("%FT%T")
+        if fraction_digits > 0
+          s << strftime(".%#{fraction_digits}N")
+        end
+        s << (utc? ? 'Z' : strftime("%:z"))
+      end
+    end
+  end
+  time = Time.now
+  utc_time = Time.now.utc
+benchmark:
+  - time.xmlschema
+  - utc_time.xmlschema
+  - time.xmlschema(6)
+  - utc_time.xmlschema(6)
+  - time.xmlschema(9)
+  - utc_time.xmlschema(9)
diff --git a/spec/ruby/core/time/iso8601_spec.rb b/spec/ruby/core/time/iso8601_spec.rb
new file mode 100644
index 0000000000..ad60c3bb32
--- /dev/null
+++ b/spec/ruby/core/time/iso8601_spec.rb
@@ -0,0 +1,6 @@
+require_relative '../../spec_helper'
+require_relative 'shared/xmlschema'
+
+describe "Time#iso8601" do
+  it_behaves_like :time_xmlschema, :iso8601
+end
diff --git a/spec/ruby/core/time/shared/xmlschema.rb b/spec/ruby/core/time/shared/xmlschema.rb
new file mode 100644
index 0000000000..d68c18df36
--- /dev/null
+++ b/spec/ruby/core/time/shared/xmlschema.rb
@@ -0,0 +1,31 @@
+describe :time_xmlschema, shared: true do
+  ruby_version_is "3.4" do
+    it "generates ISO-8601 strings in Z for UTC times" do
+      t = Time.utc(1985, 4, 12, 23, 20, 50, 521245)
+      t.send(@method).should == "1985-04-12T23:20:50Z"
+      t.send(@method, 2).should == "1985-04-12T23:20:50.52Z"
+      t.send(@method, 9).should == "1985-04-12T23:20:50.521245000Z"
+    end
+
+    it "generates ISO-8601 string with timeone offset for non-UTC times" do
+      t = Time.new(1985, 4, 12, 23, 20, 50, "+02:00")
+      t.send(@method).should == "1985-04-12T23:20:50+02:00"
+      t.send(@method, 2).should == "1985-04-12T23:20:50.00+02:00"
+    end
+
+    it "year is always at least 4 digits" do
+      t = Time.utc(12, 4, 12)
+      t.send(@method).should ==  "0012-04-12T00:00:00Z"
+    end
+
+    it "year can be more than 4 digits" do
+      t = Time.utc(40_000, 4, 12)
+      t.send(@method).should ==  "40000-04-12T00:00:00Z"
+    end
+
+    it "year can be negative" do
+      t = Time.utc(-2000, 4, 12)
+      t.send(@method).should ==  "-2000-04-12T00:00:00Z"
+    end
+  end
+end
diff --git a/spec/ruby/core/time/xmlschema_spec.rb b/spec/ruby/core/time/xmlschema_spec.rb
new file mode 100644
index 0000000000..bdf1dc7923
--- /dev/null
+++ b/spec/ruby/core/time/xmlschema_spec.rb
@@ -0,0 +1,6 @@
+require_relative '../../spec_helper'
+require_relative 'shared/xmlschema'
+
+describe "Time#xmlschema" do
+  it_behaves_like :time_xmlschema, :xmlschema
+end
diff --git a/spec/ruby/library/time/iso8601_spec.rb b/spec/ruby/library/time/iso8601_spec.rb
index 4a9eb45613..9e2607fbd0 100644
--- a/spec/ruby/library/time/iso8601_spec.rb
+++ b/spec/ruby/library/time/iso8601_spec.rb
@@ -3,5 +3,5 @@ require_relative 'shared/xmlschema'
 require 'time'
 
 describe "Time.xmlschema" do
-  it_behaves_like :time_xmlschema, :iso8601
+  it_behaves_like :time_library_xmlschema, :iso8601
 end
diff --git a/spec/ruby/library/time/shared/xmlschema.rb b/spec/ruby/library/time/shared/xmlschema.rb
index 44d33cda7e..831d8509a7 100644
--- a/spec/ruby/library/time/shared/xmlschema.rb
+++ b/spec/ruby/library/time/shared/xmlschema.rb
@@ -1,4 +1,4 @@
-describe :time_xmlschema, shared: true do
+describe :time_library_xmlschema, shared: true do
   it "parses ISO-8601 strings" do
     t = Time.utc(1985, 4, 12, 23, 20, 50, 520000)
     s = "1985-04-12T23:20:50.52Z"
diff --git a/spec/ruby/library/time/xmlschema_spec.rb b/spec/ruby/library/time/xmlschema_spec.rb
index 4279311199..ff3c864a02 100644
--- a/spec/ruby/library/time/xmlschema_spec.rb
+++ b/spec/ruby/library/time/xmlschema_spec.rb
@@ -3,5 +3,5 @@ require_relative 'shared/xmlschema'
 require 'time'
 
 describe "Time.xmlschema" do
-  it_behaves_like :time_xmlschema, :xmlschema
+  it_behaves_like :time_library_xmlschema, :xmlschema
 end
diff --git a/test/ruby/test_time.rb b/test/ruby/test_time.rb
index 2a541bbe8c..cab14eb694 100644
--- a/test/ruby/test_time.rb
+++ b/test/ruby/test_time.rb
@@ -1444,4 +1444,60 @@ class TestTime < Test::Unit::TestCase
   def test_parse_zero_bigint
     assert_equal 0, Time.new("2020-10-28T16:48:07.000Z").nsec, '[Bug #19390]'
   end
+
+  def test_xmlschema_encode
+    [:xmlschema, :iso8601].each do |method|
+      bug6100 = '[ruby-core:42997]'
+
+      t = Time.utc(2001, 4, 17, 19, 23, 17, 300000)
+      assert_equal("2001-04-17T19:23:17Z", t.__send__(method))
+      assert_equal("2001-04-17T19:23:17.3Z", t.__send__(method, 1))
+      assert_equal("2001-04-17T19:23:17.300000Z", t.__send__(method, 6))
+      assert_equal("2001-04-17T19:23:17.3000000Z", t.__send__(method, 7))
+      assert_equal("2001-04-17T19:23:17.3Z", t.__send__(method, 1.9), bug6100)
+
+      t = Time.utc(2001, 4, 17, 19, 23, 17, 123456)
+      assert_equal("2001-04-17T19:23:17.1234560Z", t.__send__(method, 7))
+      assert_equal("2001-04-17T19:23:17.123456Z", t.__send__(method, 6))
+      assert_equal("2001-04-17T19:23:17.12345Z", t.__send__(method, 5))
+      assert_equal("2001-04-17T19:23:17.1Z", t.__send__(method, 1))
+      assert_equal("2001-04-17T19:23:17.1Z", t.__send__(method, 1.9), bug6100)
+
+      t = Time.at(2.quo(3)).getlocal("+09:00")
+      assert_equal("1970-01-01T09:00:00.666+09:00", t.__send__(method, 3))
+      assert_equal("1970-01-01T09:00:00.6666666666+09:00", t.__send__(method, 10))
+      assert_equal("1970-01-01T09:00:00.66666666666666666666+09:00", t.__send__(method, 20))
+      assert_equal("1970-01-01T09:00:00.6+09:00", t.__send__(method, 1.1), bug6100)
+      assert_equal("1970-01-01T09:00:00.666+09:00", t.__send__(method, 3.2), bug6100)
+
+      t = Time.at(123456789.quo(9999999999)).getlocal("+09:00")
+      assert_equal("1970-01-01T09:00:00.012+09:00", t.__send__(method, 3))
+      assert_equal("1970-01-01T09:00:00.012345678+09:00", t.__send__(method, 9))
+      assert_equal("1970-01-01T09:00:00.0123456789+09:00", t.__send__(method, 10))
+      assert_equal("1970-01-01T09:00:00.0123456789012345678+09:00", t.__send__(method, 19))
+      assert_equal("1970-01-01T09:00:00.01234567890123456789+09:00", t.__send__(method, 20))
+      assert_equal("1970-01-01T09:00:00.012+09:00", t.__send__(method, 3.8), bug6100)
+
+      t = Time.utc(1)
+      assert_equal("0001-01-01T00:00:00Z", t.__send__(method))
+
+      begin
+        Time.at(-1)
+      rescue ArgumentError
+        # ignore
+      else
+        t = Time.utc(1960, 12, 31, 23, 0, 0, 123456)
+        assert_equal("1960-12-31T23:00:00.123456Z", t.__send__(method, 6))
+      end
+
+      assert_equal("10000-01-01T00:00:00Z", Time.utc(10000).__send__(method))
+      assert_equal("9999-01-01T00:00:00Z", Time.utc(9999).__send__(method))
+      assert_equal("0001-01-01T00:00:00Z", Time.utc(1).__send__(method)) # 1 AD
+      assert_equal("0000-01-01T00:00:00Z", Time.utc(0).__send__(method)) # 1 BC
+      assert_equal("-0001-01-01T00:00:00Z", Time.utc(-1).__send__(method)) # 2 BC
+      assert_equal("-0004-01-01T00:00:00Z", Time.utc(-4).__send__(method)) # 5 BC
+      assert_equal("-9999-01-01T00:00:00Z", Time.utc(-9999).__send__(method))
+      assert_equal("-10000-01-01T00:00:00Z", Time.utc(-10000).__send__(method))
+    end
+  end
 end
diff --git a/time.c b/time.c
index 035d32dd31..801df3658e 100644
--- a/time.c
+++ b/time.c
@@ -5215,6 +5215,86 @@ time_strftime(VALUE time, VALUE format)
     }
 }
 
+static VALUE
+time_xmlschema(int argc, VALUE *argv, VALUE time)
+{
+    long fraction_digits = 0;
+    rb_check_arity(argc, 0, 1);
+    if (argc > 0) {
+        fraction_digits = NUM2LONG(argv[0]);
+        if (fraction_digits < 0) {
+            fraction_digits = 0;
+        }
+    }
+
+    struct time_object *tobj;
+
+    GetTimeval(time, tobj);
+    MAKE_TM(time, tobj);
+
+    long year = -1;
+    if (FIXNUM_P(tobj->vtm.year)) {
+        year = FIX2LONG(tobj->vtm.year);
+    }
+    if (RB_UNLIKELY(year > 9999 || year < 0 || fraction_digits > 9)) {
+        // Slow path for uncommon dates.
+        VALUE format = rb_utf8_str_new_cstr("%FT%T");
+        if (fraction_digits > 0) {
+            rb_str_catf(format, ".%%#%ldN", fraction_digits);
+        }
+        rb_str_cat_cstr(format, TZMODE_UTC_P(tobj) ? "Z" : "%:z");
+        return rb_funcallv(time, rb_intern("strftime"), 1, &format);
+    }
+
+    long buf_size = sizeof("YYYY-MM-DDTHH:MM:SS+ZH:ZM") + fraction_digits + (fraction_digits > 0 ? 1 : 0);
+
+    VALUE str = rb_str_buf_new(buf_size);
+    rb_enc_associate_index(str, rb_utf8_encindex());
+
+    char *ptr = RSTRING_PTR(str);
+    char *start = ptr;
+    int written = snprintf(
+        ptr,
+        sizeof("YYYY-MM-DDTHH:MM:SS"),
+        "%04ld-%02d-%02dT%02d:%02d:%02d",
+        year,
+        tobj->vtm.mon,
+        tobj->vtm.mday,
+        tobj->vtm.hour,
+        tobj->vtm.min,
+        tobj->vtm.sec
+    );
+    RUBY_ASSERT(written == sizeof("YYYY-MM-DDTHH:MM:SS") - 1);
+    ptr += written;
+
+    if (fraction_digits > 0) {
+        long nsec = NUM2LONG(mulquov(tobj->vtm.subsecx, INT2FIX(1000000000), INT2FIX(TIME_SCALE)));
+        long subsec = nsec / (long)pow(10, 9 - fraction_digits);
+
+        *ptr = '.';
+        ptr++;
+
+        written = snprintf(ptr, fraction_digits + 1, "%0*ld", (int)fraction_digits, subsec); // Always allow to write \0
+        RUBY_ASSERT(written > 0);
+        ptr += written;
+    }
+
+    if (TZMODE_UTC_P(tobj)) {
+        *ptr = 'Z';
+        ptr++;
+    }
+    else {
+        long offset = NUM2LONG(rb_time_utc_offset(time));
+        int offset_hours = (int)(offset / 3600);
+        int offset_minutes = (int)((offset % 3600 / 60));
+        written = snprintf(ptr, sizeof("+ZH:ZM"), "%+03d:%02d", offset_hours, offset_minutes);
+        RUBY_ASSERT(written == sizeof("+ZH:ZM") - 1);
+        ptr += written;
+    }
+    rb_str_set_len(str, ptr -start); // We could skip coderange scanning as we know it's full ASCII.
+    return str;
+}
+
 int ruby_marshal_write_long(long x, char *buf);
 
 enum {base_dump_size = 8};
@@ -5842,6 +5922,8 @@ Init_Time(void)
     rb_define_method(rb_cTime, "subsec", time_subsec, 0);
 
     rb_define_method(rb_cTime, "strftime", time_strftime, 1);
+    rb_define_method(rb_cTime, "xmlschema", time_xmlschema, -1);
+    rb_define_alias(rb_cTime, "iso8601", "xmlschema");
 
     /* methods for marshaling */
     rb_define_private_method(rb_cTime, "_dump", time_dump, -1);