Fix divmod and modulo by infinity to match Float#divmod and Float#modulo (#429)

Author: tompng

ext/bigdecimal/bigdecimal.c

@@ -1733,17 +1733,26 @@ BigDecimal_DoDivmod(VALUE self, VALUE r, NULLABLE_BDVALUE *div, NULLABLE_BDVALUE
         *mod = (NULLABLE_BDVALUE) { nan, DATA_PTR(nan) };
         goto Done;
     }
-    if (VpIsInf(b.real)) {
-        VALUE zero = BigDecimal_positive_zero();
-        *div = (NULLABLE_BDVALUE) { zero, DATA_PTR(zero) };
-        *mod = bdvalue_nullable(a);
-        goto Done;
-    }
     if (VpIsZero(a.real)) {
         VALUE zero = BigDecimal_positive_zero();
         *div = *mod = (NULLABLE_BDVALUE) { zero, DATA_PTR(zero) };
         goto Done;
     }
+    if (VpIsInf(b.real)) {
+        if (!truncate && VpGetSign(a.real) * VpGetSign(b.real) < 0) {
+            BDVALUE minus_one = NewZeroWrap(1, BASE_FIG);
+            VpSetOne(minus_one.real);
+            VpSetSign(minus_one.real, -1);
+            RB_GC_GUARD(minus_one.bigdecimal);
+            *div = bdvalue_nullable(minus_one);
+            *mod = bdvalue_nullable(b);
+        } else {
+            VALUE zero = BigDecimal_positive_zero();
+            *div = (NULLABLE_BDVALUE) { zero, DATA_PTR(zero) };
+            *mod = bdvalue_nullable(a);
+        }
+        goto Done;
+    }
 
     a_exponent = VpExponent10(a.real);
     b_exponent = VpExponent10(b.real);

test/bigdecimal/test_bigdecimal.rb

@@ -2698,14 +2698,24 @@ def test_llong_min_gh_200
     assert_equal(BigDecimal(minus_ullong_max.to_s), BigDecimal(minus_ullong_max), "[GH-200]")
   end
 
-  def test_reminder_infinity_gh_187
-    # https://github.com/ruby/bigdecimal/issues/187
+  def test_divmod_modulo_remainder_infinity
+    pend '-4.2.divmod(Float::INFINITY) is not [-1, Infinity]' if RUBY_ENGINE == 'truffleruby'
     BigDecimal.save_exception_mode do
       BigDecimal.mode(BigDecimal::EXCEPTION_INFINITY, false)
       BigDecimal.mode(BigDecimal::EXCEPTION_NaN, false)
-      bd = BigDecimal("4.2")
-      assert_equal(bd.remainder(BigDecimal("+Infinity")), bd)
-      assert_equal(bd.remainder(BigDecimal("-Infinity")), bd)
+      pinf = BigDecimal("+Infinity")
+      minf = BigDecimal("-Infinity")
+      assert_nan(pinf.modulo(pinf))
+      assert_nan(pinf.remainder(pinf))
+      [BigDecimal("-4.2"), BigDecimal(0), BigDecimal("4.2")].each do |x|
+        assert_equal(x.to_f.divmod(pinf.to_f), x.divmod(pinf))
+        assert_equal(x.to_f.divmod(minf.to_f), x.divmod(minf))
+        assert_equal(x.to_f.modulo(pinf.to_f), x.modulo(pinf))
+        assert_equal(x.to_f.modulo(minf.to_f), x.modulo(minf))
+        # Float#remainder(plus_or_minus_infinity) returns itself in Ruby >= 3.1
+        assert_equal(x, x.remainder(pinf))
+        assert_equal(x, x.remainder(minf))
+      end
     end
   end