Add tests for NativeMath + code clean up

Author: plokhotnyuk

jsoniter-scala-core/native/src/main/resources/scala-native/multiply_high.c

@@ -15,7 +15,7 @@ unsigned long long jsoniter_scala_unsigned_multiply_high(unsigned long long x, u
 #elif defined(__SIZEOF_INT128__)
 
 long long jsoniter_scala_multiply_high(long long x, long long y) {
-  return x * (unsigned __int128) y >> 64;
+  return x * (__int128) y >> 64;
 }
 
 unsigned long long jsoniter_scala_unsigned_multiply_high(unsigned long long x, unsigned long long y) {

jsoniter-scala-core/native/src/test/scala/com/github/plokhotnyuk/jsoniter_scala/core/NativeMathSpec.scala

@@ -0,0 +1,36 @@
+package com.github.plokhotnyuk.jsoniter_scala.core
+
+import org.scalacheck.Arbitrary.arbitrary
+import org.scalatest.matchers.should.Matchers
+import org.scalatest.wordspec.AnyWordSpec
+import org.scalatestplus.scalacheck.ScalaCheckPropertyChecks
+
+class NativeMathSpec extends AnyWordSpec with Matchers with ScalaCheckPropertyChecks {
+  "NativeMath" should {
+    "have multiplyHigh consistent with Math.multiplyHigh" in {
+      def check(x: Long, y: Long): Unit = NativeMath.multiplyHigh(x, y) shouldBe Math.multiplyHigh(x, y)
+
+      check(Long.MaxValue, Long.MaxValue)
+      check(Long.MinValue, Long.MaxValue)
+      check(Long.MaxValue, Long.MinValue)
+      check(Long.MinValue, Long.MinValue)
+      forAll(arbitrary[Long], arbitrary[Long], minSuccessful(100000))(check)
+    }
+    "have unsignedMultiplyHigh consistent with Math.unsignedMultiplyHigh" in {
+      def check(x: Long, y: Long): Unit = NativeMath.unsignedMultiplyHigh(x, y) shouldBe unsignedMultiplyHigh(x, y)
+
+      check(Long.MaxValue, Long.MaxValue)
+      check(Long.MinValue, Long.MaxValue)
+      check(Long.MaxValue, Long.MinValue)
+      check(Long.MinValue, Long.MinValue)
+      forAll(arbitrary[Long], arbitrary[Long], minSuccessful(100000))(check)
+    }
+  }
+
+  def unsignedMultiplyHigh(x: Long, y: Long): Long = { // FIXME: replace by Math.unsignedMultiplyHigh after dropping JDK 17 support
+    var r = Math.multiplyHigh(x, y)
+    r += y & (x >> 63)
+    r += x & (y >> 63)
+    r
+  }
+}