LibCrypto: Add a rounding mode to UnsignedBigInteger::to_double

This allows using different options for rounding, like IEEE
roundTiesToEven, which is the mode that JS requires.

Also fix that the last word read from the bigint for the mantissa could
be shifted incorrectly leading to incorrect results.

1 files changed, 36 insertions, 2 deletions

diff --git a/Tests/LibCrypto/TestBigInteger.cpp b/Tests/LibCrypto/TestBigInteger.cpp
index 117f5e1891..3a94712739 100644
--- a/Tests/LibCrypto/TestBigInteger.cpp
+++ b/Tests/LibCrypto/TestBigInteger.cpp
@@ -750,7 +750,7 @@ EXPECT_EQUAL_TO(zero, -0.0);
 TEST_CASE(to_double)
 {
 #define EXPECT_TO_EQUAL_DOUBLE(bigint, double_value) \
-    EXPECT_EQ((bigint).to_double(), double_value)
+    EXPECT_EQ((bigint).to_double(Crypto::UnsignedBigInteger::RoundingMode::RoundTowardZero), double_value)
 
     EXPECT_TO_EQUAL_DOUBLE(Crypto::UnsignedBigInteger(0), 0.0);
     // Make sure we don't get negative zero!
@@ -825,7 +825,41 @@ TEST_CASE(to_double)
     EXPECT_TO_EQUAL_DOUBLE(Crypto::SignedBigInteger::from_base(10, "2345678901234567890"sv),
         2345678901234567680.0);
 
-    EXPECT_EQ(1234567890123456800.0, 1234567890123456768.0);
+    EXPECT_EQ(
+        Crypto::UnsignedBigInteger::from_base(16, "1fffffffffffff00"sv).to_double(Crypto::UnsignedBigInteger::RoundingMode::IEEERoundAndTiesToEvenMantissa),
+        2305843009213693696.0);
+
+    EXPECT_EQ(
+        Crypto::UnsignedBigInteger::from_base(16, "1fffffffffffff00"sv).to_double(Crypto::UnsignedBigInteger::RoundingMode::RoundTowardZero),
+        2305843009213693696.0);
+
+    EXPECT_EQ(
+        Crypto::UnsignedBigInteger::from_base(16, "1fffffffffffff80"sv).to_double(Crypto::UnsignedBigInteger::RoundingMode::IEEERoundAndTiesToEvenMantissa),
+        2305843009213693952.0);
+
+    EXPECT_EQ(Crypto::UnsignedBigInteger::from_base(16, "20000000000001"sv).to_double(Crypto::UnsignedBigInteger::RoundingMode::IEEERoundAndTiesToEvenMantissa),
+        9007199254740992.0);
+
+    EXPECT_EQ(Crypto::UnsignedBigInteger::from_base(16, "20000000000002"sv).to_double(Crypto::UnsignedBigInteger::RoundingMode::IEEERoundAndTiesToEvenMantissa),
+        9007199254740994.0);
+
+    // 2^53 = 20000000000000, +3 Rounds up because of tiesRoundToEven
+    EXPECT_EQ(Crypto::UnsignedBigInteger::from_base(16, "20000000000003"sv).to_double(Crypto::UnsignedBigInteger::RoundingMode::IEEERoundAndTiesToEvenMantissa),
+        9007199254740996.0);
+
+    // +4 is exactly 9007199254740996
+    EXPECT_EQ(Crypto::UnsignedBigInteger::from_base(16, "20000000000004"sv).to_double(Crypto::UnsignedBigInteger::RoundingMode::IEEERoundAndTiesToEvenMantissa),
+        9007199254740996.0);
+
+    // +5 rounds down because of tiesRoundToEven
+    EXPECT_EQ(Crypto::UnsignedBigInteger::from_base(16, "20000000000005"sv).to_double(Crypto::UnsignedBigInteger::RoundingMode::IEEERoundAndTiesToEvenMantissa),
+        9007199254740996.0);
+
+    EXPECT_EQ(Crypto::UnsignedBigInteger::from_base(16, "20000000000006"sv).to_double(Crypto::UnsignedBigInteger::RoundingMode::IEEERoundAndTiesToEvenMantissa),
+        9007199254740998.0);
+
+    EXPECT_EQ(Crypto::UnsignedBigInteger::from_base(10, "98382635059784269824"sv).to_double(Crypto::UnsignedBigInteger::RoundingMode::IEEERoundAndTiesToEvenMantissa),
+        bit_cast<double>(0x4415555555555555ULL));
 
 #undef EXPECT_TO_EQUAL_DOUBLE
 }