LibCrypto: Add naive implementation of {Un,}SignedBigInteger::to_double

Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.cpp

@@ -63,6 +63,14 @@ u64 SignedBigInteger::to_u64() const
     return ~(unsigned_value - 1); // equivalent to `-unsigned_value`, but doesnt trigger UBSAN
 }
 
+double SignedBigInteger::to_double() const
+{
+    double unsigned_value = m_unsigned_data.to_double();
+    if (!m_sign)
+        return unsigned_value;
+    return -unsigned_value;
+}
+
 FLATTEN SignedBigInteger SignedBigInteger::plus(const SignedBigInteger& other) const
 {
     // If both are of the same sign, just add the unsigned data and return.

Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.h

@@ -66,6 +66,7 @@ public:
     String to_base(u16 N) const;
 
     u64 to_u64() const;
+    double to_double() const;
 
     const UnsignedBigInteger& unsigned_value() const { return m_unsigned_data; }
     const Vector<u32, STARTING_WORD_SIZE> words() const { return m_unsigned_data.words(); }

Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.cpp

@@ -112,6 +112,12 @@ u64 UnsignedBigInteger::to_u64() const
     return value;
 }
 
+double UnsignedBigInteger::to_double() const
+{
+    // FIXME: I am naive
+    return static_cast<double>(to_u64());
+}
+
 void UnsignedBigInteger::set_to_0()
 {
     m_words.clear_with_capacity();

Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.h

@@ -57,6 +57,7 @@ public:
     String to_base(u16 N) const;
 
     u64 to_u64() const;
+    double to_double() const;
 
     const Vector<Word, STARTING_WORD_SIZE>& words() const { return m_words; }