[bigint] Implement .toString for power-of-2 radixes

Other radixes require Divide/Remainder to be implemented first.

Bug: v8:6791
Change-Id: I95f1fad39a0a4df556a194094805ed93bd46d0db
Reviewed-on: https://chromium-review.googlesource.com/664037
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48020}

src/objects/bigint.cc

@@ -81,14 +81,9 @@ Handle<BigInt> BigInt::BitwiseOr(Handle<BigInt> x, Handle<BigInt> y) {
 }
 
 Handle<String> BigInt::ToString(Handle<BigInt> bigint, int radix) {
-  // TODO(jkummerow): Implement this properly.
-  Factory* factory = bigint->GetIsolate()->factory();
-  if (bigint->is_zero()) return factory->NewStringFromStaticChars("0");
-  DCHECK(bigint->length() == 1);
-  int value = static_cast<uint32_t>(bigint->digit(0));
-  if (bigint->sign()) value = -value;  // This can overflow. We don't care.
-  Handle<Object> number = factory->NewNumberFromInt(value);
-  return factory->NumberToString(number);
+  // TODO(jkummerow): Support non-power-of-two radixes.
+  if (!base::bits::IsPowerOfTwo(radix)) radix = 16;
+  return bigint->ToStringBasePowerOfTwo(bigint, radix);
 }
 
 void BigInt::Initialize(int length, bool zero_initialize) {
@@ -107,6 +102,55 @@ void BigInt::Initialize(int length, bool zero_initialize) {
   }
 }
 
+// Private helpers for public methods.
+
+static const char kConversionChars[] = "0123456789abcdefghijklmnopqrstuvwxyz";
+
+// TODO(jkummerow): Add more tests for this when it is exposed on
+// BigInt.prototype.
+Handle<String> BigInt::ToStringBasePowerOfTwo(Handle<BigInt> x, int radix) {
+  STATIC_ASSERT(base::bits::IsPowerOfTwo(kDigitBits));
+  DCHECK(base::bits::IsPowerOfTwo(radix));
+  DCHECK(radix >= 2 && radix <= 32);
+  Factory* factory = x->GetIsolate()->factory();
+  // TODO(jkummerow): check in caller?
+  if (is_zero()) return factory->NewStringFromStaticChars("0");
+
+  const int len = x->length();
+  const bool sign = x->sign();
+  const int bits_per_char = base::bits::CountTrailingZeros32(radix);
+  const int char_mask = radix - 1;
+  const int chars_per_digit = kDigitBits / bits_per_char;
+  // Compute the number of chars needed to represent the most significant
+  // bigint digit.
+  int chars_for_msd = 0;
+  for (digit_t msd = x->digit(len - 1); msd != 0; msd >>= bits_per_char) {
+    chars_for_msd++;
+  }
+  // All other digits need chars_per_digit characters; a leading "-" needs one.
+  const int chars = chars_for_msd + (len - 1) * chars_per_digit + sign;
+
+  Handle<SeqOneByteString> result =
+      factory->NewRawOneByteString(chars).ToHandleChecked();
+  uint8_t* buffer = result->GetChars();
+  // Print the number into the string, starting from the last position.
+  int pos = chars - 1;
+  for (int i = 0; i < len - 1; i++) {
+    digit_t digit = x->digit(i);
+    for (int j = 0; j < chars_per_digit; j++) {
+      buffer[pos--] = kConversionChars[digit & char_mask];
+      digit >>= bits_per_char;
+    }
+  }
+  // Print the most significant digit.
+  for (digit_t msd = x->digit(len - 1); msd != 0; msd >>= bits_per_char) {
+    buffer[pos--] = kConversionChars[msd & char_mask];
+  }
+  if (sign) buffer[pos--] = '-';
+  DCHECK(pos == -1);
+  return result;
+}
+
 #ifdef OBJECT_PRINT
 void BigInt::BigIntPrint(std::ostream& os) {
   DisallowHeapAllocation no_gc;

src/objects/bigint.h

@@ -88,6 +88,9 @@ class BigInt : public HeapObject {
   static const int kHalfDigitBits = kDigitBits / 2;
   static const digit_t kHalfDigitMask = (1ull << kHalfDigitBits) - 1;
 
+  // Private helpers for public methods.
+  Handle<String> ToStringBasePowerOfTwo(Handle<BigInt> x, int radix);
+
   class LengthBits : public BitField<int, 0, kMaxLengthBits> {};
   class SignBits : public BitField<bool, LengthBits::kNext, 1> {};