The Tor Browser: mfbt/double-conversion/bignum.cc@97036ab72558 (annotated)

mfbt/double-conversion/bignum.cc@97036ab72558 (annotated)

mfbt/double-conversion/bignum.cc

Tue, 06 Jan 2015 21:39:09 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Tue, 06 Jan 2015 21:39:09 +0100
branch: TOR_BUG_9701
changeset 8: 97036ab72558
permissions: -rw-r--r--

Conditionally force memory storage according to privacy.thirdparty.isolate;
This solves Tor bug #9701, complying with disk avoidance documented in
https://www.torproject.org/projects/torbrowser/design/#disk-avoidance.

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
 //       with the distribution.
 //     * Neither the name of Google Inc. nor the names of its
 //       contributors may be used to endorse or promote products derived
 //       from this software without specific prior written permission.
 //
 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include "bignum.h"
 #include "utils.h"
 namespace double_conversion {
 Bignum::Bignum()
     : bigits_(bigits_buffer_, kBigitCapacity), used_digits_(0), exponent_(0) {
   for (int i = 0; i < kBigitCapacity; ++i) {
     bigits_[i] = 0;
   }
 }
 template<typename S>
 static int BitSize(S value) {
   return 8 * sizeof(value);
 }
 // Guaranteed to lie in one Bigit.
 void Bignum::AssignUInt16(uint16_t value) {
   ASSERT(kBigitSize >= BitSize(value));
   Zero();
   if (value == 0) return;
   EnsureCapacity(1);
   bigits_[0] = value;
   used_digits_ = 1;
 }
 void Bignum::AssignUInt64(uint64_t value) {
   const int kUInt64Size = 64;
   Zero();
   if (value == 0) return;
   int needed_bigits = kUInt64Size / kBigitSize + 1;
   EnsureCapacity(needed_bigits);
   for (int i = 0; i < needed_bigits; ++i) {
     bigits_[i] = value & kBigitMask;
     value = value >> kBigitSize;
   }
   used_digits_ = needed_bigits;
   Clamp();
 }
 void Bignum::AssignBignum(const Bignum& other) {
   exponent_ = other.exponent_;
   for (int i = 0; i < other.used_digits_; ++i) {
     bigits_[i] = other.bigits_[i];
   }
   // Clear the excess digits (if there were any).
   for (int i = other.used_digits_; i < used_digits_; ++i) {
     bigits_[i] = 0;
   }
   used_digits_ = other.used_digits_;
 }
 static uint64_t ReadUInt64(Vector<const char> buffer,
                            int from,
                            int digits_to_read) {
   uint64_t result = 0;
   for (int i = from; i < from + digits_to_read; ++i) {
     int digit = buffer[i] - '0';
     ASSERT(0 <= digit && digit <= 9);
     result = result * 10 + digit;
   }
   return result;
 }
 void Bignum::AssignDecimalString(Vector<const char> value) {
   // 2^64 = 18446744073709551616 > 10^19
   const int kMaxUint64DecimalDigits = 19;
   Zero();
   int length = value.length();
   int pos = 0;
   // Let's just say that each digit needs 4 bits.
   while (length >= kMaxUint64DecimalDigits) {
     uint64_t digits = ReadUInt64(value, pos, kMaxUint64DecimalDigits);
     pos += kMaxUint64DecimalDigits;
     length -= kMaxUint64DecimalDigits;
     MultiplyByPowerOfTen(kMaxUint64DecimalDigits);
     AddUInt64(digits);
   }
   uint64_t digits = ReadUInt64(value, pos, length);
   MultiplyByPowerOfTen(length);
   AddUInt64(digits);
   Clamp();
 }
 static int HexCharValue(char c) {
   if ('0' <= c && c <= '9') return c - '0';
   if ('a' <= c && c <= 'f') return 10 + c - 'a';
   if ('A' <= c && c <= 'F') return 10 + c - 'A';
   UNREACHABLE();
   return 0;  // To make compiler happy.
 }
 void Bignum::AssignHexString(Vector<const char> value) {
   Zero();
   int length = value.length();
   int needed_bigits = length * 4 / kBigitSize + 1;
   EnsureCapacity(needed_bigits);
   int string_index = length - 1;
   for (int i = 0; i < needed_bigits - 1; ++i) {
     // These bigits are guaranteed to be "full".
     Chunk current_bigit = 0;
     for (int j = 0; j < kBigitSize / 4; j++) {
       current_bigit += HexCharValue(value[string_index--]) << (j * 4);
     }
     bigits_[i] = current_bigit;
   }
   used_digits_ = needed_bigits - 1;
   Chunk most_significant_bigit = 0;  // Could be = 0;
   for (int j = 0; j <= string_index; ++j) {
     most_significant_bigit <<= 4;
     most_significant_bigit += HexCharValue(value[j]);
   }
   if (most_significant_bigit != 0) {
     bigits_[used_digits_] = most_significant_bigit;
     used_digits_++;
   }
   Clamp();
 }
 void Bignum::AddUInt64(uint64_t operand) {
   if (operand == 0) return;
   Bignum other;
   other.AssignUInt64(operand);
   AddBignum(other);
 }
 void Bignum::AddBignum(const Bignum& other) {
   ASSERT(IsClamped());
   ASSERT(other.IsClamped());
   // If this has a greater exponent than other append zero-bigits to this.
   // After this call exponent_ <= other.exponent_.
   Align(other);
   // There are two possibilities:
   //   aaaaaaaaaaa 0000  (where the 0s represent a's exponent)
   //     bbbbb 00000000
   //   ----------------
   //   ccccccccccc 0000
   // or
   //    aaaaaaaaaa 0000
   //  bbbbbbbbb 0000000
   //  -----------------
   //  cccccccccccc 0000
   // In both cases we might need a carry bigit.
   EnsureCapacity(1 + Max(BigitLength(), other.BigitLength()) - exponent_);
   Chunk carry = 0;
   int bigit_pos = other.exponent_ - exponent_;
   ASSERT(bigit_pos >= 0);
   for (int i = 0; i < other.used_digits_; ++i) {
     Chunk sum = bigits_[bigit_pos] + other.bigits_[i] + carry;
     bigits_[bigit_pos] = sum & kBigitMask;
     carry = sum >> kBigitSize;
     bigit_pos++;
   }
   while (carry != 0) {
     Chunk sum = bigits_[bigit_pos] + carry;
     bigits_[bigit_pos] = sum & kBigitMask;
     carry = sum >> kBigitSize;
     bigit_pos++;
   }
   used_digits_ = Max(bigit_pos, used_digits_);
   ASSERT(IsClamped());
 }
 void Bignum::SubtractBignum(const Bignum& other) {
   ASSERT(IsClamped());
   ASSERT(other.IsClamped());
   // We require this to be bigger than other.
   ASSERT(LessEqual(other, *this));
   Align(other);
   int offset = other.exponent_ - exponent_;
   Chunk borrow = 0;
   int i;
   for (i = 0; i < other.used_digits_; ++i) {
     ASSERT((borrow == 0) || (borrow == 1));
     Chunk difference = bigits_[i + offset] - other.bigits_[i] - borrow;
     bigits_[i + offset] = difference & kBigitMask;
     borrow = difference >> (kChunkSize - 1);
   }
   while (borrow != 0) {
     Chunk difference = bigits_[i + offset] - borrow;
     bigits_[i + offset] = difference & kBigitMask;
     borrow = difference >> (kChunkSize - 1);
     ++i;
   }
   Clamp();
 }
 void Bignum::ShiftLeft(int shift_amount) {
   if (used_digits_ == 0) return;
   exponent_ += shift_amount / kBigitSize;
   int local_shift = shift_amount % kBigitSize;
   EnsureCapacity(used_digits_ + 1);
   BigitsShiftLeft(local_shift);
 }
 void Bignum::MultiplyByUInt32(uint32_t factor) {
   if (factor == 1) return;
   if (factor == 0) {
     Zero();
     return;
   }
   if (used_digits_ == 0) return;
   // The product of a bigit with the factor is of size kBigitSize + 32.
   // Assert that this number + 1 (for the carry) fits into double chunk.
   ASSERT(kDoubleChunkSize >= kBigitSize + 32 + 1);
   DoubleChunk carry = 0;
   for (int i = 0; i < used_digits_; ++i) {
     DoubleChunk product = static_cast<DoubleChunk>(factor) * bigits_[i] + carry;
     bigits_[i] = static_cast<Chunk>(product & kBigitMask);
     carry = (product >> kBigitSize);
   }
   while (carry != 0) {
     EnsureCapacity(used_digits_ + 1);
     bigits_[used_digits_] = carry & kBigitMask;
     used_digits_++;
     carry >>= kBigitSize;
   }
 }
 void Bignum::MultiplyByUInt64(uint64_t factor) {
   if (factor == 1) return;
   if (factor == 0) {
     Zero();
     return;
   }
   ASSERT(kBigitSize < 32);
   uint64_t carry = 0;
   uint64_t low = factor & 0xFFFFFFFF;
   uint64_t high = factor >> 32;
   for (int i = 0; i < used_digits_; ++i) {
     uint64_t product_low = low * bigits_[i];
     uint64_t product_high = high * bigits_[i];
     uint64_t tmp = (carry & kBigitMask) + product_low;
     bigits_[i] = tmp & kBigitMask;
     carry = (carry >> kBigitSize) + (tmp >> kBigitSize) +
         (product_high << (32 - kBigitSize));
   }
   while (carry != 0) {
     EnsureCapacity(used_digits_ + 1);
     bigits_[used_digits_] = carry & kBigitMask;
     used_digits_++;
     carry >>= kBigitSize;
   }
 }
 void Bignum::MultiplyByPowerOfTen(int exponent) {
   const uint64_t kFive27 = UINT64_2PART_C(0x6765c793, fa10079d);
   const uint16_t kFive1 = 5;
   const uint16_t kFive2 = kFive1 * 5;
   const uint16_t kFive3 = kFive2 * 5;
   const uint16_t kFive4 = kFive3 * 5;
   const uint16_t kFive5 = kFive4 * 5;
   const uint16_t kFive6 = kFive5 * 5;
   const uint32_t kFive7 = kFive6 * 5;
   const uint32_t kFive8 = kFive7 * 5;
   const uint32_t kFive9 = kFive8 * 5;
   const uint32_t kFive10 = kFive9 * 5;
   const uint32_t kFive11 = kFive10 * 5;
   const uint32_t kFive12 = kFive11 * 5;
   const uint32_t kFive13 = kFive12 * 5;
   const uint32_t kFive1_to_12[] =
       { kFive1, kFive2, kFive3, kFive4, kFive5, kFive6,
         kFive7, kFive8, kFive9, kFive10, kFive11, kFive12 };
   ASSERT(exponent >= 0);
   if (exponent == 0) return;
   if (used_digits_ == 0) return;
   // We shift by exponent at the end just before returning.
   int remaining_exponent = exponent;
   while (remaining_exponent >= 27) {
     MultiplyByUInt64(kFive27);
     remaining_exponent -= 27;
   }
   while (remaining_exponent >= 13) {
     MultiplyByUInt32(kFive13);
     remaining_exponent -= 13;
   }
   if (remaining_exponent > 0) {
     MultiplyByUInt32(kFive1_to_12[remaining_exponent - 1]);
   }
   ShiftLeft(exponent);
 }
 void Bignum::Square() {
   ASSERT(IsClamped());
   int product_length = 2 * used_digits_;
   EnsureCapacity(product_length);
   // Comba multiplication: compute each column separately.
   // Example: r = a2a1a0 * b2b1b0.
   //    r =  1    * a0b0 +
   //        10    * (a1b0 + a0b1) +
   //        100   * (a2b0 + a1b1 + a0b2) +
   //        1000  * (a2b1 + a1b2) +
   //        10000 * a2b2
   //
   // In the worst case we have to accumulate nb-digits products of digit*digit.
   //
   // Assert that the additional number of bits in a DoubleChunk are enough to
   // sum up used_digits of Bigit*Bigit.
   if ((1 << (2 * (kChunkSize - kBigitSize))) <= used_digits_) {
     UNIMPLEMENTED();
   }
   DoubleChunk accumulator = 0;
   // First shift the digits so we don't overwrite them.
   int copy_offset = used_digits_;
   for (int i = 0; i < used_digits_; ++i) {
     bigits_[copy_offset + i] = bigits_[i];
   }
   // We have two loops to avoid some 'if's in the loop.
   for (int i = 0; i < used_digits_; ++i) {
     // Process temporary digit i with power i.
     // The sum of the two indices must be equal to i.
     int bigit_index1 = i;
     int bigit_index2 = 0;
     // Sum all of the sub-products.
     while (bigit_index1 >= 0) {
       Chunk chunk1 = bigits_[copy_offset + bigit_index1];
       Chunk chunk2 = bigits_[copy_offset + bigit_index2];
       accumulator += static_cast<DoubleChunk>(chunk1) * chunk2;
       bigit_index1--;
       bigit_index2++;
     }
     bigits_[i] = static_cast<Chunk>(accumulator) & kBigitMask;
     accumulator >>= kBigitSize;
   }
   for (int i = used_digits_; i < product_length; ++i) {
     int bigit_index1 = used_digits_ - 1;
     int bigit_index2 = i - bigit_index1;
     // Invariant: sum of both indices is again equal to i.
     // Inner loop runs 0 times on last iteration, emptying accumulator.
     while (bigit_index2 < used_digits_) {
       Chunk chunk1 = bigits_[copy_offset + bigit_index1];
       Chunk chunk2 = bigits_[copy_offset + bigit_index2];
       accumulator += static_cast<DoubleChunk>(chunk1) * chunk2;
       bigit_index1--;
       bigit_index2++;
     }
     // The overwritten bigits_[i] will never be read in further loop iterations,
     // because bigit_index1 and bigit_index2 are always greater
     // than i - used_digits_.
     bigits_[i] = static_cast<Chunk>(accumulator) & kBigitMask;
     accumulator >>= kBigitSize;
   }
   // Since the result was guaranteed to lie inside the number the
   // accumulator must be 0 now.
   ASSERT(accumulator == 0);
   // Don't forget to update the used_digits and the exponent.
   used_digits_ = product_length;
   exponent_ *= 2;
   Clamp();
 }
 void Bignum::AssignPowerUInt16(uint16_t base, int power_exponent) {
   ASSERT(base != 0);
   ASSERT(power_exponent >= 0);
   if (power_exponent == 0) {
     AssignUInt16(1);
     return;
   }
   Zero();
   int shifts = 0;
   // We expect base to be in range 2-32, and most often to be 10.
   // It does not make much sense to implement different algorithms for counting
   // the bits.
   while ((base & 1) == 0) {
     base >>= 1;
     shifts++;
   }
   int bit_size = 0;
   int tmp_base = base;
   while (tmp_base != 0) {
     tmp_base >>= 1;
     bit_size++;
   }
   int final_size = bit_size * power_exponent;
   // 1 extra bigit for the shifting, and one for rounded final_size.
   EnsureCapacity(final_size / kBigitSize + 2);
   // Left to Right exponentiation.
   int mask = 1;
   while (power_exponent >= mask) mask <<= 1;
   // The mask is now pointing to the bit above the most significant 1-bit of
   // power_exponent.
   // Get rid of first 1-bit;
   mask >>= 2;
   uint64_t this_value = base;
   bool delayed_multipliciation = false;
   const uint64_t max_32bits = 0xFFFFFFFF;
   while (mask != 0 && this_value <= max_32bits) {
     this_value = this_value * this_value;
     // Verify that there is enough space in this_value to perform the
     // multiplication.  The first bit_size bits must be 0.
     if ((power_exponent & mask) != 0) {
       uint64_t base_bits_mask =
           ~((static_cast<uint64_t>(1) << (64 - bit_size)) - 1);
       bool high_bits_zero = (this_value & base_bits_mask) == 0;
       if (high_bits_zero) {
         this_value *= base;
       } else {
         delayed_multipliciation = true;
       }
     }
     mask >>= 1;
   }
   AssignUInt64(this_value);
   if (delayed_multipliciation) {
     MultiplyByUInt32(base);
   }
   // Now do the same thing as a bignum.
   while (mask != 0) {
     Square();
     if ((power_exponent & mask) != 0) {
       MultiplyByUInt32(base);
     }
     mask >>= 1;
   }
   // And finally add the saved shifts.
   ShiftLeft(shifts * power_exponent);
 }
 // Precondition: this/other < 16bit.
 uint16_t Bignum::DivideModuloIntBignum(const Bignum& other) {
   ASSERT(IsClamped());
   ASSERT(other.IsClamped());
   ASSERT(other.used_digits_ > 0);
   // Easy case: if we have less digits than the divisor than the result is 0.
   // Note: this handles the case where this == 0, too.
   if (BigitLength() < other.BigitLength()) {
     return 0;
   }
   Align(other);
   uint16_t result = 0;
   // Start by removing multiples of 'other' until both numbers have the same
   // number of digits.
   while (BigitLength() > other.BigitLength()) {
     // This naive approach is extremely inefficient if `this` divided by other
     // is big. This function is implemented for doubleToString where
     // the result should be small (less than 10).
     ASSERT(other.bigits_[other.used_digits_ - 1] >= ((1 << kBigitSize) / 16));
     // Remove the multiples of the first digit.
     // Example this = 23 and other equals 9. -> Remove 2 multiples.
     result += bigits_[used_digits_ - 1];
     SubtractTimes(other, bigits_[used_digits_ - 1]);
   }
   ASSERT(BigitLength() == other.BigitLength());
   // Both bignums are at the same length now.
   // Since other has more than 0 digits we know that the access to
   // bigits_[used_digits_ - 1] is safe.
   Chunk this_bigit = bigits_[used_digits_ - 1];
   Chunk other_bigit = other.bigits_[other.used_digits_ - 1];
   if (other.used_digits_ == 1) {
     // Shortcut for easy (and common) case.
     int quotient = this_bigit / other_bigit;
     bigits_[used_digits_ - 1] = this_bigit - other_bigit * quotient;
     result += quotient;
     Clamp();
     return result;
   }
   int division_estimate = this_bigit / (other_bigit + 1);
   result += division_estimate;
   SubtractTimes(other, division_estimate);
   if (other_bigit * (division_estimate + 1) > this_bigit) {
     // No need to even try to subtract. Even if other's remaining digits were 0
     // another subtraction would be too much.
     return result;
   }
   while (LessEqual(other, *this)) {
     SubtractBignum(other);
     result++;
   }
   return result;
 }
 template<typename S>
 static int SizeInHexChars(S number) {
   ASSERT(number > 0);
   int result = 0;
   while (number != 0) {
     number >>= 4;
     result++;
   }
   return result;
 }
 static char HexCharOfValue(int value) {
   ASSERT(0 <= value && value <= 16);
   if (value < 10) return value + '0';
   return value - 10 + 'A';
 }
 bool Bignum::ToHexString(char* buffer, int buffer_size) const {
   ASSERT(IsClamped());
   // Each bigit must be printable as separate hex-character.
   ASSERT(kBigitSize % 4 == 0);
   const int kHexCharsPerBigit = kBigitSize / 4;
   if (used_digits_ == 0) {
     if (buffer_size < 2) return false;
     buffer[0] = '0';
     buffer[1] = '\0';
     return true;
   }
   // We add 1 for the terminating '\0' character.
   int needed_chars = (BigitLength() - 1) * kHexCharsPerBigit +
       SizeInHexChars(bigits_[used_digits_ - 1]) + 1;
   if (needed_chars > buffer_size) return false;
   int string_index = needed_chars - 1;
   buffer[string_index--] = '\0';
   for (int i = 0; i < exponent_; ++i) {
     for (int j = 0; j < kHexCharsPerBigit; ++j) {
       buffer[string_index--] = '0';
     }
   }
   for (int i = 0; i < used_digits_ - 1; ++i) {
     Chunk current_bigit = bigits_[i];
     for (int j = 0; j < kHexCharsPerBigit; ++j) {
       buffer[string_index--] = HexCharOfValue(current_bigit & 0xF);
       current_bigit >>= 4;
     }
   }
   // And finally the last bigit.
   Chunk most_significant_bigit = bigits_[used_digits_ - 1];
   while (most_significant_bigit != 0) {
     buffer[string_index--] = HexCharOfValue(most_significant_bigit & 0xF);
     most_significant_bigit >>= 4;
   }
   return true;
 }
 Bignum::Chunk Bignum::BigitAt(int index) const {
   if (index >= BigitLength()) return 0;
   if (index < exponent_) return 0;
   return bigits_[index - exponent_];
 }
 int Bignum::Compare(const Bignum& a, const Bignum& b) {
   ASSERT(a.IsClamped());
   ASSERT(b.IsClamped());
   int bigit_length_a = a.BigitLength();
   int bigit_length_b = b.BigitLength();
   if (bigit_length_a < bigit_length_b) return -1;
   if (bigit_length_a > bigit_length_b) return +1;
   for (int i = bigit_length_a - 1; i >= Min(a.exponent_, b.exponent_); --i) {
     Chunk bigit_a = a.BigitAt(i);
     Chunk bigit_b = b.BigitAt(i);
     if (bigit_a < bigit_b) return -1;
     if (bigit_a > bigit_b) return +1;
     // Otherwise they are equal up to this digit. Try the next digit.
   }
   return 0;
 }
 int Bignum::PlusCompare(const Bignum& a, const Bignum& b, const Bignum& c) {
   ASSERT(a.IsClamped());
   ASSERT(b.IsClamped());
   ASSERT(c.IsClamped());
   if (a.BigitLength() < b.BigitLength()) {
     return PlusCompare(b, a, c);
   }
   if (a.BigitLength() + 1 < c.BigitLength()) return -1;
   if (a.BigitLength() > c.BigitLength()) return +1;
   // The exponent encodes 0-bigits. So if there are more 0-digits in 'a' than
   // 'b' has digits, then the bigit-length of 'a'+'b' must be equal to the one
   // of 'a'.
   if (a.exponent_ >= b.BigitLength() && a.BigitLength() < c.BigitLength()) {
     return -1;
   }
   Chunk borrow = 0;
   // Starting at min_exponent all digits are == 0. So no need to compare them.
   int min_exponent = Min(Min(a.exponent_, b.exponent_), c.exponent_);
   for (int i = c.BigitLength() - 1; i >= min_exponent; --i) {
     Chunk chunk_a = a.BigitAt(i);
     Chunk chunk_b = b.BigitAt(i);
     Chunk chunk_c = c.BigitAt(i);
     Chunk sum = chunk_a + chunk_b;
     if (sum > chunk_c + borrow) {
       return +1;
     } else {
       borrow = chunk_c + borrow - sum;
       if (borrow > 1) return -1;
       borrow <<= kBigitSize;
     }
   }
   if (borrow == 0) return 0;
   return -1;
 }
 void Bignum::Clamp() {
   while (used_digits_ > 0 && bigits_[used_digits_ - 1] == 0) {
     used_digits_--;
   }
   if (used_digits_ == 0) {
     // Zero.
     exponent_ = 0;
   }
 }
 bool Bignum::IsClamped() const {
   return used_digits_ == 0 || bigits_[used_digits_ - 1] != 0;
 }
 void Bignum::Zero() {
   for (int i = 0; i < used_digits_; ++i) {
     bigits_[i] = 0;
   }
   used_digits_ = 0;
   exponent_ = 0;
 }
 void Bignum::Align(const Bignum& other) {
   if (exponent_ > other.exponent_) {
     // If "X" represents a "hidden" digit (by the exponent) then we are in the
     // following case (a == this, b == other):
     // a:  aaaaaaXXXX   or a:   aaaaaXXX
     // b:     bbbbbbX      b: bbbbbbbbXX
     // We replace some of the hidden digits (X) of a with 0 digits.
     // a:  aaaaaa000X   or a:   aaaaa0XX
     int zero_digits = exponent_ - other.exponent_;
     EnsureCapacity(used_digits_ + zero_digits);
     for (int i = used_digits_ - 1; i >= 0; --i) {
       bigits_[i + zero_digits] = bigits_[i];
     }
     for (int i = 0; i < zero_digits; ++i) {
       bigits_[i] = 0;
     }
     used_digits_ += zero_digits;
     exponent_ -= zero_digits;
     ASSERT(used_digits_ >= 0);
     ASSERT(exponent_ >= 0);
   }
 }
 void Bignum::BigitsShiftLeft(int shift_amount) {
   ASSERT(shift_amount < kBigitSize);
   ASSERT(shift_amount >= 0);
   Chunk carry = 0;
   for (int i = 0; i < used_digits_; ++i) {
     Chunk new_carry = bigits_[i] >> (kBigitSize - shift_amount);
     bigits_[i] = ((bigits_[i] << shift_amount) + carry) & kBigitMask;
     carry = new_carry;
   }
   if (carry != 0) {
     bigits_[used_digits_] = carry;
     used_digits_++;
   }
 }
 void Bignum::SubtractTimes(const Bignum& other, int factor) {
   ASSERT(exponent_ <= other.exponent_);
   if (factor < 3) {
     for (int i = 0; i < factor; ++i) {
       SubtractBignum(other);
     }
     return;
   }
   Chunk borrow = 0;
   int exponent_diff = other.exponent_ - exponent_;
   for (int i = 0; i < other.used_digits_; ++i) {
     DoubleChunk product = static_cast<DoubleChunk>(factor) * other.bigits_[i];
     DoubleChunk remove = borrow + product;
     Chunk difference = bigits_[i + exponent_diff] - (remove & kBigitMask);
     bigits_[i + exponent_diff] = difference & kBigitMask;
     borrow = static_cast<Chunk>((difference >> (kChunkSize - 1)) +
                                 (remove >> kBigitSize));
   }
   for (int i = other.used_digits_ + exponent_diff; i < used_digits_; ++i) {
     if (borrow == 0) return;
     Chunk difference = bigits_[i] - borrow;
     bigits_[i] = difference & kBigitMask;
     borrow = difference >> (kChunkSize - 1);
   }
   Clamp();
 }
 }  // namespace double_conversion

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mfbt/double-conversion/bignum.cc@97036ab72558 (annotated)

mfbt/double-conversion/bignum.cc