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+// 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 the this divided other
+ // might be 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);
+ ++i;
+ }
+ Clamp();
+}
+
+
+} // namespace double_conversion
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