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BigInteger parsing optimization for large decimal string #51953

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BigInteger parsing optimization for large decimal string #51953

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e93dde5
implement divide-and-conquer method for parsing digits
key-moon Apr 27, 2021
2fdbe45
fix argument order in Assert when x equals to 0
key-moon Apr 27, 2021
7d89c46
Apply format fix
key-moon May 26, 2021
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4 changes: 2 additions & 2 deletions src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.SquMul.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,7 @@ public static unsafe uint[] Square(uint[] value)
private static int SquareThreshold = 32;
private static int AllocationThreshold = 256;

private static unsafe void Square(uint* value, int valueLength,
internal static unsafe void Square(uint* value, int valueLength,
uint* bits, int bitsLength)
{
Debug.Assert(valueLength >= 0);
Expand Down Expand Up @@ -208,7 +208,7 @@ public static unsafe uint[] Multiply(uint[] left, uint[] right)
// Mutable for unit testing...
private static int MultiplyThreshold = 32;

private static unsafe void Multiply(uint* left, int leftLength,
internal static unsafe void Multiply(uint* left, int leftLength,
uint* right, int rightLength,
uint* bits, int bitsLength)
{
Expand Down
284 changes: 216 additions & 68 deletions src/libraries/System.Runtime.Numerics/src/System/Numerics/BigNumber.cs
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Expand Up @@ -494,35 +494,242 @@ private static bool HexNumberToBigInteger(ref BigNumberBuffer number, out BigInt
}
}

private static int s_naiveThreshold = 20000;
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Nit:

private const int NaiveThreshold = 20_000;

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Also, it'd be helpful to add a comment about how this value was selected.

private static bool NumberToBigInteger(ref BigNumberBuffer number, out BigInteger result)
{
Span<uint> stackBuffer = stackalloc uint[BigInteger.StackallocUInt32Limit];

Span<uint> currentBuffer = stackBuffer;
int currentBufferSize = 0;

int[]? arrayFromPool = null;

uint partialValue = 0;
int partialDigitCount = 0;
int totalDigitCount = 0;
int numberScale = number.scale;

const int MaxPartialDigits = 9;
const uint TenPowMaxPartial = 1000000000;

try
{
foreach (ReadOnlyMemory<char> digitsChunk in number.digits.GetChunks())
if (number.digits.Length <= s_naiveThreshold)
{
if (!ProcessChunk(digitsChunk.Span, ref currentBuffer))
uint partialValue = 0;
int partialDigitCount = 0;

foreach (ReadOnlyMemory<char> digitsChunk in number.digits.GetChunks())
{
if (!ProcessChunk(digitsChunk.Span, ref currentBuffer))
{
result = default;
return false;
}
}

if (partialDigitCount > 0)
{
MultiplyAdd(ref currentBuffer, s_uint32PowersOfTen[partialDigitCount], partialValue);
}

bool ProcessChunk(ReadOnlySpan<char> chunkDigits, ref Span<uint> currentBuffer)
{
int remainingIntDigitCount = Math.Max(numberScale - totalDigitCount, 0);
ReadOnlySpan<char> intDigitsSpan = chunkDigits.Slice(0, Math.Min(remainingIntDigitCount, chunkDigits.Length));

bool endReached = false;

// Storing these captured variables in locals for faster access in the loop.
uint _partialValue = partialValue;
int _partialDigitCount = partialDigitCount;
int _totalDigitCount = totalDigitCount;

for (int i = 0; i < intDigitsSpan.Length; i++)
{
char digitChar = chunkDigits[i];
if (digitChar == '\0')
{
endReached = true;
break;
}

_partialValue = _partialValue * 10 + (uint)(digitChar - '0');
_partialDigitCount++;
_totalDigitCount++;

// Update the buffer when enough partial digits have been accumulated.
if (_partialDigitCount == MaxPartialDigits)
{
MultiplyAdd(ref currentBuffer, TenPowMaxPartial, _partialValue);
_partialValue = 0;
_partialDigitCount = 0;
}
}

// Check for nonzero digits after the decimal point.
if (!endReached)
{
ReadOnlySpan<char> fracDigitsSpan = chunkDigits.Slice(intDigitsSpan.Length);
for (int i = 0; i < fracDigitsSpan.Length; i++)
{
char digitChar = fracDigitsSpan[i];
if (digitChar == '\0')
{
break;
}
if (digitChar != '0')
{
return false;
}
}
}

partialValue = _partialValue;
partialDigitCount = _partialDigitCount;
totalDigitCount = _totalDigitCount;

return true;
}
}
else
{
if (numberScale < 0)
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Just adding a note to myself: This is the real change in the PR. The block above is just refactoring inside an if. I'll review this PR this week. It's taking some time since I need to go through the algorithm first and then review the implementation here.

{
result = default;
return false;
}
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}
totalDigitCount = Math.Min(number.digits.Length - 1, numberScale);
var bufferSize = (totalDigitCount + MaxPartialDigits - 1) / MaxPartialDigits;
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if (partialDigitCount > 0)
{
MultiplyAdd(ref currentBuffer, s_uint32PowersOfTen[partialDigitCount], partialValue);
Span<uint> buffer = new uint[bufferSize];

int bufferInd = buffer.Length - 1;
uint currentBlock = 0;
int shiftUntil = (totalDigitCount - 1) % MaxPartialDigits;
int remainingIntDigitCount = totalDigitCount;
foreach (ReadOnlyMemory<char> digitsChunk in number.digits.GetChunks())
{
var digitsChunkSpan = digitsChunk.Span;
ReadOnlySpan<char> intDigitsSpan = digitsChunkSpan.Slice(0, Math.Min(remainingIntDigitCount, digitsChunkSpan.Length));

for (int i = 0; i < intDigitsSpan.Length; i++)
{
char digitChar = intDigitsSpan[i];
Debug.Assert(char.IsDigit(digitChar));
currentBlock = currentBlock * 10 + digitChar - '0';
if (shiftUntil == 0)
{
buffer[bufferInd] = currentBlock;
currentBlock = 0;
bufferInd--;
shiftUntil = MaxPartialDigits;
}
shiftUntil--;
}
remainingIntDigitCount -= intDigitsSpan.Length;

ReadOnlySpan<char> fracDigitsSpan = digitsChunkSpan.Slice(intDigitsSpan.Length);
for (int i = 0; i < fracDigitsSpan.Length; i++)
{
char digitChar = fracDigitsSpan[i];
if (digitChar == '\0')
{
break;
}
if (digitChar != '0')
{
result = default;
return false;
}
}
}
Debug.Assert(bufferInd == -1);

unsafe
{
Span<uint> newBuffer = new uint[bufferSize];

arrayFromPool = ArrayPool<int>.Shared.Rent(1);
Span<uint> multiplier = MemoryMarshal.Cast<int, uint>(arrayFromPool);
multiplier[0] = TenPowMaxPartial;

int blockSize = 1;
while (true)
{
fixed (uint* bufPtr = buffer, newBufPtr = newBuffer, mulPtr = multiplier)
{
uint* curBufPtr = bufPtr;
uint* curNewBufPtr = newBufPtr;
for (int i = 0; i < bufferSize; i += blockSize * 2)
{
int len = Math.Min(bufferSize - i, blockSize * 2);
int lowerLen = Math.Min(len, blockSize);
int upperLen = len - lowerLen;
if (upperLen != 0)
{
BigIntegerCalculator.Multiply(mulPtr, blockSize, curBufPtr + blockSize, upperLen, curNewBufPtr, len);
}

long carry = 0;
int j = 0;
for (; j < lowerLen; j++)
{
long digit = (curBufPtr[j] + carry) + curNewBufPtr[j];
curNewBufPtr[j] = unchecked((uint)digit);
carry = digit >> 32;
}
if (carry != 0)
{
while (true)
{
curNewBufPtr[j]++;
if (curNewBufPtr[j] != 0)
{
break;
}
j++;
}
}

curBufPtr += blockSize * 2;
curNewBufPtr += blockSize * 2;
}
}

// swap
var tmp = buffer;
buffer = newBuffer;
newBuffer = tmp;
blockSize *= 2;

if (bufferSize <= blockSize)
{
break;
}
newBuffer.Clear();
int[]? arrayToReturn = arrayFromPool;

arrayFromPool = ArrayPool<int>.Shared.Rent(blockSize);
Span<uint> newMultiplier = MemoryMarshal.Cast<int, uint>(arrayFromPool);
newMultiplier.Clear();
fixed (uint* mulPtr = multiplier, newMulPtr = newMultiplier)
{
BigIntegerCalculator.Square(mulPtr, blockSize / 2, newMulPtr, blockSize);
}
multiplier = newMultiplier;
if (arrayToReturn is not null)
{
ArrayPool<int>.Shared.Return(arrayToReturn);
}
}
}

// = log_{2^32}(10^9)
const double digitRatio = 0.934292276687070661;
currentBufferSize = Math.Min((int)(bufferSize * digitRatio) + 1, bufferSize);
while (buffer[currentBufferSize - 1] == 0)
{
currentBufferSize--;
}
currentBuffer = buffer.Slice(0, currentBufferSize);
}

int trailingZeroCount = numberScale - totalDigitCount;
Expand Down Expand Up @@ -568,65 +775,6 @@ private static bool NumberToBigInteger(ref BigNumberBuffer number, out BigIntege
}
}

bool ProcessChunk(ReadOnlySpan<char> chunkDigits, ref Span<uint> currentBuffer)
{
int remainingIntDigitCount = Math.Max(numberScale - totalDigitCount, 0);
ReadOnlySpan<char> intDigitsSpan = chunkDigits.Slice(0, Math.Min(remainingIntDigitCount, chunkDigits.Length));

bool endReached = false;

// Storing these captured variables in locals for faster access in the loop.
uint _partialValue = partialValue;
int _partialDigitCount = partialDigitCount;
int _totalDigitCount = totalDigitCount;

for (int i = 0; i < intDigitsSpan.Length; i++)
{
char digitChar = chunkDigits[i];
if (digitChar == '\0')
{
endReached = true;
break;
}

_partialValue = _partialValue * 10 + (uint)(digitChar - '0');
_partialDigitCount++;
_totalDigitCount++;

// Update the buffer when enough partial digits have been accumulated.
if (_partialDigitCount == MaxPartialDigits)
{
MultiplyAdd(ref currentBuffer, TenPowMaxPartial, _partialValue);
_partialValue = 0;
_partialDigitCount = 0;
}
}

// Check for nonzero digits after the decimal point.
if (!endReached)
{
ReadOnlySpan<char> fracDigitsSpan = chunkDigits.Slice(intDigitsSpan.Length);
for (int i = 0; i < fracDigitsSpan.Length; i++)
{
char digitChar = fracDigitsSpan[i];
if (digitChar == '\0')
{
break;
}
if (digitChar != '0')
{
return false;
}
}
}

partialValue = _partialValue;
partialDigitCount = _partialDigitCount;
totalDigitCount = _totalDigitCount;

return true;
}

void MultiplyAdd(ref Span<uint> currentBuffer, uint multiplier, uint addValue)
{
Span<uint> curBits = currentBuffer.Slice(0, currentBufferSize);
Expand Down
8 changes: 4 additions & 4 deletions src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -856,9 +856,10 @@ private static void Eval(BigInteger x, string expected)
x = -x;
}

string actual;
if (x == 0)
{
Assert.Equal("0", expected);
actual = "0";
}
else
{
Expand All @@ -869,10 +870,9 @@ private static void Eval(BigInteger x, string expected)
x = x / 10;
}
number.Reverse();
string actual = new string(number.ToArray());

Assert.Equal(expected, actual);
actual = new string(number.ToArray());
}
Assert.Equal(expected, actual);
}
}
}