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BitStack.cs
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BitStack.cs
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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
using System.Diagnostics;
using System.Runtime.CompilerServices;
namespace System.Text.Json
{
internal struct BitStack
{
// We are using a ulong to represent our nested state, so we can only
// go 64 levels deep without having to allocate.
private const int AllocationFreeMaxDepth = sizeof(ulong) * 8;
private const int DefaultInitialArraySize = 2;
private int[]? _array;
// This ulong container represents a tiny stack to track the state during nested transitions.
// The first bit represents the state of the current depth (1 == object, 0 == array).
// Each subsequent bit is the parent / containing type (object or array). Since this
// reader does a linear scan, we only need to keep a single path as we go through the data.
// This is primarily used as an optimization to avoid having to allocate an object for
// depths up to 64 (which is the default max depth).
private ulong _allocationFreeContainer;
private int _currentDepth;
public int CurrentDepth => _currentDepth;
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public void PushTrue()
{
if (_currentDepth < AllocationFreeMaxDepth)
{
_allocationFreeContainer = (_allocationFreeContainer << 1) | 1;
}
else
{
PushToArray(true);
}
_currentDepth++;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public void PushFalse()
{
if (_currentDepth < AllocationFreeMaxDepth)
{
_allocationFreeContainer <<= 1;
}
else
{
PushToArray(false);
}
_currentDepth++;
}
// Allocate the bit array lazily only when it is absolutely necessary
[MethodImpl(MethodImplOptions.NoInlining)]
private void PushToArray(bool value)
{
_array ??= new int[DefaultInitialArraySize];
int index = _currentDepth - AllocationFreeMaxDepth;
Debug.Assert(index >= 0, $"Set - Negative - index: {index}, arrayLength: {_array.Length}");
// Maximum possible array length if bitLength was int.MaxValue (i.e. 67_108_864)
Debug.Assert(_array.Length <= int.MaxValue / 32 + 1, $"index: {index}, arrayLength: {_array.Length}");
int elementIndex = Div32Rem(index, out int extraBits);
// Grow the array when setting a bit if it isn't big enough
// This way the caller doesn't have to check.
if (elementIndex >= _array.Length)
{
// This multiplication can overflow, so cast to uint first.
Debug.Assert(index >= 0 && index > (int)((uint)_array.Length * 32 - 1), $"Only grow when necessary - index: {index}, arrayLength: {_array.Length}");
DoubleArray(elementIndex);
}
Debug.Assert(elementIndex < _array.Length, $"Set - index: {index}, elementIndex: {elementIndex}, arrayLength: {_array.Length}, extraBits: {extraBits}");
int newValue = _array[elementIndex];
if (value)
{
newValue |= 1 << extraBits;
}
else
{
newValue &= ~(1 << extraBits);
}
_array[elementIndex] = newValue;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public bool Pop()
{
_currentDepth--;
bool inObject;
if (_currentDepth < AllocationFreeMaxDepth)
{
_allocationFreeContainer >>= 1;
inObject = (_allocationFreeContainer & 1) != 0;
}
else if (_currentDepth == AllocationFreeMaxDepth)
{
inObject = (_allocationFreeContainer & 1) != 0;
}
else
{
inObject = PopFromArray();
}
return inObject;
}
[MethodImpl(MethodImplOptions.NoInlining)]
private bool PopFromArray()
{
int index = _currentDepth - AllocationFreeMaxDepth - 1;
Debug.Assert(_array != null);
Debug.Assert(index >= 0, $"Get - Negative - index: {index}, arrayLength: {_array.Length}");
int elementIndex = Div32Rem(index, out int extraBits);
Debug.Assert(elementIndex < _array.Length, $"Get - index: {index}, elementIndex: {elementIndex}, arrayLength: {_array.Length}, extraBits: {extraBits}");
return (_array[elementIndex] & (1 << extraBits)) != 0;
}
private void DoubleArray(int minSize)
{
Debug.Assert(_array != null);
Debug.Assert(_array.Length < int.MaxValue / 2, $"Array too large - arrayLength: {_array.Length}");
Debug.Assert(minSize >= 0 && minSize >= _array.Length);
int nextDouble = Math.Max(minSize + 1, _array.Length * 2);
Debug.Assert(nextDouble > minSize);
Array.Resize(ref _array, nextDouble);
}
public void SetFirstBit()
{
Debug.Assert(_currentDepth == 0, "Only call SetFirstBit when depth is 0");
_currentDepth++;
_allocationFreeContainer = 1;
}
public void ResetFirstBit()
{
Debug.Assert(_currentDepth == 0, "Only call ResetFirstBit when depth is 0");
_currentDepth++;
_allocationFreeContainer = 0;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static int Div32Rem(int number, out int remainder)
{
uint quotient = (uint)number / 32;
remainder = number & (32 - 1); // equivalent to number % 32, since 32 is a power of 2
return (int)quotient;
}
}
}