private StringId AddStringToBuffer <T>(T seg) where T : struct, ICharSpan <T>
{
Contract.Requires(IsValid());
Contract.Ensures(Contract.Result <StringId>().IsValid);
// Not in the set, so get some space to hold the string
//
// Note that this call is invoked in a racy setting. It's possible for multiple calls to execute concurrently and
// try to insert the same string N times. We allow this and end up just wasting space in the table when this happens.
// In this context, m_count thus accounts for the total number of strings inserted into the table, and not the potential
// smaller of logical strings known to the table.
bool longString = seg.Length >= 255;
int space = 0;
// see if the string is pure ASCII
bool isAscii = seg.OnlyContains8BitChars;
if (isAscii)
{
// stored as bytes
space += seg.Length;
}
else
{
// stored as UTF-16
space += (seg.Length * 2) + 1; // *2 for UTF-16, +1 for the 'switch to unicode' marker byte
}
// count how many strings are in the buffer
Interlocked.Increment(ref m_count);
if (space > s_largeStringBufferThreshold && m_largeStringBuffer.TryReserveSlot(out var largeStringIndex))
{
var buffer = new byte[space];
WriteBytes(seg, isAscii, buffer, 0);
m_largeStringBuffer[largeStringIndex] = buffer;
return(ComputeStringId(LargeStringBufferNum, offset: largeStringIndex));
}
// if the length >= 255 then we store it as a marker byte followed by a 4-byte length value
space += longString ? 5 : 1;
int byteIndex;
int bufferNum;
// loop until we find a suitable location to copy the string to
while (true)
{
// get the next possible location for the string
int current = Volatile.Read(ref m_nextId);
bufferNum = (current >> BytesPerBufferBits) & NumByteBuffersMask;
byteIndex = current & BytesPerBufferMask;
// is the available space big enough?
if (space < BytesPerBuffer - byteIndex)
{
// there's room in the buffer so try to claim it
int next = (bufferNum << BytesPerBufferBits) | (byteIndex + space);
if (Interlocked.CompareExchange(ref m_nextId, next, current) == current)
{
// got some room, now go fill it in
break;
}
// go try again...
continue;
}
// the string doesn't fit in the current buffer, we need a new buffer
int newBufferSize = BytesPerBuffer;
if (space > BytesPerBuffer)
{
newBufferSize = space;
}
bufferNum++;
// Make sure we don't overflow the buffer we're indexing into
if (bufferNum >= NumByteBuffers)
{
Contract.Assert(false, $"Exceeded the number of ByteBuffers allowed in this StringTable: {bufferNum} >= {NumByteBuffers}");
}
lock (m_byteBuffers)
{
if (m_byteBuffers[bufferNum] != null)
{
// somebody racily beat us and allocated this buffer, so just retry the whole thing from scratch
continue;
}
// allocate a new buffer
m_byteBuffers[bufferNum] = new byte[newBufferSize];
if (space >= BytesPerBuffer)
{
// force writing into a fresh buffer
Volatile.Write(ref m_nextId, (bufferNum << BytesPerBufferBits) | BytesPerBufferMask);
}
else
{
// force writing into this new buffer
Volatile.Write(ref m_nextId, (bufferNum << BytesPerBufferBits) | space);
}
// go write the string at the base of the new buffer
byteIndex = 0;
break;
}
}
var stringId = ComputeStringId(bufferNum, offset: byteIndex);
// now copy the string data into the buffer
byte[] currentBuffer = m_byteBuffers[bufferNum];
if (longString)
{
currentBuffer[byteIndex++] = LongStringMarker;
Bits.WriteInt32(currentBuffer, ref byteIndex, seg.Length);
}
else
{
currentBuffer[byteIndex++] = (byte)seg.Length;
}
WriteBytes(seg, isAscii, currentBuffer, byteIndex);
return(stringId);
}