/// <summary>
/// Find the start position of the desired item index.
/// If the node knows it, the known value is returned.
/// If not, the code seeks to the nearest node and reads enough of the document to find it.
/// </summary>
/// <param name="index">Index of array element to find</param>
/// <param name="inputStreamProvider">Function which can open original file, if needed</param>
/// <returns>Absolute byte offset of the start array[index] within this array</returns>
public long FindArrayStart(int index, Func <Stream> inputStreamProvider)
{
if (index < 0 || index > this.Count)
{
throw new ArgumentOutOfRangeException("index");
}
if (index == this.Count)
{
// If item after last is requested, return array end
return(this.End);
}
else if (this.ArrayStarts != null && this.Every == 1)
{
// If we know every element position, just return it
return(this.ArrayStarts[index]);
}
else if (index % this.Every == 0)
{
// If we know this element position, just return it
return(this.ArrayStarts[index / this.Every]);
}
// Otherwise, find the closest span of the file we must read
long readFromPosition;
long readToPosition;
int startIndex;
if (this.ArrayStarts == null)
{
// If there are no array positions, we must read the whole array (it should be small)
readFromPosition = this.Start + 1;
readToPosition = this.End;
startIndex = 0;
}
else
{
// If there are array positions, read from the nearest previous element available
int startToRead = index / this.Every;
readFromPosition = this.ArrayStarts[startToRead];
readToPosition = (this.ArrayStarts.Count > startToRead + 1 ? this.ArrayStarts[startToRead + 1] : this.End);
startIndex = startToRead * this.Every;
}
using (Stream source = inputStreamProvider())
{
int lengthToRead = (int)(1 + readToPosition - readFromPosition);
byte[] buffer = new byte[lengthToRead + 1];
// Read the array slice
source.Seek(readFromPosition, SeekOrigin.Begin);
source.Read(buffer, 1, lengthToRead);
// Make it a valid array prefix (it must start with '[', which will look like the root of the Json document
buffer[0] = (byte)'[';
using (JsonPositionedTextReader reader = new JsonPositionedTextReader(() => new MemoryStream(buffer)))
{
// Find the desired array item index in the buffer
long relativePosition = reader.ReadToArrayIndex(index - startIndex);
// Convert back to an absolute position (buffer[0] was (readFromPosition - 1)
return((readFromPosition - 1) + relativePosition);
}
}
}
