public SegmentIntArrayIndexer(SegmentIntArray array)
{
_array = array;
_currentSegment = 0;
_currentBit = 0;
if (_array._segType.Length > 0)
{
_currentIndex = 0;
_currentType = _array._segType[0];
_nextIndex = _array._segLength[0];
}
else
{
// Handle the edge case where we have a completely empty array.
_currentIndex = _array.Length;
_currentType = 0;
_nextIndex = _currentIndex;
}
}
/// <summary>
/// Finds the most space efficient representation of the feature
/// (with slight slack cut for dense features). The behavior of
/// this method depends upon the static value <see cref="CompatibilityLevel"/>.
/// </summary>
/// <param name="workarray">Should be non-null if you want it to
/// consider segment arrays.</param>
/// <returns>Returns a more space efficient version of the array,
/// or the item itself if that is impossible, somehow.</returns>
public IntArray Compress(uint[] workarray = null)
{
int maxval = 0;
int zerocount = 0;
int runs = 0;
int last = -1;
int overflows = 0;
int zoverflows = 0;
int runnow = 0; // The longest run of having the same value.
int len = Length;
IIntArrayForwardIndexer ind = GetIndexer();
for (int i = 0; i < len; ++i)
{
int val = ind[i];
if (workarray != null)
{
workarray[i] = (uint)val;
}
if (val == 0)
{
zerocount++;
}
else if (val > maxval)
{
maxval = val;
}
if (last == val)
{
runs++;
if (++runnow > byte.MaxValue)
{
// We have 256 items in a row the same.
overflows++;
if (val == 0)
{
zoverflows++;
}
runnow = 0;
}
}
last = val;
}
// Estimate the costs of the available options.
IntArrayBits classicBits = IntArray.NumBitsNeeded(maxval + 1);
long denseBits = (long)classicBits * (long)Length;
long sparseBits = (long)(Math.Max((int)classicBits, 8) + 8) * (long)(Length - zerocount + zoverflows);
long rleBits = (long)(classicBits + 8) * (long)(Length - runs + overflows);
long segBits = long.MaxValue;
int segTransitions = 0;
if (workarray != null)
{
int bits = SegmentIntArray.BitsForValue((uint)maxval);
if (bits <= 21)
{
SegmentIntArray.SegmentFindOptimalPath(workarray, Length,
bits, out segBits, out segTransitions);
}
}
if ((IntArray.CompatibilityLevel & 0x4) == 0)
{
rleBits = long.MaxValue;
}
long bestCost = Math.Min(Math.Min(Math.Min(denseBits, sparseBits), rleBits), segBits);
IntArrayType bestType = IntArrayType.Dense;
if (bestCost >= denseBits * 98 / 100)
{
// Cut the dense bits a wee bit of slack.
}
else if (bestCost == sparseBits)
{
bestType = IntArrayType.Sparse;
}
else if (bestCost == rleBits)
{
bestType = IntArrayType.Repeat;
}
else
{
bestType = IntArrayType.Segmented;
}
if (bestType == Type && classicBits == BitsPerItem)
{
return(this);
}
IntArray bins = null;
if (bestType != IntArrayType.Segmented)
{
bins = IntArray.New(Length, bestType, classicBits, this);
}
else
{
bins = SegmentIntArray.FromWorkArray(workarray, Length, segBits, segTransitions);
}
return(bins);
}
