/// <summary>
/// Choose a random sample of approximately 1/8 of the rows in an ArraySelector.
/// Used to quickly sample rows when sampling is appropriate.
/// </summary>
/// <param name="selector">ArraySelector to sample</param>
/// <param name="r">Random instance to use</param>
/// <param name="remapArray">Array to put remapped indices in</param>
/// <returns>ArraySelector including approximately 1/8 of the input selector rows chosen randomly</returns>
public static ArraySelector Eighth(ArraySelector selector, Random r, ref int[] remapArray)
{
if (selector.Count == 0)
{
return(selector);
}
if (selector.IsSingleValue)
{
return(ArraySelector.Single((selector.Count / 8) + 1));
}
// Allocate an indices array for the sampled subset
Allocator.AllocateToSize(ref remapArray, selector.Count);
int sampleCount = 0;
// Choose the rows to sample
if (selector.Indices == null)
{
// If no indices, loop from start index to end index
int i = selector.StartIndexInclusive;
while (i < selector.EndIndexExclusive)
{
// Generate one random integer
int random = r.Next();
// Choose whether the next 10 rows (30 bits) are included
int end = Math.Min(i + 10, selector.EndIndexExclusive);
for (; i < end; ++i)
{
if ((random & 7) == 0)
{
remapArray[sampleCount++] = i;
}
random = random >> 3;
}
}
}
else
{
// If indices, look up the index of each row
int i = selector.StartIndexInclusive;
while (i < selector.EndIndexExclusive)
{
// Generate one random integer
int random = r.Next();
// Choose whether the next 10 rows (30 bits) are included
int end = Math.Min(i + 10, selector.EndIndexExclusive);
for (; i < end; ++i)
{
if ((random & 7) == 0)
{
remapArray[sampleCount++] = selector.Indices[i];
}
random = random >> 3;
}
}
}
return(ArraySelector.Map(remapArray, sampleCount));
}
