westonpace commented on code in PR #35342:
URL: https://github.com/apache/arrow/pull/35342#discussion_r1186523477
##########
csharp/src/Apache.Arrow/ArrowBuffer.BitmapBuilder.cs:
##########
@@ -138,6 +138,28 @@ public BitmapBuilder AppendRange(IEnumerable<bool> values)
return this;
}
+
+ /// <summary>
+ /// Append multiple bits.
+ /// </summary>
+ /// <param name="value">Value of bits to append.</param>
+ /// <param name="length">Number of times the value should be
added.</param>
+ /// <returns>Returns the builder (for fluent-style
composition).</returns>
+ public BitmapBuilder AppendRange(bool value, int length)
+ {
+ EnsureAdditionalCapacity(length);
+
+ for (int i = 0; i < length; i++)
+ {
+ BitUtility.SetBit(Span, Length, value);
+ Length++;
+ }
+
+ SetBitCount += value ? length : 0;
+
+ return this;
+ }
Review Comment:
If you really want to eke out performance you could use
[Span.Fill](https://learn.microsoft.com/en-us/dotnet/api/system.span-1.fill?view=net-7.0)
for the whole bytes in the middle (assuming `length > 8`). E.g. something
like this:
```
public BitmapBuilder AppendRange2(bool value, int length)
{
// Use simpler method if there aren't many values
if (length < 20)
{
EnsureAdditionalCapacity(length);
for (int i = 0; i < length; i++)
{
BitUtility.SetBit(Span, Length, value);
Length++;
}
SetBitCount += value ? length : 0;
return this;
}
// Otherwise do the work to figure out how to copy whole bytes
int spaceLeftInCurrentByte = (8 - (Length % 8)) % 8;
// Bits we will be using to finish up the current byte
int remainderBits = Math.Min(length, spaceLeftInCurrentByte);
// Whole bytes that will be set
int wholeBytes = (length - remainderBits) / 8;
// Bits to set after the last whole byte
int trailingBits = length - remainderBits - (wholeBytes * 8);
int newBytes = (trailingBits > 0) ? wholeBytes + 1 : wholeBytes;
EnsureAdditionalCapacity(length);
var span = Memory.Span;
for (int i = 0; i < remainderBits; i++)
{
BitUtility.SetBit(span, Length, value);
Length++;
}
if (wholeBytes > 0)
{
var fill = (byte)(value ? 0xFF : 0x00);
// Length is guaranteed to be a multiple of 8 here
span.Slice(Length / 8, wholeBytes).Fill(fill);
}
Length += wholeBytes * 8;
for (int i = 0; i < trailingBits; i++)
{
BitUtility.SetBit(span, Length, value);
Length++;
}
SetBitCount += value ? length : 0;
return this;
}
```
The results are pretty significant once `length` gets large enough:
```
|------------------ |-------- |--------------:|-----------:|-----------:|
| AppendRangeFalse | 1 | 5.546 ns | 0.0364 ns | 0.0323 ns |
| AppendRangeTrue | 1 | 5.613 ns | 0.0484 ns | 0.0429 ns |
| AppendRange2False | 1 | 5.835 ns | 0.0751 ns | 0.0666 ns |
| AppendRange2True | 1 | 5.782 ns | 0.0280 ns | 0.0248 ns |
| AppendRangeFalse | 100 | 281.145 ns | 0.6840 ns | 0.6063 ns |
| AppendRangeTrue | 100 | 278.547 ns | 0.9460 ns | 0.8849 ns |
| AppendRange2False | 100 | 17.507 ns | 0.0414 ns | 0.0346 ns |
| AppendRange2True | 100 | 17.108 ns | 0.0698 ns | 0.0653 ns |
| AppendRangeFalse | 10000 | 27,397.477 ns | 51.9248 ns | 46.0300 ns |
| AppendRangeTrue | 10000 | 26,872.102 ns | 59.5216 ns | 49.7032 ns |
| AppendRange2False | 10000 | 14.870 ns | 0.1078 ns | 0.1009 ns |
| AppendRange2True | 10000 | 19.864 ns | 0.0747 ns | 0.0698 ns |
```
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