Change string.Repeat to use SpanAction string.Create

[Unordinal]

This change minimizes string.Repeat allocations via the SpanAction string.Create overload and gives a slight speed boost.

This implementation of the string repeat extension uses the string.Create overload that takes a SpanAction to write directly into the resulting string buffer instead of allocating a new string. This shaves off a bit of time and reduces the allocations by half due to not having to create a temporary buffer.

Method	N	Mean	Error	StdDev	Ratio	RatioSD	Gen 0	Gen 1	Gen 2	Allocated
RepeatLinq	1	19.02 ns	0.214 ns	0.179 ns	1.00	0.00	0.0038	-	-	40 B
RepeatStrBuilder	1	24.51 ns	0.314 ns	0.278 ns	1.29	0.02	0.0107	-	-	112 B
RepeatArray	1	13.78 ns	0.102 ns	0.091 ns	0.73	0.01	0.0061	-	-	64 B
RepeatSpan	1	14.55 ns	0.036 ns	0.030 ns	0.77	0.01	0.0061	-	-	64 B
RepeatStringCreate	1	12.01 ns	0.070 ns	0.065 ns	0.63	0.01	0.0031	-	-	32 B
RepeatLinq	10	100.08 ns	1.158 ns	1.083 ns	1.00	0.00	0.0122	-	-	128 B
RepeatStrBuilder	10	62.08 ns	0.395 ns	0.330 ns	0.62	0.01	0.0213	-	-	224 B
RepeatArray	10	51.17 ns	0.442 ns	0.414 ns	0.51	0.01	0.0168	-	-	176 B
RepeatSpan	10	41.59 ns	0.441 ns	0.412 ns	0.42	0.01	0.0168	-	-	176 B
RepeatStringCreate	10	36.51 ns	0.245 ns	0.217 ns	0.37	0.00	0.0084	-	-	88 B
RepeatLinq	100	763.41 ns	4.178 ns	3.908 ns	1.00	0.00	0.0629	-	-	664 B
RepeatStrBuilder	100	445.93 ns	3.844 ns	3.407 ns	0.58	0.01	0.1235	0.0005	-	1296 B
RepeatArray	100	443.01 ns	5.675 ns	5.308 ns	0.58	0.01	0.1192	-	-	1248 B
RepeatSpan	100	325.17 ns	6.301 ns	6.471 ns	0.42	0.01	0.1192	-	-	1248 B
RepeatStringCreate	100	288.81 ns	1.724 ns	1.613 ns	0.38	0.00	0.0596	-	-	624 B
RepeatLinq	1000	6,973.50 ns	28.335 ns	26.505 ns	1.00	0.00	0.5722	0.0076	-	6064 B
RepeatStrBuilder	1000	4,266.35 ns	38.939 ns	36.423 ns	0.61	0.01	1.1520	0.0381	-	12096 B
RepeatArray	1000	4,393.29 ns	75.649 ns	77.686 ns	0.63	0.01	1.1520	0.0381	-	12048 B
RepeatSpan	1000	3,174.67 ns	31.122 ns	25.988 ns	0.46	0.00	1.1520	0.0229	-	12048 B
RepeatStringCreate	1000	2,751.88 ns	13.235 ns	11.733 ns	0.39	0.00	0.5760	0.0114	-	6024 B

[ardalis]

In your PR does it make sense to just replace Repeat with your span implementation, rather than having two versions?

[Unordinal]

In your PR does it make sense to just replace Repeat with your span implementation, rather than having two versions?

I did replace the current StringManipulationExtensions.Repeat(this string text, uint n) that uses just span copying with my implementation that uses string.Create and span copying.

Or do you mean replacing the benchmark of the current version (RepeatSpan) completely with the one from this PR (RepeatStringCreate)?

Edit: Ah, or maybe you meant having both the current version and the one from my PR? They both result in the same thing, so I don't think it would make sense to keep both.

[ardalis]

Oh if you did then never mind - I was just looking at it in the browser and saw there were 2 public methods.

[Unordinal]

Oh if you did then never mind - I was just looking at it in the browser and saw there were 2 public methods.

Ah, I actually had to double-check myself when I saw that as well, hah. The new RepeatStringCreate extension in the benchmarks class is superfluous and could be removed by just using the new extension implementation directly in the benchmark method, but I thought it made some sense to separate it out for consistency with the others. I can remove it if wanted.