Ordered Select

This implements deterministic select for go1.18+. Two to 9 channels, plus a default case, are suppported. It utilizes the ast package for code generation & 1.18 generics to allow reading from arbitrary channel types.

To get started:

go get jonwillia.ms/oselect

FAQ

1. Can I mix and match sends and receives?

   Use the Select family of functions.

2. Would variadic templates remove the need for generating a function for every N-terms?

   No, because there's no way to generate a select block for an arbitrary number of of channels at compile time.

3. Which functions perform best?

   The Recv/Send families appear to be faster that the general purpose Select family.

   go test -benchmem -bench '.'                                                                 main  ✭
   goos: linux
   goarch: amd64
   pkg: jonwillia.ms/oselect
   cpu: AMD Ryzen 7 2700X Eight-Core Processor         
   BenchmarkRecv4Default-16                           	 4983958	       236.3 ns/op	       0 B/op	       0 allocs/op
   BenchmarkSelect4Default_Recv-16                    	 2180769	       540.7 ns/op	      96 B/op	       4 allocs/op
   BenchmarkSelect4Default_Recv_preroll-16            	 4211283	       280.6 ns/op	       0 B/op	       0 allocs/op
   Benchmark_select_4_default-16                      	 5084107	       228.1 ns/op	       0 B/op	       0 allocs/op
   Benchmark_reflectDotSelect_4_default_preroll-16    	  592192	      2014 ns/op	     632 B/op	      23 allocs/op
   BenchmarkRecv4-16                                  	 4384048	       256.5 ns/op	       0 B/op	       0 allocs/op
   BenchmarkSelect4_Recv-16                           	 1925407	       585.1 ns/op	      96 B/op	       4 allocs/op
   BenchmarkSelect4_RecvOK-16                         	 3690008	       323.5 ns/op	       0 B/op	       0 allocs/op
   BenchmarkSelect4_Recv_preroll-16                   	 3730765	       316.2 ns/op	       0 B/op	       0 allocs/op
   Benchmark_select_4-16                              	 4705082	       255.5 ns/op	       0 B/op	       0 allocs/op
   PASS
   ok  	jonwillia.ms/oselect	14.923s
   

4. The generated code for the Select functions is ugly.

   That isn't a question!

5. Doesn't select just use selectgo under the hood anyway? Why not use reflect.Select?

   I believe it does. It looks like the compiler uses looping constructs under the hood for non-trivial cases.

   // The compiler rewrites selects that statically have
   // only 0 or 1 cases plus default into simpler constructs.

   Check out the benchmark for Benchmark_reflectDotSelect_4_default_preroll. Oof!