channel/map.go

package channel

import "runtime"

// Map applies mapper to all I's coming from in and sends their return values to out while preserving input order.
// All mappings will be done as concurrently as possible using as many threads as there are CPU cores
func Map[I, O any](source <-chan I, mapper func(I) O) (out <-chan O) {
	return MapWithRunner(source, NewLimitedRunner(runtime.NumCPU()), mapper)
}

// MapWithRunner behaves like Map but uses runner to spawn its routines
func MapWithRunner[I, O any](source <-chan I, runner Runner, mapper func(I) O) <-chan O {
	out := make(chan O, cap(source))
	outchannels := make(chan chan O, cap(source))

	// start routine for each incoming value
	go func(in <-chan I, outchannels chan chan O) {
		defer close(outchannels)
		for inputValue := range in {
			inputValue := inputValue
			outCh := make(chan O)
			outchannels <- outCh

			runner.Run(func() {
				defer close(outCh)
				outCh <- mapper(inputValue)
			})
		}
	}(source, outchannels)

	// gather all results in incoming order
	go func(out chan<- O, outchannels chan chan O) {
		defer close(out)
		for ch := range outchannels {
			for outputValue := range ch {
				out <- outputValue
			}
		}
	}(out, outchannels)

	return out
}

// MapSuccessive applies mapper to all I's coming from in and sends their return values to out while preserving input order.
// All mappings will be done successively
func MapSuccessive[I, O any](source <-chan I, mapper func(I) O) <-chan O {
	out := make(chan O, cap(source))

	go func() {
		defer close(out)
		for value := range source {
			out <- mapper(value)
		}
	}()

	return out
}