miller/pkg/transformers/bootstrap.go

package transformers

import (
	"fmt"
	"os"
	"strings"

	"github.com/johnkerl/miller/v6/pkg/cli"
	"github.com/johnkerl/miller/v6/pkg/lib"
	"github.com/johnkerl/miller/v6/pkg/types"
)

const verbNameBootstrap = "bootstrap"

var bootstrapOptions = []OptionSpec{
	{Flag: "-n", Arg: "{n}", Type: "int", Desc: "Number of samples to output. Defaults to number of input records. Must be non-negative."},
}

var BootstrapSetup = TransformerSetup{
	Verb:         verbNameBootstrap,
	UsageFunc:    transformerBootstrapUsage,
	ParseCLIFunc: transformerBootstrapParseCLI,
	IgnoresInput: false,
	Options:      bootstrapOptions,
}

func transformerBootstrapUsage(
	o *os.File,
) {
	fmt.Fprintf(o, "Usage: %s %s [options]\n", "mlr", verbNameBootstrap)
	fmt.Fprintf(o,
		`Emits an n-sample, with replacement, of the input records.
See also %s sample and %s shuffle.
`, "mlr", "mlr")
	WriteVerbOptions(o, bootstrapOptions)
}

func transformerBootstrapParseCLI(
	pargi *int,
	argc int,
	args []string,
	_ *cli.TOptions,
	doConstruct bool, // false for first pass of CLI-parse, true for second pass
) (RecordTransformer, error) {

	// Skip the verb name from the current spot in the mlr command line
	argi := *pargi
	verb := args[argi]
	argi++

	var err error
	nout := int64(-1)

	for argi < argc /* variable increment: 1 or 2 depending on flag */ {
		opt := args[argi]
		if !strings.HasPrefix(opt, "-") {
			break // No more flag options to process
		}
		if args[argi] == "--" {
			break // All transformers must do this so main-flags can follow verb-flags
		}
		argi++

		switch opt {
		case "-h", "--help":
			transformerBootstrapUsage(os.Stdout)
			return nil, cli.ErrHelpRequested

		case "-n":
			nout, err = cli.VerbGetIntArg(verb, opt, args, &argi, argc)
			if err != nil {
				return nil, err
			}

		default:
			return nil, cli.VerbErrorf(verb, "option \"%s\" not recognized", opt)
		}
	}

	*pargi = argi
	if !doConstruct { // All transformers must do this for main command-line parsing
		return nil, nil
	}

	transformer, err := NewTransformerBootstrap(nout)
	if err != nil {
		return nil, err
	}

	return transformer, nil
}

type TransformerBootstrap struct {
	recordsAndContexts []*types.RecordAndContext
	nout               int64
}

func NewTransformerBootstrap(nout int64) (*TransformerBootstrap, error) {
	tr := &TransformerBootstrap{
		recordsAndContexts: []*types.RecordAndContext{},
		nout:               nout,
	}
	return tr, nil
}

func (tr *TransformerBootstrap) Transform(
	inrecAndContext *types.RecordAndContext,
	outputRecordsAndContexts *[]*types.RecordAndContext, // list of *types.RecordAndContext
	inputDownstreamDoneChannel <-chan bool,
	outputDownstreamDoneChannel chan<- bool,
) error {
	HandleDefaultDownstreamDone(inputDownstreamDoneChannel, outputDownstreamDoneChannel)
	// Not end of input stream: retain the record, and emit nothing until end of stream.
	if !inrecAndContext.EndOfStream {
		tr.recordsAndContexts = append(tr.recordsAndContexts, inrecAndContext)
		return nil
	}

	// Else end of record stream

	// Given nin input records, we produce nout output records, but
	// sampling with replacement.
	//
	// About memory management:
	//
	// Normally in Miller transformers we pass through pointers to records.
	// Here, though, since we do sampling with replacement, a record could
	// be emitted twice or more. To avoid producing multiple records in the
	// output stream pointing to the same memory, we would have to copy the
	// second one. In the original C (single-threaded) version of this
	// code, that was the case.
	//
	// However, in Go, there is concurrent processing.  It would be
	// possible for us to emit a pointer to a particular record without
	// copying, then when emitting that same record a second time, copy it.
	// But due to concurrency, the pointed-to record could have already
	// been mutated downstream. We wouldn't be copying our input as we
	// received it -- we'd be copying something potentially modified.
	//
	// For that reason, this transformer must copy all output.

	nin := int64(len(tr.recordsAndContexts))
	nout := tr.nout
	if nout == -1 {
		nout = nin
	}

	if nout == 0 {
		// Emit the stream-terminating null record
		*outputRecordsAndContexts = append(*outputRecordsAndContexts, inrecAndContext)
		return nil
	}

	// Make an array of pointers into the input list.
	recordArray := tr.recordsAndContexts

	// Do the sample-with-replacment, reading from random indices in the input
	// array and emitting output.
	for i := int64(0); i < nout; i++ {
		index := lib.RandRange(0, nin)
		recordAndContext := recordArray[index]
		// Already emitted once; copy
		*outputRecordsAndContexts = append(*outputRecordsAndContexts, recordAndContext.Copy())
	}

	// Emit the stream-terminating null record
	*outputRecordsAndContexts = append(*outputRecordsAndContexts, inrecAndContext)
	return nil
}