package rules

import (
	"encoding/json"
	"errors"
	"flag"
	"fmt"
	"io"
	"io/fs"
)

// Program is the checking program's work (cmd/mesh-checking-program; novox/hq ADR 0644, ADR 0656): over its arguments,
// its input (read only when asked for the verdict) and the heads laid for it (nil when none are), the one JSON line it
// answers, or why it refuses. `-seats` answers the seat list of the heads (SeatsOf), `-verdict` the verdict of the
// facts in the input (Judged), refusing facts of a version it does not read.
func Program(args []string, input func() ([]byte, error), heads fs.FS) ([]byte, error) {
	set := flag.NewFlagSet("mesh-checking-program", flag.ContinueOnError)
	set.SetOutput(io.Discard)
	verdict := set.Bool("verdict", false, "answer the verdict of the facts in the input")
	seats := set.Bool("seats", false, "answer the seat list of the heads laid for it")
	if err := set.Parse(args); err != nil {
		return nil, err
	}
	switch {
	case *seats:
		if heads == nil {
			return nil, errors.New("no heads are laid for it")
		}
		answer, err := SeatsOf(heads)
		if err != nil {
			return nil, err
		}
		return json.Marshal(answer)
	case *verdict:
		data, err := input()
		if err != nil {
			return nil, fmt.Errorf("the input cannot be read: %w", err)
		}
		facts, err := DecodeFacts(data)
		if err != nil {
			return nil, err
		}
		return json.Marshal(Judged(facts))
	}
	return nil, errors.New("nothing asked: give -verdict or -seats")
}