package rootparticipation

import (
	"context"
	"errors"
	"os"
)

// MigrationCensus owns the same exclusion as Acquire. Completion never resets
// revisions or overwrites another transaction's pending marker.
func (l Ledger) MigrationCensus(ctx context.Context, want Census, verifyOnly bool) error {
	unlock, err := l.State.Lock(ctx, "transaction.lock")
	if err != nil {
		return err
	}
	defer unlock()
	raw, err := l.State.Read("census.json")
	if err == nil {
		var c Census
		if Decode(raw, &c) != nil || c.Schema != 1 || c.Box != want.Box || c.Enrollment != want.Enrollment || c.Migration != want.Migration || c.NeverEnrolled || c.Revision == 0 || c.LaunchRevision == 0 {
			return ErrUnavailable
		}
		if c.Complete && c.Pending == "" {
			return nil
		}
		if verifyOnly || c.Pending != "migration" {
			return ErrUnavailable
		}
		want.Revision = c.Revision
		want.LaunchRevision = c.LaunchRevision
	} else if !errors.Is(err, os.ErrNotExist) {
		return err
	} else if verifyOnly {
		return ErrUnavailable
	}
	if err := l.writeCensus(want); err != nil {
		if want.Complete {
			want.Complete = false
			want.Pending = "migration"
			return errors.Join(err, l.writeCensus(want))
		}
		return err
	}
	return nil
}