package hostfloor

import (
	"encoding/json"
	"os"
	"path/filepath"
)

// Begun is root's update-begun marker for a floor transaction. Begin writes
// it with the reservation ID before the first box-changing step. RolledBack
// is set by the backward paths before the journal is removed, so completion
// can tell a finished rollback from a transaction that never changed the box
// (Opus re-review M1-4).
type Begun struct {
	ID         string `json:"id"`
	RolledBack bool   `json:"rolled_back,omitempty"`
}

// ReadBegun reads the marker in dir. An absent marker is os.ErrNotExist; one
// that does not decode or names no transaction is ErrFloor.
func ReadBegun(dir string) (Begun, error) {
	raw, err := os.ReadFile(filepath.Join(dir, "update-begun.json"))
	if err != nil {
		return Begun{}, err
	}
	var b Begun
	if json.Unmarshal(raw, &b) != nil || b.ID == "" {
		return Begun{}, ErrFloor
	}
	return b, nil
}

// MarkRolledBack records durably that transaction id rolled back. It never
// creates a marker: an absent one is os.ErrNotExist, and a marker for any
// other transaction is ErrFloor and is left as it is.
func MarkRolledBack(dir, id string) error {
	b, err := ReadBegun(dir)
	if err != nil {
		return err
	}
	if b.ID != id {
		return ErrFloor
	}
	if b.RolledBack {
		return nil
	}
	b.RolledBack = true
	raw, err := json.Marshal(b)
	if err != nil {
		return err
	}
	return Durable(filepath.Join(dir, "update-begun.json"), raw, 0644)
}