package membership

import (
	"errors"
	"fmt"
)

// ErrNotFound reports that a membership record was requested but does not
// exist in the store.
var ErrNotFound = errors.New("membership not found")

// ErrConflict reports that a membership mutation could not be applied
// because the record changed concurrently or failed a compare-and-swap
// guard.
var ErrConflict = errors.New("membership conflict")

// ErrInvalidTransition is the sentinel returned when Transition rejects a
// `(from, to)` pair.
var ErrInvalidTransition = errors.New("invalid membership status transition")

// InvalidTransitionError stores the rejected `(from, to)` pair and wraps
// ErrInvalidTransition so callers can match it with errors.Is.
type InvalidTransitionError struct {
	// From stores the source status that was attempted to leave.
	From Status

	// To stores the destination status that was attempted to enter.
	To Status
}

// Error reports a human-readable summary of the rejected pair.
func (err *InvalidTransitionError) Error() string {
	return fmt.Sprintf(
		"invalid membership status transition from %q to %q",
		err.From, err.To,
	)
}

// Unwrap returns ErrInvalidTransition so errors.Is recognizes the sentinel.
func (err *InvalidTransitionError) Unwrap() error {
	return ErrInvalidTransition
}