feat: game lobby service

lobby/internal/domain/application/errors.go

@@ -0,0 +1,42 @@
+package application
+
+import (
+	"errors"
+	"fmt"
+)
+
+// ErrNotFound reports that an application record was requested but does
+// not exist in the store.
+var ErrNotFound = errors.New("application not found")
+
+// ErrConflict reports that an application mutation could not be applied.
+// It is returned for single-active-application violations and for
+// compare-and-swap mismatches on status transitions.
+var ErrConflict = errors.New("application conflict")
+
+// ErrInvalidTransition is the sentinel returned when Transition rejects a
+// `(from, to)` pair.
+var ErrInvalidTransition = errors.New("invalid application 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 application 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
+}