package session

import (
	"context"
	"errors"
	"fmt"
)

// BackendLookup describes the slice of `backendclient.RESTClient`
// SessionCache depends on. The narrow interface keeps this package free
// of any backendclient import.
type BackendLookup interface {
	LookupSession(ctx context.Context, deviceSessionID string) (Record, error)
}

// BackendCache resolves authenticated device sessions by issuing one
// synchronous REST call to backend per request. The canonical implementation replaces the
// previous Redis-backed projection with this thin wrapper; gateway no
// longer keeps a process-local snapshot. See ARCHITECTURE.md §11
// «backend (sync REST), no Redis projection».
type BackendCache struct {
	backend BackendLookup
}

// NewBackendCache constructs a Cache that delegates every Lookup to
// backend over REST. backend must not be nil.
func NewBackendCache(backend BackendLookup) (*BackendCache, error) {
	if backend == nil {
		return nil, errors.New("session.NewBackendCache: backend lookup must not be nil")
	}
	return &BackendCache{backend: backend}, nil
}

// Lookup resolves deviceSessionID via backend. ErrNotFound is forwarded
// unchanged so callers can keep using the existing equality check.
func (c *BackendCache) Lookup(ctx context.Context, deviceSessionID string) (Record, error) {
	if c == nil {
		return Record{}, errors.New("session backend cache: nil cache")
	}
	if c.backend == nil {
		return Record{}, errors.New("session backend cache: nil backend lookup")
	}
	rec, err := c.backend.LookupSession(ctx, deviceSessionID)
	if err != nil {
		return Record{}, fmt.Errorf("session backend cache: %w", err)
	}
	return rec, nil
}

var _ Cache = (*BackendCache)(nil)