// Package engine defines the Engine interface and mount registry.
// Phase 1: interface and registry only, no concrete implementations.
package engine

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
	"regexp"
	"strings"
	"sync"

	"git.wntrmute.dev/mc/metacrypt/internal/barrier"
)

// EngineType identifies a cryptographic engine type.
type EngineType string

const (
	EngineTypeCA      EngineType = "ca"
	EngineTypeSSHCA   EngineType = "sshca"
	EngineTypeTransit EngineType = "transit"
	EngineTypeUser    EngineType = "user"
)

var (
	ErrMountExists   = errors.New("engine: mount already exists")
	ErrMountNotFound = errors.New("engine: mount not found")
	ErrUnknownType   = errors.New("engine: unknown engine type")
	ErrInvalidName   = errors.New("engine: invalid name")
)

var validName = regexp.MustCompile(`^[a-zA-Z0-9][a-zA-Z0-9._-]*$`)

// ValidateName checks that a user-provided name is safe for use in barrier
// paths. Names must be 1-128 characters, start with an alphanumeric, and
// contain only alphanumerics, dots, hyphens, and underscores.
func ValidateName(name string) error {
	if name == "" || len(name) > 128 || !validName.MatchString(name) {
		return fmt.Errorf("%w: %q", ErrInvalidName, name)
	}
	return nil
}

// CallerInfo carries authentication context into engines.
type CallerInfo struct {
	Username    string
	AccountType string // "human" or "system"
	Roles       []string
	IsAdmin     bool
}

// IsUser returns true if the caller is authorized to perform user-level
// operations. Admins, system (service) accounts, and humans with the
// "user" role all qualify. Authenticated callers with no roles are treated
// as service accounts (MCIAS issues service tokens with nil roles).
func (c *CallerInfo) IsUser() bool {
	if c.IsAdmin {
		return true
	}
	if c.AccountType == "system" {
		return true
	}
	for _, r := range c.Roles {
		if r == "user" {
			return true
		}
		if r == "guest" {
			return false
		}
	}
	// Authenticated caller with no roles — service account.
	if c.Username != "" && len(c.Roles) == 0 {
		return true
	}
	return false
}

// PolicyChecker evaluates whether the caller has access to a specific resource.
// Returns the policy effect ("allow" or "deny") and whether a matching rule was found.
// When matched is false, the caller should fall back to default access rules.
type PolicyChecker func(resource, action string) (effect string, matched bool)

// Request is a request to an engine.
type Request struct {
	Data        map[string]interface{}
	CallerInfo  *CallerInfo
	CheckPolicy PolicyChecker
	Operation   string
	Path        string
}

// Response is a response from an engine.
type Response struct {
	Data map[string]interface{}
}

// Engine is the interface that all cryptographic engines must implement.
type Engine interface {
	// Type returns the engine type.
	Type() EngineType
	// Initialize sets up the engine for first use.
	Initialize(ctx context.Context, b barrier.Barrier, mountPath string, config map[string]interface{}) error
	// Unseal opens the engine using state from the barrier.
	Unseal(ctx context.Context, b barrier.Barrier, mountPath string) error
	// Seal closes the engine and zeroizes key material.
	Seal() error
	// HandleRequest processes a request.
	HandleRequest(ctx context.Context, req *Request) (*Response, error)
}

// Factory creates a new engine instance of a given type.
type Factory func() Engine

// Mount represents a mounted engine instance.
type Mount struct {
	Engine    Engine     `json:"-"`
	Name      string     `json:"name"`
	Type      EngineType `json:"type"`
	MountPath string     `json:"mount_path"`
}

// Registry manages mounted engine instances.
type Registry struct {
	barrier   barrier.Barrier
	mounts    map[string]*Mount
	factories map[EngineType]Factory
	logger    *slog.Logger
	mu        sync.RWMutex
}

// NewRegistry creates a new engine registry.
func NewRegistry(b barrier.Barrier, logger *slog.Logger) *Registry {
	return &Registry{
		mounts:    make(map[string]*Mount),
		factories: make(map[EngineType]Factory),
		barrier:   b,
		logger:    logger,
	}
}

// RegisterFactory registers a factory for the given engine type.
func (r *Registry) RegisterFactory(t EngineType, f Factory) {
	r.mu.Lock()
	defer r.mu.Unlock()
	r.logger.Debug("registering engine factory", "type", t)
	r.factories[t] = f
}

// mountMeta is persisted to the barrier so mounts survive seal/unseal cycles.
type mountMeta struct {
	Name string     `json:"name"`
	Type EngineType `json:"type"`
}

const mountsPrefix = "engine/_mounts/"

// Mount creates and initializes a new engine mount.
func (r *Registry) Mount(ctx context.Context, name string, engineType EngineType, config map[string]interface{}) error {
	if err := ValidateName(name); err != nil {
		return err
	}

	r.mu.Lock()
	defer r.mu.Unlock()

	if _, exists := r.mounts[name]; exists {
		return ErrMountExists
	}

	factory, ok := r.factories[engineType]
	if !ok {
		return fmt.Errorf("%w: %s", ErrUnknownType, engineType)
	}

	r.logger.Debug("mounting engine", "name", name, "type", engineType)
	eng := factory()
	mountPath := fmt.Sprintf("engine/%s/%s/", engineType, name)

	// Create a per-engine DEK in the barrier for this mount.
	if aesBarrier, ok := r.barrier.(*barrier.AESGCMBarrier); ok {
		dekKeyID := fmt.Sprintf("engine/%s/%s", engineType, name)
		if err := aesBarrier.CreateKey(ctx, dekKeyID); err != nil {
			return fmt.Errorf("engine: create DEK %q: %w", dekKeyID, err)
		}
	}

	if err := eng.Initialize(ctx, r.barrier, mountPath, config); err != nil {
		return fmt.Errorf("engine: initialize %q: %w", name, err)
	}

	// Persist mount metadata so it can be reloaded after unseal.
	meta, err := json.Marshal(mountMeta{Name: name, Type: engineType})
	if err != nil {
		return fmt.Errorf("engine: marshal mount metadata: %w", err)
	}
	if err := r.barrier.Put(ctx, mountsPrefix+name+".json", meta); err != nil {
		return fmt.Errorf("engine: persist mount metadata: %w", err)
	}

	r.mounts[name] = &Mount{
		Name:      name,
		Type:      engineType,
		MountPath: mountPath,
		Engine:    eng,
	}
	r.logger.Debug("engine mounted", "name", name, "type", engineType, "mount_path", mountPath)
	return nil
}

// GetEngine returns the engine for the given mount name.
func (r *Registry) GetEngine(name string) (Engine, error) {
	r.mu.RLock()
	defer r.mu.RUnlock()

	mount, exists := r.mounts[name]
	if !exists {
		return nil, ErrMountNotFound
	}
	return mount.Engine, nil
}

// GetMount returns the mount for the given name.
func (r *Registry) GetMount(name string) (*Mount, error) {
	r.mu.RLock()
	defer r.mu.RUnlock()

	mount, exists := r.mounts[name]
	if !exists {
		return nil, ErrMountNotFound
	}
	return mount, nil
}

// Unmount removes and seals an engine mount.
func (r *Registry) Unmount(ctx context.Context, name string) error {
	r.mu.Lock()
	defer r.mu.Unlock()

	mount, exists := r.mounts[name]
	if !exists {
		return ErrMountNotFound
	}

	r.logger.Debug("unmounting engine", "name", name, "type", mount.Type)
	if err := mount.Engine.Seal(); err != nil {
		return fmt.Errorf("engine: seal %q: %w", name, err)
	}

	// Remove persisted mount metadata.
	if err := r.barrier.Delete(ctx, mountsPrefix+name+".json"); err != nil {
		return fmt.Errorf("engine: delete mount metadata %q: %w", name, err)
	}

	delete(r.mounts, name)
	r.logger.Debug("engine unmounted", "name", name)
	return nil
}

// UnsealAll discovers persisted mounts from the barrier and reloads them into
// memory. It must be called after the barrier is unsealed.
//
// It first loads mounts recorded in engine/_mounts/, then does a discovery
// pass over each registered engine type to pick up any mounts that predate
// the metadata mechanism (one-time migration).
func (r *Registry) UnsealAll(ctx context.Context) error {
	r.mu.Lock()
	defer r.mu.Unlock()

	// Load mounts from persisted metadata.
	if err := r.loadFromMetadata(ctx); err != nil {
		return err
	}

	// Discovery pass: scan each registered engine type for mounts that exist
	// in the barrier but have no metadata entry yet (pre-migration mounts).
	for engineType, factory := range r.factories {
		prefix := fmt.Sprintf("engine/%s/", engineType)
		paths, err := r.barrier.List(ctx, prefix)
		if err != nil {
			continue // no entries for this type
		}

		// Extract unique mount names from paths like "myca/config.json".
		seen := make(map[string]bool)
		for _, p := range paths {
			parts := strings.SplitN(p, "/", 2)
			if len(parts) == 0 || parts[0] == "" || parts[0] == "_mounts" {
				continue
			}
			seen[parts[0]] = true
		}

		for name := range seen {
			if _, exists := r.mounts[name]; exists {
				continue // already loaded
			}

			r.logger.Debug("discovered pre-migration engine mount", "name", name, "type", engineType)
			eng := factory()
			mountPath := fmt.Sprintf("engine/%s/%s/", engineType, name)
			if err := eng.Unseal(ctx, r.barrier, mountPath); err != nil {
				return fmt.Errorf("engine: unseal (discovered) %q: %w", name, err)
			}

			// Write metadata so future restarts don't need the discovery pass.
			meta, _ := json.Marshal(mountMeta{Name: name, Type: engineType})
			_ = r.barrier.Put(ctx, mountsPrefix+name+".json", meta)

			r.mounts[name] = &Mount{
				Name:      name,
				Type:      engineType,
				MountPath: mountPath,
				Engine:    eng,
			}
		}
	}

	return nil
}

// loadFromMetadata loads mounts recorded under engine/_mounts/. Caller must hold r.mu.
func (r *Registry) loadFromMetadata(ctx context.Context) error {
	paths, err := r.barrier.List(ctx, mountsPrefix)
	if err != nil {
		return nil // no metadata written yet
	}

	for _, p := range paths {
		if !strings.HasSuffix(p, ".json") {
			continue
		}

		data, err := r.barrier.Get(ctx, mountsPrefix+p)
		if err != nil {
			return fmt.Errorf("engine: read mount metadata %q: %w", p, err)
		}

		var meta mountMeta
		if err := json.Unmarshal(data, &meta); err != nil {
			return fmt.Errorf("engine: parse mount metadata %q: %w", p, err)
		}

		factory, ok := r.factories[meta.Type]
		if !ok {
			return fmt.Errorf("%w: %s (mount %q)", ErrUnknownType, meta.Type, meta.Name)
		}

		r.logger.Debug("unsealing engine from metadata", "name", meta.Name, "type", meta.Type)
		eng := factory()
		mountPath := fmt.Sprintf("engine/%s/%s/", meta.Type, meta.Name)
		if err := eng.Unseal(ctx, r.barrier, mountPath); err != nil {
			return fmt.Errorf("engine: unseal %q: %w", meta.Name, err)
		}

		r.mounts[meta.Name] = &Mount{
			Name:      meta.Name,
			Type:      meta.Type,
			MountPath: mountPath,
			Engine:    eng,
		}
	}

	return nil
}

// ListMounts returns all current mounts.
func (r *Registry) ListMounts() []Mount {
	r.mu.RLock()
	defer r.mu.RUnlock()

	mounts := make([]Mount, 0, len(r.mounts))
	for _, m := range r.mounts {
		mounts = append(mounts, Mount{
			Name:      m.Name,
			Type:      m.Type,
			MountPath: m.MountPath,
		})
	}
	return mounts
}

// HandleRequest routes a request to the appropriate engine.
func (r *Registry) HandleRequest(ctx context.Context, mountName string, req *Request) (*Response, error) {
	r.mu.RLock()
	mount, exists := r.mounts[mountName]
	r.mu.RUnlock()

	if !exists {
		return nil, ErrMountNotFound
	}

	r.logger.Debug("routing engine request", "mount", mountName, "operation", req.Operation, "path", req.Path)
	return mount.Engine.HandleRequest(ctx, req)
}

// SealAll seals all mounted engines.
func (r *Registry) SealAll() error {
	r.mu.Lock()
	defer r.mu.Unlock()

	r.logger.Debug("sealing all engines", "count", len(r.mounts))
	for name, mount := range r.mounts {
		if err := mount.Engine.Seal(); err != nil {
			return fmt.Errorf("engine: seal %q: %w", name, err)
		}
	}
	return nil
}