// Package readback answers which applications a subject may begin to use.
//
// A readback decision is advisory. It is evaluated by each participating
// system's own versioned package, recorded like any decision, and carries
// purpose "readback" inside every request digest, so no PEP can accept it as an
// enforcement decision. Only the action ApplicationAccess is ever evaluated
// (FLEX-DEC-2026-019, docs/application-access-readback.md).
package readback

import (
	"context"
	"errors"
	"fmt"
	"slices"
	"strings"

	decisioncore "github.com/netkingdom/access-engine/internal/decision"
	"github.com/netkingdom/access-engine/internal/policy"
	"github.com/netkingdom/access-engine/internal/registry"
	"github.com/netkingdom/access-engine/pkg/api"
)

// ApplicationAccess is the only action the readback evaluates. A package takes
// part by declaring it; the rules are the package's own.
const ApplicationAccess = "application.access"

// ResourceType is the resource type of the application a readback names. The
// resource id is the protected-system id.
const ResourceType = "application"

// MaxSystems bounds one request (user-engine review, 2026-10-09).
const MaxSystems = 64

// ReasonNotOffered is the reason on a not_applicable row for a system the pin
// does not know or whose package does not declare ApplicationAccess.
const ReasonNotOffered = "application_access_not_declared"

// RevocationVisibility restates the per-input-class deadlines published in
// docs/decision-input-freshness.md for the classes the readback pin joins. It
// commits to nothing beyond them. The pin is standalone, so no directory
// class is joined.
var RevocationVisibility = map[string]string{
	"claims":         "immediate",
	"registry":       "reload_or_lifetime",
	"policy_package": "reload_or_lifetime",
}

// ErrInvalidRequest marks a request refused before evaluation.
var ErrInvalidRequest = errors.New("invalid readback request")

// Request names a subject, the claims user-engine verified for it, and the
// systems to read back. Subject and Tenant have the /v1/check shape.
type Request struct {
	Tenant  string         `json:"tenant,omitempty"`
	Subject api.SubjectRef `json:"subject"`
	Context map[string]any `json:"context,omitempty"`
	Systems []string       `json:"systems"`
}

// Result is one row, in request order. A system is never omitted.
type Result struct {
	System               string                `json:"system"`
	Effect               api.DecisionEffect    `json:"effect"`
	Reason               string                `json:"reason,omitempty"`
	DecisionID           string                `json:"decision_id,omitempty"`
	Lifetime             *api.DecisionLifetime `json:"lifetime,omitempty"`
	RevocationVisibility map[string]string     `json:"revocation_visibility,omitempty"`
	PolicyPackage        string                `json:"policy_package,omitempty"`
	PolicyPackageDigest  string                `json:"policy_package_digest,omitempty"`
}

// Response is the whole readback answer.
type Response struct {
	RegistrySnapshotDigest string   `json:"registry_snapshot_digest"`
	Results                []Result `json:"results"`
}

// System is one row of the registry projection.
type System struct {
	ID            string             `json:"id"`
	Name          string             `json:"name,omitempty"`
	ClientBinding *api.ClientBinding `json:"client_binding,omitempty"`
	Readback      bool               `json:"readback"`
}

// Systems is the read-only registry projection.
type Systems struct {
	RegistrySnapshotDigest string   `json:"registry_snapshot_digest"`
	Systems                []System `json:"systems"`
}

// Service evaluates readbacks over one merged registry and one package per
// participating system.
type Service struct {
	store    *registry.Store
	engines  map[string]*decisioncore.Engine
	declared map[string]bool
}

// New builds one engine per system over the shared store. Every system named
// in packages must exist in the store.
func New(store *registry.Store, packages map[string]*policy.Package) (*Service, error) {
	known := map[string]bool{}
	for _, system := range store.Snapshot().Systems {
		known[system.ID] = true
	}
	engines := make(map[string]*decisioncore.Engine, len(packages))
	declared := make(map[string]bool, len(packages))
	for system, pkg := range packages {
		if !known[system] {
			return nil, fmt.Errorf("readback package for %q names a system absent from the registry", system)
		}
		engine, err := decisioncore.NewEngine(store, pkg)
		if err != nil {
			return nil, fmt.Errorf("readback package for %q: %w", system, err)
		}
		engines[system] = engine
		declared[system] = slices.Contains(pkg.Metadata.Actions, ApplicationAccess)
	}
	return &Service{store: store, engines: engines, declared: declared}, nil
}

// SetDecisionLog records every engine's decisions to log.
func (s *Service) SetDecisionLog(log decisioncore.DecisionRecorder) {
	for _, engine := range s.engines {
		engine.SetDecisionLog(log)
	}
}

// Evaluate answers one row per requested system. Any evaluation or record
// error fails the whole request: the consumer renders every row unavailable,
// and no effect is released without its required record (GH-DEC-2026-024).
func (s *Service) Evaluate(ctx context.Context, request Request) (Response, error) {
	if err := validate(request); err != nil {
		return Response{}, err
	}
	response := Response{RegistrySnapshotDigest: s.store.Digest(), Results: make([]Result, 0, len(request.Systems))}
	for _, system := range request.Systems {
		engine, ok := s.engines[system]
		if !ok || !s.declared[system] {
			response.Results = append(response.Results, Result{System: system, Effect: api.DecisionEffectNotApplicable, Reason: ReasonNotOffered})
			continue
		}
		decision, err := engine.Check(ctx, api.CheckRequest{
			Tenant:   request.Tenant,
			Subject:  request.Subject,
			Action:   ApplicationAccess,
			Resource: api.ResourceRef{ID: system, Type: ResourceType, System: system},
			Context:  request.Context,
			Purpose:  api.PurposeReadback,
		})
		if err != nil {
			return Response{}, fmt.Errorf("readback %q: %w", system, err)
		}
		response.Results = append(response.Results, Result{
			System:               system,
			Effect:               decision.Effect,
			Reason:               decision.Reason,
			DecisionID:           decision.ID,
			Lifetime:             decision.Lifetime,
			RevocationVisibility: RevocationVisibility,
			PolicyPackage:        decision.Provenance.PolicyPackage,
			PolicyPackageDigest:  decision.Provenance.PolicyPackageDigest,
		})
	}
	return response, nil
}

// Systems projects the merged registry. It authorizes nothing.
func (s *Service) Systems() Systems {
	snapshot := s.store.Snapshot()
	out := Systems{RegistrySnapshotDigest: s.store.Digest(), Systems: make([]System, 0, len(snapshot.Systems))}
	for _, system := range snapshot.Systems {
		out.Systems = append(out.Systems, System{
			ID:            system.ID,
			Name:          system.Name,
			ClientBinding: system.ClientBinding,
			Readback:      s.declared[system.ID],
		})
	}
	return out
}

func validate(request Request) error {
	if strings.TrimSpace(request.Subject.ID) == "" {
		return fmt.Errorf("%w: subject.id is required", ErrInvalidRequest)
	}
	if len(request.Systems) == 0 || len(request.Systems) > MaxSystems {
		return fmt.Errorf("%w: systems must name 1 to %d systems", ErrInvalidRequest, MaxSystems)
	}
	seen := make(map[string]bool, len(request.Systems))
	for _, system := range request.Systems {
		if strings.TrimSpace(system) == "" || seen[system] {
			return fmt.Errorf("%w: systems must be non-empty and unique", ErrInvalidRequest)
		}
		seen[system] = true
	}
	return nil
}