package broker

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"os"
	"regexp"
	"strings"
	"time"
)

// The mesh runs the broker, so there is no chicken-and-egg in a node needing an account before it
// can connect: the account is created when the token is issued, and the one-time secret in that
// token IS the password. A node's first connection is already authenticated, and enrolment is
// what happens over it.
//
// novox/hq ADR 0004's *a node holds its own identity and nothing else* is why the account is per
// node rather than shared. A shared enrolment account would let any node consume another's queue,
// which is the shared-credential fault that record exists to remove, reappearing at the transport.

// ManagementVar holds the broker's management API, credentials included.
const ManagementVar = "MESH_BROKER_MANAGEMENT"

// safeName is what a node may be called at the broker.
//
// The name goes into a URL path and into permission patterns, which are regular expressions. A
// name carrying a `.` or a `*` would silently widen what that node may reach — so it is
// constrained here rather than escaped later, because an escape that is forgotten once is a node
// reading everybody's queues.
var safeName = regexp.MustCompile(`^[a-z0-9][a-z0-9-]{0,62}$`)

// Management is the broker's administrative interface.
type Management struct {
	base   *url.URL
	client *http.Client
}

// ManagementFromEnvironment reads where the management API is, if it is configured.
func ManagementFromEnvironment() (*Management, error) {
	raw := strings.TrimSpace(os.Getenv(ManagementVar))
	if raw == "" {
		return nil, ErrNotConfigured
	}
	base, err := url.Parse(raw)
	if err != nil || base.Host == "" {
		// The value carries a password, so it is not quoted back.
		return nil, fmt.Errorf("%s is not a usable URL", ManagementVar)
	}
	return &Management{base: base, client: &http.Client{Timeout: 15 * time.Second}}, nil
}

// QueueFor is the queue a node consumes from. One per node, named after it.
func QueueFor(node string) string { return "node." + node }

// ExchangeName is where nodes publish what they have to say. One exchange, and the control plane
// is the only consumer behind it (novox/hq ADR 0006 — one consumer, so two cannot silently split
// the traffic between them).
const ExchangeName = "mesh"

// BuildQueueName is where build work waits. Duplicated from `link` rather than imported, for the
// same reason QueueFor above is: this package must not depend on the one that uses it, and a
// constant that differed would be caught by the test that asserts they agree.
const BuildQueueName = "builds"

// The events bus (novox/hq ADR 0047): one topic exchange every event rides, a second for tool
// RPC kept apart, and a dead-letter home for a poison event. The substrate owns these — a module's
// account cannot declare them, only bind its own queue to the events one.
const (
	EventsExchangeName = "mesh.events"
	RPCExchangeName    = "mesh.rpc"
	DeadExchangeName   = "mesh.events.dead"
)

// ModuleQueueFor is the durable queue a module consumes its events from — one per module per node
// (novox/hq ADR 0047), named so the account that may read it is exactly this module's.
func ModuleQueueFor(node, module string) string { return node + "." + module + ".events" }

// modulePermissions is a module's authority on the bus, derived from its manifest (novox/hq
// ADR 0048): what it consumes and what it emits, and nothing else. Pure, so the scope is tested as
// patterns without a broker — the way a builder's is.
//
// A note on the limit: the broker's write permission is per exchange, not per routing key (LavinMQ
// has no topic permissions), so an emitting module is granted the events exchange whole. ADR 0047's
// origin reservation — a module publishes only under `module.<self>.*` — is stamped by the sdk, not
// enforced here; that gap is the broker's, and is recorded rather than hidden. A pure consumer like
// the audit logger is unaffected: it is granted no write to the exchange at all.
func modulePermissions(node, module string, emits, consumes []string) (configure, write, read string) {
	queue := regexp.QuoteMeta(ModuleQueueFor(node, module))
	events := regexp.QuoteMeta(EventsExchangeName)
	rpc := regexp.QuoteMeta(RPCExchangeName)
	// A module serves each of its tools on its own queue, namespaced by the module (novox/hq
	// ADR 0052) — serve.<module>.<tool> — so the account may declare, bind and read exactly its own,
	// and no other module's.
	serve := "serve\\." + regexp.QuoteMeta(module) + "\\..*"

	// Declare its own events queue and its own tool serve queues.
	configure = "^(" + queue + "|" + serve + ")$"

	// Write to bind its queue and serve queues (binding is a write on the queue), and to the RPC
	// exchange to publish replies (ADR 0052: replies ride mesh.rpc, never the default exchange, which
	// would let it publish into any queue). To the events exchange only if it emits.
	writes := []string{queue, serve, rpc}
	if len(emits) > 0 {
		writes = append(writes, events)
	}
	write = "^(" + strings.Join(writes, "|") + ")$"

	// Read its own queue and serve queues to consume them, and the RPC exchange to bind its serve
	// queues onto. The events exchange to bind onto only if it consumes.
	reads := []string{queue, serve, rpc}
	if len(consumes) > 0 {
		reads = append(reads, events)
	}
	read = "^(" + strings.Join(reads, "|") + ")$"
	return configure, write, read
}

// CreateModuleAccount gives an assigned module its own broker account, scoped by what it emits and
// consumes (novox/hq ADR 0048). The account name carries the node so the same module on two machines
// holds two accounts, each sealed to its own; the permissions carry the module so one module cannot
// read another's queue. Generic — the builder is one instance of this rule, not a separate kind.
func (m *Management) CreateModuleAccount(ctx context.Context, node, module, password string, emits, consumes []string) (string, error) {
	if !safeName.MatchString(node) {
		return "", fmt.Errorf("%q cannot be part of a broker account: it is a permission pattern", node)
	}
	if !safeName.MatchString(module) {
		return "", fmt.Errorf("%q cannot be part of a broker account: it is a permission pattern", module)
	}
	account := node + "-" + module
	if !safeName.MatchString(account) {
		return "", fmt.Errorf("%q is not a usable broker account name", account)
	}

	if err := m.put(ctx, "/api/users/"+url.PathEscape(account),
		map[string]string{"password": password, "tags": ""}); err != nil {
		return "", fmt.Errorf("cannot create the broker account for %s on %s: %w", module, node, err)
	}

	configure, write, read := modulePermissions(node, module, emits, consumes)
	if err := m.put(ctx, "/api/permissions/%2f/"+url.PathEscape(account), map[string]string{
		"configure": configure, "write": write, "read": read,
	}); err != nil {
		return "", fmt.Errorf("cannot scope the broker account for %s on %s: %w", module, node, err)
	}
	return account, nil
}

// EnsureModuleQueue declares a consuming module's queue with its dead-letter exchange, idempotently.
// The substrate declares it because a scoped module account may not: the broker refuses a queue with
// a dead-letter exchange to a non-administrator (novox/hq ADR 0048), so a consumer passively checks
// the queue the mesh made rather than declaring its own.
func (m *Management) EnsureModuleQueue(ctx context.Context, node, module string) error {
	queue := ModuleQueueFor(node, module)
	if err := m.put(ctx, "/api/queues/%2f/"+url.PathEscape(queue), map[string]any{
		"durable":   true,
		"arguments": map[string]any{"x-dead-letter-exchange": DeadExchangeName},
	}); err != nil {
		return fmt.Errorf("cannot declare the queue for %s on %s: %w", module, node, err)
	}
	return nil
}

// EnsureEventExchanges declares the bus's exchanges and the dead-letter home, idempotently. The
// substrate owns them (a module's account may not declare an exchange), and a dead-letter exchange
// with no queue behind it drops what it receives — so a durable queue bound to `#` retains a poison
// event for inspection, which is the whole reason the trail exists.
func (m *Management) EnsureEventExchanges(ctx context.Context) error {
	for _, exchange := range []string{EventsExchangeName, RPCExchangeName, DeadExchangeName} {
		if err := m.put(ctx, "/api/exchanges/%2f/"+url.PathEscape(exchange),
			map[string]any{"type": "topic", "durable": true}); err != nil {
			return fmt.Errorf("cannot declare the %s exchange: %w", exchange, err)
		}
	}
	if err := m.put(ctx, "/api/queues/%2f/"+url.PathEscape(DeadExchangeName),
		map[string]any{"durable": true}); err != nil {
		return fmt.Errorf("cannot declare the dead-letter queue: %w", err)
	}
	if err := m.post(ctx, "/api/bindings/%2f/e/"+url.PathEscape(DeadExchangeName)+
		"/q/"+url.PathEscape(DeadExchangeName), map[string]string{"routing_key": "#"}); err != nil {
		return fmt.Errorf("cannot bind the dead-letter queue: %w", err)
	}
	return nil
}

// CreateNodeAccount gives a node its own broker account, with the token's secret as the password.
//
// Scoped so a node can reach its own queue and the one exchange, and nothing else. The patterns
// are anchored: a node called `laptop` must not be able to read `laptop-of-somebody-else`.
func (m *Management) CreateNodeAccount(ctx context.Context, node, password string) error {
	if !safeName.MatchString(node) {
		return fmt.Errorf(
			"%q cannot be a broker account name: it becomes part of a permission pattern, so it "+
				"is lower-case letters, digits and dashes", node)
	}

	if err := m.put(ctx, "/api/users/"+url.PathEscape(node),
		map[string]string{"password": password, "tags": ""}); err != nil {
		return fmt.Errorf("cannot create the broker account for %s: %w", node, err)
	}

	queue := regexp.QuoteMeta(QueueFor(node))
	if err := m.put(ctx, "/api/permissions/%2f/"+url.PathEscape(node), map[string]string{
		"configure": "^" + queue + "$",
		"write":     "^(" + regexp.QuoteMeta(ExchangeName) + "|" + queue + ")$",
		"read":      "^" + queue + "$",
	}); err != nil {
		return fmt.Errorf("cannot scope the broker account for %s: %w", node, err)
	}
	return nil
}

// CreateBuilderAccount scopes an account to taking build work and answering it.
//
// **A build machine is not a node**, and giving it a node's account would let it read another
// machine's declarations. What it needs is narrower and different: read the build queue, and
// write to the exchange and to whatever temporary queue an asker is waiting on.
//
// The reply queues are the reason `write` is not simply the exchange. `RequestBuild` declares an
// exclusive queue with a generated name and waits on it, so a builder that could not write to it
// could take work and never answer — which is the failure that looks like a builder that is not
// running.
func (m *Management) CreateBuilderAccount(ctx context.Context, name, password string) error {
	if !safeName.MatchString(name) {
		return fmt.Errorf(
			"%q cannot be a broker account name: it becomes part of a permission pattern, so it "+
				"is lower-case letters, digits and dashes", name)
	}

	if err := m.put(ctx, "/api/users/"+url.PathEscape(name),
		map[string]string{"password": password, "tags": ""}); err != nil {
		return fmt.Errorf("cannot create the broker account for %s: %w", name, err)
	}

	builds := regexp.QuoteMeta(BuildQueueName)
	if err := m.put(ctx, "/api/permissions/%2f/"+url.PathEscape(name), map[string]string{
		// It declares the build queue, because whichever builder starts first must be able to —
		// and a queue nobody may declare is a queue that exists only if the control plane has
		// already run, which makes the order they start in matter.
		"configure": "^" + builds + "$",
		// The exchange, and nothing else. **Not the default exchange**: permission there is
		// granted per exchange rather than per queue, so a builder allowed to use it could
		// publish into any node's queue — the privilege a build machine most obviously should
		// not have. Answers go through the exchange, which is why they can.
		"write": "^" + regexp.QuoteMeta(ExchangeName) + "$",
		// The build queue and nothing else. Not another machine's declarations.
		"read": "^" + builds + "$",
	}); err != nil {
		return fmt.Errorf("cannot scope the broker account for %s: %w", name, err)
	}
	return nil
}

// RemoveNodeAccount withdraws a node's access.
func (m *Management) RemoveNodeAccount(ctx context.Context, node string) error {
	if !safeName.MatchString(node) {
		return fmt.Errorf("%q is not a broker account name", node)
	}
	return m.do(ctx, http.MethodDelete, "/api/users/"+url.PathEscape(node), nil)
}

// Accounts lists the broker's users, so a picture can be read from the system rather than assumed
// (novox/hq ADR 0018).
func (m *Management) Accounts(ctx context.Context) ([]string, error) {
	body, err := m.get(ctx, "/api/users")
	if err != nil {
		return nil, err
	}
	var users []struct {
		Name string `json:"name"`
	}
	if err := json.Unmarshal(body, &users); err != nil {
		return nil, err
	}
	names := make([]string, 0, len(users))
	for _, u := range users {
		names = append(names, u.Name)
	}
	return names, nil
}

func (m *Management) put(ctx context.Context, path string, body any) error {
	return m.do(ctx, http.MethodPut, path, body)
}

func (m *Management) post(ctx context.Context, path string, body any) error {
	return m.do(ctx, http.MethodPost, path, body)
}

func (m *Management) get(ctx context.Context, path string) ([]byte, error) {
	request, err := m.request(ctx, http.MethodGet, path, nil)
	if err != nil {
		return nil, err
	}
	response, err := m.client.Do(request)
	if err != nil {
		return nil, err
	}
	defer response.Body.Close()
	if response.StatusCode >= 300 {
		return nil, fmt.Errorf("the broker's management API answered %s to GET %s",
			response.Status, path)
	}
	return io.ReadAll(io.LimitReader(response.Body, 1<<20))
}

func (m *Management) do(ctx context.Context, method, path string, body any) error {
	request, err := m.request(ctx, method, path, body)
	if err != nil {
		return err
	}
	response, err := m.client.Do(request)
	if err != nil {
		return err
	}
	defer response.Body.Close()
	if response.StatusCode >= 300 {
		detail, _ := io.ReadAll(io.LimitReader(response.Body, 4096))
		return fmt.Errorf("the broker's management API answered %s to %s %s: %s",
			response.Status, method, path, strings.TrimSpace(string(detail)))
	}
	return nil
}

func (m *Management) request(ctx context.Context, method, path string, body any) (*http.Request, error) {
	var payload io.Reader
	if body != nil {
		raw, err := json.Marshal(body)
		if err != nil {
			return nil, err
		}
		payload = bytes.NewReader(raw)
	}

	// Path joined by hand rather than through url.Parse: %2f is the default vhost and must reach
	// the broker still encoded. Parsing would decode it to a slash and address a different route.
	target := strings.TrimSuffix(m.base.String(), "/")
	if user := m.base.User; user != nil {
		target = strings.TrimSuffix(m.base.Scheme+"://"+m.base.Host, "/")
	}
	request, err := http.NewRequestWithContext(ctx, method, target+path, payload)
	if err != nil {
		return nil, err
	}
	if user := m.base.User; user != nil {
		password, _ := user.Password()
		request.SetBasicAuth(user.Username(), password)
	}
	if body != nil {
		request.Header.Set("Content-Type", "application/json")
	}
	return request, nil
}