package httpapi

import (
	"context"
	"errors"
	"log/slog"
	"net/http"
	"time"

	"textmachine/platform/internal/auth"
	"textmachine/platform/internal/reqid"
)

// APIPrefix is the contract's base path.
const APIPrefix = "/v0"

// readyProbeTimeout bounds the readiness ping. The endpoint is unauthenticated, and without its own
// deadline a stuck database turns every probe into a held connection (PD-14).
const readyProbeTimeout = 2 * time.Second

// LoginSurface is the sign-in flow, as this package needs to see it.
type LoginSurface interface {
	Routes(guard func(http.Handler) http.Handler) http.Handler
}

// Prober is what readiness needs from the database: not "is it reachable" but "is it the database
// this build was made for". Reachability alone reported ready against a Postgres with no schema.
type Prober interface {
	Ready(ctx context.Context) error
}

// Deps is everything the HTTP surface is built from.
type Deps struct {
	Log *slog.Logger
	// DB is nil when no database is configured: the service still starts and still answers
	// liveness, and readiness reports why it is not ready.
	DB   Prober
	Auth *auth.Authenticator
	// TrustedOrigins are origins besides our own allowed to make unsafe requests.
	TrustedOrigins []string
	// HSTS asks browsers never to speak plain http to this host again. Off in the dev profile: the
	// policy is pinned per host and localhost would keep it long after the experiment.
	HSTS bool
	// Login, when set, is mounted at /auth/. It is handed the session guard rather than sitting
	// behind one: the surface that CREATES a session cannot require one, and the surface that ends
	// a session must.
	Login LoginSurface
	// Library and Runs are the contract surface. Both nil leaves /v0 a guarded 404, which is what a
	// service started without a database serves — the routes below all read one.
	Library Library
	Runs    Runs
	// Intake is the book upload. Nil where this deployment has nowhere to put a file or no engine to
	// cut it with, and then POST /books is a guarded 404 like every other unbuilt route: an instance
	// that accepted uploads it could only reject would be worse than one that says it takes none.
	Intake Intake
	// Upload bounds the one route that carries a file.
	Upload UploadLimits
	// Capabilities is what `GET /capabilities` answers: what this deployment can do.
	Capabilities Capabilities
	// Keys is the store of `Idempotency-Key` records. Nil means the header is accepted and ignored,
	// which is what an instance with no database can honestly offer.
	Keys IdempotencyKeys
	// Observe wraps every request in this deployment's telemetry. A plain middleware rather than an
	// interface so this package stays free of a metrics library; nil is a service that measures
	// nothing.
	Observe func(http.Handler) http.Handler
}

// UploadLimits is what the intake route is allowed to cost.
//
// Both numbers are the operator's, and both are refusals rather than sizings: a book is tens of
// megabytes and minutes of a domestic connection, and the defaults of the surface — a one-megabyte
// body and a thirty-second read — are correct for every OTHER route and would make this one
// impossible.
type UploadLimits struct {
	// MaxBytes caps the request body of the intake route (http.MaxBytesReader, the second half of
	// PD-2 and the whole of PD-72).
	MaxBytes int64
	// Deadline is how long the body of ONE upload may take to arrive. It replaces the server's
	// ReadTimeout for this route through http.ResponseController and is still a finite bound — the
	// forbidden thing is CLEARING the deadline, which puts a half-fed request back to being
	// unbounded (STACK_DECISIONS §12, register row PD-51).
	Deadline time.Duration
}

// DefaultUploadLimits are the intake's bounds when an operator chooses none: a book file large
// enough for the acceptance corpus (23 MB) with room around it, and long enough to arrive over a
// slow domestic uplink.
func DefaultUploadLimits() UploadLimits {
	return UploadLimits{MaxBytes: 64 << 20, Deadline: 10 * time.Minute}
}

// New builds the handler.
//
// Contract routes are registered on the SAME mux as the ops ones, with the version prefix written
// into the pattern, and each is wrapped in guard(). One mux is what keeps Request.Pattern
// meaningful all the way out to the access log — a nested mux behind http.StripPrefix hands the
// inner handler a copy, and the pattern the copy learns never comes back.
func New(d Deps) (http.Handler, error) {
	if d.Auth == nil {
		return nil, errors.New("httpapi: no authenticator: the API subtree may not be served unguarded")
	}
	csrf, err := auth.CSRF(d.TrustedOrigins, d.Auth.Cookies.SessionName(),
		CauseHandler(CodeForbidden, CauseOriginRejected),
		CauseHandler(CodeForbidden, CauseClientHeaderMissing))
	if err != nil {
		return nil, err
	}
	// Every API route goes through this. An anonymous caller therefore gets 401 before 404, which
	// is deliberate: the shape of the surface is not public information.
	//
	// The body limit is per ROUTE, not a blanket outer layer: MaxBytesReader wrapping an already
	// wrapped body keeps the tighter limit, so an upload route could never raise its own above a
	// shared default. The book upload is the route that needed it and registers guard(MaxBytes, …).
	guard := func(maxBody int64, h http.Handler) http.Handler {
		return LimitBody(maxBody)(csrf(d.Auth.Require(h)))
	}
	if d.Upload.MaxBytes <= 0 || d.Upload.Deadline <= 0 {
		// A zero here would be a route with no bound at all — MaxBytesReader treats a negative count
		// as zero and a zero deadline as none — so it is filled rather than trusted. The intake is the
		// one route where an unset limit is a hole and not an inconvenience.
		d.Upload = DefaultUploadLimits()
	}

	mux := http.NewServeMux()
	mux.Handle("GET /healthz", http.HandlerFunc(healthz))
	mux.Handle("GET /readyz", readyz(d.DB, d.Log))
	if d.Login != nil {
		// The subtree still gets the body cap and the CSRF check — sign-out is a POST, and a
		// cross-site page must not be able to make one. Rate limiting lives inside the flow, which
		// knows which of its endpoints is the unauthenticated one.
		mux.Handle("/auth/", LimitBody(DefaultMaxBody)(csrf(d.Login.Routes(d.Auth.Require))))
	}
	// The contract's base path is written here and nowhere else. Ops endpoints stay outside it: a
	// health check is not part of the versioned surface and must not move when the surface does.
	contractRoutes(mux, d, guard)
	// Everything else under the prefix, including the contract routes that have not been built yet,
	// answers a guarded 404: the anonymous caller still meets 401 first, so the shape of the surface
	// stays private either way.
	mux.Handle(APIPrefix+"/", guard(DefaultMaxBody, ProblemHandler(CodeNotFound)))

	// Recover sits INSIDE AccessLog: a panic converted to a 500 still produces a log line, whereas
	// a panic unwinding past the logger produces none. Telemetry sits beside the log and for the same
	// reason — it reads the route the mux matched and the status the recovery produced.
	served := http.Handler(Recover(d.Log)(mux))
	if d.Observe != nil {
		served = d.Observe(served)
	}
	return reqid.Middleware(SecurityHeaders(d.HSTS)(AccessLog(d.Log)(served))), nil
}

// healthz is liveness: the process is up and serving. It touches nothing, so a database outage
// cannot make a supervisor kill a healthy process.
func healthz(w http.ResponseWriter, _ *http.Request) {
	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
	w.WriteHeader(http.StatusOK)
	_, _ = w.Write([]byte("ok\n"))
}

// readyz is readiness: this instance can serve traffic, which means the database answers.
func readyz(db Prober, log *slog.Logger) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if db == nil {
			WriteStatusProblem(w, r, http.StatusServiceUnavailable, "Not ready", "no database configured")
			return
		}
		ctx, cancel := context.WithTimeout(r.Context(), readyProbeTimeout)
		defer cancel()
		if err := db.Ready(ctx); err != nil {
			// The reason goes to the LOG and not to the wire. Both halves are deliberate: a
			// swallowed dependency failure is a defect of its own (PD-16), and /readyz is
			// unauthenticated, so "the schema is two migrations behind" is a fact about our rollout
			// that no anonymous caller needs. An operator has the log line.
			log.ErrorContext(r.Context(), "readiness probe failed", "err", err)
			WriteStatusProblem(w, r, http.StatusServiceUnavailable, "Not ready", "database not ready")
			return
		}
		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
		w.WriteHeader(http.StatusOK)
		_, _ = w.Write([]byte("ready\n"))
	})
}