package httpx

import (
	"bufio"
	"context"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"io"
	"log/slog"
	"net"
	"net/http"
	"net/netip"
	"runtime/debug"
	"strings"
	"sync"
	"time"
)

// Middleware wraps a handler. Chain applies them so that the first listed runs
// outermost.
type Middleware func(http.Handler) http.Handler

// Chain wraps h with mw, outermost first.
func Chain(h http.Handler, mw ...Middleware) http.Handler {
	for i := len(mw) - 1; i >= 0; i-- {
		h = mw[i](h)
	}
	return h
}

type ctxKey int

const (
	ctxKeyRequestID ctxKey = iota
	ctxKeyLogState
)

// ---------------------------------------------------------------- request id

// RequestIDFrom returns the request id assigned by WithRequestID.
func RequestIDFrom(ctx context.Context) string {
	id, _ := ctx.Value(ctxKeyRequestID).(string)
	return id
}

// WithRequestID assigns each request an id, echoes it in X-Request-Id, and makes
// it available to handlers and the access log.
//
// An inbound X-Request-Id is adopted only when the peer is a trusted proxy and
// the value is short and printable: it ends up in log records, and an arbitrary
// client-controlled string there is a log-forging primitive.
func WithRequestID(trusted []netip.Prefix) Middleware {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			id := ""
			if in := r.Header.Get("X-Request-Id"); in != "" && sanitaryID(in) {
				if addr, ok := peerAddr(r); ok && inAny(addr, trusted) {
					id = in
				}
			}
			if id == "" {
				id = newID()
			}
			w.Header().Set("X-Request-Id", id)
			ctx := context.WithValue(r.Context(), ctxKeyRequestID, id)
			next.ServeHTTP(w, r.WithContext(ctx))
		})
	}
}

func newID() string {
	var b [8]byte
	if _, err := rand.Read(b[:]); err != nil {
		// crypto/rand cannot fail on any supported platform; if it somehow does,
		// an empty id is better than taking down the request.
		return ""
	}
	return hex.EncodeToString(b[:])
}

func sanitaryID(s string) bool {
	if len(s) == 0 || len(s) > 64 {
		return false
	}
	for i := 0; i < len(s); i++ {
		c := s[i]
		if c < 0x21 || c > 0x7e {
			return false
		}
	}
	return true
}

// ------------------------------------------------------------- log enrichment

// logState collects attributes that handlers discover mid-request (project,
// deployment, key id) so the single access-log line can carry them.
type logState struct {
	mu    sync.Mutex
	attrs []slog.Attr
}

// LogAttr attaches a key/value pair to this request's access-log line. It is a
// no-op outside the middleware chain, so handlers may call it unconditionally.
//
// Never pass a token, an Authorization header, or any part of either.
func LogAttr(ctx context.Context, key string, value any) {
	st, ok := ctx.Value(ctxKeyLogState).(*logState)
	if !ok {
		return
	}
	st.mu.Lock()
	st.attrs = append(st.attrs, slog.Any(key, value))
	st.mu.Unlock()
}

// -------------------------------------------------------------- access logger

// AccessLog emits exactly one record per request.
//
// It logs the method, the URL path, and nothing else from the request: headers
// are never logged (Authorization carries a bearer token) and neither is the raw
// query string. Handlers add their own context with LogAttr.
func AccessLog(log *slog.Logger, trusted []netip.Prefix) Middleware {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			st := &logState{}
			ctx := context.WithValue(r.Context(), ctxKeyLogState, st)
			r = r.WithContext(ctx)

			rec := &recorder{ResponseWriter: w, status: http.StatusOK}
			start := time.Now()
			next.ServeHTTP(rec, r)
			dur := time.Since(start)

			level := slog.LevelInfo
			switch {
			case rec.status >= 500:
				level = slog.LevelError
			case rec.status == http.StatusNotFound || rec.status == http.StatusMethodNotAllowed:
				// Ordinary outcomes on a public listener: a crawler, a stale link,
				// a missing favicon. Logging them as warnings would make warnings
				// the bulk of the file and hide the ones that mean something.
				level = slog.LevelInfo
			case rec.status >= 400:
				level = slog.LevelWarn
			case r.URL.Path == "/healthz" || r.URL.Path == "/readyz":
				level = slog.LevelDebug
			}
			if !log.Enabled(ctx, level) {
				return
			}

			st.mu.Lock()
			extra := st.attrs
			st.mu.Unlock()

			attrs := make([]slog.Attr, 0, 8+len(extra))
			attrs = append(attrs,
				slog.String("method", r.Method),
				slog.String("path", r.URL.Path),
				slog.Int("status", rec.status),
				slog.Int64("bytes", rec.written),
				slog.Float64("dur_ms", float64(dur.Microseconds())/1000),
			)
			if ip, ok := ClientIP(r, trusted); ok {
				attrs = append(attrs, slog.String("ip", ip.String()))
			}
			if id := RequestIDFrom(ctx); id != "" {
				attrs = append(attrs, slog.String("req_id", id))
			}
			attrs = append(attrs, extra...)
			log.LogAttrs(ctx, level, "request", attrs...)
		})
	}
}

// ------------------------------------------------------------------ recovery

// Recover turns a handler panic into a 500 instead of tearing down the process,
// logging the stack. The panic value itself never reaches the client.
func Recover(log *slog.Logger) Middleware {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			defer func() {
				v := recover()
				if v == nil {
					return
				}
				// ErrAbortHandler is the documented way to abort a response;
				// net/http expects to handle it and logs nothing.
				if err, ok := v.(error); ok && errors.Is(err, http.ErrAbortHandler) {
					panic(v)
				}
				log.ErrorContext(r.Context(), "handler panic",
					"panic", v,
					"method", r.Method,
					"path", r.URL.Path,
					"req_id", RequestIDFrom(r.Context()),
					"stack", string(debug.Stack()),
				)
				if rec, ok := w.(*recorder); ok && rec.wroteHeader {
					return // response already begun; nothing safe left to send
				}
				WriteJSON(w, http.StatusInternalServerError,
					newInternalEnvelope(RequestIDFrom(r.Context())))
			}()
			next.ServeHTTP(w, r)
		})
	}
}

// ---------------------------------------------------------------- client IP

// ClientIP returns the address to attribute the request to. X-Forwarded-For is
// honoured only when the direct peer is itself a trusted proxy; otherwise any
// client could forge its own source address and defeat per-IP rate limiting.
func ClientIP(r *http.Request, trusted []netip.Prefix) (netip.Addr, bool) {
	peer, ok := peerAddr(r)
	if !ok {
		return netip.Addr{}, false
	}
	if !inAny(peer, trusted) {
		return peer, true
	}
	// Walk right to left and take the first address that is not itself trusted:
	// everything to its right was appended by infrastructure we control, and
	// everything to its left may have been forged by the client.
	xff := r.Header.Values("X-Forwarded-For")
	for i := len(xff) - 1; i >= 0; i-- {
		parts := strings.Split(xff[i], ",")
		for j := len(parts) - 1; j >= 0; j-- {
			addr, err := netip.ParseAddr(strings.TrimSpace(parts[j]))
			if err != nil {
				continue
			}
			addr = addr.Unmap()
			if !inAny(addr, trusted) {
				return addr, true
			}
		}
	}
	return peer, true
}

func peerAddr(r *http.Request) (netip.Addr, bool) {
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		host = r.RemoteAddr
	}
	addr, err := netip.ParseAddr(host)
	if err != nil {
		return netip.Addr{}, false
	}
	return addr.Unmap(), true
}

func inAny(addr netip.Addr, prefixes []netip.Prefix) bool {
	for _, p := range prefixes {
		if p.Contains(addr) {
			return true
		}
	}
	return false
}

// ------------------------------------------------------------ response record

// recorder observes the status and byte count without altering behaviour.
//
// It implements Unwrap, ReadFrom, Flush and Hijack so that wrapping costs
// nothing: without ReadFrom the static handler would lose the sendfile fast
// path that io.Copy takes when the destination is the raw *http.response.
type recorder struct {
	http.ResponseWriter
	status      int
	written     int64
	wroteHeader bool
}

func (r *recorder) WriteHeader(status int) {
	if r.wroteHeader {
		return
	}
	r.status = status
	r.wroteHeader = true
	r.ResponseWriter.WriteHeader(status)
}

func (r *recorder) Write(b []byte) (int, error) {
	if !r.wroteHeader {
		r.WriteHeader(http.StatusOK)
	}
	n, err := r.ResponseWriter.Write(b)
	r.written += int64(n)
	return n, err
}

func (r *recorder) ReadFrom(src io.Reader) (int64, error) {
	if !r.wroteHeader {
		r.WriteHeader(http.StatusOK)
	}
	rf, ok := r.ResponseWriter.(io.ReaderFrom)
	if !ok {
		n, err := io.Copy(r.ResponseWriter, src)
		r.written += n
		return n, err
	}
	n, err := rf.ReadFrom(src)
	r.written += n
	return n, err
}

func (r *recorder) Unwrap() http.ResponseWriter { return r.ResponseWriter }

func (r *recorder) Flush() {
	if f, ok := r.ResponseWriter.(http.Flusher); ok {
		if !r.wroteHeader {
			r.WriteHeader(http.StatusOK)
		}
		f.Flush()
	}
}

func (r *recorder) Hijack() (net.Conn, *bufio.ReadWriter, error) {
	h, ok := r.ResponseWriter.(http.Hijacker)
	if !ok {
		return nil, nil, errors.New("httpx: ResponseWriter does not support hijacking")
	}
	return h.Hijack()
}