package gateway

import (
	"crypto/rand"
	"encoding/hex"
	"errors"
	"fmt"
	"io"
	"mime/multipart"
	"net"
	"net/http"
	"os"
	"path/filepath"
	"strconv"
	"strings"

	apperr "git.yeij.top/AskaEth/Cyrene_v2/internal/errors"
)

// maxUploadBytes caps a single uploaded image at 5 MiB. The multipart envelope
// (boundaries, part headers) gets a little headroom on top of that; the decoded
// file itself is checked against this exact limit.
const (
	maxUploadBytes       = 5 << 20
	uploadMultipartSlack = 1 << 20
	uploadNameBytes      = 16
	uploadSniffBytes     = 512
)

// uploadFilePrefix is the same-origin static route that serves stored uploads
// (registered in Handler). It lives under /api/v1/ so the gateway mux — not the
// SPA fallback — owns it, and so it is part of the versioned client contract.
const uploadFilePrefix = "/api/v1/files/"

// uploadResponse is the POST /api/v1/upload success payload. URL is an absolute
// http(s) URL built from the request authority so it can be handed straight to
// chat.image: the vision layer classifies only http(s):// as a URL and would
// otherwise treat a bare path as base64. Name is the server-generated storage
// name; the client's original filename is never echoed back or persisted.
type uploadResponse struct {
	URL  string `json:"url"`
	Size int64  `json:"size"`
	MIME string `json:"mime"`
	Name string `json:"name"`
}

// handleUpload accepts a multipart/form-data request with a single `file` part,
// validates it is a raster image within the size limit, stores it under
// <dataDir>/uploads under a cryptographically random name, audits the write and
// returns the URL to attach as chat.image. It is owner/admin only (registered in
// Handler). The client's original filename is deliberately ignored: it is
// attacker-controlled and could carry a path separator or overwrite a file.
func (s *Server) handleUpload(w http.ResponseWriter, r *http.Request) {
	r.Body = http.MaxBytesReader(w, r.Body, maxUploadBytes+uploadMultipartSlack)
	if err := r.ParseMultipartForm(1 << 20); err != nil {
		s.auditClaims(r, "upload.image", "error")
		writeUploadBodyError(w, err)
		return
	}
	file, header, err := r.FormFile("file")
	if err != nil {
		s.auditClaims(r, "upload.image", "error")
		writeError(w, apperr.Wrap(apperr.ErrBadRequest, "bad_request", "file is required"))
		return
	}
	defer func() { _ = file.Close() }()

	if header.Size <= 0 {
		s.auditClaims(r, "upload.image", "error")
		writeError(w, apperr.Wrap(apperr.ErrBadRequest, "bad_request", "file is empty"))
		return
	}
	if header.Size > maxUploadBytes {
		s.auditClaims(r, "upload.image", "error")
		writeError(w, apperr.Wrap(apperr.ErrPayloadTooLarge, "payload_too_large", "file exceeds the 5MB limit"))
		return
	}
	mime, err := sniffImageMIME(file)
	if err != nil {
		s.auditClaims(r, "upload.image", "error")
		s.log.Error("upload sniff", "err", err)
		writeError(w, apperr.Wrap(apperr.ErrInternal, "upload_failed", "failed to read upload"))
		return
	}
	ext, ok := uploadExtForMIME(mime)
	if !ok {
		s.auditClaims(r, "upload.image", "error")
		writeError(w, apperr.Wrap(apperr.ErrUnsupportedMediaType, "unsupported_media_type", "only png/jpeg/gif/webp/bmp images are accepted"))
		return
	}
	name, err := s.storeUpload(file, ext)
	if err != nil {
		s.auditClaims(r, "upload.image", "error")
		s.log.Error("upload store", "err", err)
		writeError(w, apperr.Wrap(apperr.ErrInternal, "upload_failed", "failed to store upload"))
		return
	}
	s.auditClaims(r, "upload.image", "ok")
	writeJSON(w, http.StatusCreated, uploadResponse{
		URL:  s.uploadURL(r, name),
		Size: header.Size,
		MIME: mime,
		Name: name,
	})
}

// guardUploadPaths rejects a static-upload request whose decoded path contains a
// parent-directory segment, before ServeMux can clean it and redirect the client
// to a non-upload path (which the SPA would then answer with index.html). The
// {name} route only ever matches one segment, so a `..` can only be an escape
// attempt; answering 404 here keeps every traversal shape uniformly rejected.
// Non-upload paths pass through untouched.
func guardUploadPaths(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if strings.HasPrefix(r.URL.Path, uploadFilePrefix) && hasDotDotSegment(r.URL.Path) {
			writeError(w, apperr.Wrap(apperr.ErrNotFound, "not_found", "file not found"))
			return
		}
		next.ServeHTTP(w, r)
	})
}

// hasDotDotSegment reports whether p has a literal ".." path segment.
func hasDotDotSegment(p string) bool {
	for _, seg := range strings.Split(p, "/") {
		if seg == ".." {
			return true
		}
	}
	return false
}

// handleUploadFile serves a stored upload from <dataDir>/uploads. It is
// unauthenticated by design: the returned URL is handed to chat.image and to
// <img> tags, neither of which can attach a bearer token. The random 128-bit
// name is the capability; the handler accepts only a strict hex+known-extension
// name and never lets a request path escape the uploads directory.
func (s *Server) handleUploadFile(w http.ResponseWriter, r *http.Request) {
	name := r.PathValue("name")
	if !validUploadName(name) {
		writeError(w, apperr.Wrap(apperr.ErrNotFound, "not_found", "file not found"))
		return
	}
	dir, err := s.uploadDir()
	if err != nil {
		writeError(w, apperr.Wrap(apperr.ErrNotFound, "not_found", "file not found"))
		return
	}
	path := filepath.Join(dir, name)
	if filepath.Dir(path) != filepath.Clean(dir) {
		writeError(w, apperr.Wrap(apperr.ErrNotFound, "not_found", "file not found"))
		return
	}
	mime, ok := uploadMIMEForExt(filepath.Ext(name))
	if !ok {
		writeError(w, apperr.Wrap(apperr.ErrNotFound, "not_found", "file not found"))
		return
	}
	f, err := os.Open(path)
	if err != nil {
		writeError(w, apperr.Wrap(apperr.ErrNotFound, "not_found", "file not found"))
		return
	}
	defer func() { _ = f.Close() }()
	info, err := f.Stat()
	if err != nil || info.IsDir() {
		writeError(w, apperr.Wrap(apperr.ErrNotFound, "not_found", "file not found"))
		return
	}
	w.Header().Set("Content-Type", mime)
	w.Header().Set("Cache-Control", "private, max-age=86400, immutable")
	w.Header().Set("X-Content-Type-Options", "nosniff")
	http.ServeContent(w, r, name, info.ModTime(), f)
}

// writeUploadBodyError maps a multipart parse failure to its wire error: an
// over-limit body is 413, anything else (not multipart, malformed part) is 400.
func writeUploadBodyError(w http.ResponseWriter, err error) {
	var maxErr *http.MaxBytesError
	if errors.As(err, &maxErr) {
		writeError(w, apperr.Wrap(apperr.ErrPayloadTooLarge, "payload_too_large", "file exceeds the 5MB limit"))
		return
	}
	writeError(w, apperr.Wrap(apperr.ErrBadRequest, "bad_request", "invalid multipart body"))
}

// sniffImageMIME detects the content type of the upload head with the stdlib
// sniffer (never the client-supplied Content-Type header, which is untrusted)
// and rewinds the part so the caller can copy it in full.
func sniffImageMIME(f multipart.File) (string, error) {
	buf := make([]byte, uploadSniffBytes)
	n, err := io.ReadFull(f, buf)
	if err != nil && !errors.Is(err, io.EOF) && !errors.Is(err, io.ErrUnexpectedEOF) {
		return "", fmt.Errorf("read upload head: %w", err)
	}
	if _, err := f.Seek(0, io.SeekStart); err != nil {
		return "", fmt.Errorf("rewind upload: %w", err)
	}
	return http.DetectContentType(buf[:n]), nil
}

// uploadExtForMIME maps a sniffed image MIME to the extension used on disk. The
// whitelist is deliberately raster-only: SVG (and HTML) would be served inline
// and could execute script, so they are not accepted.
func uploadExtForMIME(mime string) (string, bool) {
	switch mime {
	case "image/png":
		return ".png", true
	case "image/jpeg":
		return ".jpg", true
	case "image/gif":
		return ".gif", true
	case "image/webp":
		return ".webp", true
	case "image/bmp":
		return ".bmp", true
	default:
		return "", false
	}
}

// uploadMIMEForExt returns the Content-Type for a stored upload extension. It is
// the read-side twin of uploadExtForMIME: only whitelisted extensions resolve,
// which is what makes validUploadName strict.
func uploadMIMEForExt(ext string) (string, bool) {
	switch strings.ToLower(ext) {
	case ".png":
		return "image/png", true
	case ".jpg", ".jpeg":
		return "image/jpeg", true
	case ".gif":
		return "image/gif", true
	case ".webp":
		return "image/webp", true
	case ".bmp":
		return "image/bmp", true
	default:
		return "", false
	}
}

// storeUpload writes f to <dataDir>/uploads/<random>.<ext> and returns the
// generated name. It writes to a dot-prefixed temp file first and renames it
// into place, so a partial upload is never visible under a servable name. The
// directory is created 0700 and the file inherits the temp file's 0600 mode.
func (s *Server) storeUpload(f io.Reader, ext string) (string, error) {
	dir, err := s.uploadDir()
	if err != nil {
		return "", err
	}
	if err := os.MkdirAll(dir, 0o700); err != nil {
		return "", fmt.Errorf("create upload dir: %w", err)
	}
	name, err := randomUploadName(ext)
	if err != nil {
		return "", err
	}
	tmp, err := os.CreateTemp(dir, ".upload-*")
	if err != nil {
		return "", fmt.Errorf("create upload temp: %w", err)
	}
	tmpName := tmp.Name()
	if _, err := io.Copy(tmp, io.LimitReader(f, maxUploadBytes+1)); err != nil {
		_ = tmp.Close()
		_ = os.Remove(tmpName)
		return "", fmt.Errorf("write upload: %w", err)
	}
	if err := tmp.Close(); err != nil {
		_ = os.Remove(tmpName)
		return "", fmt.Errorf("close upload: %w", err)
	}
	if err := os.Rename(tmpName, filepath.Join(dir, name)); err != nil {
		_ = os.Remove(tmpName)
		return "", fmt.Errorf("commit upload: %w", err)
	}
	return name, nil
}

// uploadDir returns <dataDir>/uploads. The resolved data dir is injected by the
// composition root (Deps.DataDir, the same value resolveDataDir hands the DB and
// the runtime configs); the configured cfg.DataDir is the fallback so a Server
// built without the injection still resolves its uploads.
func (s *Server) uploadDir() (string, error) {
	base := strings.TrimSpace(s.dataDir)
	if base == "" {
		base = strings.TrimSpace(s.cfg.DataDir)
	}
	if base == "" {
		return "", fmt.Errorf("gateway: data directory not configured")
	}
	return filepath.Join(base, "uploads"), nil
}

// randomUploadName returns a 128-bit random hex name with ext appended. It is
// the only filename component ever used: the client's original name is dropped.
func randomUploadName(ext string) (string, error) {
	buf := make([]byte, uploadNameBytes)
	if _, err := rand.Read(buf); err != nil {
		return "", fmt.Errorf("random upload name: %w", err)
	}
	return hex.EncodeToString(buf) + ext, nil
}

// validUploadName reports whether name is exactly a 32-lowercase-hex stem plus a
// whitelisted image extension. This is the path-traversal guard for the static
// route: separators, dots, dots-dots and any other shape are rejected before the
// filesystem is touched.
func validUploadName(name string) bool {
	if name == "" || name != filepath.Base(name) || strings.ContainsAny(name, "/\\") {
		return false
	}
	stem, ext, ok := strings.Cut(name, ".")
	if !ok || len(stem) != uploadNameBytes*2 || ext == "" {
		return false
	}
	for _, c := range stem {
		if !isLowerHex(c) {
			return false
		}
	}
	_, ok = uploadMIMEForExt("." + ext)
	return ok
}

// isLowerHex reports whether c is an ASCII lowercase hex digit.
func isLowerHex(c rune) bool {
	return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f')
}

// uploadURL builds the absolute, same-origin URL for a stored upload. The scheme
// honours X-Forwarded-Proto (reverse proxy) and the authority is the request
// host, sanitized so a malformed Host header cannot smuggle path syntax into the
// returned URL; the configured listen address is the fallback.
func (s *Server) uploadURL(r *http.Request, name string) string {
	scheme := "http"
	if r.TLS != nil {
		scheme = "https"
	}
	if proto := forwardedProto(r); proto != "" {
		scheme = proto
	}
	host := sanitizeHost(r.Host)
	if host == "" {
		host = net.JoinHostPort(s.cfg.Server.Host, strconv.Itoa(s.cfg.Server.Port))
	}
	return scheme + "://" + host + uploadFilePrefix + name
}

// forwardedProto returns the first X-Forwarded-Proto value when it is http or
// https, else "".
func forwardedProto(r *http.Request) string {
	v := r.Header.Get("X-Forwarded-Proto")
	if v == "" {
		return ""
	}
	v = strings.ToLower(strings.TrimSpace(strings.Split(v, ",")[0]))
	if v == "http" || v == "https" {
		return v
	}
	return ""
}

// sanitizeHost rejects a host value containing path, userinfo or whitespace
// syntax; an empty result makes uploadURL fall back to the configured address.
func sanitizeHost(host string) string {
	host = strings.TrimSpace(host)
	if host == "" || strings.ContainsAny(host, "/\\@ \t") {
		return ""
	}
	return host
}