package api

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"errors"
	"log"
	"net/http"
	"strconv"
	"strings"
	"time"

	"milkway/internal/cache"
	"milkway/internal/config"
	"milkway/internal/models"
	"milkway/internal/ratelimit"
	"milkway/internal/store"
)

// maxBodyBytes caps request body size to defend against oversized-payload DoS.
const maxBodyBytes = 1 << 20 // 1 MiB

// cacheMaxEntries bounds the in-memory read cache. Generous for the handful of
// org-scoped entities/rates/analytics rollups we memoize.
const cacheMaxEntries = 10000

// Server holds dependencies shared by all HTTP handlers.
type Server struct {
	cfg   *config.Config
	store *store.Store
	cache cache.Cache

	// Rate limiters, per client IP. They are nil when their configured limit is
	// 0 (disabled), and the middleware treats a nil limiter as a no-op so test
	// traffic (which constructs config.Config{} with zero values) is never
	// throttled. Both use a fixed 1-minute window (the config is "per minute").
	rateLimiter  ratelimit.Limiter // global cap across all /api/* requests
	loginLimiter ratelimit.Limiter // stricter cap on POST /api/auth/login
}

func NewServer(cfg *config.Config, st *store.Store) *Server {
	// The cache is constructed here (not injected) so NewServer's signature is
	// unchanged and a Redis-backed cache.Cache can later replace NewMemory.
	s := &Server{cfg: cfg, store: st, cache: cache.NewMemory(cacheMaxEntries)}

	// Only build limiters when configured (>0). Leaving them nil keeps the
	// middleware a no-op, which is exactly what the zero-value test config wants.
	if cfg.RateLimitRPM > 0 {
		s.rateLimiter = ratelimit.NewWindow(cfg.RateLimitRPM, time.Minute)
	}
	if cfg.LoginRateLimitRPM > 0 {
		s.loginLimiter = ratelimit.NewWindow(cfg.LoginRateLimitRPM, time.Minute)
	}
	return s
}

// ---------- error envelope ----------

// Error codes returned in the error envelope. Each maps to an HTTP status.
const (
	CodeValidation      = "VALIDATION"
	CodeUnauthenticated = "UNAUTHENTICATED"
	CodeForbidden       = "FORBIDDEN"
	CodeNotFound        = "NOT_FOUND"
	CodeConflict        = "CONFLICT"
	CodeRateLimited     = "RATE_LIMITED"
	CodeInternal        = "INTERNAL"
)

// apiError is the body of every error response, wrapped as {"error": apiError}.
// Field names a specific input that failed validation; Ref correlates a 500 to
// a server-side log entry and is only set for INTERNAL errors.
type apiError struct {
	Code    string `json:"code"`
	Message string `json:"message"`
	Field   string `json:"field,omitempty"`
	Ref     string `json:"ref,omitempty"`
}

// codeForStatus derives the error code from the HTTP status.
func codeForStatus(status int) string {
	switch status {
	case http.StatusBadRequest:
		return CodeValidation
	case http.StatusUnauthorized:
		return CodeUnauthenticated
	case http.StatusForbidden:
		return CodeForbidden
	case http.StatusNotFound:
		return CodeNotFound
	case http.StatusConflict:
		return CodeConflict
	case http.StatusTooManyRequests:
		return CodeRateLimited
	default:
		return CodeInternal
	}
}

// ---------- response helpers ----------

func writeJSON(w http.ResponseWriter, status int, v any) {
	w.Header().Set("Content-Type", "application/json; charset=utf-8")
	w.WriteHeader(status)
	if v != nil {
		_ = json.NewEncoder(w).Encode(v)
	}
}

// writeError emits the error envelope, deriving the code from the HTTP status.
func writeError(w http.ResponseWriter, status int, msg string) {
	writeJSON(w, status, map[string]apiError{"error": {
		Code:    codeForStatus(status),
		Message: msg,
	}})
}

// writeFieldError emits the error envelope naming a specific input field.
func writeFieldError(w http.ResponseWriter, status int, code, message, field string) {
	writeJSON(w, status, map[string]apiError{"error": {
		Code:    code,
		Message: message,
		Field:   field,
	}})
}

// internalError logs the real error server-side with a short correlation ref and
// returns a generic message to the client, so internal/DB details never leak.
func internalError(w http.ResponseWriter, err error) {
	// A canceled request context means the client hung up before we finished —
	// e.g. the browser (or TanStack Query) aborted an in-flight fetch on navigation.
	// That is not a server fault: don't log it as one, and don't bother writing a
	// 500 nobody will read. (Deadline-exceeded is a real timeout and still logged.)
	if errors.Is(err, context.Canceled) {
		return
	}
	buf := make([]byte, 4)
	_, _ = rand.Read(buf)
	ref := hex.EncodeToString(buf)
	log.Printf("internal error [ref=%s]: %v", ref, err)
	writeJSON(w, http.StatusInternalServerError, map[string]apiError{"error": {
		Code:    CodeInternal,
		Message: "internal error",
		Ref:     ref,
	}})
}

// handleStoreErr maps store errors to HTTP responses. Returns true if handled.
// ErrNotFound becomes a 404; any other error is logged server-side and surfaced
// to the client only as a generic message (no raw DB strings).
func handleStoreErr(w http.ResponseWriter, err error) bool {
	if err == nil {
		return false
	}
	if errors.Is(err, store.ErrNotFound) {
		writeError(w, http.StatusNotFound, "not found")
		return true
	}
	internalError(w, err)
	return true
}

// decodeJSON reads a size-capped JSON body into dst. On any failure it returns a
// generic message (parser internals are not leaked to the client).
func decodeJSON(w http.ResponseWriter, r *http.Request, dst any) bool {
	r.Body = http.MaxBytesReader(w, r.Body, maxBodyBytes)
	if err := json.NewDecoder(r.Body).Decode(dst); err != nil {
		writeError(w, http.StatusBadRequest, "invalid request body")
		return false
	}
	return true
}

// pathID parses the {id} path value as int64.
func pathID(w http.ResponseWriter, r *http.Request) (int64, bool) {
	id, err := strconv.ParseInt(r.PathValue("id"), 10, 64)
	if err != nil {
		writeError(w, http.StatusBadRequest, "invalid id")
		return 0, false
	}
	return id, true
}

func queryInt(r *http.Request, key string, fallback int) int {
	return config.MustAtoi(r.URL.Query().Get(key), fallback)
}

func queryInt64(r *http.Request, key string) int64 {
	v, _ := strconv.ParseInt(r.URL.Query().Get(key), 10, 64)
	return v
}

// pageParams parses the shared pagination/search query params for list
// endpoints: ?q= (trimmed text search, optional), ?limit= (default 50, clamped
// to 1..200) and ?offset= (default 0, floored at 0).
func pageParams(r *http.Request) (q string, limit, offset int) {
	q = strings.TrimSpace(r.URL.Query().Get("q"))
	limit = queryInt(r, "limit", 50)
	if limit < 1 {
		limit = 1
	}
	if limit > 200 {
		limit = 200
	}
	offset = queryInt(r, "offset", 0)
	if offset < 0 {
		offset = 0
	}
	return q, limit, offset
}

// audit records a mutating action against the authenticated user/org. It is
// best-effort: a logging failure never affects the client response. entityID 0
// is recorded as an empty entity_id.
func (s *Server) audit(r *http.Request, action, entity string, entityID int64) {
	c, ok := claimsFrom(r)
	if !ok {
		return
	}
	a := models.AuditLog{
		OrgID:  c.OrgID,
		UserID: &c.UserID,
		Action: action,
		Entity: entity,
	}
	if entityID != 0 {
		a.EntityID = strconv.FormatInt(entityID, 10)
	}
	_ = s.store.InsertAuditLog(r.Context(), a)
}