// Package proxy contains the HTTP handlers for both gpulock listeners:
// reverse proxies to Ollama and ComfyUI with GPU lock arbitration in front
// of the endpoints that load models.
package proxy

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
	"net/http"
	"net/http/httputil"
	"net/url"
	"time"

	"gpu-turnstile/internal/comfy"
	"gpu-turnstile/internal/lock"
	"gpu-turnstile/internal/metrics"
	"gpu-turnstile/internal/ollama"
)

// captureLimit bounds how much of a /prompt response body is buffered while
// looking for prompt_id. The body still passes through to the client
// unchanged regardless of size.
const captureLimit = 64 * 1024

// Config wires a Server.
type Config struct {
	OllamaURL string
	ComfyURL  string

	Lock    *lock.Lock
	Ollama  *ollama.Client
	Comfy   *comfy.Client
	Metrics *metrics.Metrics
	Log     *slog.Logger

	LLMWaitTimeout time.Duration
	UnloadTimeout  time.Duration
	JobTimeout     time.Duration
	WarmModel      string
}

// Server serves both gpulock listeners.
type Server struct {
	cfg Config
	log *slog.Logger

	ollamaProxy *httputil.ReverseProxy
	comfyProxy  *httputil.ReverseProxy
}

// New builds a Server, validating the upstream URLs.
func New(cfg Config) (*Server, error) {
	ollamaURL, err := url.Parse(cfg.OllamaURL)
	if err != nil || ollamaURL.Scheme == "" || ollamaURL.Host == "" {
		return nil, fmt.Errorf("invalid OLLAMA_URL %q", cfg.OllamaURL)
	}
	comfyURL, err := url.Parse(cfg.ComfyURL)
	if err != nil || comfyURL.Scheme == "" || comfyURL.Host == "" {
		return nil, fmt.Errorf("invalid COMFY_URL %q", cfg.ComfyURL)
	}
	log := cfg.Log
	if log == nil {
		log = slog.Default()
	}
	return &Server{
		cfg:         cfg,
		log:         log,
		ollamaProxy: newReverseProxy(ollamaURL, log.With("upstream", "ollama")),
		comfyProxy:  newReverseProxy(comfyURL, log.With("upstream", "comfy")),
	}, nil
}

func newReverseProxy(target *url.URL, log *slog.Logger) *httputil.ReverseProxy {
	return &httputil.ReverseProxy{
		Rewrite: func(pr *httputil.ProxyRequest) {
			pr.SetURL(target)
			pr.SetXForwarded()
		},
		// Flush after every write so NDJSON/SSE streams and websocket
		// upgrades pass through unbuffered.
		FlushInterval: -1,
		ErrorHandler: func(w http.ResponseWriter, r *http.Request, err error) {
			log.Warn("upstream error", "path", r.URL.Path, "err", err)
			http.Error(w, "upstream unavailable", http.StatusBadGateway)
		},
	}
}

func (s *Server) writeHealthz(w http.ResponseWriter) {
	state, n, pending := s.cfg.Lock.Snapshot()
	w.Header().Set("Content-Type", "application/json")
	json.NewEncoder(w).Encode(map[string]any{
		"state":         state,
		"llm_inflight":  n,
		"image_pending": pending,
	})
}

func (s *Server) writeMetrics(w http.ResponseWriter) {
	state, n, pending := s.cfg.Lock.Snapshot()
	w.Header().Set("Content-Type", "text/plain; version=0.0.4")
	s.cfg.Metrics.Render(w, string(state), n, pending)
}

// llmPaths are the Ollama endpoints that load models into VRAM and therefore
// take the LLM lock. Everything else passes through unlocked.
var llmPaths = map[string]bool{
	"/api/generate":        true,
	"/api/chat":            true,
	"/api/embed":           true,
	"/api/embeddings":      true,
	"/v1/chat/completions": true,
	"/v1/completions":      true,
	"/v1/embeddings":       true,
}

func isLLMRequest(r *http.Request) bool {
	return r.Method == http.MethodPost && llmPaths[r.URL.Path]
}

// OllamaHandler serves the Ollama-facing listener.
func (s *Server) OllamaHandler() http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		switch r.URL.Path {
		case "/healthz":
			s.writeHealthz(w)
			return
		case "/metrics":
			s.writeMetrics(w)
			return
		}
		if !isLLMRequest(r) {
			s.ollamaProxy.ServeHTTP(w, r)
			return
		}

		start := time.Now()
		wctx, cancel := context.WithTimeout(r.Context(), s.cfg.LLMWaitTimeout)
		err := s.cfg.Lock.AcquireLLM(wctx)
		cancel()
		s.cfg.Metrics.ObserveLockWait("llm", time.Since(start).Seconds())
		if err != nil {
			if errors.Is(err, context.DeadlineExceeded) && r.Context().Err() == nil {
				http.Error(w, "GPU busy: timed out waiting for the lock", http.StatusServiceUnavailable)
			}
			return
		}
		defer s.cfg.Lock.ReleaseLLM()
		s.ollamaProxy.ServeHTTP(w, r)
	})
}

// ComfyHandler serves the ComfyUI-facing listener.
func (s *Server) ComfyHandler() http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.URL.Path == "/healthz" {
			s.writeHealthz(w)
			return
		}
		if r.Method == http.MethodPost && r.URL.Path == "/prompt" {
			s.handlePrompt(w, r)
			return
		}
		s.comfyProxy.ServeHTTP(w, r)
	})
}

// captureWriter passes the response through unchanged while recording the
// status code and the first captureLimit bytes of the body.
type captureWriter struct {
	http.ResponseWriter
	status int
	buf    bytes.Buffer
}

func (w *captureWriter) WriteHeader(code int) {
	w.status = code
	w.ResponseWriter.WriteHeader(code)
}

func (w *captureWriter) Write(p []byte) (int, error) {
	if w.buf.Len() < captureLimit {
		w.buf.Write(p)
	}
	return w.ResponseWriter.Write(p)
}

func (w *captureWriter) Flush() {
	if f, ok := w.ResponseWriter.(http.Flusher); ok {
		f.Flush()
	}
}

func (w *captureWriter) Unwrap() http.ResponseWriter { return w.ResponseWriter }

// handlePrompt implements the image job flow from the spec: acquire the
// image lock, unload Ollama, forward to ComfyUI, then track the job in the
// background and free VRAM before releasing the lock.
func (s *Server) handlePrompt(w http.ResponseWriter, r *http.Request) {
	log := s.log.With("op", "image")

	start := time.Now()
	if err := s.cfg.Lock.AcquireImage(r.Context()); err != nil {
		if errors.Is(err, context.DeadlineExceeded) {
			http.Error(w, "GPU busy: timed out waiting for the lock", http.StatusServiceUnavailable)
		}
		return
	}
	s.cfg.Metrics.ObserveLockWait("image", time.Since(start).Seconds())
	log.Info("image lock acquired")

	uctx, ucancel := context.WithTimeout(r.Context(), s.cfg.UnloadTimeout)
	elapsed, uerr := s.cfg.Ollama.UnloadAll(uctx)
	ucancel()
	s.cfg.Metrics.ObserveUnload(elapsed.Seconds())
	switch {
	case r.Context().Err() != nil:
		s.cfg.Lock.ReleaseImage()
		return
	case uerr != nil:
		// Degrade, don't fail the user's request on a misbehaving neighbour.
		log.Warn("ollama unload incomplete; continuing", "err", uerr)
	default:
		log.Info("ollama models unloaded", "seconds", elapsed.Seconds())
	}

	cw := &captureWriter{ResponseWriter: w, status: http.StatusOK}
	s.comfyProxy.ServeHTTP(cw, r)

	var accepted struct {
		PromptID string `json:"prompt_id"`
	}
	if cw.status == http.StatusOK {
		_ = json.Unmarshal(cw.buf.Bytes(), &accepted)
	}
	if accepted.PromptID == "" {
		log.Info("prompt not accepted; releasing image lock", "status", cw.status)
		s.cfg.Lock.ReleaseImage()
		return
	}
	s.cfg.Metrics.IncImageJobs()
	go s.finishImageJob(accepted.PromptID)
}

// finishImageJob runs after the prompt has been accepted by ComfyUI: wait
// for the job to finish, free ComfyUI's models, release the lock, and
// optionally warm the chat model.
func (s *Server) finishImageJob(promptID string) {
	log := s.log.With("prompt_id", promptID)
	log.Info("image job running")

	ctx, cancel := context.WithTimeout(context.Background(), s.cfg.JobTimeout)
	err := s.cfg.Comfy.WaitJob(ctx, promptID)
	cancel()
	if err != nil {
		log.Warn("image job did not complete cleanly; releasing lock anyway", "err", err)
	} else {
		log.Info("image job completed")
	}

	freeCtx, freeCancel := context.WithTimeout(context.Background(), 30*time.Second)
	if err := s.cfg.Comfy.Free(freeCtx); err != nil {
		log.Warn("failed to free ComfyUI models", "err", err)
	}
	freeCancel()

	s.cfg.Lock.ReleaseImage()
	log.Info("image lock released")

	if s.cfg.WarmModel != "" {
		if state, _, _ := s.cfg.Lock.Snapshot(); state == lock.StateIdle {
			wctx, wcancel := context.WithTimeout(context.Background(), 2*time.Minute)
			if err := s.cfg.Ollama.Warm(wctx, s.cfg.WarmModel); err != nil {
				log.Warn("warm model reload failed", "model", s.cfg.WarmModel, "err", err)
			} else {
				log.Info("warm model reloaded", "model", s.cfg.WarmModel)
			}
			wcancel()
		}
	}
}