package handler

import (
	"bufio"
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"strings"
	"time"

	"github.com/git-pkgs/proxy/internal/config/cargo"
	"github.com/git-pkgs/purl"
)

const (
	cargoIndexLen1 = 1
	cargoIndexLen2 = 2
	cargoIndexLen3 = 3
)

// CargoHandler handles cargo registry protocol requests.
type CargoHandler struct {
	proxy       *Proxy
	path        string
	indexURL    string
	downloadURL string
	proxyURL    string
}

// NewCargoHandler creates a new cargo protocol handler.
func NewCargoHandler(proxy *Proxy, proxyURL string, cfg cargo.RouteConfig) *CargoHandler {
	return &CargoHandler{
		proxy:       proxy,
		path:        cfg.Path,
		indexURL:    cfg.Upstream[0].Index,
		downloadURL: cfg.Upstream[0].Crates,
		proxyURL:    strings.TrimSuffix(proxyURL, "/"),
	}
}

func (h *CargoHandler) Path() string {
	return h.path
}

// Routes returns the HTTP handler for cargo requests.
// Mount this at /cargo on your router.
func (h *CargoHandler) Routes() http.Handler {
	mux := http.NewServeMux()

	// Config endpoint
	mux.HandleFunc("GET /config.json", h.handleConfig)

	// Sparse index endpoints
	// Crate names 1-2 chars: /1/{name} or /2/{name}
	// Crate names 3 chars: /3/{first_char}/{name}
	// Crate names 4+ chars: /{first_two}/{second_two}/{name}
	mux.HandleFunc("GET /1/{name}", h.handleIndex)
	mux.HandleFunc("GET /2/{name}", h.handleIndex)
	mux.HandleFunc("GET /3/{a}/{name}", h.handleIndex)
	mux.HandleFunc("GET /{a}/{b}/{name}", h.handleIndex)

	// Download endpoint
	mux.HandleFunc("GET /crates/{name}/{version}/download", h.handleDownload)

	return mux
}

// CargoConfig is the registry configuration returned by config.json.
type CargoConfig struct {
	DL  string `json:"dl"`
	API string `json:"api,omitempty"`
}

// handleConfig returns the registry configuration.
func (h *CargoHandler) handleConfig(w http.ResponseWriter, r *http.Request) {
	config := CargoConfig{
		DL: h.proxyURL + h.path + "/crates/{crate}/{version}/download",
	}

	w.Header().Set("Content-Type", "application/json")
	_ = json.NewEncoder(w).Encode(config)
}

// handleIndex proxies the crate index from upstream.
func (h *CargoHandler) handleIndex(w http.ResponseWriter, r *http.Request) {
	name := r.PathValue("name")
	if name == "" {
		http.Error(w, "not found", http.StatusNotFound)
		return
	}

	h.proxy.Logger.Info("cargo index request", "crate", name)

	indexPath := h.buildIndexPath(name)
	upstreamURL := fmt.Sprintf("%s/%s", h.indexURL, indexPath)

	body, contentType, err := h.proxy.FetchOrCacheMetadata(r.Context(), "cargo", name, upstreamURL, "text/plain")
	if err != nil {
		if errors.Is(err, ErrUpstreamNotFound) {
			http.Error(w, "not found", http.StatusNotFound)
			return
		}
		h.proxy.Logger.Error("failed to fetch upstream index", "error", err)
		http.Error(w, "failed to fetch from upstream", http.StatusBadGateway)
		return
	}

	if contentType == "" {
		contentType = "text/plain; charset=utf-8"
	}

	w.Header().Set("Content-Type", contentType)
	w.WriteHeader(http.StatusOK)
	h.applyCooldownFiltering(w, body)
}

type crateIndexEntry struct {
	Name        string `json:"name"`
	Version     string `json:"vers"`
	PublishTime string `json:"pubtime,omitempty"`
}

func (h *CargoHandler) applyCooldownFiltering(downstreamResponse http.ResponseWriter, body []byte) {
	if h.proxy.Cooldown == nil || !h.proxy.Cooldown.Enabled() {
		_, _ = downstreamResponse.Write(body)
		return
	}

	scanner := bufio.NewScanner(strings.NewReader(string(body)))

	for scanner.Scan() {
		line := scanner.Text()

		var crate crateIndexEntry
		err := json.Unmarshal([]byte(line), &crate)

		if err != nil {
			h.proxy.Logger.Error("failed to parse json entry in index", "error", err)
			continue
		}

		publishedAt, err := time.Parse(time.RFC3339, crate.PublishTime)

		if crate.PublishTime == "" || err != nil {
			_, _ = downstreamResponse.Write([]byte(line + "\n"))
			continue
		}

		cratePURL := purl.MakePURLString("cargo", crate.Name, "")

		if !h.proxy.Cooldown.IsAllowed("cargo", cratePURL, publishedAt) {
			h.proxy.Logger.Info("cooldown: filtering cargo version",
				"crate", crate.Name, "version", crate.Version,
				"published", crate.PublishTime)
			continue
		}

		_, _ = downstreamResponse.Write([]byte(line + "\n"))
	}

	if err := scanner.Err(); err != nil {
		h.proxy.Logger.Error("error reading index response", "error", err)
	}
}

// buildIndexPath builds the sparse index path for a crate name.
func (h *CargoHandler) buildIndexPath(name string) string {
	name = strings.ToLower(name)

	switch len(name) {
	case cargoIndexLen1:
		return fmt.Sprintf("1/%s", name)
	case cargoIndexLen2:
		return fmt.Sprintf("2/%s", name)
	case cargoIndexLen3:
		return fmt.Sprintf("3/%c/%s", name[0], name)
	default:
		return fmt.Sprintf("%s/%s/%s", name[0:2], name[2:4], name)
	}
}

// handleDownload serves a crate file, fetching and caching from upstream if needed.
func (h *CargoHandler) handleDownload(w http.ResponseWriter, r *http.Request) {
	name := r.PathValue("name")
	version := r.PathValue("version")

	if name == "" || version == "" {
		http.Error(w, "invalid request", http.StatusBadRequest)
		return
	}

	filename := fmt.Sprintf("%s-%s.crate", name, version)

	h.proxy.Logger.Info("cargo download request",
		"crate", name, "version", version, "filename", filename)

	result, err := h.proxy.GetOrFetchArtifact(r.Context(), "cargo", name, version, filename)
	if err != nil {
		h.proxy.Logger.Error("failed to get artifact", "error", err)
		http.Error(w, "failed to fetch crate", http.StatusBadGateway)
		return
	}

	ServeArtifact(w, result)
}