package proxy

import (
	"net/http"
	"net/http/httputil"
	"net/url"
	"strings"
	"time"

	"github.com/gin-gonic/gin"
)

// Upstream holds a reverse proxy for one backend service.
type Upstream struct {
	Name string
	rp   *httputil.ReverseProxy
}

// New creates an Upstream reverse proxy pointing at target (e.g. "http://localhost:5010").
func New(name, target string) *Upstream {
	u, err := url.Parse(target)
	if err != nil {
		panic("invalid upstream URL for " + name + ": " + err.Error())
	}
	rp := httputil.NewSingleHostReverseProxy(u)
	rp.Transport = &http.Transport{
		MaxIdleConns:       100,
		IdleConnTimeout:    90 * time.Second,
		DisableCompression: false,
	}
	// Custom error handler so we return JSON rather than Go's default HTML
	rp.ErrorHandler = func(w http.ResponseWriter, r *http.Request, err error) {
		w.Header().Set("Content-Type", "application/json")
		w.WriteHeader(http.StatusBadGateway)
		_, _ = w.Write([]byte(`{"code":"bad_gateway","message":"upstream unavailable"}`))
	}
	// Rewrite the request before forwarding
	orig := rp.Director
	rp.Director = func(req *http.Request) {
		orig(req)
		// Normalise away a trailing slash (gin's RedirectTrailingSlash adds one when a
		// route is registered as catch-all, e.g. /v1/nodes/*path, but upstream services
		// register the canonical no-slash form and redirect /nodes/ → /nodes — without
		// this the two redirects form an infinite loop).
		if len(req.URL.Path) > 1 && strings.HasSuffix(req.URL.Path, "/") {
			req.URL.Path = strings.TrimRight(req.URL.Path, "/")
		}
		req.Header.Set("X-Forwarded-Host", req.Host)
		req.Header.Del("X-Forwarded-For") // let httputil re-add it properly
		req.Host = u.Host
	}
	return &Upstream{Name: name, rp: rp}
}

// Handler returns a gin.HandlerFunc that proxies the request.
func (up *Upstream) Handler() gin.HandlerFunc {
	return func(c *gin.Context) {
		up.rp.ServeHTTP(c.Writer, c.Request)
	}
}

// StripPrefixHandler proxies after stripping a URL prefix (e.g. "/api" → "").
func (up *Upstream) StripPrefixHandler(prefix string) gin.HandlerFunc {
	return func(c *gin.Context) {
		c.Request.URL.Path = strings.TrimPrefix(c.Request.URL.Path, prefix)
		if c.Request.URL.Path == "" {
			c.Request.URL.Path = "/"
		}
		up.rp.ServeHTTP(c.Writer, c.Request)
	}
}