package proxy

import (
	"net/http"
	"net/http/httputil"
	"net/url"
	"path"
	"strings"
)

// NewReverseProxy builds a reverse proxy to target base URL with path rewriting:
// stripPrefix (/api/{alias}) + pathPrefix (/v1) + remainder, prepended to target.Path
// (so base_url https://host/api/ yields upstream /api/v1/...).
func NewReverseProxy(target *url.URL, stripPrefix, pathPrefix string, setAuth string) *httputil.ReverseProxy {
	proxy := httputil.NewSingleHostReverseProxy(target)
	orig := proxy.Director
	targetQuery := target.RawQuery
	proxy.Director = func(req *http.Request) {
		p := req.URL.Path
		if !strings.HasPrefix(p, stripPrefix) {
			orig(req)
			if setAuth != "" {
				req.Header.Set("Authorization", setAuth)
			}
			return
		}
		rest := strings.TrimPrefix(p, stripPrefix)
		rest = strings.TrimPrefix(rest, "/")
		upPath := buildUpstreamPath(target.Path, pathPrefix, rest)
		req.URL.Scheme = target.Scheme
		req.URL.Host = target.Host
		req.URL.Path = upPath
		req.URL.RawPath = ""
		if targetQuery == "" || req.URL.RawQuery == "" {
			req.URL.RawQuery = targetQuery + req.URL.RawQuery
		} else {
			req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
		}
		if setAuth != "" {
			req.Header.Set("Authorization", setAuth)
		}
	}
	return proxy
}

func buildUpstreamPath(targetPath, pathPrefix, rest string) string {
	base := strings.TrimSuffix(targetPath, "/")
	p := strings.Trim(pathPrefix, "/")
	r := strings.Trim(rest, "/")
	var segs []string
	if base != "" {
		segs = append(segs, base)
	}
	if p != "" {
		segs = append(segs, p)
	}
	if r != "" {
		segs = append(segs, r)
	}
	if len(segs) == 0 {
		return "/"
	}
	out := path.Join(segs...)
	if !strings.HasPrefix(out, "/") {
		out = "/" + out
	}
	return out
}