Examples in Go

package main

import (
	"encoding/json"
	"net/http"
	"strings"
)

const linkBase = "https://link.example.com"
const plateID = "1"

type resolveEnvelope struct {
	Data struct {
		Destination string `json:"destination"`
	} `json:"data"`
}

func main() {
	http.HandleFunc("/u/", func(w http.ResponseWriter, r *http.Request) {
		path := strings.TrimPrefix(r.URL.Path, "/u/")
		parts := strings.Split(path, "/")
		if len(parts) == 0 || strings.TrimSpace(parts[0]) == "" {
			http.Error(w, "missing id", http.StatusBadRequest)
			return
		}
		id := parts[0]
		tail := ""
		if len(parts) > 1 {
			tail = strings.Join(parts[1:], "/")
		}

		resolveURL := linkBase + "/" + plateID + "/resolve/" + id
		if tail != "" {
			resolveURL += "/" + tail
		}
		if r.URL.RawQuery != "" {
			resolveURL += "?" + r.URL.RawQuery
		}

		resp, err := http.Get(resolveURL)
		if err != nil {
			http.Error(w, "resolve failed", http.StatusBadGateway)
			return
		}
		defer resp.Body.Close()
		if resp.StatusCode >= 400 {
			http.Error(w, "link not found", resp.StatusCode)
			return
		}

		var resolved resolveEnvelope
		if err := json.NewDecoder(resp.Body).Decode(&resolved); err != nil || strings.TrimSpace(resolved.Data.Destination) == "" {
			http.Error(w, "invalid resolve payload", http.StatusBadGateway)
			return
		}

		http.Redirect(w, r, resolved.Data.Destination, http.StatusTemporaryRedirect)
	})

	_ = http.ListenAndServe(":8080", nil)
}