package server

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

// ReadinessChecker is a function that reports whether a dependency is ready.
// Return nil if healthy, or an error describing the problem.
type ReadinessChecker func() error

// HealthHandler returns an http.Handler that exposes liveness and readiness
// endpoints:
//
//   - GET /healthz — liveness check, always returns 200 OK
//   - GET /readyz  — readiness check, returns 200 if all checkers pass, 503 otherwise
func HealthHandler(checkers ...ReadinessChecker) http.Handler {
	mux := http.NewServeMux()

	mux.HandleFunc("GET /healthz", func(w http.ResponseWriter, _ *http.Request) {
		w.Header().Set("Content-Type", "application/json")
		w.WriteHeader(http.StatusOK)
		_ = json.NewEncoder(w).Encode(healthResponse{Status: "ok"})
	})

	mux.HandleFunc("GET /readyz", func(w http.ResponseWriter, _ *http.Request) {
		w.Header().Set("Content-Type", "application/json")

		var errs []string
		for _, check := range checkers {
			if err := check(); err != nil {
				errs = append(errs, err.Error())
			}
		}

		if len(errs) > 0 {
			w.WriteHeader(http.StatusServiceUnavailable)
			_ = json.NewEncoder(w).Encode(healthResponse{
				Status: "unavailable",
				Errors: errs,
			})
			return
		}

		w.WriteHeader(http.StatusOK)
		_ = json.NewEncoder(w).Encode(healthResponse{Status: "ok"})
	})

	return mux
}

type healthResponse struct {
	Status string   `json:"status"`
	Errors []string `json:"errors,omitempty"`
}