package config

import (
	"fmt"
	"net/url"
	"strings"
)

// ValidateConfig validates a loaded Config and returns a list of human-readable
// error messages. An empty slice means the config is valid.
// Called at startup — the server must exit 1 if any errors are returned.
func ValidateConfig(cfg *Config) []string {
	var errs []string

	// Issuer must be a valid URL with an http(s) scheme.
	if cfg.Issuer == "" {
		errs = append(errs, "issuer: must not be empty")
	} else {
		u, err := url.Parse(cfg.Issuer)
		if err != nil || u.Scheme == "" || u.Host == "" {
			errs = append(errs, fmt.Sprintf("issuer: %q is not a valid URL (must include scheme and host)", cfg.Issuer))
		} else if u.Scheme != "http" && u.Scheme != "https" {
			errs = append(errs, fmt.Sprintf("issuer: scheme must be http or https, got %q", u.Scheme))
		}
	}

	// Port must be in the valid TCP range.
	if cfg.Port < 1 || cfg.Port > 65535 {
		errs = append(errs, fmt.Sprintf("port: must be between 1 and 65535, got %d", cfg.Port))
	}

	// At least one client must be registered.
	if len(cfg.Clients) == 0 {
		errs = append(errs, "clients: at least one client must be defined")
	}

	// Each client must have at least one redirect URI and a non-empty clientId.
	for i, c := range cfg.Clients {
		prefix := fmt.Sprintf("clients[%d] (%s)", i, c.ClientID)
		if c.ClientID == "" {
			prefix = fmt.Sprintf("clients[%d]", i)
			errs = append(errs, prefix+": clientId must not be empty")
		}
		if len(c.RedirectURIs) == 0 {
			errs = append(errs, prefix+": redirect_uri: at least one redirectUri must be registered")
		}
		// Warn about wildcard redirect URIs (they are blocked at runtime anyway).
		for _, uri := range c.RedirectURIs {
			if strings.ContainsAny(uri, "*?") {
				errs = append(errs, prefix+fmt.Sprintf(": redirect_uri %q must not contain wildcards", uri))
			}
		}
	}

	// Private key PEM path must be provided (existence is checked at startup).
	if cfg.PrivateKeyPEM == "" {
		errs = append(errs, "privateKeyPem: path must not be empty")
	}

	return errs
}