package main

import (
	"fmt"
	"os"
	"os/exec"
	"sort"
	"strings"
)

// renderDnsmasqConfig builds the content of the generated dnsmasq include
// file from the current host list. Every host gets a host-record (forward +
// reverse DNS). Hosts with a MAC additionally get a dhcp-host static lease.
func renderDnsmasqConfig(hosts []Host) string {
	sorted := make([]Host, len(hosts))
	copy(sorted, hosts)
	sort.Slice(sorted, func(i, j int) bool {
		return strings.ToLower(sorted[i].Name) < strings.ToLower(sorted[j].Name)
	})

	var b strings.Builder
	b.WriteString("# Generated by ipadm -u — DO NOT EDIT MANUALLY.\n")
	b.WriteString("# Edit hosts with `ipadm` and re-run `ipadm -u` to regenerate this file.\n\n")
	for _, h := range sorted {
		if h.Comment != "" {
			fmt.Fprintf(&b, "# %s: %s\n", h.Name, h.Comment)
		}
		fmt.Fprintf(&b, "host-record=%s,%s\n", h.Name, h.IP)
		if h.MAC != "" {
			fmt.Fprintf(&b, "dhcp-host=%s,%s,%s,infinite\n", h.MAC, h.IP, h.Name)
		}
		b.WriteString("\n")
	}
	return b.String()
}

// applyDnsmasqConfig writes the generated config to path, validates the full
// dnsmasq configuration, and on success reloads the dnsmasq service. On
// validation failure the previous file content is restored and the service
// is left untouched.
func applyDnsmasqConfig(path string, content string) error {
	var backup []byte
	hadFile := false
	if b, err := os.ReadFile(path); err == nil {
		backup = b
		hadFile = true
	} else if !os.IsNotExist(err) {
		return fmt.Errorf("cannot read existing %s: %w", path, err)
	}

	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
		return fmt.Errorf("cannot write %s: %w", path, err)
	}

	if err := runDnsmasqTest(); err != nil {
		// roll back so a bad generated file never lingers
		if hadFile {
			os.WriteFile(path, backup, 0644)
		} else {
			os.Remove(path)
		}
		return fmt.Errorf("dnsmasq config invalid, change rolled back: %w", err)
	}

	if err := reloadDnsmasq(); err != nil {
		return fmt.Errorf("%s written, but reload failed: %w", path, err)
	}
	return nil
}

func runDnsmasqTest() error {
	out, err := exec.Command("dnsmasq", "--test").CombinedOutput()
	if err != nil {
		return fmt.Errorf("%s", strings.TrimSpace(string(out)))
	}
	return nil
}

func reloadDnsmasq() error {
	out, err := exec.Command("systemctl", "reload", "dnsmasq").CombinedOutput()
	if err != nil {
		return fmt.Errorf("%s", strings.TrimSpace(string(out)))
	}
	return nil
}