// Copyright (c) 2026, Pim van Pelt <pim@ipng.ch>

package config

import (
	"fmt"
	"net"
	"os"
	"regexp"
	"strconv"
	"strings"
	"time"

	"gopkg.in/yaml.v3"
)

// Config is the top-level parsed and validated configuration.
type Config struct {
	HealthChecker HealthCheckerConfig
	HealthChecks  map[string]HealthCheck
	Backends      map[string]Backend
	Frontends     map[string]Frontend
}

// HealthCheckerConfig holds global health checker settings.
type HealthCheckerConfig struct {
	TransitionHistory int
	Netns             string // network namespace for probes; "" = current netns
}

// HealthCheck describes how to probe a backend.
type HealthCheck struct {
	Type         string
	Port         uint16      // destination port; required for tcp/http/https
	HTTP         *HTTPParams // non-nil for type http and https
	TCP          *TCPParams  // non-nil for type tcp
	ProbeIPv4Src net.IP      // source address for IPv4 probes; nil = OS picks
	ProbeIPv6Src net.IP      // source address for IPv6 probes; nil = OS picks
	Interval     time.Duration
	FastInterval time.Duration // optional; used while health counter is degraded
	DownInterval time.Duration // optional; used while fully down
	Timeout      time.Duration
	Rise         int // default 2
	Fall         int // default 3
}

// HTTPParams holds validated parameters for http/https health checks.
type HTTPParams struct {
	Path               string
	Host               string         // Host header; defaults to backend IP if empty
	ResponseCodeMin    int            // inclusive lower bound; default 200
	ResponseCodeMax    int            // inclusive upper bound; default 200
	ResponseRegexp     *regexp.Regexp // nil if not configured
	ServerName         string         // TLS SNI; falls back to Host if empty (https only)
	InsecureSkipVerify bool           // skip TLS certificate verification (https only)
}

// TCPParams holds validated parameters for tcp health checks.
type TCPParams struct {
	SSL                bool
	ServerName         string
	InsecureSkipVerify bool
}

// Backend is a single named backend server.
type Backend struct {
	Address     net.IP
	HealthCheck string // name reference into Config.HealthChecks; "" = no probing, assume healthy
	Enabled     bool   // default true; false = exclude from serving entirely
}

// PoolBackend is a backend reference within a pool, with pool-local weight.
type PoolBackend struct {
	Weight int // 0-100, default 100
}

// Pool is an ordered tier of backends within a frontend.
type Pool struct {
	Name     string
	Backends map[string]PoolBackend // keyed by backend name
}

// Frontend is a single virtual IP entry.
type Frontend struct {
	Description string
	Address     net.IP
	Protocol    string // "tcp", "udp", or "" (all traffic)
	Port        uint16 // 0 means omitted (all ports)
	Pools       []Pool // ordered tiers; first pool with any up backend is active
}

// ---- raw YAML types --------------------------------------------------------

type rawConfig struct {
	Maglev rawMaglev `yaml:"maglev"`
}

type rawMaglev struct {
	HealthChecker rawHealthCheckerCfg       `yaml:"healthchecker"`
	HealthChecks  map[string]rawHealthCheck `yaml:"healthchecks"`
	Backends      map[string]rawBackend     `yaml:"backends"`
	Frontends     map[string]rawFrontend    `yaml:"frontends"`
}

type rawHealthCheckerCfg struct {
	TransitionHistory int    `yaml:"transition-history"`
	Netns             string `yaml:"netns"`
}

type rawHealthCheck struct {
	Type         string    `yaml:"type"`
	Port         uint16    `yaml:"port"`
	Params       rawParams `yaml:"params"`
	ProbeIPv4Src string    `yaml:"probe-ipv4-src"`
	ProbeIPv6Src string    `yaml:"probe-ipv6-src"`
	Interval     string    `yaml:"interval"`
	FastInterval string    `yaml:"fast-interval"`
	DownInterval string    `yaml:"down-interval"`
	Timeout      string    `yaml:"timeout"`
	Rise         int       `yaml:"rise"`
	Fall         int       `yaml:"fall"`
}

type rawParams struct {
	// HTTP / HTTPS
	Path               string `yaml:"path"`
	Host               string `yaml:"host"`
	ResponseCode       string `yaml:"response-code"`
	ResponseRegexp     string `yaml:"response-regexp"`
	ServerName         string `yaml:"server-name"`
	InsecureSkipVerify bool   `yaml:"insecure-skip-verify"`
	// TCP
	SSL bool `yaml:"ssl"`
}

type rawBackend struct {
	Address     string `yaml:"address"`
	HealthCheck string `yaml:"healthcheck"`
	Enabled     *bool  `yaml:"enabled"` // nil → default true
}

type rawPoolBackend struct {
	Weight *int `yaml:"weight"` // nil → default 100
}

type rawPool struct {
	Name     string                    `yaml:"name"`
	Backends map[string]rawPoolBackend `yaml:"backends"`
}

type rawFrontend struct {
	Description string    `yaml:"description"`
	Address     string    `yaml:"address"`
	Protocol    string    `yaml:"protocol"`
	Port        uint16    `yaml:"port"`
	Pools       []rawPool `yaml:"pools"`
}

// ---- Load ------------------------------------------------------------------

// Load reads and validates the config file at path.
func Load(path string) (*Config, error) {
	data, err := os.ReadFile(path)
	if err != nil {
		return nil, fmt.Errorf("read config %q: %w", path, err)
	}
	return parse(data)
}

func parse(data []byte) (*Config, error) {
	var raw rawConfig
	if err := yaml.Unmarshal(data, &raw); err != nil {
		return nil, fmt.Errorf("parse yaml: %w", err)
	}
	return convert(&raw.Maglev)
}

func convert(r *rawMaglev) (*Config, error) {
	cfg := &Config{}

	// ---- healthchecker --------------------------------------------------------
	cfg.HealthChecker.Netns = r.HealthChecker.Netns
	cfg.HealthChecker.TransitionHistory = r.HealthChecker.TransitionHistory
	if cfg.HealthChecker.TransitionHistory == 0 {
		cfg.HealthChecker.TransitionHistory = 5
	}
	if cfg.HealthChecker.TransitionHistory < 1 {
		return nil, fmt.Errorf("healthchecker.transition-history must be >= 1")
	}

	// ---- healthchecks ---------------------------------------------------------
	cfg.HealthChecks = make(map[string]HealthCheck, len(r.HealthChecks))
	for name, rh := range r.HealthChecks {
		hc, err := convertHealthCheck(&rh)
		if err != nil {
			return nil, fmt.Errorf("healthcheck %q: %w", name, err)
		}
		cfg.HealthChecks[name] = hc
	}

	// ---- backends -------------------------------------------------------------
	cfg.Backends = make(map[string]Backend, len(r.Backends))
	for name, rb := range r.Backends {
		b, err := convertBackend(name, &rb, cfg.HealthChecks)
		if err != nil {
			return nil, fmt.Errorf("backend %q: %w", name, err)
		}
		cfg.Backends[name] = b
	}

	// ---- frontends ------------------------------------------------------------
	cfg.Frontends = make(map[string]Frontend, len(r.Frontends))
	for name, rf := range r.Frontends {
		fe, err := convertFrontend(name, &rf, cfg.Backends)
		if err != nil {
			return nil, fmt.Errorf("frontend %q: %w", name, err)
		}
		cfg.Frontends[name] = fe
	}

	return cfg, nil
}

func convertHealthCheck(r *rawHealthCheck) (HealthCheck, error) {
	h := HealthCheck{Type: r.Type, Port: r.Port}

	switch r.Type {
	case "icmp":
		// ICMP does not use ports.
		if r.Port != 0 {
			return HealthCheck{}, fmt.Errorf("type icmp does not use a port")
		}
	case "tcp":
		if r.Port == 0 {
			return HealthCheck{}, fmt.Errorf("type tcp requires port")
		}
		h.TCP = &TCPParams{
			SSL:                r.Params.SSL,
			ServerName:         r.Params.ServerName,
			InsecureSkipVerify: r.Params.InsecureSkipVerify,
		}
	case "http", "https":
		if r.Port == 0 {
			return HealthCheck{}, fmt.Errorf("type %s requires port", r.Type)
		}
		if r.Params.Path == "" {
			return HealthCheck{}, fmt.Errorf("type http requires params.path")
		}
		min, max, err := parseCodeRange(r.Params.ResponseCode, 200)
		if err != nil {
			return HealthCheck{}, err
		}
		hp := &HTTPParams{
			Path:               r.Params.Path,
			Host:               r.Params.Host,
			ResponseCodeMin:    min,
			ResponseCodeMax:    max,
			InsecureSkipVerify: r.Params.InsecureSkipVerify,
		}
		// TLS SNI: server-name takes precedence, falls back to host.
		hp.ServerName = r.Params.ServerName
		if hp.ServerName == "" {
			hp.ServerName = r.Params.Host
		}
		if r.Params.ResponseRegexp != "" {
			re, err := regexp.Compile(r.Params.ResponseRegexp)
			if err != nil {
				return HealthCheck{}, fmt.Errorf("invalid response-regexp %q: %w", r.Params.ResponseRegexp, err)
			}
			hp.ResponseRegexp = re
		}
		h.HTTP = hp
	default:
		return HealthCheck{}, fmt.Errorf("type must be \"icmp\", \"tcp\", \"http\", or \"https\", got %q", r.Type)
	}

	var err error
	if r.ProbeIPv4Src != "" {
		if h.ProbeIPv4Src, err = parseOptionalIPFamily(r.ProbeIPv4Src, 4, "probe-ipv4-src"); err != nil {
			return HealthCheck{}, err
		}
	}
	if r.ProbeIPv6Src != "" {
		if h.ProbeIPv6Src, err = parseOptionalIPFamily(r.ProbeIPv6Src, 6, "probe-ipv6-src"); err != nil {
			return HealthCheck{}, err
		}
	}

	if r.Interval == "" {
		return HealthCheck{}, fmt.Errorf("interval is required")
	}
	if h.Interval, err = time.ParseDuration(r.Interval); err != nil || h.Interval <= 0 {
		return HealthCheck{}, fmt.Errorf("interval %q must be a positive duration", r.Interval)
	}
	if r.FastInterval != "" {
		if h.FastInterval, err = time.ParseDuration(r.FastInterval); err != nil || h.FastInterval <= 0 {
			return HealthCheck{}, fmt.Errorf("fast-interval %q must be a positive duration", r.FastInterval)
		}
	}
	if r.DownInterval != "" {
		if h.DownInterval, err = time.ParseDuration(r.DownInterval); err != nil || h.DownInterval <= 0 {
			return HealthCheck{}, fmt.Errorf("down-interval %q must be a positive duration", r.DownInterval)
		}
	}
	if r.Timeout == "" {
		return HealthCheck{}, fmt.Errorf("timeout is required")
	}
	if h.Timeout, err = time.ParseDuration(r.Timeout); err != nil || h.Timeout <= 0 {
		return HealthCheck{}, fmt.Errorf("timeout %q must be a positive duration", r.Timeout)
	}

	h.Fall = r.Fall
	if h.Fall == 0 {
		h.Fall = 3
	}
	if h.Fall < 1 {
		return HealthCheck{}, fmt.Errorf("fall must be >= 1")
	}
	h.Rise = r.Rise
	if h.Rise == 0 {
		h.Rise = 2
	}
	if h.Rise < 1 {
		return HealthCheck{}, fmt.Errorf("rise must be >= 1")
	}

	return h, nil
}

func convertBackend(name string, r *rawBackend, hcs map[string]HealthCheck) (Backend, error) {
	ip := net.ParseIP(r.Address)
	if ip == nil {
		return Backend{}, fmt.Errorf("invalid address %q", r.Address)
	}

	b := Backend{
		Address:     ip,
		HealthCheck: r.HealthCheck,
		Enabled:     boolDefault(r.Enabled, true),
	}

	if b.HealthCheck != "" {
		if _, ok := hcs[b.HealthCheck]; !ok {
			return Backend{}, fmt.Errorf("healthcheck %q not defined", b.HealthCheck)
		}
	}

	return b, nil
}

func convertFrontend(name string, r *rawFrontend, backends map[string]Backend) (Frontend, error) {
	fe := Frontend{
		Description: r.Description,
		Protocol:    r.Protocol,
		Port:        r.Port,
	}

	ip := net.ParseIP(r.Address)
	if ip == nil {
		return Frontend{}, fmt.Errorf("invalid address %q", r.Address)
	}
	fe.Address = ip

	switch r.Protocol {
	case "", "tcp", "udp":
	default:
		return Frontend{}, fmt.Errorf("protocol must be \"tcp\", \"udp\", or omitted, got %q", r.Protocol)
	}
	if r.Port != 0 && r.Protocol == "" {
		return Frontend{}, fmt.Errorf("port requires protocol to be set")
	}
	if r.Protocol != "" && r.Port == 0 {
		return Frontend{}, fmt.Errorf("protocol %q requires port to be set (1-65535)", r.Protocol)
	}

	if len(r.Pools) == 0 {
		return Frontend{}, fmt.Errorf("pools must not be empty")
	}
	var firstFamily int
	firstBackend := true
	for pi, rp := range r.Pools {
		if rp.Name == "" {
			return Frontend{}, fmt.Errorf("pools[%d].name must not be empty", pi)
		}
		if len(rp.Backends) == 0 {
			return Frontend{}, fmt.Errorf("pool %q backends must not be empty", rp.Name)
		}
		pool := Pool{Name: rp.Name, Backends: make(map[string]PoolBackend, len(rp.Backends))}
		for bName, rpb := range rp.Backends {
			b, ok := backends[bName]
			if !ok {
				return Frontend{}, fmt.Errorf("pool %q backend %q not defined", rp.Name, bName)
			}
			fam := ipFamily(b.Address)
			if firstBackend {
				firstFamily = fam
				firstBackend = false
			} else if fam != firstFamily {
				return Frontend{}, fmt.Errorf("pool %q backend %q has different address family than first backend", rp.Name, bName)
			}
			w := intDefault(rpb.Weight, 100)
			if w < 0 || w > 100 {
				return Frontend{}, fmt.Errorf("pool %q backend %q weight %d out of range [0, 100]", rp.Name, bName, w)
			}
			pool.Backends[bName] = PoolBackend{Weight: w}
		}
		fe.Pools = append(fe.Pools, pool)
	}

	return fe, nil
}

// ---- helpers ---------------------------------------------------------------

func parseOptionalIPFamily(s string, family int, field string) (net.IP, error) {
	if s == "" {
		return nil, nil
	}
	ip := net.ParseIP(s)
	if ip == nil {
		return nil, fmt.Errorf("%s %q is not a valid IP address", field, s)
	}
	if ipFamily(ip) != family {
		return nil, fmt.Errorf("%s %q must be an IPv%d address", field, s, family)
	}
	return ip, nil
}

func ipFamily(ip net.IP) int {
	if ip.To4() != nil {
		return 4
	}
	return 6
}

func parseCodeRange(s string, defaultCode int) (min, max int, err error) {
	if s == "" {
		return defaultCode, defaultCode, nil
	}
	if idx := strings.IndexByte(s, '-'); idx > 0 {
		min, err = strconv.Atoi(s[:idx])
		if err != nil {
			return 0, 0, fmt.Errorf("invalid response-code range %q", s)
		}
		max, err = strconv.Atoi(s[idx+1:])
		if err != nil {
			return 0, 0, fmt.Errorf("invalid response-code range %q", s)
		}
		return min, max, nil
	}
	min, err = strconv.Atoi(s)
	if err != nil {
		return 0, 0, fmt.Errorf("invalid response-code %q", s)
	}
	return min, min, nil
}

func boolDefault(p *bool, def bool) bool {
	if p == nil {
		return def
	}
	return *p
}

func intDefault(p *int, def int) int {
	if p == nil {
		return def
	}
	return *p
}