// SPDX-License-Identifier: MIT

// Package config loads, validates and persists the prx user configuration.
//
// It also exposes the small set of validators that both the unprivileged CLI
// and the privileged helper rely on. Sharing them keeps a single source of
// truth for the values that end up in nftables scripts, ip commands and
// systemctl invocations.
package config

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"net"
	"os"
	"path/filepath"
	"regexp"
	"strings"

	"prx/internal/fsx"
)

// Config is the on-disk prx configuration.
type Config struct {
	Engine        Engine        `json:"engine"`
	Network       Network       `json:"network"`
	Marks         Marks         `json:"marks"`
	Routing       Routing       `json:"routing"`
	Bypass        Bypass        `json:"bypass"`
	ProcessBypass ProcessBypass `json:"process_bypass"`
	Health        Health        `json:"health"`
	State         StateSettings `json:"state"`
}

// Engine describes the proxy engine prx orchestrates.
type Engine struct {
	Type        string `json:"type"`
	ConfigPath  string `json:"config_path"`
	ServiceName string `json:"service_name"`
	Manage      bool   `json:"manage"`
	StopOnOff   bool   `json:"stop_on_off"`
}

// Network holds optional overrides for interface detection.
type Network struct {
	WANInterface string `json:"wan_interface"`
	WANGateway   string `json:"wan_gateway"`
	WANGateway6  string `json:"wan_gateway6"`
	DNSKeepalive bool   `json:"dns_keepalive"`
}

// Marks holds the conntrack mark used by the routing rules.
type Marks struct {
	InboundMark uint32 `json:"inbound_mark"`
}

// Routing holds the policy routing identifiers.
type Routing struct {
	TableV4    int `json:"table_v4"`
	TableV6    int `json:"table_v6"`
	PriorityV4 int `json:"priority_v4"`
	PriorityV6 int `json:"priority_v6"`

	BypassTableV4 int `json:"bypass_table_v4"`
	BypassTableV6 int `json:"bypass_table_v6"`
	BypassPrioV4  int `json:"bypass_priority_v4"`
	BypassPrioV6  int `json:"bypass_priority_v6"`
}

// Bypass lists destinations that must never go through the proxy.
type Bypass struct {
	AlwaysCIDRs []string `json:"always_cidrs"`
	ExtraCIDRs  []string `json:"extra_cidrs"`
	Domains     []string `json:"domains"`
}

// ProcessBypass lists process names the engine should bypass.
type ProcessBypass struct {
	Names []string `json:"names"`
}

// Health configures the post-apply connectivity probe.
type Health struct {
	// Probe is a host:port dialed after enabling to confirm connectivity.
	// An empty probe disables the check.
	Probe string `json:"probe"`
}

// StateSettings points at the runtime state snapshot.
type StateSettings struct {
	Path string `json:"path"`
}

const (
	// DefaultStatePath is where the helper records its snapshot.
	DefaultStatePath = "/run/prx/state.json"
	// DefaultEngineConfig is the sing-box configuration path.
	DefaultEngineConfig = "/etc/sing-box/config.json"
	// DefaultServiceName is the sing-box systemd unit name.
	DefaultServiceName = "sing-box"
	// DefaultMark is the conntrack mark (0x100).
	DefaultMark uint32 = 0x100
	// DefaultHealthProbe is the connectivity probe dialed after applying.
	DefaultHealthProbe = "1.1.1.1:443"
	// MaxTableID is the largest usable custom route table identifier.
	MaxTableID = 2147483646
	// MaxPriority is the largest usable ip rule priority.
	MaxPriority = 32765
)

// reservedRouteTables are the route table identifiers reserved by the kernel.
var reservedRouteTables = map[int]bool{0: true, 253: true, 254: true, 255: true}

var (
	ifaceRE   = regexp.MustCompile(`^[A-Za-z0-9_.:@-]{1,15}$`)
	serviceRE = regexp.MustCompile(`^[A-Za-z0-9_.@:-]{1,128}$`)
	// domainRE accepts a DNS name, optionally with a leading wildcard label.
	domainRE = regexp.MustCompile(`^(\*\.)?([A-Za-z0-9]([A-Za-z0-9-]{0,61}[A-Za-z0-9])?\.)+[A-Za-z]{2,63}$`)
)

// ValidInterfaceName reports whether name is a safe network interface name.
func ValidInterfaceName(name string) bool { return ifaceRE.MatchString(name) }

// ValidServiceName reports whether name is a safe systemd unit name.
func ValidServiceName(name string) bool { return serviceRE.MatchString(name) }

// ValidDomain reports whether d is a plausible DNS name (optionally wildcard).
func ValidDomain(d string) bool { return domainRE.MatchString(strings.TrimSpace(d)) }

// CheckTableID validates a route table identifier for the named field.
func CheckTableID(field string, id int) error {
	if id < 1 || id > MaxTableID {
		return fmt.Errorf("%s must be between 1 and %d, got %d", field, MaxTableID, id)
	}
	if reservedRouteTables[id] {
		return fmt.Errorf("%s must not use reserved table %d", field, id)
	}
	return nil
}

// CheckPriority validates an ip rule priority for the named field.
func CheckPriority(field string, p int) error {
	if p < 1 || p > MaxPriority {
		return fmt.Errorf("%s must be between 1 and %d, got %d", field, MaxPriority, p)
	}
	return nil
}

// CheckDomain validates a bypass domain for the named field.
func CheckDomain(field, d string) error {
	if strings.TrimSpace(d) == "" {
		return fmt.Errorf("%s must not be empty", field)
	}
	if !ValidDomain(d) {
		return fmt.Errorf("%s %q is not a valid domain name", field, d)
	}
	return nil
}

// Default returns the built-in configuration.
func Default() *Config {
	return &Config{
		Engine: Engine{
			Type:        "sing-box",
			ConfigPath:  DefaultEngineConfig,
			ServiceName: DefaultServiceName,
			Manage:      true,
			StopOnOff:   false,
		},
		Network: Network{DNSKeepalive: true},
		Marks:   Marks{InboundMark: DefaultMark},
		Routing: Routing{
			TableV4:       100,
			TableV6:       101,
			PriorityV4:    100,
			PriorityV6:    100,
			BypassTableV4: 102,
			BypassTableV6: 103,
			BypassPrioV4:  50,
			BypassPrioV6:  50,
		},
		Bypass: Bypass{
			AlwaysCIDRs: []string{
				"127.0.0.0/8",
				"10.0.0.0/8",
				"172.16.0.0/12",
				"192.168.0.0/16",
				"169.254.0.0/16",
				"::1/128",
				"fc00::/7",
				"fe80::/10",
			},
			ExtraCIDRs: []string{},
			Domains:    []string{},
		},
		ProcessBypass: ProcessBypass{Names: []string{"ssh", "scp", "sftp"}},
		Health:        Health{Probe: DefaultHealthProbe},
		State:         StateSettings{Path: DefaultStatePath},
	}
}

// DefaultPath resolves the configuration file location.
func DefaultPath() string {
	if p := os.Getenv("PRX_CONFIG"); p != "" {
		return p
	}
	base := os.Getenv("XDG_CONFIG_HOME")
	if base == "" {
		home, err := os.UserHomeDir()
		if err == nil && home != "" {
			base = filepath.Join(home, ".config")
		} else {
			base = "."
		}
	}
	return filepath.Join(base, "prx", "config.json")
}

// Exists reports whether a configuration file is present at path.
func Exists(path string) bool {
	_, err := os.Stat(path)
	return err == nil
}

// Load reads the configuration from path. A missing file yields defaults.
//
// Unknown fields are rejected so typos surface immediately instead of being
// silently ignored.
func Load(path string) (*Config, error) {
	raw, err := os.ReadFile(path)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			return Default(), nil
		}
		return nil, fmt.Errorf("read config %s: %w", path, err)
	}
	cfg := Default()
	dec := json.NewDecoder(bytes.NewReader(raw))
	dec.DisallowUnknownFields()
	if err := dec.Decode(cfg); err != nil {
		return nil, fmt.Errorf("parse config %s: %w", path, err)
	}
	if err := cfg.Validate(); err != nil {
		return nil, fmt.Errorf("config %s: %w", path, err)
	}
	return cfg, nil
}

// Save validates and atomically writes the configuration to path.
func Save(path string, cfg *Config) error {
	if err := cfg.Validate(); err != nil {
		return err
	}
	raw, err := json.MarshalIndent(cfg, "", "  ")
	if err != nil {
		return fmt.Errorf("encode config: %w", err)
	}
	raw = append(raw, '\n')
	if err := fsx.WriteFileAtomic(path, raw, 0o644); err != nil {
		return fmt.Errorf("write config %s: %w", path, err)
	}
	return nil
}

// Validate checks the configuration for internally consistent, safe values.
func (c *Config) Validate() error {
	if c.Engine.Type == "" {
		return errors.New("engine.type must not be empty")
	}
	if c.Engine.Manage {
		if c.Engine.ServiceName == "" {
			return errors.New("engine.service_name must not be empty when engine.manage is true")
		}
		if !ValidServiceName(c.Engine.ServiceName) {
			return fmt.Errorf("engine.service_name %q contains invalid characters", c.Engine.ServiceName)
		}
		if c.Engine.ConfigPath != "" && !filepath.IsAbs(c.Engine.ConfigPath) {
			return fmt.Errorf("engine.config_path %q must be absolute", c.Engine.ConfigPath)
		}
	}
	if c.Network.WANInterface != "" && !ValidInterfaceName(c.Network.WANInterface) {
		return fmt.Errorf("network.wan_interface %q is not a valid interface name", c.Network.WANInterface)
	}
	if c.Network.WANGateway != "" && net.ParseIP(c.Network.WANGateway) == nil {
		return fmt.Errorf("network.wan_gateway %q is not a valid IP address", c.Network.WANGateway)
	}
	if c.Network.WANGateway6 != "" {
		ip := net.ParseIP(c.Network.WANGateway6)
		if ip == nil || ip.To4() != nil {
			return fmt.Errorf("network.wan_gateway6 %q is not a valid IPv6 address", c.Network.WANGateway6)
		}
	}
	if c.Marks.InboundMark == 0 {
		return errors.New("marks.inbound_mark must be non-zero")
	}
	for _, check := range []struct {
		field string
		value int
	}{
		{"routing.table_v4", c.Routing.TableV4},
		{"routing.table_v6", c.Routing.TableV6},
		{"routing.bypass_table_v4", c.Routing.BypassTableV4},
		{"routing.bypass_table_v6", c.Routing.BypassTableV6},
	} {
		if err := CheckTableID(check.field, check.value); err != nil {
			return err
		}
	}
	for _, check := range []struct {
		field string
		value int
	}{
		{"routing.priority_v4", c.Routing.PriorityV4},
		{"routing.priority_v6", c.Routing.PriorityV6},
		{"routing.bypass_priority_v4", c.Routing.BypassPrioV4},
		{"routing.bypass_priority_v6", c.Routing.BypassPrioV6},
	} {
		if err := CheckPriority(check.field, check.value); err != nil {
			return err
		}
	}
	for _, group := range [][]string{c.Bypass.AlwaysCIDRs, c.Bypass.ExtraCIDRs} {
		for _, cidr := range group {
			if _, err := ParseCIDR(cidr); err != nil {
				return fmt.Errorf("bypass: %w", err)
			}
		}
	}
	for _, d := range c.Bypass.Domains {
		if err := CheckDomain("bypass.domains", d); err != nil {
			return err
		}
	}
	if c.Health.Probe != "" {
		host, port, err := net.SplitHostPort(c.Health.Probe)
		if err != nil {
			return fmt.Errorf("health.probe %q must be host:port: %w", c.Health.Probe, err)
		}
		if host == "" {
			return fmt.Errorf("health.probe %q must include a host", c.Health.Probe)
		}
		if n, err := net.LookupPort("tcp", port); err != nil || n < 1 {
			return fmt.Errorf("health.probe %q has an invalid port", c.Health.Probe)
		}
	}
	if c.State.Path == "" {
		return errors.New("state.path must not be empty")
	}
	if !filepath.IsAbs(c.State.Path) {
		return fmt.Errorf("state.path %q must be absolute", c.State.Path)
	}
	return nil
}

// ParseCIDR validates and normalises a CIDR or bare IP into CIDR form.
func ParseCIDR(v string) (string, error) {
	v = strings.TrimSpace(v)
	if v == "" {
		return "", errors.New("empty address")
	}
	if ip, ipnet, err := net.ParseCIDR(v); err == nil {
		ones, _ := ipnet.Mask.Size()
		return fmt.Sprintf("%s/%d", ip.String(), ones), nil
	}
	if ip := net.ParseIP(v); ip != nil {
		if ip.To4() != nil {
			return ip.String() + "/32", nil
		}
		return ip.String() + "/128", nil
	}
	return "", fmt.Errorf("invalid CIDR or IP %q", v)
}

// AddExtraCIDR appends a normalised CIDR to the extra bypass list, reporting
// whether the list changed.
func (c *Config) AddExtraCIDR(v string) (bool, error) {
	cidr, err := ParseCIDR(v)
	if err != nil {
		return false, err
	}
	if err := guardBypassCIDR(cidr); err != nil {
		return false, err
	}
	for _, existing := range c.Bypass.ExtraCIDRs {
		if existing == cidr {
			return false, nil
		}
	}
	c.Bypass.ExtraCIDRs = append(c.Bypass.ExtraCIDRs, cidr)
	return true, nil
}

// RemoveExtraCIDR removes a normalised CIDR from the extra bypass list.
func (c *Config) RemoveExtraCIDR(v string) (bool, error) {
	cidr, err := ParseCIDR(v)
	if err != nil {
		return false, err
	}
	out := c.Bypass.ExtraCIDRs[:0]
	changed := false
	for _, existing := range c.Bypass.ExtraCIDRs {
		if existing == cidr {
			changed = true
			continue
		}
		out = append(out, existing)
	}
	c.Bypass.ExtraCIDRs = out
	return changed, nil
}

// AddDomain appends a validated domain to the bypass list.
func (c *Config) AddDomain(d string) (bool, error) {
	d = strings.TrimSpace(d)
	if err := CheckDomain("domain", d); err != nil {
		return false, err
	}
	for _, existing := range c.Bypass.Domains {
		if strings.EqualFold(existing, d) {
			return false, nil
		}
	}
	c.Bypass.Domains = append(c.Bypass.Domains, d)
	return true, nil
}

// RemoveDomain removes a domain (case-insensitively) from the bypass list.
func (c *Config) RemoveDomain(d string) bool {
	d = strings.TrimSpace(d)
	out := c.Bypass.Domains[:0]
	changed := false
	for _, existing := range c.Bypass.Domains {
		if strings.EqualFold(existing, d) {
			changed = true
			continue
		}
		out = append(out, existing)
	}
	c.Bypass.Domains = out
	return changed
}

// guardBypassCIDR refuses values that would blackhole essential traffic.
func guardBypassCIDR(cidr string) error {
	switch cidr {
	case "0.0.0.0/0", "::/0":
		return fmt.Errorf("refusing to add default route %s to bypass list", cidr)
	}
	return nil
}

// IsCIDR reports whether v parses as a CIDR or bare IP.
func IsCIDR(v string) bool {
	_, err := ParseCIDR(v)
	return err == nil
}