// Package dial builds an HTTP client pinned to one config URL.
// The agent cannot name the target; only the operator-configured host:port is reachable.
package dial

import (
	"context"
	"crypto/tls"
	"fmt"
	"net"
	"net/http"
	"net/netip"
	"net/url"
	"strings"
	"sync"
	"syscall"
	"time"

	"git.wntrmute.dev/kyle/sift/internal/guard"
)

const DefaultRefresh = 10 * time.Minute

// Lookup resolves a hostname. Tests inject a fake.
type Lookup func(ctx context.Context, host string) ([]netip.Addr, error)

type Options struct {
	// Loopback permits loopback (and only loopback) addresses. Public sources
	// must leave this false: every record must pass guard.CheckAddr.
	Loopback bool
	Lookup   Lookup
	Refresh  time.Duration
	Timeout  time.Duration
}

// Client is an http.Client that may only connect to the config URL's host:port.
type Client struct {
	URL  *url.URL
	HTTP *http.Client
	Port string

	loopback bool
	lookup   Lookup
	refresh  time.Duration

	mu       sync.Mutex
	ips      []netip.Addr
	resolved time.Time
}

func New(raw string, opt Options) (*Client, error) {
	u, err := parseConfigURL(raw)
	if err != nil {
		return nil, err
	}
	port := u.Port()
	if port == "" {
		if u.Scheme == "https" {
			port = "443"
		} else {
			port = "80"
		}
	}
	if opt.Refresh <= 0 {
		opt.Refresh = DefaultRefresh
	}
	if opt.Timeout <= 0 {
		opt.Timeout = 8 * time.Second
	}
	lookup := opt.Lookup
	if lookup == nil {
		lookup = defaultLookup
	}
	c := &Client{
		URL:      u,
		Port:     port,
		loopback: opt.Loopback,
		lookup:   lookup,
		refresh:  opt.Refresh,
	}
	_ = c.resolve(context.Background()) // DNS down: keep the client, retry in Search
	d := &net.Dialer{
		Timeout: opt.Timeout,
		Control: c.control,
	}
	tr := &http.Transport{
		Proxy:                 nil,
		DialContext:           d.DialContext,
		ForceAttemptHTTP2:     true,
		TLSHandshakeTimeout:   10 * time.Second,
		ResponseHeaderTimeout: opt.Timeout,
		TLSClientConfig:       &tls.Config{MinVersion: tls.VersionTLS12}, // system roots; InsecureSkipVerify stays false
		DisableCompression:    false,
	}
	c.HTTP = &http.Client{
		Timeout:       opt.Timeout + time.Second,
		Transport:     tr,
		CheckRedirect: c.checkRedirect,
	}
	return c, nil
}

func parseConfigURL(raw string) (*url.URL, error) {
	u, err := url.Parse(raw)
	if err != nil {
		return nil, err
	}
	if u.Scheme != "http" && u.Scheme != "https" {
		return nil, fmt.Errorf("search upstream: scheme %q", u.Scheme)
	}
	if u.User != nil {
		return nil, fmt.Errorf("search upstream: userinfo not allowed")
	}
	if u.Hostname() == "" {
		return nil, fmt.Errorf("search upstream: empty host")
	}
	return u, nil
}

func defaultLookup(ctx context.Context, host string) ([]netip.Addr, error) {
	if ip, err := netip.ParseAddr(host); err == nil {
		return []netip.Addr{ip}, nil
	}
	return net.DefaultResolver.LookupNetIP(ctx, "ip", host)
}

func (c *Client) resolve(ctx context.Context) error {
	addrs, err := c.lookup(ctx, c.URL.Hostname())
	if err != nil {
		return fmt.Errorf("search resolve: %w", err)
	}
	if len(addrs) == 0 {
		return fmt.Errorf("search resolve: no addresses")
	}
	for i, a := range addrs {
		addrs[i] = a.Unmap()
		if err := c.checkAddr(addrs[i]); err != nil {
			return err
		}
	}
	c.mu.Lock()
	c.ips = addrs
	c.resolved = time.Now()
	c.mu.Unlock()
	return nil
}

func (c *Client) checkAddr(a netip.Addr) error {
	if c.loopback {
		if !a.IsLoopback() {
			return fmt.Errorf("search loopback source got non-loopback %s", a)
		}
		return nil
	}
	if err := guard.CheckAddr(a); err != nil {
		return fmt.Errorf("search public source: %w", err)
	}
	return nil
}

func (c *Client) allow(ip netip.Addr, port string) bool {
	if port != c.Port {
		return false
	}
	ip = ip.Unmap()
	c.mu.Lock()
	defer c.mu.Unlock()
	for _, a := range c.ips {
		if a == ip {
			return true
		}
	}
	return false
}

func (c *Client) control(network, address string, _ syscall.RawConn) error {
	switch network {
	case "tcp", "tcp4", "tcp6":
	default:
		return fmt.Errorf("search dial: network %s", network)
	}
	host, port, err := net.SplitHostPort(address)
	if err != nil {
		return fmt.Errorf("search dial: %w", err)
	}
	ip, err := netip.ParseAddr(host)
	if err != nil {
		return fmt.Errorf("search dial: not an IP")
	}
	if !c.allow(ip, port) {
		return fmt.Errorf("search dial refused")
	}
	return nil
}

func (c *Client) checkRedirect(req *http.Request, via []*http.Request) error {
	if len(via) > 1 {
		return fmt.Errorf("search redirect: extra hop")
	}
	if len(via) == 1 && !strings.EqualFold(req.URL.Hostname(), via[0].URL.Hostname()) {
		return fmt.Errorf("search redirect: different host")
	}
	return nil
}

// RefreshIfStale re-resolves when unpinned or the pin is older than Refresh.
// A failed refresh leaves existing pins in place.
func (c *Client) RefreshIfStale(ctx context.Context) error {
	c.mu.Lock()
	need := len(c.ips) == 0 || time.Since(c.resolved) >= c.refresh
	c.mu.Unlock()
	if !need {
		return nil
	}
	return c.resolve(ctx)
}

// Ensure resolves if needed. Error only when still unpinned afterwards.
func (c *Client) Ensure(ctx context.Context) error {
	if c == nil {
		return fmt.Errorf("search dial: nil")
	}
	if err := c.RefreshIfStale(ctx); err != nil && !c.Ready() {
		return err
	}
	return nil
}

func (c *Client) Ready() bool {
	if c == nil {
		return false
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	return len(c.ips) > 0
}

func (c *Client) IPs() []netip.Addr {
	c.mu.Lock()
	defer c.mu.Unlock()
	out := make([]netip.Addr, len(c.ips))
	copy(out, c.ips)
	return out
}

func (c *Client) TLSConfig() *tls.Config {
	tr, _ := c.HTTP.Transport.(*http.Transport)
	if tr == nil {
		return nil
	}
	return tr.TLSClientConfig
}