// Package dns provides the low-level DNS query primitives used by ExploreDNS.
//
// It wraps the github.com/miekg/dns library to provide retrying, TCP fallback,
// EDNS0 buffer size negotiation, and root server discovery.  The package is
// intentionally narrow: it sends iterative (non-recursive) queries and returns
// the raw responses for the traversal engine to interpret.
package dns

import (
	"context"
	"fmt"
	"net"
	"time"

	"github.com/miekg/dns"
)

type QueryConfig struct {
	UDPSize  int
	Timeout  time.Duration
	Retries  int
	UseTCP   bool
	AllowTCP bool
}

func DefaultQueryConfig() *QueryConfig {
	return &QueryConfig{
		UDPSize:  DefaultEDNS0UDPSize(),
		Timeout:  5 * time.Second,
		Retries:  3,
		UseTCP:   false,
		AllowTCP: true,
	}
}

type ExchangeFunc func(ctx context.Context, server string, msg *dns.Msg, useTCP bool) (*dns.Msg, error)

func Query(ctx context.Context, server net.IP, name string, qtype uint16, cfg *QueryConfig) (*dns.Msg, error) {
	if cfg == nil {
		cfg = DefaultQueryConfig()
	}

	if cfg.UDPSize <= 0 {
		cfg.UDPSize = DefaultEDNS0UDPSize()
	}
	if cfg.Timeout <= 0 {
		cfg.Timeout = 5 * time.Second
	}

	return QueryWithExchange(ctx, server, name, qtype, cfg, realExchange)
}

func realExchange(ctx context.Context, server string, msg *dns.Msg, useTCP bool) (*dns.Msg, error) {
	addr := net.JoinHostPort(server, "53")

	var c *dns.Client
	if useTCP {
		c = &dns.Client{
			Net:          "tcp",
			ReadTimeout:  5 * time.Second,
			WriteTimeout: 5 * time.Second,
		}
	} else {
		c = &dns.Client{
			Net:          "udp",
			ReadTimeout:  5 * time.Second,
			WriteTimeout: 5 * time.Second,
		}
	}

	if deadline, ok := ctx.Deadline(); ok {
		c.ReadTimeout = time.Until(deadline)
		c.WriteTimeout = time.Until(deadline)
	}

	r, _, err := c.ExchangeContext(ctx, msg, addr)
	if err != nil {
		return nil, fmt.Errorf("dns exchange (%s) with %s: %w", c.Net, addr, err)
	}

	return r, nil
}

func QueryWithExchange(ctx context.Context, server net.IP, name string, qtype uint16, cfg *QueryConfig, exchangeFn ExchangeFunc) (*dns.Msg, error) {
	if cfg == nil {
		cfg = DefaultQueryConfig()
	}
	if cfg.UDPSize <= 0 {
		cfg.UDPSize = DefaultEDNS0UDPSize()
	}

	msg := buildQuery(name, qtype, cfg.UDPSize)
	serverStr := server.String()

	var lastErr error

	for attempt := 0; attempt < cfg.Retries; attempt++ {
		if attempt > 0 {
			select {
			case <-ctx.Done():
				return nil, fmt.Errorf("query retries cancelled: %w", ctx.Err())
			case <-time.After(backoffDelay(attempt)):
			}
		}

		if cfg.UseTCP {
			resp, err := exchangeFn(ctx, serverStr, msg, true)
			if err != nil {
				lastErr = err
				continue
			}
			return resp, nil
		}

		resp, err := exchangeFn(ctx, serverStr, msg, false)
		if err != nil {
			lastErr = err
			continue
		}

		if resp.Truncated && cfg.AllowTCP {
			resp, err = exchangeFn(ctx, serverStr, msg, true)
			if err != nil {
				lastErr = err
				continue
			}
			return resp, nil
		}

		return resp, nil
	}

	return nil, fmt.Errorf("query %s %s failed after %d retries: %w", name, QNameType(qtype), cfg.Retries, lastErr)
}

func IterativeQuery(ctx context.Context, server net.IP, name string, qtype uint16, cfg *QueryConfig) (*dns.Msg, error) {
	if cfg == nil {
		cfg = DefaultQueryConfig()
	}
	if cfg.UDPSize <= 0 {
		cfg.UDPSize = DefaultEDNS0UDPSize()
	}
	return IterativeQueryWithExchange(ctx, server, name, qtype, cfg, realExchange)
}

func IterativeQueryWithExchange(ctx context.Context, server net.IP, name string, qtype uint16, cfg *QueryConfig, exchangeFn ExchangeFunc) (*dns.Msg, error) {
	if cfg == nil {
		cfg = DefaultQueryConfig()
	}
	if cfg.UDPSize <= 0 {
		cfg.UDPSize = DefaultEDNS0UDPSize()
	}

	msg := buildQuery(name, qtype, cfg.UDPSize)
	msg.RecursionDesired = false
	serverStr := server.String()

	var lastErr error

	for attempt := 0; attempt < cfg.Retries; attempt++ {
		if attempt > 0 {
			select {
			case <-ctx.Done():
				return nil, fmt.Errorf("query retries cancelled: %w", ctx.Err())
			case <-time.After(backoffDelay(attempt)):
			}
		}

		if cfg.UseTCP {
			resp, err := exchangeFn(ctx, serverStr, msg, true)
			if err != nil {
				lastErr = err
				continue
			}
			return resp, nil
		}

		resp, err := exchangeFn(ctx, serverStr, msg, false)
		if err != nil {
			lastErr = err
			continue
		}

		if resp == nil {
			lastErr = fmt.Errorf("nil response")
			continue
		}

		if resp.Truncated && cfg.AllowTCP {
			resp, err = exchangeFn(ctx, serverStr, msg, true)
			if err != nil {
				lastErr = err
				continue
			}
			return resp, nil
		}

		return resp, nil
	}

	return nil, fmt.Errorf("iterative query %s %s failed after %d retries: %w", name, QNameType(qtype), cfg.Retries, lastErr)
}

// backoffDelay computes the wait duration before the given retry attempt (1-indexed).
// Delays: attempt=1 → 100ms, attempt=2 → 200ms, attempt=3 → 400ms, capped at 2s.
func backoffDelay(attempt int) time.Duration {
	if attempt <= 0 {
		return 0
	}
	delay := time.Duration(uint(1)<<uint(attempt-1)) * 100 * time.Millisecond
	const maxDelay = 2 * time.Second
	if delay > maxDelay {
		return maxDelay
	}
	return delay
}

func buildQuery(name string, qtype uint16, udpSize int) *dns.Msg {
	m := new(dns.Msg)
	m.SetQuestion(dns.Fqdn(name), qtype)
	m.RecursionDesired = true
	m.SetEdns0(uint16(udpSize), false)
	return m
}