// Package blocking turns blocklists and block/allow rules into BIND
// response policy zones (RPZ), and answers which rule or list matches a
// name.
//
// A policy zone holds one rule per owner name: "ads.example CNAME ."
// answers NXDOMAIN for ads.example, "*.ads.example CNAME ." for every name
// below it, and "CNAME rpz-passthru." answers normally (an exception).
// named looks names up in a policy zone like in any zone, so wildcards
// follow DNS rules: a wildcard doesn't apply below a name that exists in
// the zone. The writers here add the rules that keep "the most specific
// rule wins" true despite that (see RulesZone).
package blocking

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"net/netip"
	"strings"
	"unicode/utf8"

	"jarl/internal/dnsname"
)

// Flags say what a list entry covers.
const (
	Exact uint8 = 1 << iota // the name itself
	Below                   // every name below it
)

// List is a parsed blocklist.
type List struct {
	// Domains maps names (lowercase, without the final dot) to flags.
	Domains map[string]uint8
	// Lines is the number of lines with an entry (not comments or blank
	// lines); Skipped how many of them were not usable.
	Lines, Skipped int
}

// Limits bound what Parse accepts.
type Limits struct {
	MaxEntries int // distinct domains (0: no limit)
	MaxNameLen int // longest name in characters (0: 253)
}

// ErrTooManyEntries is returned when a list has more than MaxEntries
// domains.
var ErrTooManyEntries = errors.New("too many entries")

// Parse reads a blocklist. Each line is one of:
//
//	||ads.example^          Adblock style: the name and everything below it
//	*.ads.example           the name and everything below it
//	0.0.0.0 ads.example     hosts file (any address; several names allowed)
//	ads.example             just this name
//
// Comments (#, !), Adblock headers ([Adblock Plus]) and blank lines are
// ignored. Lines that can't be used for DNS blocking (exceptions,
// cosmetic and URL rules, addresses, single-label names, invalid names)
// are counted as skipped.
func Parse(r io.Reader, lim Limits) (*List, error) {
	maxLen := lim.MaxNameLen
	if maxLen <= 0 || maxLen > 253 {
		maxLen = 253
	}
	l := &List{Domains: map[string]uint8{}}
	full := false // a new name was refused because of MaxEntries
	add := func(name string, flags uint8) bool {
		name, ok := normalize(name, maxLen)
		if !ok {
			return false
		}
		if _, seen := l.Domains[name]; !seen && lim.MaxEntries > 0 && len(l.Domains) >= lim.MaxEntries {
			full = true
			return false
		}
		l.Domains[name] |= flags
		return true
	}
	sc := bufio.NewScanner(r)
	sc.Buffer(make([]byte, 64*1024), 1024*1024)
	first := true
	for sc.Scan() {
		line := sc.Text()
		if first {
			line = strings.TrimPrefix(line, "\uFEFF")
			first = false
		}
		line = strings.TrimSpace(line)
		if line == "" || line[0] == '#' || line[0] == '!' || line[0] == '[' {
			continue
		}
		l.Lines++
		ok := false
		switch {
		case strings.HasPrefix(line, "||"):
			ok = add(adblockDomain(line[2:]), Exact|Below)
		case strings.ContainsAny(line, "|/$^@") || strings.Contains(line, "##"):
			// Exceptions (@@), URL and cosmetic rules: not for DNS.
		default:
			if i := strings.Index(line, "#"); i >= 0 {
				line = strings.TrimSpace(line[:i])
			}
			fields := strings.Fields(line)
			switch {
			case len(fields) == 1 && strings.HasPrefix(fields[0], "*."):
				ok = add(fields[0][2:], Exact|Below)
			case len(fields) == 1:
				ok = add(fields[0], Exact)
			case len(fields) > 1 && isAddr(fields[0]):
				for _, f := range fields[1:] {
					if add(f, Exact) {
						ok = true
					}
				}
			}
		}
		if full {
			return l, fmt.Errorf("%w: the list has more than %d domains", ErrTooManyEntries, lim.MaxEntries)
		}
		if !ok {
			l.Skipped++
		}
	}
	if err := sc.Err(); err != nil {
		return l, err
	}
	return l, nil
}

// adblockDomain returns the domain of an Adblock rule's body
// ("ads.example^" or "ads.example^$important"), or "" when the rule
// does more than block a domain (other modifiers, paths, wildcards).
func adblockDomain(s string) string {
	d, rest, _ := strings.Cut(s, "^")
	if rest != "" && rest != "$important" {
		return ""
	}
	if strings.ContainsAny(d, "*/:|$") {
		return ""
	}
	return d
}

func isAddr(s string) bool {
	_, err := netip.ParseAddr(s)
	return err == nil
}

// localNames are hosts-file names that aren't blocklist entries.
var localNames = map[string]bool{"localhost.localdomain": true, "ip6-localhost": true, "ip6-loopback": true}

// normalize lowercases a name, removes the final dot, converts
// international names to their ASCII form, and checks it is usable: at
// least two labels of letters, digits, hyphens and underscores, and not an
// address.
func normalize(name string, maxLen int) (string, bool) {
	name = strings.TrimSuffix(strings.ToLower(strings.TrimSpace(name)), ".")
	if name == "" || localNames[name] || isAddr(name) {
		return "", false
	}
	for i := 0; i < len(name); i++ {
		if name[i] >= utf8.RuneSelf {
			a, err := dnsname.ToASCII(name)
			if err != nil {
				return "", false
			}
			name = strings.TrimSuffix(strings.ToLower(a), ".")
			break
		}
	}
	if len(name) > maxLen || !strings.Contains(name, ".") {
		return "", false
	}
	for _, label := range strings.Split(name, ".") {
		if label == "" || len(label) > 63 {
			return "", false
		}
		for i := 0; i < len(label); i++ {
			c := label[i]
			if !(c >= 'a' && c <= 'z' || c >= '0' && c <= '9' || c == '-' || c == '_') {
				return "", false
			}
		}
	}
	return name, true
}

// NormalizeName checks and normalizes a domain the way list entries are
// (for rules entered by hand). The name must have at least two labels.
func NormalizeName(name string) (string, error) {
	n, ok := normalize(name, 253-len(RulesZone)-2)
	if !ok {
		return "", fmt.Errorf("%q is not a domain name that can be blocked (it needs at least two labels of letters, digits, hyphens or underscores, like ads.example.com)", name)
	}
	return n, nil
}