package blacklist

import (
	"bufio"
	"io"
	"net/netip"
	"strings"

	"github.com/owen/vibedns/internal/models"
	"github.com/owen/vibedns/internal/validate"
)

// ParsedDomain is one domain extracted from an import.
type ParsedDomain struct {
	Domain          string
	MatchSubdomains bool
}

// ParseOptions tunes bulk import behaviour.
type ParseOptions struct {
	// DefaultMatchSubdomains sets the match mode for entries that do not carry
	// explicit wildcard syntax.
	DefaultMatchSubdomains bool
	// MaxInvalidSamples caps how many rejected lines are reported back.
	MaxInvalidSamples int
}

// hostsPlaceholders are the addresses a hosts-file blocklist points at. Lines
// using any other address are still parsed, but these are the common ones.
var localhostNames = map[string]bool{
	"localhost":             true,
	"localhost.localdomain": true,
	"local":                 true,
	"ip6-localhost":         true,
	"ip6-loopback":          true,
	"ip6-localnet":          true,
	"ip6-mcastprefix":       true,
	"ip6-allnodes":          true,
	"ip6-allrouters":        true,
	"ip6-allhosts":          true,
	"broadcasthost":         true,
}

// Parse reads a domain list and returns the normalised, de-duplicated domains
// alongside a summary of what happened to every line.
//
// It accepts three shapes, mixed freely in one file:
//
//	example.com                    plain list
//	0.0.0.0 ads.example.com        hosts file
//	||tracker.example.net^         Adblock-style host rule
//
// Comments (#, ;, !), blank lines, IP-only lines and localhost entries are
// ignored. Everything that survives is normalised to lowercase without a
// trailing dot.
func Parse(r io.Reader, opts ParseOptions) ([]ParsedDomain, models.ImportSummary) {
	if opts.MaxInvalidSamples <= 0 {
		opts.MaxInvalidSamples = 10
	}
	var summary models.ImportSummary
	// Pre-size for a large list; growth is amortised anyway.
	out := make([]ParsedDomain, 0, 1024)
	seen := make(map[string]struct{}, 1024)

	sc := bufio.NewScanner(r)
	// Blocklist lines are short, but a stray long line must not abort the scan.
	sc.Buffer(make([]byte, 0, 64*1024), 1024*1024)

	for sc.Scan() {
		summary.LinesProcessed++
		line := strings.TrimSpace(sc.Text())

		if line == "" || isComment(line) {
			summary.Ignored++
			continue
		}
		line = stripTrailingComment(line)
		if line == "" {
			summary.Ignored++
			continue
		}

		candidates, wildcard, ok := extractCandidates(line)
		if !ok {
			summary.Ignored++
			continue
		}
		if len(candidates) == 0 {
			summary.Invalid++
			addSample(&summary, line, opts.MaxInvalidSamples)
			continue
		}

		anyValid := false
		for _, c := range candidates {
			domain, err := validate.NormaliseDomain(c)
			if err != nil {
				continue
			}
			if localhostNames[domain] || !strings.Contains(domain, ".") {
				// Single-label names are hosts-file noise, not blockable domains.
				continue
			}
			if isIPLiteral(domain) {
				continue
			}
			anyValid = true
			if _, dup := seen[domain]; dup {
				summary.Duplicates++
				continue
			}
			seen[domain] = struct{}{}
			out = append(out, ParsedDomain{
				Domain:          domain,
				MatchSubdomains: wildcard || opts.DefaultMatchSubdomains,
			})
			summary.Imported++
		}
		if !anyValid {
			summary.Invalid++
			addSample(&summary, line, opts.MaxInvalidSamples)
		}
	}
	if err := sc.Err(); err != nil {
		// A read failure still returns what was parsed so far; the caller
		// decides whether a partial import is acceptable.
		return out, summary
	}
	return out, summary
}

func addSample(s *models.ImportSummary, line string, max int) {
	if len(s.InvalidSamples) >= max {
		return
	}
	if len(line) > 120 {
		line = line[:120] + "..."
	}
	s.InvalidSamples = append(s.InvalidSamples, line)
}

func isComment(line string) bool {
	switch line[0] {
	case '#', ';':
		return true
	case '!':
		// Adblock comment, but "!" never starts a host rule.
		return true
	case '[':
		// Adblock header such as [Adblock Plus 2.0]
		return true
	}
	return false
}

// stripTrailingComment removes an inline comment while leaving the rest.
func stripTrailingComment(line string) string {
	if i := strings.IndexAny(line, "#;"); i >= 0 {
		line = line[:i]
	}
	return strings.TrimSpace(line)
}

// extractCandidates pulls the domain-shaped tokens out of one line.
//
// The bool return reports whether the line should count as "ignored" rather
// than "invalid" — used for rules this importer deliberately does not support,
// such as Adblock element-hiding or exception rules.
func extractCandidates(line string) (domains []string, wildcard bool, supported bool) {
	// Adblock-style rules.
	if strings.HasPrefix(line, "@@") {
		return nil, false, false // exception rule: not a block entry
	}
	if strings.HasPrefix(line, "||") {
		rest := strings.TrimPrefix(line, "||")
		rest = strings.TrimSuffix(rest, "^")
		rest = strings.TrimSuffix(rest, "^$all")
		if i := strings.IndexAny(rest, "/^$*"); i >= 0 {
			// A path or option makes this a URL rule, which DNS cannot express.
			if i == 0 {
				return nil, false, false
			}
			rest = rest[:i]
		}
		return []string{rest}, true, true
	}
	if strings.ContainsAny(line, "/$") && !strings.HasPrefix(line, "0.0.0.0") {
		// dnsmasq address=/example.com/0.0.0.0
		if strings.HasPrefix(line, "address=/") || strings.HasPrefix(line, "server=/") {
			parts := strings.Split(line, "/")
			if len(parts) >= 2 && parts[1] != "" {
				return []string{parts[1]}, true, true
			}
			return nil, false, false
		}
		return nil, false, false
	}

	fields := strings.Fields(line)
	if len(fields) == 0 {
		return nil, false, false
	}

	// Hosts-file syntax: the first field is an IP address, the rest are names.
	if isIPLiteral(fields[0]) {
		if len(fields) == 1 {
			return nil, false, false // an address on its own carries no domain
		}
		return fields[1:], false, true
	}

	// Plain list: one domain per line. Extra fields are treated as noise.
	first := fields[0]
	if strings.HasPrefix(first, "*.") {
		return []string{strings.TrimPrefix(first, "*.")}, true, true
	}
	return []string{first}, false, true
}

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

// ParseString is a convenience wrapper for textarea input.
func ParseString(s string, opts ParseOptions) ([]ParsedDomain, models.ImportSummary) {
	return Parse(strings.NewReader(s), opts)
}