package extract

import (
	"bufio"
	"context"
	"errors"
	"io"
	"regexp"
)

type Mode int

const (
	ModeIP Mode = iota
	ModeMAC
)

type Options struct {
	Mode Mode
	IPv6 bool
}

// Grep scans r line-by-line and emits raw matches via emit.
// It avoids reading the full input into memory.
func Grep(ctx context.Context, r io.Reader, opts Options, emit func(string)) error {
	if emit == nil {
		return errors.New("emit func is nil")
	}

	var re *regexp.Regexp
	switch opts.Mode {
	case ModeMAC:
		re = macRE
	case ModeIP:
		if opts.IPv6 {
			re = ipAnyRE
		} else {
			re = ipv4RE
		}
	default:
		return errors.New("unknown mode")
	}

	sc := bufio.NewScanner(r)
	// Large-ish buffer for long lines (logs, JSON blobs, etc.).
	buf := make([]byte, 64*1024)
	sc.Buffer(buf, 1024*1024)

	for sc.Scan() {
		select {
		case <-ctx.Done():
			return ctx.Err()
		default:
		}
		line := sc.Bytes()
		matches := re.FindAll(line, -1)
		for _, m := range matches {
			emit(string(m))
		}
	}
	if err := sc.Err(); err != nil {
		return err
	}
	return nil
}

// Note: These regexes intentionally find candidates only; callers should
// validate/normalize as needed.
var (
	// \b based candidate matcher, validated later.
	ipv4RE = regexp.MustCompile(`\b(?:\d{1,3}\.){3}\d{1,3}\b`)

	// Rough IPv6 candidate matcher (validation happens later if caller chooses to).
	ipv6RE = regexp.MustCompile(`(?i)\b(?:[0-9a-f]{0,4}:){2,7}[0-9a-f]{0,4}\b`)

	ipAnyRE = regexp.MustCompile(`(?i)(?:` + `\b(?:\d{1,3}\.){3}\d{1,3}\b` + `|` + `\b(?:[0-9a-f]{0,4}:){2,7}[0-9a-f]{0,4}\b` + `)`)

	macRE = regexp.MustCompile(`(?i)\b(?:[0-9a-f]{2}[:-]){5}[0-9a-f]{2}\b|\b(?:[0-9a-f]{4}\.){2}[0-9a-f]{4}\b`)
)