package grep

import (
	"bufio"
	"bytes"
	"context"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"regexp"
	"strings"

	"github.com/go-git/go-git/v5/plumbing/format/gitignore"
)

const (
	keep     = 64 << 10
	lineKeep = 500
	lineMax  = 1 << 20
	sniff    = 8000
)

type Options struct {
	Glob       string
	IgnoreCase bool
	FilesOnly  bool
}

type search struct {
	ctx      context.Context
	re       *regexp.Regexp
	opts     Options
	given    string
	root     string
	base     string
	patterns []gitignore.Pattern
	reader   *bufio.Reader
	out      strings.Builder
	full     bool
}

func Search(ctx context.Context, pattern, dir, path string, opts Options) (string, error) {
	expr := pattern
	if opts.IgnoreCase {
		expr = "(?i)" + expr
	}
	re, err := regexp.Compile(expr)
	if err != nil {
		return "", fmt.Errorf("pattern: %w", err)
	}
	_, err = filepath.Match(opts.Glob, "")
	if err != nil {
		return "", fmt.Errorf("glob %q: %w", opts.Glob, err)
	}
	root := path
	if !filepath.IsAbs(root) {
		root = filepath.Join(dir, root)
	}
	root, err = filepath.Abs(root)
	if err != nil {
		return "", err
	}
	info, err := os.Stat(root)
	if err != nil {
		return "", err
	}

	top := root
	if !info.IsDir() {
		top = filepath.Dir(root)
	}
	s := &search{ctx: ctx, re: re, opts: opts, given: path, root: root, base: repoRoot(top)}
	s.load(filepath.Join(gitDir(s.base), "info", "exclude"), nil)
	rel, err := filepath.Rel(s.base, top)
	if err != nil {
		return "", err
	}
	parts := split(rel)
	for i := range parts {
		s.load(filepath.Join(s.base, filepath.Join(parts[:i]...), ".gitignore"), parts[:i])
	}
	if len(parts) > 0 && gitignore.NewMatcher(s.patterns).Match(parts, true) {
		s.patterns = nil
	}

	err = filepath.WalkDir(root, s.visit)
	if err != nil {
		return "", err
	}
	if s.full {
		s.out.WriteString("(output cut off at 64 KiB; narrow the pattern, path or glob)\n")
	}
	if s.out.Len() == 0 {
		return "No matches.", nil
	}
	return s.out.String(), nil
}

func repoRoot(dir string) string {
	for d := dir; ; d = filepath.Dir(d) {
		_, err := os.Lstat(filepath.Join(d, ".git"))
		if err == nil {
			return d
		}
		if filepath.Dir(d) == d {
			return dir
		}
	}
}

func gitDir(base string) string {
	dot := filepath.Join(base, ".git")
	data, err := os.ReadFile(dot)
	if err != nil {
		return dot
	}
	dir, ok := strings.CutPrefix(strings.TrimSpace(string(data)), "gitdir: ")
	if !ok {
		return dot
	}
	if !filepath.IsAbs(dir) {
		dir = filepath.Join(base, dir)
	}
	common, err := os.ReadFile(filepath.Join(dir, "commondir"))
	if err != nil {
		return dir
	}
	c := strings.TrimSpace(string(common))
	if !filepath.IsAbs(c) {
		c = filepath.Join(dir, c)
	}
	return c
}

func split(rel string) []string {
	if rel == "." {
		return nil
	}
	return strings.Split(filepath.ToSlash(rel), "/")
}

func (s *search) load(file string, domain []string) {
	data, err := os.ReadFile(file)
	if err != nil {
		return
	}
	for line := range strings.SplitSeq(string(data), "\n") {
		line = strings.TrimSuffix(line, "\r")
		if strings.HasPrefix(line, "#") || strings.TrimSpace(line) == "" {
			continue
		}
		s.patterns = append(s.patterns, gitignore.ParsePattern(line, domain))
	}
}

func (s *search) visit(p string, d fs.DirEntry, err error) error {
	if err != nil {
		if p == s.root {
			return err
		}
		return nil
	}
	if s.ctx.Err() != nil {
		return s.ctx.Err()
	}
	if s.full {
		return fs.SkipAll
	}
	rel, err := filepath.Rel(s.base, p)
	if err != nil {
		return err
	}
	parts := split(rel)
	if p != s.root && (d.Name() == ".git" || gitignore.NewMatcher(s.patterns).Match(parts, d.IsDir())) {
		if d.IsDir() {
			return fs.SkipDir
		}
		return nil
	}
	if d.IsDir() {
		s.load(filepath.Join(p, ".gitignore"), parts)
		return nil
	}
	if !d.Type().IsRegular() {
		return nil
	}
	if s.opts.Glob != "" {
		ok, _ := filepath.Match(s.opts.Glob, d.Name())
		if !ok {
			return nil
		}
	}
	return s.file(p)
}

func (s *search) file(p string) error {
	f, err := os.Open(p)
	if err != nil {
		return nil
	}
	defer f.Close()
	if s.reader == nil {
		s.reader = bufio.NewReaderSize(f, lineMax)
	}
	r := s.reader
	r.Reset(f)
	head, _ := r.Peek(sniff)
	if bytes.IndexByte(head, 0) >= 0 {
		return nil
	}
	shown := s.given
	if p != s.root {
		rel, err := filepath.Rel(s.root, p)
		if err != nil {
			return err
		}
		shown = filepath.Join(s.given, rel)
	}

	for n := 1; ; n++ {
		if s.ctx.Err() != nil {
			return s.ctx.Err()
		}
		line, err := r.ReadSlice('\n')
		if errors.Is(err, bufio.ErrBufferFull) {
			s.write(fmt.Sprintf("%s:%d: line over 1 MiB, not searched\n", shown, n))
			for errors.Is(err, bufio.ErrBufferFull) {
				if s.ctx.Err() != nil {
					return s.ctx.Err()
				}
				_, err = r.ReadSlice('\n')
			}
			if err != nil || s.full {
				return nil
			}
			continue
		}
		if len(line) == 0 {
			return nil
		}
		line = bytes.TrimSuffix(bytes.TrimSuffix(line, []byte("\n")), []byte("\r"))
		if s.re.Match(line) {
			if s.opts.FilesOnly {
				s.write(shown + "\n")
				return nil
			}
			text := string(line)
			if len(text) > lineKeep {
				text = strings.ToValidUTF8(text[:lineKeep], "") + "…"
			}
			s.write(fmt.Sprintf("%s:%d:%s\n", shown, n, text))
		}
		if err != nil || s.full {
			return nil
		}
	}
}

func (s *search) write(text string) {
	if s.out.Len()+len(text) > keep {
		s.full = true
		return
	}
	s.out.WriteString(text)
}