package agentwatch

import (
	"context"
	"crypto/sha256"
	"encoding/binary"
	"errors"
	"fmt"
	"hash"
	"io"
	"os"
	"path/filepath"
	"regexp"
	"sort"
	"strings"
)

const (
	maxConfigFiles      = 512
	maxConfigBytes      = 64 << 20
	maxFileBytes        = 16 << 20
	maxIncludeDepth     = 16
	maxDirectoryEntries = 4096
)

var errMainNotReady = errors.New("main bird.conf is not ready")

// Only standalone literal include statements are hashable. Other valid BIRD
// grammar falls back to the legacy configure-on-every-tick path.
var includeLine = regexp.MustCompile(`(?m)^[\t ]*include[\t ]+"([^"\r\n]+)"[\t ]*;[\t ]*(?:#[^\r\n]*)?\r?$`)

type configScanner struct {
	ctx   context.Context
	root  string
	h     hash.Hash
	seen  map[string]bool
	files int
	bytes int64
}

func hashConfigTree(ctx context.Context, activeDir string) ([sha256.Size]byte, error) {
	var zero [sha256.Size]byte
	root, err := filepath.Abs(activeDir)
	if err != nil {
		return zero, fmt.Errorf("agentwatch: active directory: %w", err)
	}
	root, err = filepath.EvalSymlinks(root)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			return zero, fmt.Errorf("agentwatch: %w: %v", errMainNotReady, err)
		}
		return zero, fmt.Errorf("agentwatch: resolve active directory: %w", err)
	}
	s := &configScanner{ctx: ctx, root: filepath.Clean(root), h: sha256.New(), seen: make(map[string]bool)}
	if err := s.visit(filepath.Join(s.root, "bird.conf"), 0); err != nil {
		return zero, err
	}
	var digest [sha256.Size]byte
	copy(digest[:], s.h.Sum(nil))
	return digest, nil
}

func (s *configScanner) visit(path string, depth int) error {
	if err := s.ctx.Err(); err != nil {
		return err
	}
	if depth > maxIncludeDepth {
		return fmt.Errorf("agentwatch: include depth exceeds %d", maxIncludeDepth)
	}
	path = filepath.Clean(path)
	rel, err := filepath.Rel(s.root, path)
	if err != nil || rel == ".." || strings.HasPrefix(rel, ".."+string(os.PathSeparator)) {
		return fmt.Errorf("agentwatch: include %q escapes active directory", path)
	}
	if s.seen[path] {
		return nil
	}
	info, err := os.Lstat(path)
	if err != nil {
		if depth == 0 && errors.Is(err, os.ErrNotExist) {
			return fmt.Errorf("agentwatch: %w: %v", errMainNotReady, err)
		}
		return fmt.Errorf("agentwatch: inspect %q: %w", path, err)
	}
	if !info.Mode().IsRegular() {
		return fmt.Errorf("agentwatch: include %q is not a regular file", path)
	}
	resolved, err := filepath.EvalSymlinks(path)
	if err != nil {
		return fmt.Errorf("agentwatch: resolve %q: %w", path, err)
	}
	if _, err := s.withinRoot(resolved); err != nil {
		return err
	}
	if info.Size() > maxFileBytes {
		return fmt.Errorf("agentwatch: include %q exceeds %d bytes", path, maxFileBytes)
	}
	if s.files >= maxConfigFiles || s.bytes+info.Size() > maxConfigBytes {
		return fmt.Errorf("agentwatch: configuration tree exceeds %d files or %d bytes", maxConfigFiles, maxConfigBytes)
	}
	f, err := os.Open(path)
	if err != nil {
		return fmt.Errorf("agentwatch: open %q: %w", path, err)
	}
	defer func() { _ = f.Close() }()
	limit := int64(maxFileBytes)
	if remaining := int64(maxConfigBytes) - s.bytes; remaining < limit {
		limit = remaining
	}
	content, err := readBounded(f, limit)
	if err != nil {
		return fmt.Errorf("agentwatch: read %q: %w", path, err)
	}
	s.seen[path] = true
	s.files++
	s.bytes += int64(len(content))
	writeHash(s.h, filepath.ToSlash(rel), content)
	patterns, err := includePatterns(string(content))
	if err != nil {
		return fmt.Errorf("agentwatch: %s: %w", rel, err)
	}
	for _, pattern := range patterns {
		if strings.ContainsAny(pattern, `[\`) {
			return fmt.Errorf("agentwatch: unsupported escaped include %q", pattern)
		}
		candidate := pattern
		if !filepath.IsAbs(candidate) {
			candidate = filepath.Join(filepath.Dir(path), filepath.FromSlash(candidate))
		}
		candidate = filepath.Clean(candidate)
		// The pattern itself changes the digest even when its glob currently matches nothing.
		writeHash(s.h, "include", []byte(filepath.ToSlash(candidate)))
		if _, err := s.withinRoot(candidate); err != nil {
			return err
		}
		if strings.ContainsAny(candidate, "*?") {
			if strings.ContainsAny(filepath.Dir(candidate), "*?") {
				return fmt.Errorf("agentwatch: unsupported include directory glob %q", pattern)
			}
			resolvedDir, err := filepath.EvalSymlinks(filepath.Dir(candidate))
			if err != nil {
				return fmt.Errorf("agentwatch: resolve include directory %q: %w", pattern, err)
			}
			if _, err := s.withinRoot(resolvedDir); err != nil {
				return err
			}
			matches, err := boundedGlob(candidate)
			if err != nil {
				return fmt.Errorf("agentwatch: include glob %q: %w", pattern, err)
			}
			for _, match := range matches {
				if err := s.visit(match, depth+1); err != nil {
					return err
				}
			}
			continue
		}
		if err := s.visit(candidate, depth+1); err != nil {
			return err
		}
	}
	return nil
}

func readBounded(r io.Reader, limit int64) ([]byte, error) {
	content, err := io.ReadAll(io.LimitReader(r, limit+1))
	if err != nil {
		return nil, err
	}
	if int64(len(content)) > limit {
		return nil, fmt.Errorf("configuration tree changed or exceeded size limit")
	}
	return content, nil
}

// boundedGlob only handles * and ? in a filename. Broader BIRD glob grammar
// falls back to every-tick configure instead of risking a false unchanged hash.
func boundedGlob(pattern string) ([]string, error) {
	dir, base := filepath.Dir(pattern), filepath.Base(pattern)
	f, err := os.Open(dir)
	if err != nil {
		return nil, err
	}
	defer func() { _ = f.Close() }()
	names, err := f.Readdirnames(maxDirectoryEntries + 1)
	if err != nil && !errors.Is(err, io.EOF) {
		return nil, err
	}
	if len(names) > maxDirectoryEntries {
		return nil, fmt.Errorf("directory exceeds %d entries", maxDirectoryEntries)
	}
	matches := make([]string, 0)
	for _, name := range names {
		match, err := filepath.Match(base, name)
		if err != nil {
			return nil, err
		}
		if match {
			matches = append(matches, filepath.Join(dir, name))
			if len(matches) > maxConfigFiles {
				return nil, fmt.Errorf("glob exceeds %d matches", maxConfigFiles)
			}
		}
	}
	sort.Strings(matches)
	return matches, nil
}

func (s *configScanner) withinRoot(path string) (string, error) {
	rel, err := filepath.Rel(s.root, path)
	if err != nil || rel == ".." || strings.HasPrefix(rel, ".."+string(os.PathSeparator)) {
		return "", fmt.Errorf("agentwatch: include %q escapes active directory", path)
	}
	return rel, nil
}

func writeHash(h hash.Hash, name string, content []byte) {
	var size [8]byte
	binary.LittleEndian.PutUint64(size[:], uint64(len(name)))
	_, _ = h.Write(size[:])
	_, _ = h.Write([]byte(name))
	binary.LittleEndian.PutUint64(size[:], uint64(len(content)))
	_, _ = h.Write(size[:])
	_, _ = h.Write(content)
}

func includePatterns(content string) ([]string, error) {
	tokens, unsupportedQuotedText := includeTokenOffsets(content)
	if unsupportedQuotedText {
		return nil, fmt.Errorf("unsupported quoted text or escape")
	}
	tokenSet := make(map[int]bool, len(tokens))
	for _, token := range tokens {
		tokenSet[token] = true
	}
	var patterns []string
	for _, match := range includeLine.FindAllStringSubmatchIndex(content, -1) {
		keyword := match[0] + strings.Index(content[match[0]:match[1]], "include")
		if !tokenSet[keyword] {
			continue
		} // text inside a block comment
		delete(tokenSet, keyword)
		patterns = append(patterns, content[match[2]:match[3]])
	}
	if len(tokenSet) != 0 {
		return nil, fmt.Errorf("unsupported include syntax")
	}
	return patterns, nil
}

// Recognize include keywords outside BIRD comments and quoted strings.
func includeTokenOffsets(config string) ([]int, bool) {
	var offsets []int
	quote := byte(0)
	lineComment, blockComment := false, false
	for i := 0; i < len(config); {
		c := config[i]
		if lineComment {
			if c == '\n' {
				lineComment = false
			}
			i++
			continue
		}
		if blockComment {
			if c == '*' && i+1 < len(config) && config[i+1] == '/' {
				blockComment = false
				i += 2
			} else {
				i++
			}
			continue
		}
		if quote != 0 {
			if c == '\\' {
				return offsets, true
			}
			if c == quote {
				quote = 0
			}
			i++
			continue
		}
		if c == '#' || (c == '/' && i+1 < len(config) && config[i+1] == '/') {
			lineComment = true
			i++
			continue
		}
		if c == '/' && i+1 < len(config) && config[i+1] == '*' {
			blockComment = true
			i += 2
			continue
		}
		if c == '"' || c == '\'' {
			quote = c
			i++
			continue
		}
		if (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_' {
			start := i
			for i < len(config) && ((config[i] >= 'a' && config[i] <= 'z') || (config[i] >= 'A' && config[i] <= 'Z') || (config[i] >= '0' && config[i] <= '9') || config[i] == '_') {
				i++
			}
			if config[start:i] == "include" {
				offsets = append(offsets, start)
			}
			continue
		}
		i++
	}
	return offsets, quote != 0 || blockComment
}