package middleware

import (
	"archive/zip"
	"bytes"
	"errors"
	"io"
)

var (
	// ErrZipBombDetected indicates a zip bomb (excessive compression ratio) was detected.
	ErrZipBombDetected = errors.New("zip bomb detected: compression ratio exceeds safe limit")
	// ErrZipTooManyEntries indicates a ZIP archive contains too many files (zip fragment attack).
	ErrZipTooManyEntries = errors.New("zip archive contains too many files, possible zip fragment attack")
	// ErrZipEntryTooLarge indicates a single entry within a ZIP exceeds the uncompressed size limit.
	ErrZipEntryTooLarge = errors.New("zip entry exceeds maximum uncompressed size limit")
)

// inspectZipArchive reads through a ZIP archive to detect zip bombs and zip fragment attacks.
// It checks:
//   - Number of entries (zip fragment / decompression bomb with many small files)
//   - Compression ratio (zip bomb with extreme compression)
//   - Individual entry uncompressed size
//
// The file seek position after this call is indeterminate; callers should not rely on it.
func inspectZipArchive(f io.ReadSeeker, compressedSize int64, cfg *UploadSecurityConfig) error {
	// Seek to beginning so zip.NewReader can scan the whole file
	if _, err := f.Seek(0, io.SeekStart); err != nil {
		return errors.New("cannot seek to beginning of zip file")
	}

	// Read entire file into memory so we can use zip.NewReader which requires io.ReaderAt
	// The file size is already validated against MaxFileSize, so this is bounded
	allBytes, err := io.ReadAll(f)
	if err != nil {
		return errors.New("cannot read zip file content")
	}

	r, err := zip.NewReader(bytes.NewReader(allBytes), compressedSize)
	if err != nil {
		return errors.New("cannot parse zip file")
	}

	// Check number of entries
	if len(r.File) > int(cfg.MaxZipEntries) {
		return ErrZipTooManyEntries
	}

	// Compute total uncompressed size and check compression ratio
	var totalUncompressed int64
	for _, zf := range r.File {
		totalUncompressed += int64(zf.UncompressedSize64)

		// Check individual entry size — each entry should not exceed MaxFileSize
		if int64(zf.UncompressedSize64) > cfg.MaxFileSize {
			return ErrZipEntryTooLarge
		}
	}

	// Check compression ratio
	if compressedSize > 0 {
		ratio := float64(totalUncompressed) / float64(compressedSize)
		if ratio > cfg.MaxZipCompressionRatio {
			return ErrZipBombDetected
		}
	}

	return nil
}