package app

import (
	"archive/zip"
	"bufio"
	"io"
	"io/fs"
	"os"
	"path/filepath"
)

// blackboxArchiveCompareChunk is the buffer size used when diffing the newly
// built local zip against the last one successfully written to removable
// media, to find how many leading bytes are unchanged.
const blackboxArchiveCompareChunk = 256 * 1024

// buildZipArchive walks root (a locally-staged, fast-storage copy of the
// blackbox tree — never the slow removable-media mountpoint) and writes a
// single deterministic zip to destPath: same input tree -> byte-identical
// output, so two cycles that changed nothing produce identical archives and
// patchArchiveOnTarget (below) can skip re-writing the unchanged prefix to
// the slow target. Determinism relies on fs.WalkDir's guaranteed lexical
// order and each entry's Modified time coming from the source file's mtime
// (stable across cycles for files nothing touched).
func buildZipArchive(root, destPath string) error {
	if err := os.MkdirAll(filepath.Dir(destPath), 0755); err != nil {
		return err
	}
	f, err := os.OpenFile(destPath, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0644)
	if err != nil {
		return err
	}
	zw := zip.NewWriter(f)

	walkErr := filepath.WalkDir(root, func(path string, d fs.DirEntry, err error) error {
		if err != nil {
			return err
		}
		if d.IsDir() {
			return nil
		}
		rel, err := filepath.Rel(root, path)
		if err != nil {
			return err
		}
		info, err := d.Info()
		if err != nil {
			return err
		}
		header, err := zip.FileInfoHeader(info)
		if err != nil {
			return err
		}
		header.Name = filepath.ToSlash(rel)
		header.Method = zip.Deflate
		w, err := zw.CreateHeader(header)
		if err != nil {
			return err
		}
		src, err := os.Open(path)
		if err != nil {
			return err
		}
		defer src.Close()
		_, err = io.Copy(w, src)
		return err
	})
	if walkErr != nil {
		_ = zw.Close()
		_ = f.Close()
		return walkErr
	}
	if err := zw.Close(); err != nil {
		_ = f.Close()
		return err
	}
	return f.Close()
}

// commonPrefixLen returns how many leading bytes two files share. Used to
// find how much of a freshly-rebuilt local zip is identical to the previous
// cycle's, so patchArchiveOnTarget only has to write the changed suffix to
// removable media.
func commonPrefixLen(pathA, pathB string) (int64, error) {
	fa, err := os.Open(pathA)
	if err != nil {
		return 0, err
	}
	defer fa.Close()
	fb, err := os.Open(pathB)
	if err != nil {
		return 0, err
	}
	defer fb.Close()

	ra := bufio.NewReaderSize(fa, blackboxArchiveCompareChunk)
	rb := bufio.NewReaderSize(fb, blackboxArchiveCompareChunk)
	bufA := make([]byte, blackboxArchiveCompareChunk)
	bufB := make([]byte, blackboxArchiveCompareChunk)

	var total int64
	for {
		na, errA := io.ReadFull(ra, bufA)
		nb, errB := io.ReadFull(rb, bufB)
		n := na
		if nb < n {
			n = nb
		}
		for i := 0; i < n; i++ {
			if bufA[i] != bufB[i] {
				return total + int64(i), nil
			}
		}
		total += int64(n)
		if na != nb || isEOFLike(errA) || isEOFLike(errB) {
			return total, nil
		}
		if errA != nil {
			return total, errA
		}
		if errB != nil {
			return total, errB
		}
	}
}

func isEOFLike(err error) bool {
	return err == io.EOF || err == io.ErrUnexpectedEOF
}

// patchArchiveOnTarget makes targetPath (on removable media, possibly
// FUSE-mounted with synchronous writes) byte-identical to newLocalZipPath (on
// fast local storage), writing only the changed suffix instead of the whole
// file. cachedPath is our own local record of what we last wrote to
// targetPath; if targetPath's size doesn't match what cachedPath implies
// (first run, external tampering, a previous crash mid-write), it falls back
// to writing the whole archive rather than risk corrupting it with a wrong
// truncate point.
func patchArchiveOnTarget(targetPath, newLocalZipPath, cachedPath string) error {
	newInfo, err := os.Stat(newLocalZipPath)
	if err != nil {
		return err
	}

	var prefixLen int64
	if cachedInfo, err := os.Stat(cachedPath); err == nil {
		if targetInfo, err := os.Stat(targetPath); err == nil && targetInfo.Size() == cachedInfo.Size() {
			prefixLen, err = commonPrefixLen(cachedPath, newLocalZipPath)
			if err != nil {
				prefixLen = 0
			}
		}
	}
	if prefixLen > newInfo.Size() {
		prefixLen = 0
	}

	target, err := os.OpenFile(targetPath, os.O_CREATE|os.O_RDWR, 0644)
	if err != nil {
		return err
	}
	defer target.Close()

	if err := target.Truncate(prefixLen); err != nil {
		return err
	}
	if _, err := target.Seek(prefixLen, io.SeekStart); err != nil {
		return err
	}

	src, err := os.Open(newLocalZipPath)
	if err != nil {
		return err
	}
	defer src.Close()
	if _, err := src.Seek(prefixLen, io.SeekStart); err != nil {
		return err
	}
	if _, err := io.Copy(target, src); err != nil {
		return err
	}
	return target.Sync()
}