// SPDX-License-Identifier: GPL-2.0-or-later
// SCFF, the SchOS Compressed File Format: a block image cut into chunks, each
// compressed on its own, with an index in front.
//
// It is what the live root is shipped as. The kernel carries the compressed
// image and decompresses a chunk the first time any block in it is read, so
// the kernel and the ISO are the compressed size, and the memory the root
// takes is what has been touched, not the whole of it. See kernel/dev/ramdisk.c
// for the reader and tools/mkscff.c for the writer; this header is the one
// definition of the layout both of them use.
//
//   offset 0           struct scff_header
//   index_off          uint64_t off[nchunks + 1], from the start of the file
//   off[i]..off[i+1]   chunk i
//
// A chunk is chunk_bytes of the image (the last one may be shorter). Its
// stored length says how it is kept:
//
//   0                  every byte is zero, and nothing is stored
//   its full length    stored as it is: it did not compress
//   anything else      one zstd frame
//
// Little-endian throughout, which is every machine SchOS runs on.

#ifndef SCHOS_FS_SCFF_H
#define SCHOS_FS_SCFF_H

#include <stdint.h>

#define SCFF_MAGIC      "SCFF"
#define SCFF_VERSION    1
#define SCFF_CODEC_ZSTD 1

// The kernel decompresses one chunk at a time into a buffer this big, so it
// is the largest chunk a reader accepts. 128 KB is squashfs's default and
// the point past which a bigger chunk buys little ratio for a lot of
// decompression on every miss.
#define SCFF_CHUNK_MAX  (1u << 20)

struct scff_header {
  char     magic[4];        // "SCFF"
  uint16_t version;         // SCFF_VERSION
  uint16_t codec;           // SCFF_CODEC_ZSTD
  uint32_t chunk_bytes;     // a power of two, a multiple of 4096, <= SCFF_CHUNK_MAX
  uint32_t nchunks;
  uint64_t image_bytes;     // the uncompressed image
  uint64_t index_off;
  uint64_t stored_bytes;    // the whole file, header and index included
  uint8_t  reserved[24];
};

_Static_assert(sizeof(struct scff_header) == 64, "scff_header is 64 bytes");

#endif