(** Zstandard format constants (RFC 8878) *)

(** Magic numbers *)
let zstd_magic_number = 0xFD2FB528l
let dict_magic_number = 0xEC30A437l
let skippable_magic_start = 0x184D2A50l
let skippable_magic_mask = 0xFFFFFFF0l
let skippable_header_size = 8

(** Block size limits *)
let block_size_max = 128 * 1024  (* 128 KB *)
let max_literals_size = block_size_max

(** Maximum values *)
let max_window_log = 31
let min_window_log = 10
let max_huffman_bits = 11
let max_fse_accuracy_log = 15
let max_huffman_symbols = 256
let max_fse_symbols = 256

(** Block types *)
type block_type =
  | Raw_block
  | RLE_block
  | Compressed_block
  | Reserved_block

let block_type_of_int = function
  | 0 -> Raw_block
  | 1 -> RLE_block
  | 2 -> Compressed_block
  | _ -> Reserved_block

(* Block type integer values for encoding *)
let block_raw = 0
let block_rle = 1
let block_compressed = 2

(** Literals block types *)
type literals_block_type =
  | Raw_literals
  | RLE_literals
  | Compressed_literals
  | Treeless_literals

let literals_block_type_of_int = function
  | 0 -> Raw_literals
  | 1 -> RLE_literals
  | 2 -> Compressed_literals
  | _ -> Treeless_literals

(** Sequence compression modes *)
type seq_mode =
  | Predefined_mode
  | RLE_mode
  | FSE_mode
  | Repeat_mode

let seq_mode_of_int = function
  | 0 -> Predefined_mode
  | 1 -> RLE_mode
  | 2 -> FSE_mode
  | _ -> Repeat_mode

(** Default FSE distribution tables for predefined mode *)

(* Literals length default distribution (accuracy log 6, 64 states) *)
let ll_default_distribution = [|
  4; 3; 2; 2; 2; 2; 2; 2; 2; 2; 2; 2; 2; 1; 1; 1;
  2; 2; 2; 2; 2; 2; 2; 2; 2; 3; 2; 1; 1; 1; 1; 1;
  -1; -1; -1; -1
|]
let ll_default_accuracy_log = 6
let ll_max_accuracy_log = 9

(* Match length default distribution (accuracy log 6, 64 states) *)
let ml_default_distribution = [|
  1; 4; 3; 2; 2; 2; 2; 2; 2; 1; 1; 1; 1; 1; 1; 1;
  1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
  1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; -1; -1;
  -1; -1; -1; -1; -1
|]
let ml_default_accuracy_log = 6
let ml_max_accuracy_log = 9

(* Offset default distribution (accuracy log 5, 32 states) *)
let of_default_distribution = [|
  1; 1; 1; 1; 1; 1; 2; 2; 2; 1; 1; 1; 1; 1; 1; 1;
  1; 1; 1; 1; 1; 1; 1; 1; -1; -1; -1; -1; -1
|]
let of_default_accuracy_log = 5
let of_max_accuracy_log = 8

(** Sequence code baselines and extra bits *)

(* Literals length: code 0-35 *)
let ll_baselines = [|
  0; 1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11;
  12; 13; 14; 15; 16; 18; 20; 22; 24; 28; 32; 40;
  48; 64; 128; 256; 512; 1024; 2048; 4096; 8192; 16384; 32768; 65536
|]
let ll_extra_bits = [|
  0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0;
  0; 0; 0; 0; 1; 1; 1; 1; 2; 2; 3; 3;
  4; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16
|]
let ll_max_code = 35

(* Match length: code 0-52 *)
let ml_baselines = [|
  3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16;
  17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30;
  31; 32; 33; 34; 35; 37; 39; 41; 43; 47; 51; 59; 67; 83;
  99; 131; 259; 515; 1027; 2051; 4099; 8195; 16387; 32771; 65539
|]
let ml_extra_bits = [|
  0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0;
  0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0; 0;
  0; 0; 0; 0; 1; 1; 1; 1; 2; 2; 3; 3; 4; 4;
  5; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16
|]
let ml_max_code = 52

(* Offset codes: the code is the number of bits to read *)
let of_max_code = 31

(** Initial repeat offsets *)
let initial_repeat_offsets = [| 1; 4; 8 |]

(** Error types *)
type error =
  | Invalid_magic_number
  | Invalid_frame_header
  | Invalid_block_type
  | Invalid_block_size
  | Invalid_literals_header
  | Invalid_huffman_table
  | Invalid_fse_table
  | Invalid_sequence_header
  | Invalid_offset
  | Invalid_match_length
  | Truncated_input
  | Output_too_small
  | Checksum_mismatch
  | Dictionary_mismatch
  | Corruption

exception Zstd_error of error

let error_message = function
  | Invalid_magic_number -> "Invalid magic number"
  | Invalid_frame_header -> "Invalid frame header"
  | Invalid_block_type -> "Invalid block type"
  | Invalid_block_size -> "Invalid block size"
  | Invalid_literals_header -> "Invalid literals header"
  | Invalid_huffman_table -> "Invalid Huffman table"
  | Invalid_fse_table -> "Invalid FSE table"
  | Invalid_sequence_header -> "Invalid sequence header"
  | Invalid_offset -> "Invalid offset"
  | Invalid_match_length -> "Invalid match length"
  | Truncated_input -> "Truncated input"
  | Output_too_small -> "Output buffer too small"
  | Checksum_mismatch -> "Checksum mismatch"
  | Dictionary_mismatch -> "Dictionary mismatch"
  | Corruption -> "Data corruption detected"