(*---------------------------------------------------------------------------
  Copyright (c) 2025 Anil Madhavapeddy <anil@recoil.org>. All rights reserved.
  SPDX-License-Identifier: ISC
 ---------------------------------------------------------------------------*)

(** Character encoding detection and handling *)

type t = [
  | `Utf8
  | `Utf16be
  | `Utf16le
  | `Utf32be
  | `Utf32le
]

let to_string = function
  | `Utf8 -> "UTF-8"
  | `Utf16be -> "UTF-16BE"
  | `Utf16le -> "UTF-16LE"
  | `Utf32be -> "UTF-32BE"
  | `Utf32le -> "UTF-32LE"

let pp fmt t =
  Format.pp_print_string fmt (to_string t)

(** Detect encoding from BOM or first bytes.
    Returns (encoding, bom_length) *)
let detect s =
  let len = String.length s in
  if len = 0 then (`Utf8, 0)
  else
    let b0 = Char.code s.[0] in
    let b1 = if len > 1 then Char.code s.[1] else 0 in
    let b2 = if len > 2 then Char.code s.[2] else 0 in
    let b3 = if len > 3 then Char.code s.[3] else 0 in
    match (b0, b1, b2, b3) with
    (* BOM patterns *)
    | (0xEF, 0xBB, 0xBF, _) -> (`Utf8, 3)
    | (0xFE, 0xFF, _, _) -> (`Utf16be, 2)
    | (0xFF, 0xFE, 0x00, 0x00) -> (`Utf32le, 4)
    | (0xFF, 0xFE, _, _) -> (`Utf16le, 2)
    | (0x00, 0x00, 0xFE, 0xFF) -> (`Utf32be, 4)
    (* Content pattern detection (no BOM) *)
    | (0x00, 0x00, 0x00, b3) when b3 <> 0x00 -> (`Utf32be, 0)
    | (b0, 0x00, 0x00, 0x00) when b0 <> 0x00 -> (`Utf32le, 0)
    | (0x00, b1, _, _) when b1 <> 0x00 -> (`Utf16be, 0)
    | (b0, 0x00, _, _) when b0 <> 0x00 -> (`Utf16le, 0)
    | _ -> (`Utf8, 0)

let equal a b = a = b