open Core

(* Reading files requires checking most calls for errors.
   This helper will streamline our error checks below. *)
let check_exn result =
  match result with
  | Ok x -> x
  | Error e -> raise (Unix.Unix_error (e, "", ""))

let () =
  (* Perhaps the most basic file reading task is
     slurping a file's entire contents into memory. *)
  let dat = In_channel.read_all "/tmp/dat" in
  printf "%s" dat;

  (* You'll often want more control over how and what
     parts of a file are read. For these tasks, start
     by opening a file to obtain an In_channel value. *)
  let ic = In_channel.create "/tmp/dat" in

  (* Read some bytes from the beginning of the file.
     Allow up to 5 to be read but also note how many
     actually were read. *)
  let b1 = Bytes.create 5 in
  let n1 = check_exn (In_channel.input ic ~buf:b1 ~pos:0 ~len:5) in
  printf "%d bytes: %s\n" n1 (Bytes.sub_string b1 0 n1);

  (* You can also seek to a known location in the file
     and read from there. *)
  check_exn (In_channel.seek ic 6L);
  let b2 = Bytes.create 2 in
  let n2 = check_exn (In_channel.input ic ~buf:b2 ~pos:0 ~len:2) in
  printf "%d bytes @ %d: %s\n" n2 6 (Bytes.to_string b2);

  (* Other methods of seeking are relative to the
     current cursor position, *)
  check_exn (In_channel.seek ic (Int64.of_int 4));

  (* and relative to the end of the file. *)
  check_exn (In_channel.seek ic (Int64.of_int (-10)));

  (* The Core library provides some functions that may
     be helpful for file reading. For example, reads
     like the ones above can be more robustly
     implemented with In_channel.really_input. *)
  check_exn (In_channel.seek ic 6L);
  let b3 = Bytes.create 2 in
  check_exn (In_channel.really_input ic b3 0 2);
  printf "%d bytes @ %d: %s\n" 2 6 (Bytes.to_string b3);

  (* There is no built-in rewind, but
     seek to 0 accomplishes this. *)
  check_exn (In_channel.seek ic 0L);

  (* The Core library also implements a buffered
     reader that may be useful both for its efficiency
     with many small reads and because of the additional
     reading methods it provides. *)
  let reader = Stdio.In_channel.create "/tmp/dat" in
  let line = Stdio.In_channel.input_line_exn reader in
  printf "First line: %s\n" line;

  (* Close the file when you're done (usually this would
     be scheduled immediately after opening with
     `finally`). *)
  In_channel.close ic;
  Stdio.In_channel.close reader