(* buf_write.ml - Low-level buffer writing primitives for HTTP response generation *)

open Base

let char dst ~off c =
  Bigarray.Array1.unsafe_set dst off c;
  off + 1
;;

let string dst ~off s =
  let len = String.length s in
  for i = 0 to len - 1 do
    Bigarray.Array1.unsafe_set dst (off + i) (String.unsafe_get s i)
  done;
  off + len
;;

let crlf dst ~off =
  Bigarray.Array1.unsafe_set dst off '\r';
  Bigarray.Array1.unsafe_set dst (off + 1) '\n';
  off + 2
;;

(* Count digits in a positive integer *)
let count_digits n =
  let temp = ref n in
  let digits = ref 0 in
  while !temp > 0 do
    Int.incr digits;
    temp := !temp / 10
  done;
  !digits
;;

let int dst ~off n =
  if n = 0 then (
    Bigarray.Array1.unsafe_set dst off '0';
    off + 1
  ) else (
    let digits = count_digits n in
    let p = ref (off + digits - 1) in
    let remaining = ref n in
    while !remaining > 0 do
      Bigarray.Array1.unsafe_set dst !p (Char.of_int_exn (48 + Int.rem !remaining 10));
      remaining := !remaining / 10;
      Int.decr p
    done;
    off + digits
  )
;;

let int64 dst ~off n =
  if Int64.(n = 0L) then (
    Bigarray.Array1.unsafe_set dst off '0';
    off + 1
  ) else (
    (* Count digits *)
    let temp = ref n in
    let digits = ref 0 in
    while Int64.(!temp > 0L) do
      Int.incr digits;
      temp := Int64.(!temp / 10L)
    done;
    (* Write digits in reverse *)
    let p = ref (off + !digits - 1) in
    let remaining = ref n in
    while Int64.(!remaining > 0L) do
      let digit = Int64.(!remaining % 10L) |> Int64.to_int_exn in
      Bigarray.Array1.unsafe_set dst !p (Char.of_int_exn (48 + digit));
      remaining := Int64.(!remaining / 10L);
      Int.decr p
    done;
    off + !digits
  )
;;

let hex_chars = "0123456789abcdef"

let hex dst ~off n =
  if n = 0 then (
    Bigarray.Array1.unsafe_set dst off '0';
    off + 1
  ) else (
    let temp = ref n in
    let digits = ref 0 in
    while !temp > 0 do
      Int.incr digits;
      temp := !temp lsr 4
    done;
    let p = ref (off + !digits - 1) in
    let remaining = ref n in
    while !remaining > 0 do
      Bigarray.Array1.unsafe_set dst !p (String.unsafe_get hex_chars (!remaining land 0xf));
      remaining := !remaining lsr 4;
      Int.decr p
    done;
    off + !digits
  )
;;

let digit2 dst ~off n =
  Bigarray.Array1.unsafe_set dst off (Char.of_int_exn (48 + n / 10));
  Bigarray.Array1.unsafe_set dst (off + 1) (Char.of_int_exn (48 + n % 10));
  off + 2
;;

let digit4 dst ~off n =
  Bigarray.Array1.unsafe_set dst off (Char.of_int_exn (48 + n / 1000));
  Bigarray.Array1.unsafe_set dst (off + 1) (Char.of_int_exn (48 + (n / 100) % 10));
  Bigarray.Array1.unsafe_set dst (off + 2) (Char.of_int_exn (48 + (n / 10) % 10));
  Bigarray.Array1.unsafe_set dst (off + 3) (Char.of_int_exn (48 + n % 10));
  off + 4
;;