Source file io_array.ml

(* The MIT License

   Copyright (c) 2019 Craig Ferguson <craig@tarides.com>
                      Thomas Gazagnaire <thomas@tarides.com>
                      Ioana Cristescu <ioana@tarides.com>
                      Clément Pascutto <clement@tarides.com>

   Permission is hereby granted, free of charge, to any person obtaining a copy
   of this software and associated documentation files (the "Software"), to deal
   in the Software without restriction, including without limitation the rights
   to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   copies of the Software, and to permit persons to whom the Software is
   furnished to do so, subject to the following conditions:

   The above copyright notice and this permission notice shall be included in
   all copies or substantial portions of the Software. *)

open! Import

module type ELT = sig
  type t

  val encoded_size : int
  val decode : string -> int -> t
end

module type S = sig
  include Search.ARRAY

  type io

  val v : io -> t
end

module Make (IO : Io.S) (Elt : ELT) :
  S with type io = IO.t and type elt = Elt.t = struct
  module Elt = struct
    include Elt

    let encoded_sizeL = Int63.of_int encoded_size
  end

  type io = IO.t
  type elt = Elt.t
  type buffer = { buf : bytes; low_off : int63; high_off : int63 }
  type t = { io : IO.t; mutable buffer : buffer option }

  let v io = { io; buffer = None }

  let get_entry_from_io io off =
    let buf = Bytes.create Elt.encoded_size in
    let n = IO.read io ~off ~len:Elt.encoded_size buf in
    assert (n = Elt.encoded_size);
    Elt.decode (Bytes.unsafe_to_string buf) 0

  let get_entry_from_buffer buf off =
    let buf_off = Int63.(to_int_exn (off - buf.low_off)) in
    assert (buf_off <= Bytes.length buf.buf);
    Elt.decode (Bytes.unsafe_to_string buf.buf) buf_off

  let is_in_buffer t off =
    match t.buffer with
    | None -> false
    | Some b ->
        Int63.compare off b.low_off >= 0 && Int63.compare off b.high_off <= 0

  let get t i =
    let off = Int63.(i * Elt.encoded_sizeL) in
    match t.buffer with
    | Some b when is_in_buffer t off -> (
        try get_entry_from_buffer b off with _ -> assert false)
    | _ -> get_entry_from_io t.io off

  let length t = Int63.div (IO.offset t.io) Elt.encoded_sizeL

  let max_buffer_size =
    (* The prefetched area should not exceed 4096 in most cases, thanks to the
       fan out. However, if the hash function is not well distributed, some
       exceptions might happen where the prefetched area actually exceeds 4096.
       As long as this excess is reasonable (x8), we still want to prefetch. *)
    8 * 4096

  let buf = Bytes.create max_buffer_size

  let set_buffer t ~low ~high =
    let range = Elt.encoded_size * (1 + Int63.(to_int_exn (high - low))) in
    let low_off = Int63.mul low Elt.encoded_sizeL in
    let high_off = Int63.mul high Elt.encoded_sizeL in
    let n = IO.read t.io ~off:low_off ~len:range buf in
    assert (n = range);
    t.buffer <- Some { buf; low_off; high_off }

  let pre_fetch t ~low ~high =
    let range = Elt.encoded_size * (1 + Int63.(to_int_exn (high - low))) in
    if Int63.compare low high > 0 then
      Log.warn (fun m ->
          m "Requested pre-fetch region is empty: [%a, %a]" Int63.pp low
            Int63.pp high)
    else if range > max_buffer_size then
      Log.warn (fun m ->
          m "Requested pre-fetch [%a, %a] is larger than %d" Int63.pp low
            Int63.pp high max_buffer_size)
    else
      match t.buffer with
      | Some b ->
          let low_buf, high_buf =
            Int63.
              (div b.low_off Elt.encoded_sizeL, div b.high_off Elt.encoded_sizeL)
          in
          if low >= low_buf && high <= high_buf then
            Log.debug (fun m ->
                m
                  "Pre-existing buffer [%a, %a] encloses requested pre-fetch \
                   [%a, %a]"
                  Int63.pp low_buf Int63.pp high_buf Int63.pp low Int63.pp high)
          else (
            Log.warn (fun m ->
                m
                  "Current buffer [%a, %a] insufficient. Prefetching in range \
                   [%a, %a]"
                  Int63.pp low_buf Int63.pp high_buf Int63.pp low Int63.pp high);
            set_buffer t ~low ~high)
      | None ->
          Log.debug (fun m ->
              m "No existing buffer. Prefetching in range [%a, %a]" Int63.pp low
                Int63.pp high);
          set_buffer t ~low ~high
end