res / README

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                            Distribution of RES
            Automatically Resizing Contiguous Memory for OCaml

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                               Prerequisites

             YOU WILL NEED GNU-MAKE TO COMPILE THE SYSTEM WITH
                         THE DISTRIBUTED MAKEFILES

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                       Contents of this distribution

Changes             - History of code changes.

INSTALL             - Short notes on compiling and installing the library

LICENSE             - A copy of the "GNU LESSER GENERAL PUBLIC LICENSE"

Makefile            - Top Makefile

OcamlMakefile       - Makefile for easy handling of compilation of not
                      so easy OCaml-projects.  It generates dependencies
                      of Ocaml-files automatically, is able to handle
                      "ocamllex"-, "ocamlyacc"-, IDL- and C-files and
                      generates native- or byte-code, as executable or
                      as library - with thread-support if you want!

README              - this file

examples/

lib/                - Implementation of the RES-library.

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                                What is it?

This OCaml-library consists of a set of modules which implement
automatically resizing (= reallocating) datastructures that consume
a contiguous part of memory. This allows appending and removing of
elements to/from arrays (both boxed and unboxed), strings (-> buffers),
bit strings and weak arrays while still maintaining fast constant-time
access to elements.

There are also functors that allow the generation of similar modules
which use different reallocation strategies.

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                      Why should you use it?

For several reasons:

  * Fast constant-time access to indexed elements (e.g. in arrays and
    strings) is often a prerequisite for short execution times of
    programs.

    Still, operations like adding and/or removing elements to/from the
    end of such datastructures are often needed.  Unfortunately, having
    both properties at the same time sometimes requires reallocating
    this contiguous part of memory.

    This module does not eliminate this problem, but hides the process
    of reallocation from the user, i.e. it happens automatically.

    Thus, the user is liberated from this bug-attracting (e.g. index
    errors) task.

  * This library allows the user to parameterize allocation strategies at
    runtime. This is a very important feature, because it is impossible
    for any allocation algorithm to perform optimally without having
    knowledge about the user program.

    For example, the programmer might know that a consecutive series
    of operations will alternately add and remove large amounts of
    elements. In such a case it would be wise to keep a high reserve
    of available slots in the datastructure, because otherwise it will
    resize very often during this procedure which requires a significant
    amount of time.

    By raising a corresponding threshold in appropriate places at runtime,
    the programmer can fine-tune the behaviour of e.g. his buffers for
    optimal performance and set this parameter back later to save memory.

  * Because reallocation strategies themselves may be quite complicated,
    it was also a design goal to have the user supply his own ones
    (if required).

    By using functors the user can parameterize these datastructures
    with his own reallocation strategies, giving him even more control
    over how and when reallocations are triggered.

  * It is possible that the user wants to add support for additional
    low-level implementations that require reallocations. Even in this
    case it is fairly easy to create new modules by using functors.

  * The library implements a large interface of functions, all of which
    are completely independent of the reallocation strategy and the
    low-level implementation.

    All the interfaces of the corresponding low-level implementations
    of datastructures (e.g. array, string) are fully supported and have
    been extended with further functionality. There is even a new buffer
    module which can be used in every context of the standard one.

  * OCaml makes a distinction between unboxed and boxed arrays. If
    the type of an array is "float", the representation will be unboxed
    in cases in which the array is not used in a polymorphic context
    (native code only).

    To benefit from these much faster representations there are
    specialized versions of automatically resizing arrays in the
    distribution.

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                        Documentation of Functions

The preparameterized modules (default strategy) and the functors for
mapping strategy-implementations to this kind of modules are contained
and documented in file "lib/res.mli".

For examples of how to use the functors to implement new strategies and/or
low-level representations, take a look at the implementation of "res.ml".

Their function interface, however, is documented in files
"lib/pres_intf.mli" (for parameterized "low-level" types like e.g. normal
arrays) and in "lib/nopres_intf.mli" (for non-parameterized "low-level"
types like e.g. int arrays, strings (buffers), ...).

This was done so as not do confuse people with too large an interface
description in one file.

It is advisable to keep all of these files at some specific location at
hand for quick information.

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                          Hints for Convenient Use

It should be noted that it is possible to use the standard notation for
accessing elements (e.g. "ar.(42)") with resizable arrays (and even with
"Buffer", "Bits", etc...). This requires a short explanation of how OCaml
treats such syntactic sugar:

All that OCaml does is that it replaces each such structure to an appropriate
"Array.get" or "Array.set". This may be *any* module that happens to be bound
to this name in the current scope! The same principle is true for the
String-module and the '.[]'-operator.

Thus, the following works:

  module Array = Res.Bits
  module String = Res.Buffer

  let _ =
    let ar = Array.empty () in
    Array.add_one ar true;
    print_endline (string_of_bool ar.(0));
    let str = String.empty () in
    String.add_one str 'x';
    print_char str.[0]; print_newline ()

Do not forget that it is even possible to open modules locally! Example:

  let _ =
    let module Array = Res.Array in
    Printf.printf "%d\n" (Array.init 10 (fun x -> x * x)).(7)


If you want to change one of your files to make use of resizable arrays
instead of standard ones without much trouble, you should read the
following:

You should "save" the standard Array-module and its type for later access:

  module StdArray = Array
  type 'a std_array = 'a array

Make the resizable implementation (includes the index operators!)
available:

  open Res

or explicitely:

  module Array = Res.Array

or if you want to use a specific "Array"-implementation:

  module Array = Res.Bits

Then set the type:

  type 'a array = 'a Array.t

If you create standard arrays with the builtin syntax, change lines like

  let ar = [| 1; 2; 3; 4 |] in

to

  let ar = Array.of_array [| 1; 2; 3; 4 |] in

This should allow all of your sources to compile out-of-the-box with the
additional functionality. In places where you still need the standard
implementation you should have no problems to use the rebound module
and type to do so.

This trick works similarly for the old and the new Buffer-module. You
might also want to replace the "String" module in this fashion. The
latter one, however, supports a number of functions like e.g. "escape",
which are not available then.

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Up-to-date information (newest release of distribution) can always be
found at:

  http://www.ocaml.info/home/ocaml_sources.html

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Enjoy!

Vienna, 2002-09-11
Markus Mottl

e-mail: markus.mottl@gmail.com
WWW:    http://www.ocaml.info
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