Hu Chunming / sip_server

[/
    Copyright (c) 2009-2009 Joachim Faulhaber

    Distributed under the Boost Software License, Version 1.0.
    (See accompanying file LICENSE_1_0.txt or copy at
    http://www.boost.org/LICENSE_1_0.txt)
]

[section Projects]

['*Projects*] are examples on the usage of interval containers
that go beyond small toy snippets of code. The code presented
here addresses more serious applications that approach the
quality of real world programming. At the same time it aims to
guide the reader more deeply into various aspects of 
the library. In order not to overburden the reader with implementation
details, the code in ['*projects*] tries to be ['*minimal*]. It has a focus on
the main aspects of the projects and is not intended to be complete 
and mature like the library code itself. Cause it's minimal,
project code lives in `namespace mini`.

[/ 
[section Overview]

[table Overview over Icl Examples
    [[level]   [example] [classes] [features]]
    [[basic]   [[link boost_icl.examples.large_bitset Large Bitset]]
        [__itv_map__][The most simple application of an interval map: 
                      Counting the overlaps of added intervals.]]
]

[endsect][/ Overview IN Projects]
]

[section Large Bitset][/ IN Projects]

Bitsets are just sets. Sets of unsigned integrals, 
to be more precise. 
The prefix ['*bit*] usually only indicates,
that the representation of those sets is organized in a compressed
form that exploits the fact, that we can switch on an off single
bits in machine words. Bitsets are therefore known to be very small
and thus efficient.
The efficiency of bitsets is usually coupled to the 
precondition that the range of values of elements  
is relatively small, like [0..32) or [0..64), values that can
be typically represented in single or a small number of
machine words. If we wanted to represent a set containing
two values {1, 1000000}, we would be much better off using
other sets like e.g. an `std::set`.

Bitsets compress well, if elements spread over narrow ranges
only. Interval sets compress well, if many elements are clustered
over intervals. They can span large sets very efficiently then.
In project ['*Large Bitset*] we want to ['*combine the bit compression
and the interval compression*] to achieve a set implementation, 
that is capable of spanning large chunks of contiguous elements 
using intervals and also to represent more narrow ['nests] of varying 
bit sequences using bitset compression.
As we will see, this can be achieved using only a small
amount of code because most of the properties we need
are provided by an __itv_map__ of `bitsets`:

``
typedef interval_map<IntegralT, SomeBitSet<N>, partial_absorber,
                     std::less, inplace_bit_add, inplace_bit_and> IntervalBitmap;
``

Such an `IntervalBitmap` represents `k*N` bits for every segment. 
``
[a, a+k)->'1111....1111' // N bits associated: Represents a total of k*N bits.  
``

For the interval `[a, a+k)` above all bits are set.
But we can also have individual ['nests] or ['clusters]
of bitsequences.

``
[b,  b+1)->'01001011...1'
[b+1,b+2)->'11010001...0'
. . .
``

and we can span intervals of equal bit sequences that represent
periodic patterns.

``
[c,d)->'010101....01'  // Every second bit is set              in range [c,d)
[d,e)->'001100..0011'  // Every two bits alterate              in range [d,e)
[e,f)->'bit-sequence'  // 'bit-sequence' reoccurs every N bits in range [e,f)
``

An `IntervalBitmap` can represent
`N*(2^M)` elements, if `M` is the number of bits of the 
integral type `IntegralT`. Unlike bitsets, that usually
represent ['*unsigned*] integral numbers, large_bitset may 
range over negative numbers as well.
There are fields where such large
bitsets implementations are needed. E.g. for the compact
representation of large file allocation tables.
What remains to be done for project *Large Bitset* is
to code a wrapper `class large_bitset` around `IntervalBitmap`
so that `large_bitset` looks and feels like a usual
set class.

[section Using large_bitset]

To quicken your appetite for a look at the implementation
here are a few use cases first. Within the examples that follow,
we will use `nat`[^['k]] for unsigned integrals 
and `bits`[^['k]] for bitsets containing [^['k]] bits.

Let's start large.
In the first example . . . 

[import ../example/large_bitset_/large_bitset.cpp]
[large_bitset_test_large_set_all]

. . . we are testing the limits. First we set all bits and
then we switch off the very last bit.

[large_bitset_test_large_erase_last]

Program output (/a little beautified/):
``
----- Test function test_large() -----------------------------------------------
We have just turned on the awesome amount of 18,446,744,073,709,551,616 bits ;-)
[                 0, 288230376151711744) -> 1111111111111111111111111111111111111111111111111111111111111111
---- Let's swich off the very last bit -----------------------------------------
[                 0, 288230376151711743) -> 1111111111111111111111111111111111111111111111111111111111111111
[288230376151711743, 288230376151711744) -> 1111111111111111111111111111111111111111111111111111111111111110
---- Venti is plenty ... let's do something small: A tall ----------------------
``

More readable is a smaller version of `large_bitset`.
In function `test_small()` we apply a few more operations . . .

[large_bitset_test_small]

. . . producing this output:
``
----- Test function test_small() -----------
-- Switch on all bits in range [0,64] ------
[0,8)->11111111
[8,9)->10000000
--------------------------------------------
-- Turn off bits: 25,27,28 -----------------
[0,3)->11111111
[3,4)->10100111
[4,8)->11111111
[8,9)->10000000
--------------------------------------------
-- Flip bits in range [24,30) --------------
[0,3)->11111111
[3,4)->01011011
[4,8)->11111111
[8,9)->10000000
--------------------------------------------
-- Remove the first 10 bits ----------------
[1,2)->00111111
[2,3)->11111111
[3,4)->01011011
[4,8)->11111111
[8,9)->10000000
-- Remove even bits in range [0,72) --------
[1,2)->00010101
[2,3)->01010101
[3,4)->01010001
[4,8)->01010101
--    Set odd  bits in range [0,72) --------
[0,9)->01010101
--------------------------------------------
``

Finally, we present a little /picturesque/ example, 
that demonstrates that `large_bitset` can also serve
as a self compressing bitmap, that we can 'paint' with.

[large_bitset_test_picturesque]

Note that we have two `large_bitsets` for the /outline/ 
and the /interior/. Both parts are compressed but we can
compose both by `operator +`, because the right /positions/
are provided. This is the program output:

``
----- Test function test_picturesque() -----
-------- empty face:       3 intervals -----
********
*      *
*      *
*      *
*      *
 ******
-------- compressed smile: 2 intervals -----
  *  *
  ****
-------- staring bitset:   6 intervals -----
********
*      *
* *  * *
*      *
* **** *
 ******
--------------------------------------------
``

So, may be you are curious how this class template
is coded on top of __itv_map__ using only about 250 lines 
of code. This is shown in the sections that follow.

[endsect][/ Usage of a large_bitset]

[section The interval_bitmap]

To begin, let's look at the basic data type again, that
will be providing the major functionality:

``
typedef interval_map<DomainT, BitSetT, partial_absorber,
                     std::less, inplace_bit_add, inplace_bit_and> IntervalBitmap;
``

`DomainT` is supposed to be an integral
type, the bitset type `BitSetT` will be a wrapper class around an
unsigned integral type. `BitSetT` has to implement bitwise operators
that will be called by the functors `inplace_bit_add<BitSetT>` 
and `inplace_bit_and<BitSetT>`.
The type trait of interval_map is `partial_absorber`, which means
that it is /partial/ and that empty `BitSetTs` are not stored in
the map. This is desired and keeps the `interval_map` minimal, storing
only bitsets, that contain at least one bit switched on. 
Functor template `inplace_bit_add` for parameter `Combine` indicates that we
do not expect `operator +=` as addition but the bitwise operator
`|=`. For template parameter `Section` which is instaniated by 
`inplace_bit_and` we expect the bitwise `&=` operator.

[endsect][/ section The interval_bitmap]

[section A class implementation for the bitset type]

The code of the project is enclosed in a `namespace mini`.
The name indicates, that the implementation is a /minimal/
example implementation. The name of the bitset class will
be `bits` or `mini::bits` if qualified.

To be used as a codomain parameter of class template __itv_map__,
`mini::bits` has to implement all the functions that are required
for a codomain_type in general, which are the default constructor `bits()`
and an equality `operator==`. Moreover `mini::bits` has to implement operators
required by the instantiations for parameter `Combine` and `Section`
which are `inplace_bit_add` and `inplace_bit_and`. From functors
`inplace_bit_add` and `inplace_bit_and` there are inverse functors
`inplace_bit_subtract` and `inplace_bit_xor`. Those functors 
use operators `|= &= ^=` and `~`. Finally if we want to apply
lexicographical and subset comparison on large_bitset, we also
need an `operator <`. All the operators that we need can be implemented
for `mini::bits` on a few lines:

[import ../example/large_bitset_/bits.hpp]
[mini_bits_class_bits]

Finally there is one important piece of meta information, we have 
to provide: `mini::bits` has to be recognized as a `Set` by the
icl code. Otherwise we can not exploit the fact that a map of
sets is model of `Set` and the resulting large_bitset would not
behave like a set. So we have to say that `mini::bits` shall be sets:

[mini_bits_is_set]

This is done by adding a partial template specialization to
the type trait template `icl::is_set`.
For the extension of this type trait template and the result
values of inclusion_compare we need these `#includes` for the
implementation of `mini::bits`:
 
[mini_bits_includes]
 
[endsect][/ A class implementation for the bitset type]

[section Implementation of a large bitset]

Having finished our `mini::bits` implementation, we can start to
code the wrapper class that hides the efficient interval map of mini::bits
and exposes a simple and convenient set behavior to the world of users.

Let's start with the required `#include`s this time:

[import ../example/large_bitset_/large_bitset.hpp]
[large_bitset_includes]

Besides `boost/icl/interval_map.hpp` and `bits.hpp` the most important
include here is `boost/operators.hpp`. We use this library in order
to further minimize the code and to provide pretty extensive operator
functionality using very little code.

For a short and concise naming of the most important unsigned integer
types and the corresponding `mini::bits` we define this:

[large_bitset_natural_typedefs]

[section large_bitset: Public interface][/ ------------------------------------]

And now let's code `large_bitset`. 

[large_bitset_class_template_header]

The first template parameter `DomainT` will be instantiated with
an integral type that defines the kind of numbers that can
be elements of the set. Since we want to go for a large set we
use `nat64` as default which is a 64 bit unsigned integer ranging
from `0` to `2^64-1`. As bitset parameter we also choose a 64-bit
default. Parameters `Combine` and `Interval` are necessary to
be passed to dependent type expressions. An allocator can be
chosen, if desired.

The nested list of private inheritance contains groups of template
instantiations from 
[@http://www.boost.org/doc/libs/1_39_0/libs/utility/operators.htm Boost.Operator], 
that provides derivable operators from more fundamental once.
Implementing the fundamental operators, we get the derivable ones
/for free/. Below is a short overview of what we get using Boost.Operator,
where __S stands for `large_bitset`, __i for it's `interval_type` 
and __e for it's `domain_type` or `element_type`.

[table
[[Group                    ][fundamental]      [derivable     ]]
[[Equality, ordering       ][`==`]             [`!=`          ]]
[[                         ][`<` ]             [`> <= >=`     ]]
[[Set operators (__S x __S)][`+= |= -= &= ^=` ][`+ | - & ^`   ]]
[[Set operators (__S x __e)][`+= |= -= &= ^=` ][`+ | - & ^`   ]]
[[Set operators (__S x __i)][`+= |= -= &= ^=` ][`+ | - & ^`   ]]
]

There is a number of associated types

[large_bitset_associated_types]

most importantly the implementing `interval_bitmap_type` that is used
for the implementing container.

[large_bitmap_impl_map]

In order to use
[@http://www.boost.org/doc/libs/1_39_0/libs/utility/operators.htm Boost.Operator]
we have to implement the fundamental operators as class members. This can be
done quite schematically.

[large_bitset_operators]

As we can see, the seven most important operators that work on the
class type `large_bitset` can be directly implemented by propagating
the operation to the implementing `_map`
of type `interval_bitmap_type`. For the operators that work on segment and
element types, we use member functions `add`, `subtract`, `intersect` and
`flip`. As we will see only a small amount of adaper code is needed
to couple those functions with the functionality of the implementing
container.

Member functions `add`, `subtract`, `intersect` and `flip`, 
that allow to combine ['*intervals*] to `large_bitsets` can
be uniformly implemented using a private function
`segment_apply` that applies /addition/, /subtraction/,
/intersection/ or /symmetric difference/, after having
translated the interval's borders into the right bitset
positions.  

[large_bitset_fundamental_functions]

In the sample programs, that we will present to demonstrate
the capabilities of `large_bitset` we will need a few additional
functions specifically output functions in two different
flavors.

[large_bitset_demo_functions]

* The first one, `show_segments()` shows the container 
content as it is implemented, in the compressed form.

* The second function `show_matrix` shows the complete
matrix of bits that is represented by the container.

[endsect][/ large_bitset: Public interface] 
[section large_bitset: Private implementation]  [/ -------------------------------------]

In order to implement operations like the addition of 
an element say `42` to
the large bitset, we need to translate the /value/ to the
/position/ of the associated *bit* representing `42` in the
interval container of bitsets. As an example, suppose we
use a
``
large_bitset<nat, mini::bits8> lbs;
``
that carries small bitsets of 8 bits only. 
The first four interval of `lbs` are assumed to
be associated with some bitsets. Now we want to add the interval
`[a,b]==[5,27]`. This will result in the following situation:
``
   [0,1)->   [1,2)->   [2,3)->   [3,4)->   
   [00101100][11001011][11101001][11100000]
+       [111  11111111  11111111  1111]      [5,27] as bitset
         a                           b
         
=> [0,1)->   [1,3)->   [3,4)->
   [00101111][11111111][11110000]
``
So we have to convert values 5 and 27 into a part that
points to the interval and a part that refers to the
position within the interval, which is done by a
/division/ and a /modulo/ computation. (In order to have a
consistent representation of the bitsequences across the containers,
within this project, bitsets are denoted with the
['*least significant bit on the left!*])

``
A = a/8 =  5/8 = 0 // refers to interval 
B = b/8 = 27/8 = 3
R = a%8 =  5%8 = 5 // refers to the position in the associated bitset.
S = b%8 = 27%8 = 3
``

All /division/ and /modulo operations/ needed here are always done
using a divisor `d` that is a power of `2`: `d = 2^x`. Therefore
division and modulo can be expressed by bitset operations.
The constants needed for those bitset computations are defined here:
 
[large_bitset_impl_constants]

Looking at the example again, we can see that we have to identify the
positions of the beginning and ending of the interval [5,27] that is
to insert, and then ['*subdivide*] that range of bitsets into three partitions.

# The bitset where the interval starts.
# the bitset where the interval ends
# The bitsets that are completely overlapped by the interval

``
combine interval [5,27] to large_bitset lbs w.r.t. some operation o

   [0,1)->   [1,2)->   [2,3)->   [3,4)->   
   [00101100][11001011][11101001][11100000]
o       [111  11111111  11111111  1111]
         a                           b
subdivide:
   [first!  ][mid_1] . . .[mid_n][   !last]
   [00000111][1...1] . . .[1...1][11110000]
``

After subdividing, we perform the operation `o` as follows:

# For the first bitset: Set all bits from ther starting bit (`!`)
  to the end of the bitset to `1`. All other bits are `0`. Then
  perform operation `o`: `_map o= ([0,1)->00000111)`
  
# For the last bitset: Set all bits from the beginning of the bitset
  to the ending bit (`!`) to `1`. All other bits are `0`. Then
  perform operation `o`: `_map o= ([3,4)->11110000)`
  
# For the range of bitsets in between the staring and ending one,
  perform operation `o`: `_map o= ([1,3)->11111111)`

The algorithm, that has been outlined and illustrated by the 
example, is implemented by the private member function
`segment_apply`. To make the combiner operation a variable
in this algorithm, we use a /pointer to member function type/

[large_bitset_segment_combiner]

as first function argument. We will pass member functions `combine_` here, 
``
combine_(first_of_interval, end_of_interval, some_bitset);
``
that take the beginning and ending of an interval and a bitset and combine
them to the implementing `interval_bitmap_type _map`. Here are these functions:

[large_bitmap_combiners]

Finally we can code function `segment_apply`, that does the partitioning
and subsequent combining:

[large_bitset_segment_apply]

The functions that help filling bitsets to and from a given bit are 
implemented here:
[large_bitset_bitset_filler]

This completes the implementation of class template `large_bitset`.
Using only a small amount of mostly schematic code,
we have been able to provide a pretty powerful, self compressing
and generally usable set type for all integral domain types. 

[endsect][/ large_bitset: Private implementation]

[endsect][/ Implementation of a large bitset]

[endsect][/ Large Bitsets IN Projects]


[endsect][/ Projects]