It was improved the documentation of BMZ and CHM algorithms
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BMZ.t2t
162
BMZ.t2t
@ -4,46 +4,176 @@ BMZ Algorithm
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%!includeconf: CONFIG.t2t
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----------------------------------------
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**History**
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==History==
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At the end of 2003, professor [Nivio Ziviani http://www.dcc.ufmg.br/~nivio] was
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finishing the second edition of his book.
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During the book writing, professor Nivio studied the problem of generating minimal perfect hash
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finishing the second edition of his [book http://www.dcc.ufmg.br/algoritmos/].
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During the [book http://www.dcc.ufmg.br/algoritmos/] writing,
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professor [Nivio Ziviani http://www.dcc.ufmg.br/~nivio] studied the problem of generating minimal perfect hash
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functions (if you are not familiarized with this problem, see [1][2]).
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Professor Nivio coded a modified version of the [CHM algorithm chm.html], which was proposed by
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Czech, Havas and Majewski and put it in his book.
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Professor [Nivio Ziviani http://www.dcc.ufmg.br/~nivio] coded a modified version of
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the [CHM algorithm chm.html], which was proposed by
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Czech, Havas and Majewski and put it in his [book http://www.dcc.ufmg.br/algoritmos/].
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The [CHM algorithm chm.html] is based on acyclic random graphs to generate order preserving
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minimal perfect hash functions in linear time. Professor Nivio argued himself, why must the random graph
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be acyclic? In the modified version availalbe in his book he got rid of such restriction.
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minimal perfect hash functions in linear time. Professor [Nivio Ziviani http://www.dcc.ufmg.br/~nivio]
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argued himself, why must the random graph
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be acyclic? In the modified version availalbe in his [book http://www.dcc.ufmg.br/algoritmos/] he got rid of such restriction.
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The modification presented a problem, it was impossible to generate minimal perfect hash functions
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for sets with more than 1000 keys.
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At the same time, [Fabiano C. Botelho http://www.dcc.ufmg.br/~fbotelho],
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a master degree student at [Departament of Computer Science http://www.dcc.ufmg.br] in
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[Federal University of Minas Gerais http://www.ufmg.br],
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started to be advised by Nivio who presented the problem to Fabiano.
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started to be advised by [Nivio Ziviani http://www.dcc.ufmg.br/~nivio] who presented the problem
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to [Fabiano http://www.dcc.ufmg.br/~fbotelho].
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During the master, Fabiano and Nivio faced lots of problems.
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Talking with a friend of mine (David Menoti) about our problems, many ideas
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appeared and after of implementing them, we got a very fast algorithm to generate
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minimal perfect hash functions that does not preserve order.
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During the master, [Fabiano http://www.dcc.ufmg.br/~fbotelho] and
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[Nivio Ziviani http://www.dcc.ufmg.br/~nivio] faced lots of problems.
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In april of 2004, [Fabiano http://www.dcc.ufmg.br/~fbotelho] was talking with a
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friend of him (David Menoti) about the problems
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and many ideas appeared.
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The ideas were implemented and we noticed that a very fast algorithm to generate
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minimal perfect hash functions had been designed.
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We refer the algorithm to as **BMZ**, because it was conceived by Fabiano C. **B**otelho
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David **M**enoti and Nivio **Z**iviani. The algorithm is described in [1].
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To analyse BMZ algorithm we needed some results from the random graph theory, so
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we invite professor [Yoshiharu Kohayakawa http://www.ime.usp.br/~yoshi] to help us.
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The final description and analysis of BMZ algorithm is presented in [2].
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----------------------------------------
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==The Algorithm==
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Let us show how the minimal perfect hash function [figs/img7.png] will be constructed.
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We make use of two auxiliary random functions [figs/img41.png] and [figs/img55.png],
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where [figs/img56.png] for some suitably chosen integer [figs/img57.png],
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where [figs/img58.png].We build a random graph [figs/img59.png] on [figs/img60.png],
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whose edge set is [figs/img61.png]. There is an edge in [figs/img32.png] for each
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key in the set of keys [figs/img20.png].
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In what follows, we shall be interested in the //2-core// of
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the random graph [figs/img32.png], that is, the maximal subgraph
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of [figs/img32.png] with minimal degree at
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least 2 (see, e.g., [2] for details).
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Because of its importance in our context, we call the 2-core the
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//critical// subgraph of [figs/img32.png] and denote it by [figs/img63.png].
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The vertices and edges in [figs/img63.png] are said to be //critical//.
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We let [figs/img64.png] and [figs/img65.png].
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Moreover, we let [figs/img66.png] be the set of //non-critical//
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vertices in [figs/img32.png].
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We also let [figs/img67.png] be the set of all critical
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vertices that have at least one non-critical vertex as a neighbour.
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Let [figs/img68.png] be the set of //non-critical// edges in [figs/img32.png].
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Finally, we let [figs/img69.png] be the //non-critical// subgraph
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of [figs/img32.png.
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The non-critical subgraph [figs/img70.png] corresponds to the //acyclic part//
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of [figs/img32.png].
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We have [figs/img71.png].
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We then construct a suitable labelling [figs/img72.png] of the vertices
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of [figs/img32.png]: we choose [figs/img73.png] for each [figs/img74.png] in such
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a way that [figs/img75.png] ([figs/img18.png]) is a
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minimal perfect hash function for [figs/img20.png].
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We will see later on that this labelling [figs/img37.png] can be found in linear time
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if the number of edges in [figs/img63.png] is at most [figs/img76.png].
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Figure 2 presents a pseudo code for the algorithm.
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The procedure GenerateMPHF ([figs/img20.png], [figs/img37.png]) receives as input the set of
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keys [figs/img20.png] and produces the labelling [figs/img37.png].
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The method uses a mapping, ordering and searching approach.
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We now describe each step.
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| procedure GenerateMPHF ([figs/img20.png], [figs/img37.png])
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| Mapping ([figs/img20.png], [figs/img32.png]);
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| Ordering ([figs/img32.png], [figs/img63.png], [figs/img70.png]);
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| Searching ([figs/img32.png], [figs/img63.png], [figs/img70.png], [figs/img37.png]);
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**Figure 2**: Main steps of the algorithm for constructing a minimal perfect hash function
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===Mapping Step===
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===Ordering Step===
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===Searching Step===
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====Assignment of Values to Critical Vertices====
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====Assignment of Values to Non-Critical Vertices====
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----------------------------------------
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==The Heuristic==
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----------------------------------------
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==Memory Consumption==
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Now we detail the memory consumption to generate and to store minimal perfect hash functions
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using the BMZ algorithm. The structures responsible for memory consumption are in the
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following:
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- Graph:
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+ **first**: is a vector that stores //cn// integer numbers, each one representing
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the first edge (index in the vector edges) in the list of
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edges of each vertex.
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The integer numbers are 4 bytes long. Therefore,
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the vector first is stored in //4cn// bytes.
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+ **edges**: is a vector to represent the edges of the graph. As each edge
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is compounded by a pair of vertices, each entry stores two integer numbers
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of 4 bytes that represent the vertices. As there are //n// edges, the
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vector edges is stored in //8n// bytes.
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+ **next**: given a vertex //v//, we can discover the edges that contain //v//
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following its list of edges, which starts on first[//v//] and the next
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edges are given by next[...first[//v//]...]. Therefore, the vectors first and next represent
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the linked lists of edges of each vertex. As there are two vertices for each edge,
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when an edge is iserted in the graph, it must be inserted in the two linked lists
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of the vertices in its composition. Therefore, there are //2n// entries of integer
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numbers in the vector next, so it is stored in //4*2n = 8n// bytes.
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+ **critical vertices(critical_nodes vector)**: is a vector of //cn// bits,
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where each bit indicates if a vertex is critical (1) or non-critical (0).
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Therefore, the critical and non-critical vertices are represented in //cn/8// bytes.
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+ **critical edges (used_edges vector)**: is a vector of //n// bits, where each
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bit indicates if an edge is critical (1) or non-critical (0). Therefore, the
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critical and non-critical edges are represented in //n/8// bytes.
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- Other auxiliary structures
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+ **queue**: is a queue of integer numbers used in the breadth-first search of the
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assignment of values to critical vertices. There is an entry in the queue for
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each two critical vertices. Let //|Vcrit|// be the expected number of critical
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vertices. Therefore, the queue is stored in //4*0.5*|Vcrit|=2|Vcrit|//.
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+ **visited**: is a vector of //cn// bits, where each bit indicates if the g value of
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a given vertex was already defined. Therefore, the vector visited is stored
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in //cn/8// bytes.
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+ **function //g//**: is represented by a vector of //cn// integer numbers.
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As each integer number is 4 bytes long, the function //g// is stored in
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//4cn// bytes.
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**The Algorithm**
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Thus, the total memory consumption of BMZ algorithm for generating a minimal
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perfect hash function (MPHF) is: //(8.25c + 16.125)n +2|Vcrit| + O(1)// bytes.
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As the value of constant //c// may be 1.15 and 0.93 we have:
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|| //c// | //|Vcrit|// | Memory consumption to generate a MPHF |
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| 0.93 | //0.497n// | //24.80n + O(1)// |
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| 1.15 | //0.401n// | //26.42n + O(1)// |
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The values of |Vcrit| were calculated using Eq.(1) presented in [2].
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**The Heuristic**
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Now we present the memory consumption to store the resulting function.
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We only need to store the //g// function. Thus, we need //4cn// bytes.
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Again we have:
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|| //c// | Memory consumption to store a MPHF |
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| 0.93 | //3.72n// |
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| 1.15 | //4.60n// |
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**Papers**
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----------------------------------------
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==Papers==
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+ [F. C. Botelho http://www.dcc.ufmg.br/~fbotelho], D. Menoti, [N. Ziviani http://www.dcc.ufmg.br/~nivio]. [A New algorithm for constructing minimal perfect hash functions papers/bmz_tr004_04.ps], Technical Report TR004/04, Department of Computer Science, Federal University of Minas Gerais, 2004.
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+ [F. C. Botelho http://www.dcc.ufmg.br/~fbotelho], Y. Kohayakawa, and [N. Ziviani http://www.dcc.ufmg.br/~nivio]. [A Practical Minimal Perfect Hashing Method papers/bmz_wea2005.ps], 4th International Workshop on Efficient and Experimental Algorithms (WEA), 2005.(submitted)
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+ [F. C. Botelho http://www.dcc.ufmg.br/~fbotelho], Y. Kohayakawa, and [N. Ziviani http://www.dcc.ufmg.br/~nivio]. [A Practical Minimal Perfect Hashing Method papers/bmz_wea2005.ps] (Submitted).
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----------------------------------------
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Before Width: | Height: | Size: 2.6 KiB After Width: | Height: | Size: 9.3 KiB |
51
CHM.t2t
51
CHM.t2t
@ -4,12 +4,57 @@ CHM Algorithm
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%!includeconf: CONFIG.t2t
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----------------------------------------
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==The Algorithm==
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**History**
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==Memory Consumption==
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**The Algorithm**
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Now we detail the memory consumption to generate and to store minimal perfect hash functions
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using the CHM algorithm. The structures responsible for memory consumption are in the
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following:
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- Graph:
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+ **first**: is a vector that stores //cn// integer numbers, each one representing
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the first edge (index in the vector edges) in the list of
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edges of each vertex.
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The integer numbers are 4 bytes long. Therefore,
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the vector first is stored in //4cn// bytes.
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**Papers**
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+ **edges**: is a vector to represent the edges of the graph. As each edge
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is compounded by a pair of vertices, each entry stores two integer numbers
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of 4 bytes that represent the vertices. As there are //n// edges, the
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vector edges is stored in //8n// bytes.
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+ **next**: given a vertex //v//, we can discover the edges that contain //v//
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following its list of edges, which starts on first[//v//] and the next
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edges are given by next[...first[//v//]...]. Therefore, the vectors first and next represent
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the linked lists of edges of each vertex. As there are two vertices for each edge,
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when an edge is iserted in the graph, it must be inserted in the two linked lists
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of the vertices in its composition. Therefore, there are //2n// entries of integer
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numbers in the vector next, so it is stored in //4*2n = 8n// bytes.
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- Other auxiliary structures
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+ **visited**: is a vector of //cn// bits, where each bit indicates if the g value of
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a given vertex was already defined. Therefore, the vector visited is stored
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in //cn/8// bytes.
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+ **function //g//**: is represented by a vector of //cn// integer numbers.
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As each integer number is 4 bytes long, the function //g// is stored in
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//4cn// bytes.
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Thus, the total memory consumption of CHM algorithm for generating a minimal
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perfect hash function (MPHF) is: //(8.125c + 16)n + O(1)// bytes.
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As the value of constant //c// must be at least 2.09 we have:
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|| //c// | Memory consumption to generate a MPHF |
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| 2.09 | //33.00n + O(1)// |
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Now we present the memory consumption to store the resulting function.
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We only need to store the //g// function. Thus, we need //4cn// bytes.
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Again we have:
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|| //c// | Memory consumption to store a MPHF |
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| 2.09 | //8.36n// |
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==Papers==
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+ Z.J. Czech, G. Havas, and B.S. Majewski. [An optimal algorithm for generating minimal perfect hash functions. papers/chm92.pdf], Information Processing Letters, 43(5):257-264, 1992.
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@ -5,14 +5,14 @@ Comparison Between BMZ And CHM Algorithms
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----------------------------------------
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**Features**
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==Features==
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**Constructing Minimal Perfect Hash Functions**
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==Constructing Minimal Perfect Hash Functions==
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**Memory Consumption**
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==Memory Consumption==
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**Run times**
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==Run times==
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----------------------------------------
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[Home index.html]
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@ -1,2 +1,4 @@
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%! PreProc(html): '^%html% ' ''
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%! PreProc(txt): '^%txt% ' ''
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%! PostProc(html): "&" "&"
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%! PostProc(txt): " " " "
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22
README.t2t
22
README.t2t
@ -5,7 +5,7 @@ CMPH - C Minimal Perfect Hashing Library
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-------------------------------------------------------------------
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**Description**
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==Description==
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C Minimal Perfect Hashing Library is a portable LGPLed library to create and
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to work with minimal perfect hash functions. The cmph library encapsulates the newest
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@ -31,35 +31,35 @@ of the distinguishable features of cmph:
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----------------------------------------
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**Supported Algorithms**
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==Supported Algorithms==
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%html% - [BMZ Algorithm bmz.html].
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%txt% - BMZ Algorithm.
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A very fast algorithm based on cyclic random graphs to construct minimal
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perfect hash functions in linear time. The resulting functions are not order preserving and
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can be stored in only 4cn bytes, where c is between 0.93 and 1.15.
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can be stored in only //4cn// bytes, where //c// is between 0.93 and 1.15.
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%html% - [CHM Algorithm chm.html].
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%txt% - CHM Algorithm.
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An algorithm based on acyclic random graphs to construct minimal
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perfect hash functions in linear time. The resulting functions are order preserving and
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are stored in 4cn bytes, where c is greater than 2.
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are stored in //4cn// bytes, where //c// is greater than 2.
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%html% [Click Here comparison.html] to see a comparison of the supported algorithms.
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----------------------------------------
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**News for version 0.3**
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==News for version 0.3==
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- New heuristic added to the bmz algorithm permits to generate a mphf with only
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24.61*n + O(1) bytes. The resulting function can be stored in 3.72*n bytes.
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//24.6n + O(1)// bytes. The resulting function can be stored in //3.72n// bytes.
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%html% [click here bmz.html] for details.
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----------------------------------------
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**Examples**
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==Examples==
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Using cmph is quite simple. Take a look.
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@ -113,7 +113,7 @@ Using cmph is quite simple. Take a look.
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```
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--------------------------------------
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**The cmph application**
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==The cmph application==
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cmph is the name of both the library and the utility
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application that comes with this package. You can use the cmph
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@ -157,16 +157,16 @@ utility.
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keysfile line separated file with keys
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```
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**Additional Documentation**
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==Additional Documentation==
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[FAQ faq.html]
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**Downloads**
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==Downloads==
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Use the project page at sourceforge: http://sf.net/projects/cmph
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**License Stuff**
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==License Stuff==
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Code is under the LGPL.
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----------------------------------------
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