turbonss/CHM.t2t

CHM Algorithm


%!includeconf: CONFIG.t2t

----------------------------------------
==The Algorithm==

==Memory Consumption==

Now we detail the memory consumption to generate and to store minimal perfect hash functions
using the CHM algorithm. The structures responsible for memory consumption are in the 
following:
- Graph:
  + **first**: is a vector that stores //cn// integer numbers, each one representing 
    the first edge (index in the vector edges) in the list of 
    edges of each vertex. 
    The integer numbers are 4 bytes long. Therefore,
    the vector first is stored in //4cn// bytes.
    
  + **edges**: is a vector to represent the edges of the graph. As each edge
    is compounded by a pair of vertices, each entry stores two integer numbers 
    of 4 bytes that represent the vertices. As there are //n// edges, the 
    vector edges is stored in //8n// bytes. 
    
  + **next**: given a vertex //v//, we can discover the edges that contain //v// 
    following its list of edges, which starts on first[//v//] and the next
    edges are given by next[...first[//v//]...]. Therefore, the vectors first and next represent 
    the linked lists of edges of each vertex. As there are two vertices for each edge,
    when an edge is iserted in the graph, it must be inserted in the two linked lists 
    of the vertices in its composition. Therefore, there are //2n// entries of integer
    numbers in the vector next, so it is stored in //4*2n = 8n// bytes.
    
- Other auxiliary structures    
  + **visited**: is a vector of //cn// bits, where each bit indicates if the g value of 
    a given vertex was already defined. Therefore, the vector visited is stored
    in //cn/8// bytes.
    
  + **function //g//**: is represented by a vector of //cn// integer numbers.
    As each integer number is 4 bytes long, the function //g// is stored in
    //4cn// bytes. 

    
Thus, the total memory consumption of CHM algorithm for generating a minimal 
perfect hash function (MPHF) is: //(8.125c + 16)n + O(1)// bytes.
As the value of constant //c// must be at least 2.09 we have:
 || //c// |  Memory consumption to generate a MPHF |
  | 2.09  |          //33.00n + O(1)//             |

Now we present the memory consumption to store the resulting function.
We only need to store the //g// function. Thus, we need //4cn// bytes.
Again we have:
 || //c// | Memory consumption to store a MPHF |
  | 2.09  |             //8.36n//              |
  
  
==Papers==

+ 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.

+ Z.J. Czech, G. Havas, and B.S. Majewski. Fundamental study perfect hashing.
  Theoretical Computer Science, 182:1-143, 1997.

+ B.S. Majewski, N.C. Wormald, G. Havas, and Z.J. Czech. A family of perfect hashing methods.
  The Computer Journal, 39(6):547--554, 1996.


----------------------------------------
[Home index.html]
----------------------------------------

%!include: FOOTER.t2t