04-11-2011
Some of the early NAT language packages for C used compression exploiting the null terminated string, finding short strings that were suffixes of other strings, so "1234" might be stored but "234", "34", "4" and "" were just offset pointers into "1234". While not that great for compressing long strings, it was great for sets with many short strings.
I was working on high performance container since a while back, and came up with a byte-tree, where the first byte was a lookup into an array of pointers, or similar structure, to quickly travers an invariant tree one byte of key at a time. Various alternate nodes dealt with compression, like a 'next-n-bytes-must-be' to swallow invariant areas in a key, or a truncated array of less than 256 cells, with a base and size, or a dumb list lookup leveraging strchr(), a string of random key letters, and a like-length array of pointers, or a N-copies-of for duplicates. The advantages: quick insert, sorted access, no rebalancing, quick access. Linear hash is cute, but if you are not sure of the data's key distribution, it is dicey to go all the way to one key per bucket, so how much linear search do you want?
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bsearch(3C) bsearch(3C)
NAME
bsearch() - binary search a sorted table
SYNOPSIS
DESCRIPTION
is a binary search routine generalized from Knuth (6.2.1) Algorithm B. It returns a pointer into a table indicating where a datum may be
found. The table must be previously sorted in increasing order according to a provided comparison function. key points to a datum
instance to be sought in the table. base points to the element at the base of the table. nel is the number of elements in the table.
size is the size of each element in the table. compar is the name of the comparison function, which is called with two arguments that
point to the elements being compared. The function must return an integer less than, equal to, or greater than zero indicating that the
first argument (the key) is to be considered less than, equal to, or greater than the second argument (the array element).
NOTES
The pointers to the key and the element at the base of the table should be of type pointer-to-element, and cast to type pointer-to-void.
The comparison function need not compare every byte, so arbitrary data can be contained in the elements in addition to the values being
compared.
Although declared as type pointer-to-void, the value returned should be cast into type pointer-to-element.
RETURN VALUE
A NULL pointer is returned if the key cannot be found in the table.
EXAMPLES
The example below searches a table containing pointers to nodes consisting of a string and its length. The table is ordered alphabetically
on the string in the node pointed to by each entry.
This code fragment reads in strings and either finds the corresponding node and prints out the string and its length, or prints an error
message.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define TABSIZE 1000
struct node { /* these are stored in the table */
char *string;
int length;
};
struct node table[TABSIZE]; /* table to be searched */
.
.
.
{
struct node *node_ptr, node;
/* routine to compare 2 nodes */
int node_compare(const void *, const void *);
char str_space[20]; /* space to read string into */
.
.
.
node.string = str_space;
while (scanf("%s", node.string) != EOF) {
node_ptr = (struct node *)bsearch((void *)(&node),
(void *)table, TABSIZE,
sizeof(struct node), node_compare);
if (node_ptr != NULL) {
(void)printf("string = %20s, length = %d
",
node_ptr->string, node_ptr->length);
} else {
(void)printf("not found: %s
", node.string);
}
}
}
/* This routine compares two nodes based on an
alphabetical ordering of the string field.
*/
int
node_compare(const void *node1, const void *node2)
struct node *node1, *node2;
{
return strcoll(((const struct node *)node1)->string,
((const struct node *)node2)->string);
}
WARNINGS
If the table being searched contains two or more entries that match the selection criteria, a random entry is returned by as determined by
the search algorithm.
SEE ALSO
hsearch(3C), lsearch(3C), qsort(3C), tsearch(3C), thread_safety(5).
STANDARDS CONFORMANCE
bsearch(3C)