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String Hash (shash)

A string hash function based on the djb2 algorithm, used for hash table key calculation and switch-based string matching, among other scenarios.

Algorithm​

The djb2 algorithm, designed by Daniel J. Bernstein:

hash = ((hash << 5) + hash) + c

Equivalent to hash = hash * 33 + c, with initial value 5381. Simple implementation, uniform distribution, fast computation.

API​

uint32_t shash(const char * s);

Compute a 32-bit hash value for a string. Passing NULL returns 5381 (initial value). The function is static inline, with zero call overhead.

Typical Scenario: Switch-Based String Matching​

C does not support switching on strings. Using shash to convert strings to integers enables this:

switch(shash(name))
{
case 0x41f60f4b: /* "aliceblue" */
    c->r = 0xf0; c->g = 0xf8; c->b = 0xff;
    break;
case 0x0f294442: /* "black" */
    c->r = 0x00; c->g = 0x00; c->b = 0x00;
    break;
case 0x7c94a78d: /* "blue" */
    c->r = 0x00; c->g = 0x00; c->b = 0xff;
    break;
default:
    break;
}

No need to worry about hash collisions in this scenario — if two strings produce the same hash value, duplicate case labels will cause a compile error, allowing the issue to be caught at compile time. Comments annotate the original strings for readability.

Generating Case Values​

Using the shash Command-Line Tool​

shash is a command-line tool included with the project (source at developments/shash/). It computes string hash values directly in the terminal, making it easy to look up values when writing switch-case code:

$ shash aliceblue black blue
0x41f60f4b: /* "aliceblue" */
0x0f294442: /* "black" */
0x7c94a78d: /* "blue" */

The output format matches case labels and can be copied directly into your code.

Generating in Code​

printf("case 0x%08x: /* \"%s\" */\n", shash("aliceblue"), "aliceblue");
/* Output: case 0x41f60f4b: "aliceblue" */

Usage Examples​

Command Dispatch​

switch(shash(cmd))
{
case 0x106149d3: /* "start" */
    do_start();
    break;
case 0x7c9e1b4b: /* "stop" */
    do_stop();
    break;
}