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/*
* Additive Lagged Fibonacci Generator (ALFG) Implementation
* Author: Fabian Druschke
* Date: 2024-03-13
*
* This is an implementation of the Additive Lagged Fibonacci Generator (ALFG),
* a pseudorandom number generator known for its simplicity and good statistical properties
* for a wide range of applications. ALFGs are particularly noted for their long periods
* and efficiency in generating sequences of random numbers. However, like many other PRNGs,
* they are not suitable for cryptographic purposes due to their predictability.
*
* As the author of this implementation, I, Fabian Druschke, hereby release this work into
* the public domain. I dedicate any and all copyright interest in this work to the public
* domain, making it free to use for anyone for any purpose without any conditions, unless
* such conditions are required by law.
*
* This software is provided "as is", without warranty of any kind, express or implied,
* including but not limited to the warranties of merchantability, fitness for a particular
* purpose, and noninfringement. In no event shall the authors be liable for any claim,
* damages, or other liability, whether in an action of contract, tort, or otherwise, arising
* from, out of, or in connection with the software or the use or other dealings in the software.
*
* Note: This implementation is designed for non-cryptographic applications and should not be
* used where cryptographic security is required.
*/
#include "add_lagg_fibonacci_prng.h"
#include <stdint.h>
#include <string.h>
#define STATE_SIZE 64 // Size of the state array, sufficient for a high period
#define LAG_BIG 55 // Large lag, e.g., 55
#define LAG_SMALL 24 // Small lag, e.g., 24
#define MODULUS ( 1ULL << 48 ) // Modulus for the operations, here 2^48 for simple handling
void add_lagg_fibonacci_init( add_lagg_fibonacci_state_t* state, uint64_t init_key[], unsigned long key_length )
{
// Simple initialization: Fill the state with the key values and then with a linear combination of them
for( unsigned long i = 0; i < STATE_SIZE; i++ )
{
if( i < key_length )
{
state->s[i] = init_key[i];
}
else
{
// Simple method to generate further state values. Should be improved for serious applications.
state->s[i] = ( 6364136223846793005ULL * state->s[i - 1] + 1 ) % MODULUS;
}
}
state->index = 0; // Initialize the index for the first generation
}
void add_lagg_fibonacci_genrand_uint256_to_buf( add_lagg_fibonacci_state_t* state, unsigned char* bufpos )
{
uint64_t* buf_as_uint64 = (uint64_t*) bufpos; // Interprets bufpos as a uint64_t array for direct assignment
int64_t result; // Use signed integer to handle potential negative results from subtraction
for (int i = 0; i < 4; i++) {
// Subtract the two previous numbers in the sequence
result = (int64_t)state->s[(state->index + LAG_BIG) % STATE_SIZE] - (int64_t)state->s[(state->index + LAG_SMALL) % STATE_SIZE];
// Handle borrow if result is negative
if (result < 0) {
result += MODULUS;
// Optionally set a borrow flag or adjust the next operation based on borrow logic
}
// Store the result (after adjustment) back into the state, ensuring it's positive and within range
state->s[state->index] = (uint64_t)result;
// Write the result into buf_as_uint64
buf_as_uint64[i] = state->s[state->index];
// Update the index for the next round
state->index = (state->index + 1) % STATE_SIZE;
}
}
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