lfsr.h File Reference

#include <stdint.h>

Description

A linear-feedback shift register pseudo-random number generator.

Author

Scott Fazackerley

This code implements a simple random number generator that generates a subset of possible integral values (16 bit unsigned) such that there are no collisions within the cycle generated.

Copyright

Copyright 2017 The University of British Columbia, IonDB Project Contributors (see AUTHORS.md)

Redistribution and use in source and binary forms, with or without

modification, are permitted provided that the following conditions are met:

1.Redistributions of source code must retain the above copyright notice,

this list of conditions and the following disclaimer.

2.Redistributions in binary form must reproduce the above copyright notice,

this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

3.Neither the name of the copyright holder nor the names of its contributors

may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"

AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Definition in file lfsr.h.

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Classes
struct	lfsr
	An instance type for the random number generator. More...

Typedefs
typedef struct lfsr	lfsr_t
	An instance type for the random number generator. More...

Functions
void	lfsr_init_start_state (uint16_t seed, lfsr_t *instance)
	Initialize the seed for the random number generator. More...
uint16_t	lfsr_get_next (lfsr_t *instance)
	Generate the next random number. More...
void	lfsr_reset (lfsr_t *instance)
	Reset the random number generator to it's initial state. More...

Typedef Documentation

typedef struct lfsr lfsr_t

An instance type for the random number generator.

Function Documentation

uint16_t lfsr_get_next

(

lfsr_t *

instance

)

Generate the next random number.

Parameters

instance

A pointer to the random number generator object that we wish to use to generate the next random item.

Returns

The next random integer from the random number generator.

Definition at line 55 of file lfsr.c.

  {
    unsigned lsb = instance->lfsr_value & 1;/* Get LSB (i.e., the output bit). */

    instance->lfsr_value >>= 1; /* Shift register */

    if (lsb == 1) {
        /* Only apply toggle mask if output bit is 1 */
        instance->lfsr_value ^= 0xB400u;/* Apply toggle mask, value has 1 at bits corresponding
                                                    * to taps, 0 elsewhere. */
    }

    return instance->lfsr_value;
}

void lfsr_init_start_state	(	uint16_t	seed,
		lfsr_t *	instance
	)

Initialize the seed for the random number generator.

Parameters

seed	The initial start state represented as an integer for the random number generator.
instance	A pointer to the random number generator object to be initialized. This is caller allocated.

Definition at line 43 of file lfsr.c.

  {
    instance->start_state   = seed;
    instance->lfsr_value    = seed;
}

void lfsr_reset

(

lfsr_t *

instance

)

Reset the random number generator to it's initial state.

Parameters

instance

A pointer to the random number generator object to reset.

Definition at line 75 of file lfsr.c.

  {
    instance->lfsr_value = instance->start_state;
}