CM-5

/* written by iskunk (Daniel Richard G.)
   http://www.housedillon.com/?p=1272 (check the Wayback Machine)
   emulates CM-5's 'random and pleasing' LED panel mode (rev. 3)
   FILE IS IN THE PUBLIC DOMAIN */

#include <stdio.h>
#include <string.h> /* defines memset */
#include <stdint.h> /* defines fixed-size 2-byte integer uint16_t */
#include <unistd.h> /* defines usleep function */

#define SLEEP 222222 /* was 250000 */
#define NUM_ROWS 64  /* unique rows - was 32 */
#define NUM_ROWS_DISPLAYED 64 /* displayed rows - was 106 on the CM-5 */
#define CHAR '▒' /* ■ ▀ ▄ █ ▓ ▒ ░ - they all print out the same char */

int main(void)
{
   /* uint16_t is used as integer sizes can vary between machines */
   uint16_t gen, i, gen_bit, row_bit, rows[NUM_ROWS], m, pos;
   char output[17];
   memset(rows, 0, sizeof(rows)); /* zeroes out the rows array */
   fputs("\033[0;31m", stdout);   /* set forground color to red */
   gen = 1; /* non-random seed for LFSR - other numbers also work */
   for (;;) /* main loop - each iteration fills out the entire grid */
   {
      fputs("\033[2J\033[1;1H", stdout); /* ANSI to clear terminal */
      for (i = 0; i < NUM_ROWS; i++) /* run for num unique rows */
      {
         /* see: wikipedia.org/wiki/Linear_feedback_shift_register
            a way to generate pseudo-random nums from a non-random seed
            operation is deterministic, has a finite number of states,
            will eventually repeat itself, can have a long cycle
            primitive polynomial: x^16 + x^12 + x^3 + x^1 + 1 */
         /* C bitwise operators used - all have L to R associativity:
             & = AND     | = OR     ^ = XOR
            >> = right-shift     << = left-shift     */
         gen_bit = ((gen >> 0)^(gen >> 4)^(gen >> 13)^(gen >> 15)) & 1;
         row_bit = ((gen >> 0) & (gen >> 1)) & 1;
         gen = (gen_bit << 15) | (gen >> 1);
         /* the 4 defines the row block height */
         if (i & 4)
            rows[i] = (row_bit << 15) | (rows[i] >> 1);
         else
            rows[i] = (rows[i] << 1) | row_bit;
      }
      /* run 64 times - number of displayed rows */
      for (i = 0; i < NUM_ROWS_DISPLAYED; i++)
      {
         /* why is m being setup this way? it's equal to 32768 */
         for (m = 1 << 15, pos = 0; m != 0; m >>= 1, pos++)
            output[pos] = ((rows[i & 31]) & m) ? CHAR : ' ';
         output[16] = '\0';
         puts(output);
      }
      usleep(SLEEP); /* pause between each loop iteration */
   }
}