Attempt at CM-5 pattern emulation (rev. 6)

/* http://www.housedillon.com/?p=1272
 * Written by iskunk (Daniel Richard G.) 2016 April
 * printf("<%s@%s.%s>\n", "skunk", "iskunk", "org");
 * THIS FILE IS IN THE PUBLIC DOMAIN
 *
 * Program to emulate a subset of the Thinking Machines Corp. CM-5 front
 * LED panel display modes (revision 6)
 */

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>
#include <assert.h>

#define NUM_ROWS 32     /* unique rows */
#define NUM_ROWS_HALF 16    /* NUM_ROWS divided by 2 */
#define NUM_ROWS_DISPLAYED 106  /* total rows in front panel display */

#define RNUM_SEED 0xBAD  /* :-) */

static uint16_t rnum = RNUM_SEED;
static uint16_t rnum_8741 = RNUM_SEED;

/* A Galois LFSR requires a different seed to produce the same output
 */
static uint16_t rnum_galois = 0x917D;

/* Note: rows[0] is the top row; most significant bit is at left;
 * a zero bit corresponds to a lit LED
 */
static uint16_t rows[NUM_ROWS];

/* Uninitialized bits, displayed briefly at the start of mode 7
 */
static const uint16_t rows_glitch[NUM_ROWS] = {
    0x8F10, 0x9112, 0x9314, 0x9516, 0x18E9, 0x5899, 0x38D9, 0x78B9,
    0x9F20, 0xA122, 0xA324, 0xA526, 0x14E5, 0x5495, 0x34D5, 0x74B5,
    0xAF30, 0xB132, 0xB334, 0xB536, 0x1CED, 0x5C9D, 0x3CDD, 0x7CBD,
    0xBF40, 0xC142, 0xC344, 0xC546, 0x12E3, 0x5293, 0x32D3, 0x72B3
};

/* This is a rough translation of Jim's Intel 8741 assembler code into C.
 * His original code and (lightly edited) comments are retained below.
 */
static uint16_t get_random_bit_8741(void)
{
    uint8_t RNUM   = rnum_8741 & 0xFF;  /* low-order byte */
    uint8_t RNUMp1 = rnum_8741 >> 8;    /* high-order byte */
    uint8_t A, C, tmp;

#define b(var, n) ((var >> n) & 1)  /* gets Nth bit */
#define cpl(flag) flag = flag ^ 1
#define jnb(val, label) if (!val) goto label
#define orl(flag, val) flag |= val
#define rrc(reg) tmp = reg & 1; reg = (C << 7) | (reg >> 1); C = tmp

    /* ;This subroutine implements a 16 bit random number generator based
     * ;on the primitive polynomial:
     * ;
     * ;    1 + X + X^3 + X^12 + X^16
     * ;
     * ;The value is stored in memory as RNUM.  This subroutine returns
     * ;with the low order byte of the RNUM in the accumulator and a random
     * ;bit value in the Carry (C) bit.
     * ;
     */
    C = b(RNUM,0);      /* mov C, RNUM.0    ;get the units value of the PP      */
    jnb(b(RNUM,1), rand1);  /* jnb RNUM.1, rand1    ;jmp if X = 0               */
    cpl(C);         /* cpl C        ;else compliment C (xor)        */
rand1:  jnb(b(RNUM,3), rand2);  /* jnb RNUM.3, rand2    ;jmp if X^3 = 0             */
    cpl(C);         /* cpl C        ;else compliment C (xor)        */
rand2:  jnb(b(RNUMp1,4), rand3);  /* jnb (RNUM+1).4, rand3  ;jmp if X^12 = 0            */
    cpl(C);         /* cpl C        ;else compliment C (xor)        */
rand3:  A = RNUMp1;     /* mov A, RNUM+1    ;get high byte of RNUM          */
    rrc(A);         /* rrc A        ;and rotate down (thru accumulator) */
    RNUMp1 = A;     /* mov RNUM+1, A    ;save it back to memory         */
    A = RNUM;       /* mov A, RNUM      ;get low byte of RNUM           */
    rrc(A);         /* rrc A        ;and rotate down (thru accumulator) */
    RNUM = A;       /* mov RNUM, A      ;save it back to memory         */
    orl(C, b(RNUM,1));  /* orl C, RNUM.1    ;set C 75 percent of the time       */
                /* ret          ;return                 */

#undef b
#undef cpl
#undef jnb
#undef orl
#undef rrc

    rnum_8741 = RNUM | (RNUMp1 << 8);

    return C;
}

/* "In a software implementation of an LFSR, the Galois form is more
 * efficient as the XOR operations can be implemented a word at a
 * time: only the output bit must be examined individually."
 * -- https://en.wikipedia.org/wiki/Linear_feedback_shift_register
 */
static uint16_t get_random_bit_galois(void)
{
#define X rnum_galois

    uint16_t out_bit = X & 1;
    uint16_t rand_bit = (X | (X >> 2)) & 1;

    X >>= 1;
    X ^= (-out_bit) & 0xD008;

#undef X

    return rand_bit;
}

static uint16_t get_random_bit(void)
{
#define X rnum

    /* https://en.wikipedia.org/wiki/Linear_feedback_shift_register
     * Primitive polynomial: x^16 + x^15 + x^13 + x^4 + 1
     */
    uint16_t lfsr_bit = ((X >> 0) ^ (X >> 1) ^ (X >> 3) ^ (X >> 12)) & 1;

    uint16_t rand_bit = (X | (X >> 2)) & 1;

    X = (lfsr_bit << 15) | (X >> 1);

#undef X

#ifndef NDEBUG
    /* Compare two alternative implementations of the pseudo-random bit
     * generator function */
    {
    uint16_t rand_bit_8741   = get_random_bit_8741();
    uint16_t rand_bit_galois = get_random_bit_galois();

    assert(rand_bit == rand_bit_8741);
    assert(rand_bit_galois == rand_bit);

    assert(rnum == rnum_8741);
    }
#endif

    return rand_bit;
}

static void print_row(uint16_t x)
{
    uint16_t m;
    int pos;
    char v[17];

    /* MSB at left, LSB at right
     * 0 -> LED on, 1 -> LED off
     */
    for (m = 1 << 15, pos = 0; m != 0; m >>= 1, pos++)
        v[pos] = (x & m) ? '-' : 'O';

    v[16] = '\0';

    puts(v);
}

static void print_panel(void)
{
    int i;

    /* ANSI escape sequence to clear screen
     */
    fputs("\033[2J\033[1;1H", stdout);

    for (i = 0; i < NUM_ROWS_DISPLAYED; i++)
        print_row(rows[i & 31]);
}

int main(int argc, char **argv)
{
    int mode = 7;
    int i, j;
    int toggle = 0;

    if (argc == 2)
    {
        mode = (int)strtol(argv[1], NULL, 16);

        switch (mode)
        {
            case 5:
            case 7:
            case 9:
            case 0xA:
            case 0xB:
            break;

            default:
            printf("error: invalid mode \"%s\"\n", argv[1]);
            printf("usage: %s [MODE]\n", argv[0]);
            puts("valid options for MODE: 5 7 9 A B");
            return 1;
        }
    }

    /* Initial state: all but 3 LEDs lit
     */
    memset(rows, 0, sizeof(rows));
    rows[0] = 0x9400;
    print_panel();
    fflush(stdout);
    usleep(600000); /* 600 ms */

    /* Initialize rows with glitch pattern
     */
    memcpy(rows, rows_glitch, sizeof(rows));

    for (;;)
    {
        switch (mode)
        {
            /* "random and pleasing" */
            case 5:
            for (i = 0; i < NUM_ROWS_HALF; i++)
            {
                for (j = 0; j < 16; j++)
                {
                    /* Note that the upper half of the
                     * panel is one JTAG chain, and the
                     * lower half is another */

                    uint16_t bit_lower = get_random_bit();
                    uint16_t bit_upper = get_random_bit();

                    rows[i] <<= 1;
                    rows[i] |= bit_upper;

                    rows[i + NUM_ROWS_HALF] <<= 1;
                    rows[i + NUM_ROWS_HALF] |= bit_lower;
                }
            }
            break;

            /* "interleaved display of random and pleasing" */
            case 7:
            for (i = NUM_ROWS - 1; i >= 0; i--)
            {
                uint16_t bit = get_random_bit();

                if (i & 4)
                    rows[i] = (bit << 15) | (rows[i] >> 1);
                else
                    rows[i] = (rows[i] << 1) | bit;
            }
            break;

            /* "all leds on" */
            case 9:
            memset(rows, 0, sizeof(rows));
            break;

            /* "all leds off" */
            case 0xA:
            memset(rows, 0xFF, sizeof(rows));
            break;

            /* "continuous blink leds on then off" */
            case 0xB:
            memset(rows, toggle ? 0 : 0xFF, sizeof(rows));
            toggle ^= 1;
            break;
        }

        print_panel();

        fflush(stdout);
        usleep(200000); /* 200 ms */
    }

    return 0;
}

/* EOF */