//
// sr -- Special Relativity (time dilation) calculator for low speeds.
//
// 30-Apr-2006 tvb www.LeapSecond.com/tools
//

#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define INCH_TO_METER(x) ( (x) * 2.54 / 100.0 )
#define FOOT_TO_METER(x) ( INCH_TO_METER((x) * 12) )
#define MILE_TO_METER(x) ( FOOT_TO_METER((x) * 5280) )
#define KILO_TO_METER(x) ( (x) * 1000 )
#define kt_to_kmph(v)    ( (v) * 1.852 )
#define kmph_to_kt(v)    ( (v) / 1.852 )
#define mach_to_ms(v)    ( (v) * 340.3 )
#define ms_to_mach(v)    ( (v) / 340.3 )

#define ALL_UNITS "mph, kt, fps, kph, m/s"

void main (int argc, char *argv[])
{
    double c = 299792458.0;
    double speed, v, df;
    char *units;
    int debug;

    if (argc < 3) {
        fprintf(stderr, "Usage: %s [speed] [units]\n", argv[0]);
        fprintf(stderr, "(speed units are one of: %s)\n", ALL_UNITS);
        exit(1);
    }
    debug = (argc > 3);

    speed = atof(argv[1]);
    units = argv[2];
    if (strcmp(units, "mph") == 0) {
        v = MILE_TO_METER(speed) / 3600;
    }
    else if (strcmp(units, "fps") == 0) {
        v = FOOT_TO_METER(speed);
    }
    else if (strcmp(units, "kph") == 0) {
        v = KILO_TO_METER(speed) / 3600;
    }
    else if (strcmp(units, "kt") == 0) {
        v = KILO_TO_METER(kt_to_kmph(speed)) / 3600;
    }
    else if (strcmp(units, "m/s") == 0) {
        v = speed;
    }
    else {
        fprintf(stderr, "Unit %s unknown. Valid units: %s\n", units, ALL_UNITS);
        exit(1);
    }

    if (v < 0) {
        fprintf(stderr, "Velocity too low for this tool.\n");
        exit(1);
    }

    printf("%16.6lf mph (miles per hour)\n", 1/MILE_TO_METER(1/v) * 3600);
    printf("%16.6lf kph (kilometers/hour)\n", 1/KILO_TO_METER(1/v) * 3600);
    printf("%16.6lf fps (feet per second)\n", 1/FOOT_TO_METER(1/v));
    printf("%16.6lf m/s (meters per second)\n", v);
    {  double kmph = 1/KILO_TO_METER(1/v) * 3600;
       printf("%16.6lf kt  (knots)\n", kmph_to_kt(kmph));
    }
    printf("%16.6lf ma  (mach)\n", ms_to_mach(v));

    printf("%16.6lf m/s c\n", c);
    printf("%16.6le v/c\n", v/c);

    if (v > 5000 && !debug) {
        fprintf(stderr, "Velocity too high for this tool.\n");
        exit(1);
    }

    df = -0.5 * (v*v) / (c*c);
    printf("%16.6le -1/2*v^2/c^2\n", df);
    printf("%16.6lf ps/hour\n", df * 1e12 * 3600);
    printf("%16.6lf ns/day\n", df * 1e9 * 86400);

    if (debug) {
        double v2 = v * v;
        double c2 = c * c;
        double x = 1 - (v2/c2);
        printf("\n");
        printf("%.16le          v2/c2\n",             v2/c2   );
        printf("%.16le        1-v2/c2\n",           1-v2/c2   );
        printf("0x%.8lx,%.8lx            1-v2/c2 (hex)\n",
               ((long *)&x)[1], ((long *)&x)[0]);
        printf("%.16le   sqrt(1-v2/c2)\n",     sqrt(1-v2/c2)  );
        printf("%.16le 1/sqrt(1-v2/c2)\n",   1/sqrt(1-v2/c2)  );
        printf("%.16le 1/sqrt(1-v2/c2)-1\n", 1/sqrt(1-v2/c2)-1);
        printf("%.16le 1-sqrt(1-v2/c2)\n",   1-sqrt(1-v2/c2)  );
        printf("%.16le 0.5*v2/c2\n", 0.5*v2/c2);
    }
}