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// PERMUTE_ARGS:
// EXECUTE_ARGS: 1000
import core.stdc.stdio;
import core.stdc.time;
const int LONG_TIME=4000;
byte[] p;
byte[] t;
int q;
int main(char[][] args)
{
time_t startime, endtime;
int i;
if (args.length == 2) {
sscanf(&args[1][0],"%d",&q);
} else {
printf("Usage: pi [precision]\n");
return 1;
}
if (q < 0)
{
printf("Precision was too low, running with precision of 0.\n");
q = 0;
}
if (q > LONG_TIME)
{
printf("Be prepared to wait a while...\n");
}
// Compute one more digit than we display to compensate for rounding
q++;
p.length = q + 1;
t.length = q + 1;
/* compute pi */
time(&startime);
arctan(2);
arctan(3);
mul4();
time(&endtime);
// Return to the number of digits we want to display
q--;
/* print pi */
printf("pi = %d.",cast(int)(p[0]));
for (i = 1; i <= q; i++)
printf("%d",cast(int)(p[i]));
printf("\n");
printf("%lld seconds to compute pi with a precision of %d digits.\n", cast(long)(endtime-startime),q);
return 0;
}
void arctan(int s)
{
int n;
t[0] = 1;
div(s); /* t[] = 1/s */
add();
n = 1;
do {
mul(n);
div(s * s);
div(n += 2);
if (((n-1) / 2) % 2 == 0)
add();
else
sub();
} while (!tiszero());
}
void add()
{
int j;
for (j = q; j >= 0; j--)
{
if (t[j] + p[j] > 9) {
p[j] += t[j] - 10;
p[j-1] += 1;
} else
p[j] += t[j];
}
}
void sub()
{
int j;
for (j = q; j >= 0; j--)
if (p[j] < t[j]) {
p[j] -= t[j] - 10;
p[j-1] -= 1;
} else
p[j] -= t[j];
}
void mul(int multiplier)
{
int b;
int i;
int carry = 0, digit = 0;
for (i = q; i >= 0; i--) {
b = (t[i] * multiplier + carry);
digit = b % 10;
carry = b / 10;
t[i] = cast(byte)digit;
}
}
/* t[] /= l */
void div(int divisor)
{
int i, b;
int quotient, remainder = 0;
foreach (ref x; t)
{
b = (10 * remainder + x);
quotient = b / divisor;
remainder = b % divisor;
x = cast(byte)quotient;
}
}
void div4()
{
int i, c, d = 0;
for (i = 0; i <= q; i++) {
c = (10 * d + p[i]) / 4;
d = (10 * d + p[i]) % 4;
p[i] = cast(byte)c;
}
}
void mul4()
{
int i, c, d;
d = c = 0;
for (i = q; i >= 0; i--) {
d = (p[i] * 4 + c) % 10;
c = (p[i] * 4 + c) / 10;
p[i] = cast(byte)d;
}
}
int tiszero()
{
int k;
for (k = 0; k <= q; k++)
if (t[k] != 0)
return false;
return true;
}
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