pidigits.go

package main

import (
    big "gmp";
    "flag";
    "fmt";
    "strconv";
)

var n = 0
var silent = false

var (
    tmp1  = big.NewInt(0);
    tmp2  = big.NewInt(0);
    numer = big.NewInt(1);
    accum = big.NewInt(0);
    denom = big.NewInt(1);
    ten   = big.NewInt(10);
)

func extractDigit() int64 {
    if big.CmpInt(numer, accum) > 0 {
        return -1
    }
    tmp1.Lsh(numer, 1).Add(tmp1, numer).Add(tmp1, accum);
    big.DivModInt(tmp1, tmp2, tmp1, denom);
    tmp2.Add(tmp2, numer);
    if big.CmpInt(tmp2, denom) >= 0 {
        return -1
    }
    return tmp1.Int64();
}

func nextTerm(k int64) {
    y2 := k*2 + 1;
    accum.Add(accum, tmp1.Lsh(numer, 1));
    accum.Mul(accum, tmp1.SetInt64(y2));
    numer.Mul(numer, tmp1.SetInt64(k));
    denom.Mul(denom, tmp1.SetInt64(y2));
}

func eliminateDigit(d int64) {
    accum.Sub(accum, tmp1.Mul(denom, tmp1.SetInt64(d)));
    accum.Mul(accum, ten);
    numer.Mul(numer, ten);
}

func printf(s string, arg ...) {
    if !silent {
        fmt.Printf(s, arg);
    }
}

func main() {
    flag.Parse();
    if flag.NArg() > 0 { n,_ = strconv.Atoi( flag.Arg(0) ) }

    var m int;   // 0 <= m < 10
    for i, k := 0, int64(0); ; {
        d := int64(-1);
        for d < 0 {
            k++;
            nextTerm(k);
            d = extractDigit();
        }
        
        printf("%c", d + '0');
        
        i++;
        m = i%10;
        if m == 0 {
            printf("\t:%d\n", i);
        }
        if i >= n {
            break;
        }
        eliminateDigit(d);
    }
    
    if m > 0 {
        printf("%s\t:%d\n", "          "[m : 10], n);
    }
}