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      1 /*
      2 Redistribution and use in source and binary forms, with or without
      3 modification, are permitted provided that the following conditions are met:
      4 
      5     * Redistributions of source code must retain the above copyright
      6     notice, this list of conditions and the following disclaimer.
      7 
      8     * Redistributions in binary form must reproduce the above copyright
      9     notice, this list of conditions and the following disclaimer in the
     10     documentation and/or other materials provided with the distribution.
     11 
     12     * Neither the name of "The Computer Language Benchmarks Game" nor the
     13     name of "The Computer Language Shootout Benchmarks" nor the names of
     14     its contributors may be used to endorse or promote products derived
     15     from this software without specific prior written permission.
     16 
     17 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
     18 AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     19 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     20 ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
     21 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
     22 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
     23 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
     24 INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
     25 CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
     26 ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
     27 POSSIBILITY OF SUCH DAMAGE.
     28 */
     29 
     30 /* The Computer Language Benchmarks Game
     31  * http://shootout.alioth.debian.org/
     32  *
     33  * contributed by The Go Authors.
     34  * based on pidigits.c (by Paolo Bonzini & Sean Bartlett,
     35  *                      modified by Michael Mellor)
     36  */
     37 
     38 package main
     39 
     40 import (
     41 	"flag"
     42 	"fmt"
     43 	"math/big"
     44 )
     45 
     46 var n = flag.Int("n", 27, "number of digits")
     47 var silent = flag.Bool("s", false, "don't print result")
     48 
     49 var (
     50 	tmp1  = big.NewInt(0)
     51 	tmp2  = big.NewInt(0)
     52 	tmp3  = big.NewInt(0)
     53 	y2    = big.NewInt(0)
     54 	bigk  = big.NewInt(0)
     55 	numer = big.NewInt(1)
     56 	accum = big.NewInt(0)
     57 	denom = big.NewInt(1)
     58 	ten   = big.NewInt(10)
     59 )
     60 
     61 func extract_digit() int64 {
     62 	if numer.Cmp(accum) > 0 {
     63 		return -1
     64 	}
     65 
     66 	// Compute (numer * 3 + accum) / denom
     67 	tmp1.Lsh(numer, 1)
     68 	tmp1.Add(tmp1, numer)
     69 	tmp1.Add(tmp1, accum)
     70 	tmp1.DivMod(tmp1, denom, tmp2)
     71 
     72 	// Now, if (numer * 4 + accum) % denom...
     73 	tmp2.Add(tmp2, numer)
     74 
     75 	// ... is normalized, then the two divisions have the same result.
     76 	if tmp2.Cmp(denom) >= 0 {
     77 		return -1
     78 	}
     79 
     80 	return tmp1.Int64()
     81 }
     82 
     83 func next_term(k int64) {
     84 	y2.SetInt64(k*2 + 1)
     85 	bigk.SetInt64(k)
     86 
     87 	tmp1.Lsh(numer, 1)
     88 	accum.Add(accum, tmp1)
     89 	accum.Mul(accum, y2)
     90 	numer.Mul(numer, bigk)
     91 	denom.Mul(denom, y2)
     92 }
     93 
     94 func eliminate_digit(d int64) {
     95 	tmp3.SetInt64(d)
     96 	accum.Sub(accum, tmp3.Mul(denom, tmp3))
     97 	accum.Mul(accum, ten)
     98 	numer.Mul(numer, ten)
     99 }
    100 
    101 func printf(s string, arg ...interface{}) {
    102 	if !*silent {
    103 		fmt.Printf(s, arg...)
    104 	}
    105 }
    106 
    107 func main() {
    108 	flag.Parse()
    109 
    110 	var m int // 0 <= m < 10
    111 	for i, k := 0, int64(0); ; {
    112 		d := int64(-1)
    113 		for d < 0 {
    114 			k++
    115 			next_term(k)
    116 			d = extract_digit()
    117 		}
    118 
    119 		printf("%c", d+'0')
    120 
    121 		i++
    122 		m = i % 10
    123 		if m == 0 {
    124 			printf("\t:%d\n", i)
    125 		}
    126 		if i >= *n {
    127 			break
    128 		}
    129 		eliminate_digit(d)
    130 	}
    131 
    132 	if m > 0 {
    133 		printf("%s\t:%d\n", "          "[m:10], *n)
    134 	}
    135 }
    136