Problem #26

Problem link
Solution:

package main

import (
    "big"
)

//using Fermat's little theorem we need to find the largest prime less than 1000
//see details: http://en.wikipedia.org/wiki/Repeating_decimal#Fractions_with_prime_denominators

func main() {
    //find primes
    arr := make([]bool, 1000)
    arr[0], arr[1] = true, true
    prime := 3
    var k int
    for {
        for k = prime * 2; k < len(arr); k += prime {
            arr[k] = true
        }
        for k = prime + 2; k < len(arr) && arr[k]; k += 2 {
        }
        if k < len(arr) {
            prime = k
        } else {
            break
        }
    }

    //iterate over primes in reverse direction
    b := big.NewInt(0)
    var i int
    for k = prime; k > 0; k -= 2 {
        if !arr[k] && k%2 != 0 {
        inner:
            for i = 1; i < k; i++ {
                if b.Mod(b.Sub(b.Exp(big.NewInt(10), big.NewInt(int64(i)), nil), big.NewInt(1)), big.NewInt(int64(k))).Int64() == 0 {
                    break inner
                }
            }
            if k-i == 1 {
                println(k)
                return
            }
        }
    }
}

Result: 983
Time: 0m0.041s