largest_product_in_a_series.rs

pub fn solve() -> i64 {
    let num = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843\
               8586156078911294949545950173795833195285320880551112540698747158523863050715693290963295227443043557\
               6689664895044524452316173185640309871112172238311362229893423380308135336276614282806444486645238749\
               3035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776\
               6572733300105336788122023542180975125454059475224352584907711670556013604839586446706324415722155397\
               5369781797784617406495514929086256932197846862248283972241375657056057490261407972968652414535100474\
               8216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586\
               1786645835912456652947654568284891288314260769004224219022671055626321111109370544217506941658960408\
               0719840385096245544436298123098787992724428490918884580156166097919133875499200524063689912560717606\
               0588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450";
    let digits = num.as_bytes();
    let mut product: u64 = 1;
    let mut result = product;

    // We know that the number has 1000 digits. Slide a window of length 13.
    // Keep track of how many times 0 is present in the window at any time.
    let mut zeros = 0;
    for i in 0..1000 {
        let incoming = (digits[i] - b'0') as u64;
        if incoming == 0 {
            zeros += 1;
        } else {
            product *= incoming;
        }
        let outgoing = if i < 13 { 1 } else { digits[i - 13] - b'0' } as u64;
        if outgoing == 0 {
            zeros -= 1;
        } else {
            product /= outgoing;
        }
        if zeros == 0 {
            result = std::cmp::max(result, product);
        }
    }

    assert_eq!(result, 23514624000);
    result as i64
}