Advent of Code 2022 - Day 6 #2

use std::collections::VecDeque;

fn find_start_of_next_packet(packet: &str, distinct: usize) -> usize {
    let mut position = 0;
    // count of each letter found in distinct range
    let mut letters: [u32; 26] = [0; 26];
    // queue containing the current window of characters
    let mut queue: VecDeque<char> = VecDeque::with_capacity(distinct);

    let mut distinct_count: usize = 0;

    let mut packet_iter = packet.chars();

    // Load the holding deque and char counts
    for _ in 0..distinct {
        if let Some(c) = packet_iter.next() {
            queue.push_back(c);
            if letters[(c as usize) - 97usize] == 0 {
                distinct_count += 1;
            }
            letters[(c as usize) - 97usize] += 1;

            position += 1;
        }
    }

    // loop through chars to find break point
    while let Some(c) = packet_iter.next() {
        if distinct_count == distinct {
            break;
        } else {
            let remove = queue.pop_front().unwrap();
            letters[(remove as usize) - 97usize] -= 1;
            if letters[(remove as usize) - 97usize] == 0 {
                distinct_count -= 1;
            }
            queue.push_back(c);
            if letters[(c as usize) - 97usize] == 0 {
                distinct_count += 1;
            }
            letters[(c as usize) - 97usize] += 1;
            position += 1;
        }
    }

    position
}