use anyhow::{bail, Error};

const INPUT: &str = include_str!("../../input/06/input.txt");

/// Read in the input by converting each character to a `Signal` and returning
/// the list.
pub fn read() -> Vec<Signal> {
    INPUT.trim().chars().flat_map(Signal::try_from).collect()
}

/// Represents a single signal received on our device. Each character from the
/// input string can be represented as a `u32` with a single set bit, offset
/// from the left in alphabetic order. For example:
///  
/// 'a' -> Signal(00000000000000000000000000000001)
/// 'b' -> Signal(00000000000000000000000000000010)
/// 'x' -> Signal(00000000100000000000000000000000)
/// 'z' -> Signal(00000010000000000000000000000000)
#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
struct Signal(u32);

impl TryFrom<char> for Signal {
    type Error = Error;

    fn try_from(value: char) -> Result<Self, Self::Error> {
        /// Anything besides a lowercase letter cannot make a Signal
        if !value.is_ascii_lowercase() {
            bail!("Cannot convert {value} to a `Signal`!");
        }

        /// Set a single bit and shift it left by the letter offset, return
        /// as a Signal
        let shift = (value as u32) - 97;
        Ok(Signal(1 << shift))
    }
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn name() {
        let input = read();
        assert_eq!(input.len(), 4095);

        let first_signal = *input.first().unwrap();
        let first_expected = Signal::try_from('d').unwrap();
        assert_eq!(first_signal, first_expected);

        let last_signal = *input.last().unwrap();
        let last_expected = Signal::try_from('j').unwrap();
        assert_eq!(last_signal, last_expected);
    }
}

use std::ops::{BitAnd, BitOrAssign};

/// Solve Day 6, Part 1
fn solve(input: &Input) -> u32 {
    // Instantiate a detector for sequences of length 4
    let mut detector: SequenceDetector<4> = SequenceDetector::new();

    // Pass each `Signal` in the input to the detector. Return early
    // with the index (plus one) if a unique sequence is detected.
    for (idx, signal) in input.iter().enumerate() {
        if detector.detect(*signal) { return (idx as u32 + 1); }
    }
    panic!("No start-of-packet marker detected!")
}

/// Implement bitwise _and_ for `Signal`s
impl BitAnd for Signal {
    type Output = Signal;

    fn bitand(self, rhs: Self) -> Self::Output {
        Signal(self.0 & rhs.0)
    }
}

/// Implement bitwise _or-assign_ for `Signal`s
impl BitOrAssign for Signal {
    fn bitor_assign(&mut self, rhs: Self) {
        self.0 |= rhs.0;
    }
}

/// Represents a 'detector' for unique sequences of `Signal`s of a constant length. 
/// Holds a buffer of the signals encountered so far and inserts new signals into
/// that buffer by wrapping around the length of the buffer.
#[derive(Debug)]
struct SequenceDetector<const N: usize> {
    buffer: [Signal; N],
    mark: usize,
}

impl<const N: usize> SequenceDetector<N> {
    /// Create a new `SequenceDetector` with an empty buffer and a marker set to the
    /// first index of the buffer.
    fn new() -> Self {
        let buffer = [Signal::default(); N];
        let mark = 0;
        Self { buffer, mark }
    }

    /// Given a `Signal`, indicates whether the most recent `N` signals detected
    /// comprises a unique sequence.
    fn detect(&mut self, signal: Signal) -> bool {
        // Add the signal to the buffer and bump the marker over by one to 
        // receive the next signal.
        self.buffer[self.mark] = signal;
        self.mark = (self.mark + 1) % N;

        // If the marker points to an empty signal in the buffer, this means
        // the buffer isn't full yet and we definitely haven't found `N` unique
        // signal inputs.
        if self.buffer[self.mark] == Signal(0) { return false; }

        // Check the buffer for unique signals. If any duplicate `Signal`s are
        // detected, return false early. If all `Signal`s _are_ unique, return true.
        let mut bits = Signal(0);
        for bit in self.buffer.iter() {
            if *bit & bits > Signal(0) { return false; }
            bits |= *bit;
        }
        true
    }
}

/// Solve Day 6, Part 2
pub fn solve(input: &Input) -> u32 {
    // Instantiate a detector for sequences of length 14
    let mut detector: SequenceDetector<14> = SequenceDetector::new();

    // Pass each `Signal` in the input to the detector. Return early
    // with the index (plus one) if a unique sequence is detected.
    for (idx, signal) in input.iter().enumerate() {
        if detector.detect(*signal) { return (idx as u32 + 1).into(); }
    }
    panic!("No start-of-message marker detected!")
}