use {
    anyhow::{
        anyhow,
        Context,
        Result,
    },
    std::{
        env::args,
        fs::File,
        io::{
            BufRead,
            BufReader,
        },
        path::Path,
    },
};

fn read_input<T: AsRef<Path>>(filename: T) -> Result<Vec<usize>> {
    let reader = BufReader::new(File::open(filename)?);

    reader.lines().map(
        |v| v?.parse().map_err(|err| anyhow!("Could not parse input: {}", err))
    ).collect()
}

#[derive(PartialEq)]
enum Change {
    NoPrevVal,
    Increased,
    Decreased,
    Identical,
}

fn count_increases(values: &[usize]) -> usize {
    let changes: Vec<Change> = values.iter().scan(None, |state, val| {
        let next = match state {
            None => Change::NoPrevVal,
            Some(prev) if val < prev => Change::Decreased,
            Some(prev) if val > prev => Change::Increased,
            Some(prev) if val == prev => Change::Identical,
            Some(_) => unreachable!(),
        };
        *state = Some(*val);
        Some(next)
    }).collect();

    changes.iter().fold(0, |count, change|
        if *change == Change::Increased { count + 1 } else { count }
    )
}

fn part1(input: &[usize]) -> Result<usize> {
    let count = count_increases(input);

    Ok(count)
}

fn part2(input: &[usize]) -> Result<usize> {
    let slidesums: Vec<_> = input.iter().zip(input[1..].iter()).zip(input[2..].iter()).map(|n| {
        let ((first, second), third) = n;
        first + second + third
    }).collect();
    let count = count_increases(&slidesums);

    Ok(count)
}

fn main() -> Result<()> {
    let ( do_part_1, do_part_2 ) = aoc::do_parts();

    let filename = args().nth(1).ok_or(anyhow!("Missing input filename"))?;
    let input = read_input(filename).context("Could not read input")?;
    if do_part_1 {
        let solution = part1(&input).context("No solution for part 1")?;
        println!("Part1, number of increases found to be: {}", solution);
    }
    if do_part_2 {
        let solution = part2(&input).context("No solution for part 2")?;
        println!("Part2, number of increases found to be: {}", solution);
    }
    Ok(())
}