adventofcode2024/day_02/src/main.rs

// https://adventofcode.com/2024/day/2
//
// run command: `cargo run ./input.txt`

use std::env;
use std::error::Error;
use std::fs::File;
use std::io::{BufReader, prelude::*};
use std::str::FromStr;

/// Parse a report into a Vec<T>
fn parse_report<T>(line: &str) -> Result<Vec<T>, <T as FromStr>::Err>
where
    T: FromStr,
{
    let parsed_line = line
        .split(char::is_whitespace)
        .map(|v| v.parse())
        .collect::<Result<Vec<T>, _>>()?;
    Ok(parsed_line)
}

#[derive(Eq, PartialEq, Copy, Clone, Debug)]
enum TransitionState {
    Good,        // Good state
    Bad,         // Bad state
    Dampened,    // Could be removed to make a good state
    DampenedEnd, // Could be removed to make a good state, end or start of report
}

#[derive(Eq, PartialEq, Copy, Clone, Debug)]
enum TransitionDirection {
    Asc,  // level is increasing
    Desc, // level is decreasing
}

#[derive(Debug)]
struct LevelTransition {
    state: TransitionState,
    direction: TransitionDirection,
}
/// Given two level inputs, return one of the following:
///   - `LevelTransitionState::Bad` if the difference between
///     the levels is either 0 or great than 3
///   - `LevelTransitionState::Asc` if the value from curr_level to next_level is increasing and it's not BAD
///   - `LevelTransitionState::Desc` if the value from curr_level to next_level is decreasing and it's not BAD
fn compare_levels(curr_level: i64, next_level: i64) -> LevelTransition {
    let level_diff = next_level - curr_level;

    // If the diff is 0 or greater than 3, return bad state
    let state = if level_diff == 0 || level_diff.abs() > 3 {
        TransitionState::Bad
    } else {
        TransitionState::Good
    };

    // If great than zero they are ascending
    let direction = if level_diff > 0 {
        TransitionDirection::Asc
    } else {
        TransitionDirection::Desc
    };

    return LevelTransition { state, direction };
}

fn is_report_safe_no_dampening(report: Vec<i64>) -> bool {
    // Turn the report into a bunch of states for each adjacent pair in the report
    let report: Vec<LevelTransition> = report
        .windows(2)
        .map(|l| compare_levels(l[0], l[1]))
        .collect();

    // Setup the loop by checking the first start as a special case
    let mut prev_transition = &report[0];
    if prev_transition.state != TransitionState::Good {
        return false;
    }

    // Check each state
    for level_transition in report[1..].iter() {
        // If bad, then it's just bad
        if level_transition.state != TransitionState::Good {
            return false;
        }

        // If the states aren't the same as the prev, it must
        // have changed direction
        if level_transition.direction != prev_transition.direction {
            return false;
        }

        // Setup for next iteration
        prev_transition = level_transition;
    }
    return true;
}

fn is_report_safe_with_dampening(report: Vec<i64>) -> bool {
    let mut modified_report: Vec<Option<i64>> = vec![None];
    let mut tmp_report: Vec<Option<i64>> = report.iter().map(|v| Some(*v)).collect();
    modified_report.append(&mut tmp_report);
    modified_report.push(None);

    let mut transitions = Vec::new();
    for window in modified_report.windows(3) {
        let window_tuple = (window[0], window[1], window[2]);
        let state = match window_tuple {
            // We loop through in 3 value windows. We padded a None on
            // either side of the vec to detect the ends of the report for special handling.
            //
            // If the middle value from the window could be removed to make that section of repo SAFE,
            //mark as Dempened. We will use this later to determine if there's only a single dampened
            //value we can remove to meet the dampening criteria.
            (None, Some(curr), Some(next)) | (Some(curr), Some(next), None) => {
                // Matched first or last element, which means we only have one check to do
                // and we always return Dampened instead of Bad
                let local_transition = compare_levels(curr, next);
                if local_transition.state == TransitionState::Bad {
                    // shadow bad with dampened since can always
                    // remove the first and last element.
                    LevelTransition {
                        state: TransitionState::DampenedEnd,
                        ..local_transition
                    }
                } else {
                    local_transition
                }
            }
            (Some(prev), Some(curr), Some(next)) => {
                let first_transition = compare_levels(prev, curr);
                let second_transition = compare_levels(curr, next);
                if first_transition.state != TransitionState::Good
                    || second_transition.state != TransitionState::Good
                    || first_transition.direction != second_transition.direction
                {
                    let jump_transition = compare_levels(prev, next);
                    if jump_transition.state == TransitionState::Good {
                        //println!("Dampening value window: {:?}", window_tuple);
                        // If we removed the middle value, we would be in a SAFE state, mark as DAMPENED
                        LevelTransition {
                            state: TransitionState::Dampened,
                            ..jump_transition
                        }
                    } else {
                        // Removing the middle value does not help us. mark as BAD
                        LevelTransition {
                            state: TransitionState::Bad,
                            ..jump_transition
                        }
                    }
                } else {
                    // Both transitions are good and moving in the same directions, so we can return either.
                    second_transition
                }
            }
            _ => panic!("This should never happen!"),
        };
        transitions.push(state);
    }

    let (mut dampened_cnt, mut prev_direction) = match transitions[0].state {
        TransitionState::Good => (0, transitions[0].direction),
        TransitionState::Dampened | TransitionState::DampenedEnd => (1, transitions[1].direction),
        TransitionState::Bad => {
            panic!("This should never happen!");
        }
    };
    for transition in &transitions {
        match transition.state {
            TransitionState::Good => {
                if transition.direction != prev_direction {
                    println!(
                        "Failed for non matching direction: {:?} -> {:?}",
                        report, transitions
                    );
                    return false;
                }
            }
            TransitionState::Dampened => {
                dampened_cnt += 1;
                if transition.direction != prev_direction {
                    println!(
                        "Failed for non matching direction: {:?} -> {:?}",
                        report, transitions
                    );
                    return false;
                }
            }
            TransitionState::DampenedEnd => {
                dampened_cnt += 1;
            }
            TransitionState::Bad => {
                // If it's marked as bad we can't fix it by dampening, return false
                return false;
            }
        }
        // We can only dampen one value
        if dampened_cnt > 1 {
            println!(
                "Failed for too much dampening: {:?} -> {:?}",
                report, transitions
            );
            return false;
        }
        prev_direction = transition.direction;
    }
    true
}

fn main() -> Result<(), Box<dyn Error>> {
    // Handle command input
    let args: Vec<String> = env::args().collect();
    if args.len() != 2 {
        panic!(
            "{} must be run with a single argument of files name!",
            &args[0]
        )
    }
    let input_file_string = &args[1];

    let input_file = File::open(input_file_string)?;
    let input_reader = BufReader::new(input_file);

    let mut accumulator_no_dampening = 0;
    let mut accumulator_with_dampening = 0;
    for line in input_reader.lines() {
        let line = line?;
        let parsed_report: Vec<i64> = parse_report(&line)?;
        if is_report_safe_no_dampening(parsed_report.clone()) {
            accumulator_no_dampening += 1;
        }
        if is_report_safe_with_dampening(parsed_report) {
            accumulator_with_dampening += 1;
        }
    }

    println!(
        "test: {}",
        is_report_safe_with_dampening(vec![1, 3, 2, 4, 5])
    );

    println!(
        "Number of SAFE reports (No Dampening - Part 1): {}",
        accumulator_no_dampening
    );
    println!(
        "Number of SAFE reports (With Dampening - Part 2): {}",
        accumulator_with_dampening
    );

    Ok(())
}