3
\$\begingroup\$

This is a continuation of Advent of Code 2016: Day 1, Part 1.

This time, instead of the distance at the end of the instruction string, the answer is the distance of the first location which is visited twice. I implemented this by adding a terminate_on_revisited_location parameter to the blocks_away method, tracking visited locations with a HashSet, and tracking movement steps individually.

lib.rs

#![feature(conservative_impl_trait)]
#![feature(try_from)]

mod day_01;

pub fn day_01() {
    let terminate_on_revisited_location = false;
    let day_01_answer_part_one = day_01::blocks_away(include_str!("day_01_input"),
                                                     terminate_on_revisited_location);
    assert_eq!(day_01_answer_part_one, Ok(279));

    let terminate_on_revisited_location = true;
    let day_01_answer_part_two = day_01::blocks_away(include_str!("day_01_input"),
                                                     terminate_on_revisited_location);
    assert_eq!(day_01_answer_part_two, Ok(163));
}

day_01.rs

use std::str::FromStr;
use std::collections::HashSet;

type Error = &'static str;
type Result<T> = ::std::result::Result<T, Error>;

pub fn blocks_away(instructions: &str, terminate_on_revisited_location: bool) -> Result<i16> {
    let mut x: i16 = 0;
    let mut y: i16 = 0;
    let mut direction = Direction::North;
    let mut visited = HashSet::new();
    for instruction in Instruction::try_many_from(instructions) {
        let instruction = instruction?;
        direction = direction.turn(instruction.turn);
        let blocks = instruction.blocks as i16;
        for _ in 0..blocks {
            if terminate_on_revisited_location {
                if visited.contains(&(x, y)) {
                    return Ok(x.abs() + y.abs());
                }
                visited.insert((x, y));
            }
            match direction {
                Direction::North => y += 1,
                Direction::East => x += 1,
                Direction::South => y -= 1,
                Direction::West => x -= 1,
            }
        }
    }
    if terminate_on_revisited_location {
        Err("Never visted same location twice")
    } else {
        Ok(x.abs() + y.abs())
    }
}

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
enum Direction {
    North,
    East,
    South,
    West,
}

impl Direction {
    fn turn(&self, turn: Turn) -> Self {
        use self::Direction::*;
        use self::Turn::*;

        match (*self, turn) {
            (North, Left) | (South, Right) => West,
            (North, Right) | (South, Left) => East,
            (East, Left) | (West, Right) => North,
            (East, Right) | (West, Left) => South,
        }
    }
}

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
enum Turn {
    Left,
    Right,
}

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
struct Instruction {
    turn: Turn,
    blocks: u16,
}

impl Instruction {
    #[cfg_attr(feature = "cargo-clippy", allow(needless_lifetimes))]
    fn try_many_from<'a>(s: &'a str) -> impl Iterator<Item = Result<Self>> + 'a {
        s.split(", ").map(str::parse)
    }
}

impl FromStr for Instruction {
    type Err = Error;

    fn from_str(s: &str) -> Result<Self> {
        let mut chars = s.chars();
        let turn = match chars.next() {
            Some('L') => Turn::Left,
            Some('R') => Turn::Right,
            Some(_) => return Err("Turn character invalid"),
            None => return Err("Instruction string is empty"),
        };
        let blocks = try!(chars.as_str().parse().map_err(|_| "Could not parse blocks"));
        Ok(Instruction {
            turn: turn,
            blocks: blocks,
        })
    }
}

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

    #[test]
    fn test_parse_instruction_success() {
        let instruction = "L1".parse::<Instruction>();
        assert_eq!(instruction,
                   Ok(Instruction {
                       turn: Turn::Left,
                       blocks: 1,
                   }));
        let instruction = "R2".parse::<Instruction>();
        assert_eq!(instruction,
                   Ok(Instruction {
                       turn: Turn::Right,
                       blocks: 2,
                   }));
    }

    #[test]
    fn test_parse_instruction_invalid_turn_character() {
        let instruction = "S1".parse::<Instruction>();
        assert_eq!(instruction, Err("Turn character invalid"));
    }

    #[test]
    fn test_parse_instruction_empty_string() {
        let instruction = "".parse::<Instruction>();
        assert_eq!(instruction, Err("Instruction string is empty"));
    }

    #[test]
    fn test_parse_instruction_missing_blocks_digit() {
        let instruction = "L".parse::<Instruction>();
        assert_eq!(instruction, Err("Could not parse blocks"));
    }

    #[test]
    fn test_parse_instruction_invalid_blocks_digit() {
        let instruction = "LL".parse::<Instruction>();
        assert_eq!(instruction, Err("Could not parse blocks"));
    }

    #[test]
    fn test_parse_instructions() {
        let instructions = Instruction::try_many_from("L1, R2").collect::<Result<Vec<_>>>();
        assert_eq!(instructions,
                   Ok(vec![Instruction {
                               turn: Turn::Left,
                               blocks: 1,
                           },
                           Instruction {
                               turn: Turn::Right,
                               blocks: 2,
                           }]));
    }

    // Assert that parsing failure returns the first error
    #[test]
    fn test_parse_instructions_with_error() {
        let instructions = Instruction::try_many_from("L1, , R2, S2").collect::<Result<Vec<_>>>();
        assert_eq!(instructions, Err("Instruction string is empty"));
    }

    #[test]
    fn test_blocks_away_part_one() {
        let blocks_away = blocks_away("L4, L1, L1", false);
        assert_eq!(blocks_away, Ok(4));
    }

    #[test]
    fn test_blocks_away_part_one_advent_input() {
        let day_01_answer = blocks_away(include_str!("day_01_input"), false);
        assert_eq!(day_01_answer, Ok(279));
    }

    #[test]
    fn test_blocks_away_part_two() {
        let blocks_away = blocks_away("R8, R4, R4, R8", true);
        assert_eq!(blocks_away, Ok(4));
    }

    #[test]
    fn test_blocks_away_part_two_advent_input() {
        let day_01_answer = blocks_away(include_str!("day_01_input"), true);
        assert_eq!(day_01_answer, Ok(163));
    }
}
\$\endgroup\$

1 Answer 1

1
\$\begingroup\$
  1. The code has compiler warnings:

    warning: unused or unknown feature, #[warn(unused_features)] on by default
     --> src/lib.rs:2:12
      |
    2 | #![feature(try_from)]
      |            ^^^^^^^^
    
  2. The code fails one of the tests:

    test day_01::test::test_blocks_away_part_one_advent_input ... FAILED
    
  3. You can avoid performing two hash operations by using the result value of insert:

    let did_insert = visited.insert((x, y));
    if !did_insert {
        return Ok(x.abs() + y.abs());
    }
    
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.