//! Day 05 of 2021.

use crate::prelude::*;

/// Get the solution for day 05 of 2021.
pub fn solution() -> Solution {
  Solution::new(Day::new(5, 2021), part_1, part_2).with_expected(6687, 19851)
}

/// A vector of two sets of coordinates.
#[derive(Debug)]
struct Vector {
  /// The starting coordinate.
  a: (isize, isize),
  /// The ending coordinate.
  b: (isize, isize),
}

impl Vector {
  /// Calculate the covered diagonal coordinates of the [`Vector`].
  fn diagonal_coordinates(&self) -> Vec<(isize, isize)> {
    let x_coordinates = {
      let x_amount = (self.a.0 - self.b.0).abs();
      if self.a.0 < self.b.0 {
        (0..=x_amount).map(|x| self.a.0 + x).collect::<Vec<_>>()
      } else {
        (0..=x_amount).map(|x| self.a.0 - x).collect::<Vec<_>>()
      }
    };

    let y_coordinates = {
      let y_amount = (self.a.1 - self.b.1).abs();
      if self.a.1 < self.b.1 {
        (0..=y_amount).map(|x| self.a.1 + x).collect::<Vec<_>>()
      } else {
        (0..=y_amount).map(|x| self.a.1 - x).collect::<Vec<_>>()
      }
    };

    x_coordinates.into_iter().zip(y_coordinates).collect()
  }

  /// Calculate the covered horizontal coordinates of the [`Vector`].
  fn horizontal_coordinates(&self) -> Vec<(isize, isize)> {
    let y = self.a.1;

    let (x1, x2) = if self.a.0 < self.b.0 {
      (self.a.0, self.b.0)
    } else {
      (self.b.0, self.a.0)
    };

    (x1..=x2).map(|x| (x, y)).collect()
  }

  /// Calculate the covered vertical coordinates of the [`Vector`].
  fn vertical_coordinates(&self) -> Vec<(isize, isize)> {
    let x = self.a.0;

    let (y1, y2) = if self.a.1 < self.b.1 {
      (self.a.1, self.b.1)
    } else {
      (self.b.1, self.a.1)
    };

    (y1..=y2).map(|y| (x, y)).collect()
  }
}

impl FromStr for Vector {
  type Err = Error;

  fn from_str(s: &str) -> Result<Self, Self::Err> {
    let numbers = s
      .replace(" -> ", ",")
      .split(',')
      .map(str::parse)
      .collect::<Result<Vec<_>, _>>()?;

    if numbers.len() == 4 {
      Ok(Self {
        a: (numbers[0], numbers[1]),
        b: (numbers[2], numbers[3]),
      })
    } else {
      Err(eyre!("Invalid input: {}", s))
    }
  }
}

/// The logic to solve part one.
fn part_1(input: &str) -> Result<String> {
  let vectors = input
    .lines()
    .filter_map(|line| {
      if let Ok(vector) = Vector::from_str(line) {
        if vector.a.0 == vector.b.0 || vector.a.1 == vector.b.1 {
          return Some(vector);
        }
      }

      None
    })
    .collect::<Vec<_>>();

  let mut travelled_coordinates = HashMap::<(isize, isize), isize>::new();
  for vector in vectors {
    let coordinates = if vector.a.0 == vector.b.0 {
      vector.vertical_coordinates()
    } else {
      vector.horizontal_coordinates()
    };

    for coordinate in coordinates {
      let amount = travelled_coordinates.entry(coordinate).or_default();
      *amount += 1;
    }
  }

  Ok(count_result(travelled_coordinates).to_string())
}

/// The logic to solve part two.
fn part_2(input: &str) -> Result<String> {
  let vectors = input
    .lines()
    .map(str::parse)
    .collect::<Result<Vec<Vector>>>()?;

  let mut travelled_coordinates = HashMap::<(isize, isize), isize>::new();
  for vector in vectors {
    let coordinates = if vector.a.0 == vector.b.0 {
      vector.vertical_coordinates()
    } else if vector.a.1 == vector.b.1 {
      vector.horizontal_coordinates()
    } else {
      vector.diagonal_coordinates()
    };

    for coordinate in coordinates {
      let amount = travelled_coordinates.entry(coordinate).or_default();
      *amount += 1;
    }
  }

  Ok(count_result(travelled_coordinates).to_string())
}

/// Count the results of the coordinates that overlap at least two lines.
fn count_result(coordinates: HashMap<(isize, isize), isize>) -> usize {
  coordinates
    .into_iter()
    .filter(|(_, amount)| amount >= &2)
    .count()
}