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//! A library for hexmap operations.
use std::ops::{Add, Sub};


/// A hex coordinate using a cubic coordinate scheme.
///
/// See http://www.redblobgames.com/grids/hexagons/#coordinates for more detail.
///
/// All three coordinates must sum to zero.
#[derive(Debug,PartialEq,PartialOrd,Clone,Copy)]
pub struct Coordinate {
    x: i64,
    y: i64,
    z: i64,
}

impl Coordinate {
    /// Create a new Coordinate at 0, 0, 0.
    ///
    /// This returns a `Result` to match `at`, but always succeeds.
    pub fn new() -> Result<Self, &'static str> {
        Ok(Coordinate { x: 0, y: 0, z: 0 })
    }

    /// Create a new Coordinate at the specified location, if that location is
    /// valid.
    pub fn at(x: i64, y: i64, z: i64) -> Result<Self, &'static str> {
        if x + y + z == 0 {
            Ok(Coordinate { x: x, y: y, z: z })
        } else {
            Err("Invalid cubic coordinates")
        }
    }

    /// Get the six neighbors of a given Coordinate.
    pub fn neighbors(&self) -> Vec<Coordinate> {
        vec![
            Coordinate::at(self.x + 1, self.y, self.z - 1).unwrap(),
            Coordinate::at(self.x + 1, self.y - 1, self.z).unwrap(),
            Coordinate::at(self.x, self.y - 1, self.z + 1).unwrap(),
            Coordinate::at(self.x, self.y + 1, self.z - 1).unwrap(),
            Coordinate::at(self.x - 1, self.y, self.z + 1).unwrap(),
            Coordinate::at(self.x - 1, self.y + 1, self.z).unwrap(),
        ]
    }

    /// Get the neighbor in `direction`. Valid directions are in [0, 5].
    pub fn neighbor(&self, direction: usize) -> Result<Coordinate, &'static str> {
        let neighbors = self.neighbors();
        match neighbors.get(direction) {
            Some(val) => Ok(val.to_owned()),
            None => Err("Invalid direction"),
        }
    }

    /// Get the distance between two coordinates, in grid tiles.
    pub fn distance_to(&self, other: Coordinate) -> i64 {
        (
            (self.x - other.x).abs()
            + (self.y - other.y).abs()
            + (self.z - other.z).abs()
        ) / 2
    }
}

impl Add for Coordinate {
    type Output = Coordinate;

    fn add(self, other: Coordinate) -> Coordinate {
        Coordinate {
            x: self.x + other.x,
            y: self.y + other.y,
            z: self.z + other.z,
        }
    }
}

impl Sub for Coordinate {
    type Output = Coordinate;

    fn sub(self, other: Coordinate) -> Coordinate {
        Coordinate {
            x: self.x - other.x,
            y: self.y - other.y,
            z: self.z - other.z,
        }
    }
}


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

    #[test]
    fn it_makes_a_new_one() {
        let coord = Coordinate::new().unwrap();
        assert_eq!(coord.x, 0);
        assert_eq!(coord.y, 0);
        assert_eq!(coord.z, 0);
    }

    #[test]
    fn it_accepts_args_in_constructor() {
        let coord = Coordinate::at(-3, -1, 4).unwrap();
        assert_eq!(coord.x, -3);
        assert_eq!(coord.y, -1);
        assert_eq!(coord.z, 4);
    }

    #[test]
    fn it_rejects_invalid_cube_coordinates() {
        let coord = Coordinate::at(3, 1, 4);
        assert!(coord.is_err());
    }

    #[test]
    fn it_generates_a_list_of_neighbors() {
        let coord = Coordinate::new().unwrap();
        let expected = vec![
            Coordinate::at(1, 0, -1).unwrap(),
            Coordinate::at(1, -1, 0).unwrap(),
            Coordinate::at(0, -1, 1).unwrap(),
            Coordinate::at(0, 1, -1).unwrap(),
            Coordinate::at(-1, 0, 1).unwrap(),
            Coordinate::at(-1, 1, 0).unwrap(),
        ];
        assert_eq!(coord.neighbors(), expected);
    }

    #[test]
    fn it_calcuates_distances() {
        let coord_a = Coordinate::at(-3, -1, 4).unwrap();
        let coord_b = Coordinate::at(2, 7, -9).unwrap();
        assert_eq!(coord_a.distance_to(coord_b), 13);
    }

    #[test]
    fn it_supports_addition() {
        let coord_a = Coordinate::at(-3, -1, 4).unwrap();
        let coord_b = Coordinate::at(2, 7, -9).unwrap();
        let expected = Coordinate::at(-1, 6, -5).unwrap();
        assert_eq!(coord_a + coord_b, expected);
    }

    #[test]
    fn it_supports_subtraction() {
        let coord_a = Coordinate::at(-3, -1, 4).unwrap();
        let coord_b = Coordinate::at(2, 7, -9).unwrap();
        let expected = Coordinate::at(-5, -8, 13).unwrap();
        assert_eq!(coord_a - coord_b, expected);
    }

    #[test]
    fn it_produces_neighbors_given_a_direction() {
        let coord = Coordinate::at(-3, -1, 4).unwrap();
        let expected = Ok(Coordinate::at(-2, -1, 3).unwrap());
        assert_eq!(coord.neighbor(0), expected);
    }

    #[test]
    fn it_produces_no_neighbors_given_a_bad_direction() {
        let coord = Coordinate::at(-3, -1, 4).unwrap();
        let expected = Err("Invalid direction");
        assert_eq!(coord.neighbor(6), expected);
    }
}