Advent of Rust 2024 Day 4 — niceness

niceness.rs

//! # [*Advent of Rust 2024* Day 4]: Kids and Niceness
//! Bart Massey 2024
//!
//! This code is licensed under the "MIT License". License terms
//! are [here](https://opensource.org/license/mit).
//!
//! This code is a demo of how to solve the given problem with reasonable
//! programming style and correctness,
//!
//! [*Advent of Rust 2024* Day 4]: https://www.rustfinity.com/practice/rust/challenges/aor-2024-4

// XXX Making these integers f32 is probably a bad idea from
// a numerical precision point of view.
/// Weight of good deeds.
pub const GOOD_WEIGHT: f32 = 1.0;
/// Weight of bad deeds.
pub const BAD_WEIGHT: f32 = 2.0;
/// Threshold for weighted goodness at which a kid is nice
/// instead of naughty.
// XXX This should be rational.
pub const NICE_THRESHOLD: f32 = 0.75;

/// Niceness of a kid.
#[derive(Debug, PartialEq)]
pub enum Niceness {
    /// Kid is nice, with the given number of good deeds.
    Nice(u32),
    /// Kid is naughty.
    Naughty,
}

impl Niceness {
    /// Given `g` good deeds and `b` bad ones, return a [Niceness].
    pub fn new(g: u32, b: u32) -> Self {
        let gw = GOOD_WEIGHT;
        let bw = BAD_WEIGHT;
        // XXX None of this should be a floating-point
        // computation. We could punish a kid with a
        // rounding error!
        //
        // Weighted goodness.
        let wg = g as f32 * gw;
        // Weighted total.
        let wt = wg + b as f32 * bw;
        let is_nice = g + b > 0 && wg >= wt * NICE_THRESHOLD;
        match is_nice {
            true => Niceness::Nice(g),
            false => Niceness::Naughty,
        }
    }
}

/// Record for a particular kid.
pub struct Kid {
    /// Kid's name.
    pub name: String,
    /// Kid's niceness.
    pub niceness: Niceness,
}

impl Kid {
    /// Make a new kid record.
    pub fn new(name: String, good_deeds: u32, bad_deeds: u32) -> Kid {
        let niceness = Niceness::new(good_deeds, bad_deeds);
        Kid { name, niceness }
    }
}