From 7beca553eeb033611951917dec0f135c6144e1f1 Mon Sep 17 00:00:00 2001
From: Xu Shaohua <shaohua@biofan.org>
Date: Tue, 1 Aug 2023 08:37:19 +0800
Subject: [PATCH] physics: Add horizontal projectile motion

---
 physics/src/horizontal_projectile_motion.rs | 74 +++++++++++++++++++++
 physics/src/lib.rs                          |  1 +
 2 files changed, 75 insertions(+)
 create mode 100644 physics/src/horizontal_projectile_motion.rs

diff --git a/physics/src/horizontal_projectile_motion.rs b/physics/src/horizontal_projectile_motion.rs
new file mode 100644
index 00000000..c576b840
--- /dev/null
+++ b/physics/src/horizontal_projectile_motion.rs
@@ -0,0 +1,74 @@
+// Copyright (c) 2023 Xu Shaohua <shaohua@biofan.org>. All rights reserved.
+// Use of this source is governed by General Public License that can be found
+// in the LICENSE file.
+
+//! Horizontal Projectile Motion problem in physics.
+//!
+//! [Projectile motion](https://en.wikipedia.org/wiki/Projectile_motion)
+
+/// Acceleration Constant on Earth (unit m/s^2)
+pub const G: f64 = 9.80665;
+
+// Check that the arguments are valid
+#[must_use]
+fn check_args(init_velocity: f64, angle: f64) -> bool {
+    init_velocity >= 0.0 && (1.0..=90.0).contains(&angle)
+}
+
+/// Returns the horizontal distance that the object cover
+#[must_use]
+pub fn horizontal_distance(init_velocity: f64, angle: f64) -> f64 {
+    debug_assert!(check_args(init_velocity, angle));
+    let radians = (2.0 * angle).to_radians();
+    init_velocity.powi(2) * (radians).sin() / G
+}
+
+/// Returns the maximum height that the object reach
+#[must_use]
+pub fn max_height(init_velocity: f64, angle: f64) -> f64 {
+    debug_assert!(check_args(init_velocity, angle));
+    let radians = angle.to_radians();
+    init_velocity.powi(2) * radians.sin().powi(2) / (2.0 * G)
+}
+
+/// Returns total time of the motion
+#[must_use]
+pub fn total_time(init_velocity: f64, angle: f64) -> f64 {
+    debug_assert!(check_args(init_velocity, angle));
+    let radians = angle.to_radians();
+    2.0 * init_velocity * radians.sin() / G
+}
+
+pub trait Round2 {
+    #[must_use]
+    fn round2(self) -> Self;
+}
+
+impl Round2 for f64 {
+    fn round2(self) -> Self {
+        (self * 100.0).round() / 100.0
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{horizontal_distance, max_height, total_time, Round2};
+
+    #[test]
+    fn test_horizontal_distance() {
+        assert_eq!(horizontal_distance(30.0, 45.0).round2(), 91.77);
+        assert_eq!(horizontal_distance(100.0, 78.0).round2(), 414.76);
+    }
+
+    #[test]
+    fn test_max_height() {
+        assert_eq!(max_height(30.0, 45.0).round2(), 22.94);
+        assert_eq!(max_height(100.0, 78.0).round2(), 487.82);
+    }
+
+    #[test]
+    fn test_total_time() {
+        assert_eq!(total_time(30.0, 45.0).round2(), 4.33);
+        assert_eq!(total_time(100.0, 78.0).round2(), 19.95);
+    }
+}
diff --git a/physics/src/lib.rs b/physics/src/lib.rs
index a2e336d3..a64c106d 100644
--- a/physics/src/lib.rs
+++ b/physics/src/lib.rs
@@ -14,6 +14,7 @@ pub mod archimedes_principle;
 pub mod casimir_effect;
 pub mod centripetal_force;
 pub mod grahams_law;
+pub mod horizontal_projectile_motion;
 pub mod hubble_parameter;
 pub mod ideal_gas_law;
 pub mod kinetic_energy;