//! This example displays each contributor to the bevy source code as a bouncing bevy-ball.

use bevy::{math::bounding::Aabb2d, prelude::*};
use rand::{Rng, SeedableRng};
use rand_chacha::ChaCha8Rng;
use std::{
    collections::HashMap,
    env::VarError,
    hash::{DefaultHasher, Hash, Hasher},
    io::{self, BufRead, BufReader},
    process::Stdio,
};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .init_resource::<SelectionTimer>()
        .init_resource::<SharedRng>()
        .add_systems(Startup, (setup_contributor_selection, setup))
        // Systems are chained for determinism only
        .add_systems(Update, (gravity, movement, collisions, selection).chain())
        .run();
}

type Contributors = Vec<(String, usize)>;

#[derive(Resource)]
struct ContributorSelection {
    order: Vec<Entity>,
    idx: usize,
}

#[derive(Resource)]
struct SelectionTimer(Timer);

impl Default for SelectionTimer {
    fn default() -> Self {
        Self(Timer::from_seconds(
            SHOWCASE_TIMER_SECS,
            TimerMode::Repeating,
        ))
    }
}

#[derive(Component)]
struct ContributorDisplay;

#[derive(Component)]
struct Contributor {
    name: String,
    num_commits: usize,
    hue: f32,
}

#[derive(Component)]
struct Velocity {
    translation: Vec3,
    rotation: f32,
}

// We're using a shared seeded RNG here to make this example deterministic for testing purposes.
// This isn't strictly required in practical use unless you need your app to be deterministic.
#[derive(Resource, Deref, DerefMut)]
struct SharedRng(ChaCha8Rng);
impl Default for SharedRng {
    fn default() -> Self {
        Self(ChaCha8Rng::seed_from_u64(10223163112))
    }
}

const GRAVITY: f32 = 9.821 * 100.0;
const SPRITE_SIZE: f32 = 75.0;

const SELECTED: Hsla = Hsla::hsl(0.0, 0.9, 0.7);
const DESELECTED: Hsla = Hsla::new(0.0, 0.3, 0.2, 0.92);

const SELECTED_Z_OFFSET: f32 = 100.0;

const SHOWCASE_TIMER_SECS: f32 = 3.0;

const CONTRIBUTORS_LIST: &[&str] = &["Carter Anderson", "And Many More"];

fn setup_contributor_selection(
    mut commands: Commands,
    asset_server: Res<AssetServer>,
    mut rng: ResMut<SharedRng>,
) {
    let contribs = contributors_or_fallback();

    let texture_handle = asset_server.load("branding/icon.png");

    let mut contributor_selection = ContributorSelection {
        order: Vec::with_capacity(contribs.len()),
        idx: 0,
    };

    for (name, num_commits) in contribs {
        let transform = Transform::from_xyz(
            rng.gen_range(-400.0..400.0),
            rng.gen_range(0.0..400.0),
            rng.gen(),
        );
        let dir = rng.gen_range(-1.0..1.0);
        let velocity = Vec3::new(dir * 500.0, 0.0, 0.0);
        let hue = name_to_hue(&name);

        // Some sprites should be flipped for variety
        let flipped = rng.gen();

        let entity = commands
            .spawn((
                Contributor {
                    name,
                    num_commits,
                    hue,
                },
                Velocity {
                    translation: velocity,
                    rotation: -dir * 5.0,
                },
                Sprite {
                    image: texture_handle.clone(),
                    custom_size: Some(Vec2::splat(SPRITE_SIZE)),
                    color: DESELECTED.with_hue(hue).into(),
                    flip_x: flipped,
                    ..default()
                },
                transform,
            ))
            .id();

        contributor_selection.order.push(entity);
    }

    commands.insert_resource(contributor_selection);
}

fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
    commands.spawn(Camera2d);

    let text_style = TextFont {
        font: asset_server.load("fonts/FiraSans-Bold.ttf"),
        font_size: 60.0,
        ..default()
    };

    commands
        .spawn((
            Text::new("Contributor showcase"),
            text_style.clone(),
            ContributorDisplay,
            Node {
                position_type: PositionType::Absolute,
                top: Val::Px(12.),
                left: Val::Px(12.),
                ..default()
            },
        ))
        .with_child((
            TextSpan::default(),
            TextFont {
                font_size: 30.,
                ..text_style
            },
        ));
}

/// Finds the next contributor to display and selects the entity
fn selection(
    mut timer: ResMut<SelectionTimer>,
    mut contributor_selection: ResMut<ContributorSelection>,
    contributor_root: Single<Entity, (With<ContributorDisplay>, With<Text>)>,
    mut query: Query<(&Contributor, &mut Sprite, &mut Transform)>,
    mut writer: TextUiWriter,
    time: Res<Time>,
) {
    if !timer.0.tick(time.delta()).just_finished() {
        return;
    }

    // Deselect the previous contributor

    let entity = contributor_selection.order[contributor_selection.idx];
    if let Ok((contributor, mut sprite, mut transform)) = query.get_mut(entity) {
        deselect(&mut sprite, contributor, &mut transform);
    }

    // Select the next contributor

    if (contributor_selection.idx + 1) < contributor_selection.order.len() {
        contributor_selection.idx += 1;
    } else {
        contributor_selection.idx = 0;
    }

    let entity = contributor_selection.order[contributor_selection.idx];

    if let Ok((contributor, mut sprite, mut transform)) = query.get_mut(entity) {
        let entity = *contributor_root;
        select(
            &mut sprite,
            contributor,
            &mut transform,
            entity,
            &mut writer,
        );
    }
}

/// Change the tint color to the "selected" color, bring the object to the front
/// and display the name.
fn select(
    sprite: &mut Sprite,
    contributor: &Contributor,
    transform: &mut Transform,
    entity: Entity,
    writer: &mut TextUiWriter,
) {
    sprite.color = SELECTED.with_hue(contributor.hue).into();

    transform.translation.z += SELECTED_Z_OFFSET;

    writer.text(entity, 0).clone_from(&contributor.name);
    *writer.text(entity, 1) = format!(
        "\n{} commit{}",
        contributor.num_commits,
        if contributor.num_commits > 1 { "s" } else { "" }
    );
    writer.color(entity, 0).0 = sprite.color;
}

/// Change the tint color to the "deselected" color and push
/// the object to the back.
fn deselect(sprite: &mut Sprite, contributor: &Contributor, transform: &mut Transform) {
    sprite.color = DESELECTED.with_hue(contributor.hue).into();

    transform.translation.z -= SELECTED_Z_OFFSET;
}

/// Applies gravity to all entities with a velocity.
fn gravity(time: Res<Time>, mut velocity_query: Query<&mut Velocity>) {
    let delta = time.delta_secs();

    for mut velocity in &mut velocity_query {
        velocity.translation.y -= GRAVITY * delta;
    }
}

/// Checks for collisions of contributor-birbs.
///
/// On collision with left-or-right wall it resets the horizontal
/// velocity. On collision with the ground it applies an upwards
/// force.
fn collisions(
    window: Single<&Window>,
    mut query: Query<(&mut Velocity, &mut Transform), With<Contributor>>,
    mut rng: ResMut<SharedRng>,
) {
    let window_size = window.size();

    let collision_area = Aabb2d::new(Vec2::ZERO, (window_size - SPRITE_SIZE) / 2.);

    // The maximum height the birbs should try to reach is one birb below the top of the window.
    let max_bounce_height = (window_size.y - SPRITE_SIZE * 2.0).max(0.0);
    let min_bounce_height = max_bounce_height * 0.4;

    for (mut velocity, mut transform) in &mut query {
        // Clamp the translation to not go out of the bounds
        if transform.translation.y < collision_area.min.y {
            transform.translation.y = collision_area.min.y;

            // How high this birb will bounce.
            let bounce_height = rng.gen_range(min_bounce_height..=max_bounce_height);

            // Apply the velocity that would bounce the birb up to bounce_height.
            velocity.translation.y = (bounce_height * GRAVITY * 2.).sqrt();
        }

        // Birbs might hit the ceiling if the window is resized.
        // If they do, bounce them.
        if transform.translation.y > collision_area.max.y {
            transform.translation.y = collision_area.max.y;
            velocity.translation.y *= -1.0;
        }

        // On side walls flip the horizontal velocity
        if transform.translation.x < collision_area.min.x {
            transform.translation.x = collision_area.min.x;
            velocity.translation.x *= -1.0;
            velocity.rotation *= -1.0;
        }
        if transform.translation.x > collision_area.max.x {
            transform.translation.x = collision_area.max.x;
            velocity.translation.x *= -1.0;
            velocity.rotation *= -1.0;
        }
    }
}

/// Apply velocity to positions and rotations.
fn movement(time: Res<Time>, mut query: Query<(&Velocity, &mut Transform)>) {
    let delta = time.delta_secs();

    for (velocity, mut transform) in &mut query {
        transform.translation += delta * velocity.translation;
        transform.rotate_z(velocity.rotation * delta);
    }
}

#[derive(Debug, thiserror::Error)]
enum LoadContributorsError {
    #[error("An IO error occurred while reading the git log.")]
    Io(#[from] io::Error),
    #[error("The CARGO_MANIFEST_DIR environment variable was not set.")]
    Var(#[from] VarError),
    #[error("The git process did not return a stdout handle.")]
    Stdout,
}

/// Get the names and commit counts of all contributors from the git log.
///
/// This function only works if `git` is installed and
/// the program is run through `cargo`.
fn contributors() -> Result<Contributors, LoadContributorsError> {
    let manifest_dir = std::env::var("CARGO_MANIFEST_DIR")?;

    let mut cmd = std::process::Command::new("git")
        .args(["--no-pager", "log", "--pretty=format:%an"])
        .current_dir(manifest_dir)
        .stdout(Stdio::piped())
        .spawn()?;

    let stdout = cmd.stdout.take().ok_or(LoadContributorsError::Stdout)?;

    // Take the list of commit author names and collect them into a HashMap,
    // keeping a count of how many commits they authored.
    let contributors = BufReader::new(stdout).lines().map_while(Result::ok).fold(
        HashMap::new(),
        |mut acc, word| {
            *acc.entry(word).or_insert(0) += 1;
            acc
        },
    );

    Ok(contributors.into_iter().collect())
}

/// Get the contributors list, or fall back to a default value if
/// it's unavailable or we're in CI
fn contributors_or_fallback() -> Contributors {
    let get_default = || {
        CONTRIBUTORS_LIST
            .iter()
            .cycle()
            .take(1000)
            .map(|name| (name.to_string(), 1))
            .collect()
    };

    if cfg!(feature = "bevy_ci_testing") {
        return get_default();
    }

    contributors().unwrap_or_else(|_| get_default())
}

/// Give each unique contributor name a particular hue that is stable between runs.
fn name_to_hue(s: &str) -> f32 {
    let mut hasher = DefaultHasher::new();
    s.hash(&mut hasher);
    hasher.finish() as f32 / u64::MAX as f32 * 360.
}