settled-reach/server/src/simulation/ticker.rs

//! News ticker pool for The Last Shift bar (#591).
//!
//! Holds the loaded headline pool and tracks which headline is currently
//! displayed. Rotates every `TICKER_ROTATION_TICKS` ticks using `SimRng`
//! for determinism (D-010 principle 4).
//!
//! Zone detection: headlines are only emitted when the player is in
//! `LAST_SHIFT_ZONE_ID` (The Last Shift bar tile region).

use bevy_ecs::prelude::*;
use rand::Rng;

use crate::bridge::types::TickerLine;

/// Zone ID assigned to The Last Shift bar tiles (#591, D-077).
///
/// Must match the zone_id used in the production location YAML
/// when the transit district ZoneMap is populated.
pub const LAST_SHIFT_ZONE_ID: u16 = 1;

/// How often the displayed headline rotates, in ticks.
/// 200 ticks = 20 game-minutes at 10 ticks/game-minute (D-031).
pub const TICKER_ROTATION_TICKS: u64 = 200;

/// Resource: loaded ticker pool for the news feed (#591).
///
/// Built at startup from `ticker/the-last-shift.yaml`.
/// Holds all headlines and tracks the current display index.
/// Rotation is deterministic — always use `SimRng`, never system randomness.
#[derive(Resource, Debug, Default)]
pub struct TickerPool {
    lines: Vec<TickerLine>,
    current_index: usize,
    last_rotated_tick: u64,
}

impl TickerPool {
    /// Build a pool from the given headline list.
    pub fn from_lines(lines: Vec<TickerLine>) -> Self {
        Self {
            lines,
            current_index: 0,
            last_rotated_tick: 0,
        }
    }

    /// True if the pool has at least one headline.
    pub fn is_empty(&self) -> bool {
        self.lines.is_empty()
    }

    /// The currently active headline, or `None` if the pool is empty.
    pub fn current_line(&self) -> Option<&TickerLine> {
        self.lines.get(self.current_index)
    }

    /// Advance to a new headline using SimRng if `TICKER_ROTATION_TICKS` have elapsed.
    ///
    /// Called each tick by `tick_news_ticker`. Deterministic: same seed → same rotation.
    pub fn maybe_rotate(&mut self, current_tick: u64, rng: &mut crate::simulation::rng::SimRng) {
        if self.lines.is_empty() {
            return;
        }
        if current_tick > 0
            && current_tick.saturating_sub(self.last_rotated_tick) >= TICKER_ROTATION_TICKS
        {
            self.current_index = rng.rng.random_range(0..self.lines.len());
            self.last_rotated_tick = current_tick;
        }
    }
}

/// System: advance ticker rotation each tick (#591, D-010).
///
/// Runs before `compute_observer_snapshot` so the correct headline
/// is available when the snapshot is assembled.
pub fn tick_news_ticker(
    time: Res<crate::simulation::time::SimulationTime>,
    mut pool: ResMut<TickerPool>,
    mut rng: ResMut<crate::simulation::rng::SimRng>,
) {
    pool.maybe_rotate(time.tick, &mut rng);
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::simulation::rng::SimRng;

    fn make_pool(count: usize) -> TickerPool {
        let lines: Vec<TickerLine> = (0..count)
            .map(|i| TickerLine {
                id: format!("ticker_{:03}", i),
                text: format!("Headline {}", i),
                category: "test".to_string(),
            })
            .collect();
        TickerPool::from_lines(lines)
    }

    #[test]
    fn empty_pool_returns_none() {
        let pool = TickerPool::default();
        assert!(pool.current_line().is_none());
    }

    #[test]
    fn non_empty_pool_returns_first_line() {
        let pool = make_pool(5);
        assert_eq!(pool.current_line().unwrap().id, "ticker_000");
    }

    #[test]
    fn rotation_advances_after_interval() {
        let mut pool = make_pool(30);
        let mut rng = SimRng::new(42);

        // No rotation at tick 0
        pool.maybe_rotate(0, &mut rng);
        assert_eq!(pool.current_index, 0);

        // Rotation fires at tick TICKER_ROTATION_TICKS
        pool.maybe_rotate(TICKER_ROTATION_TICKS, &mut rng);
        // index changed (RNG picks something; just verify it's in range)
        assert!(pool.current_index < 30);
    }

    #[test]
    fn rotation_is_deterministic() {
        let mut pool_a = make_pool(30);
        let mut rng_a = SimRng::new(42);
        pool_a.maybe_rotate(TICKER_ROTATION_TICKS, &mut rng_a);
        let idx_a = pool_a.current_index;

        let mut pool_b = make_pool(30);
        let mut rng_b = SimRng::new(42);
        pool_b.maybe_rotate(TICKER_ROTATION_TICKS, &mut rng_b);
        let idx_b = pool_b.current_index;

        assert_eq!(idx_a, idx_b, "Same seed must produce same rotation");
    }

    #[test]
    fn no_double_rotation_in_same_window() {
        let mut pool = make_pool(30);
        let mut rng = SimRng::new(42);

        pool.maybe_rotate(TICKER_ROTATION_TICKS, &mut rng);
        let idx_after_first = pool.current_index;

        // One tick later — within same window, should not rotate
        pool.maybe_rotate(TICKER_ROTATION_TICKS + 1, &mut rng);
        assert_eq!(
            pool.current_index, idx_after_first,
            "Should not rotate within the same window"
        );
    }
}