elevator_core/dispatch/
scan.rs

1//! SCAN (elevator) dispatch algorithm — sweeps end-to-end before reversing.
2//!
3//! Originally described for disk-arm scheduling in Denning, P. J. (1967),
4//! "Effects of Scheduling on File Memory Operations", *Proc. AFIPS Spring
5//! Joint Computer Conference*, 9–21. The same sweep discipline is the
6//! textbook "elevator" algorithm.
7
8use std::collections::HashMap;
9
10use crate::entity::EntityId;
11use crate::world::World;
12
13use super::sweep::{self, SweepDirection, SweepMode};
14use super::{DispatchManifest, DispatchStrategy, ElevatorGroup, RankContext, pair_can_do_work};
15
16/// Elevator dispatch using the SCAN (elevator) algorithm.
17///
18/// Each car tracks a sweep direction; stops demanded in that direction
19/// receive a positional cost while stops behind are excluded. When
20/// nothing remains ahead, the sweep reverses and the car considers the
21/// reverse side. Direction and mode are resolved once per pass in
22/// [`DispatchStrategy::prepare_car`] so ranking is independent of the
23/// iteration order over stops.
24#[derive(serde::Serialize, serde::Deserialize)]
25pub struct ScanDispatch {
26    /// Per-elevator sweep direction. Persisted across dispatch passes
27    /// (reversed once a sweep exhausts demand ahead) and round-tripped
28    /// through [`DispatchStrategy::snapshot_config`] so a restored sim
29    /// continues the current sweep instead of defaulting to `Up` for
30    /// every car.
31    direction: HashMap<EntityId, SweepDirection>,
32    /// Per-elevator accept mode for the current dispatch pass.
33    /// Overwritten in full by `prepare_car` every pass, so no round-
34    /// trip is needed; `#[serde(skip)]` keeps snapshot bytes compact
35    /// and deterministic across process runs.
36    #[serde(skip)]
37    mode: HashMap<EntityId, SweepMode>,
38}
39
40impl ScanDispatch {
41    /// Create a new `ScanDispatch` with no initial direction state.
42    #[must_use]
43    pub fn new() -> Self {
44        Self {
45            direction: HashMap::new(),
46            mode: HashMap::new(),
47        }
48    }
49
50    /// Sweep direction for `car`, defaulting to `Up` for first-time callers.
51    fn direction_for(&self, car: EntityId) -> SweepDirection {
52        self.direction
53            .get(&car)
54            .copied()
55            .unwrap_or(SweepDirection::Up)
56    }
57
58    /// Accept mode for `car` in the current pass, defaulting to `Strict`.
59    fn mode_for(&self, car: EntityId) -> SweepMode {
60        self.mode.get(&car).copied().unwrap_or(SweepMode::Strict)
61    }
62}
63
64impl Default for ScanDispatch {
65    fn default() -> Self {
66        Self::new()
67    }
68}
69
70impl DispatchStrategy for ScanDispatch {
71    fn prepare_car(
72        &mut self,
73        car: EntityId,
74        car_position: f64,
75        group: &ElevatorGroup,
76        manifest: &DispatchManifest,
77        world: &World,
78    ) {
79        let current = self.direction_for(car);
80        if sweep::strict_demand_ahead(current, car_position, group, manifest, world) {
81            self.mode.insert(car, SweepMode::Strict);
82        } else {
83            self.direction.insert(car, current.reversed());
84            self.mode.insert(car, SweepMode::Lenient);
85        }
86    }
87
88    fn rank(&mut self, ctx: &RankContext<'_>) -> Option<f64> {
89        // Reject un-servable pairs so a full car with only
90        // over-capacity waiting demand at its own stop can't open/close
91        // doors indefinitely. SCAN's direction reversal normally lifts
92        // it out of the self-stop within one tick (strict-ahead excludes
93        // the current position), but the Lenient transition tick would
94        // still rank the self-pair at cost 0 without this guard.
95        if !pair_can_do_work(ctx) {
96            return None;
97        }
98        sweep::rank(
99            self.mode_for(ctx.car),
100            self.direction_for(ctx.car),
101            ctx.car_position,
102            ctx.stop_position,
103        )
104    }
105
106    fn notify_removed(&mut self, elevator: EntityId) {
107        self.direction.remove(&elevator);
108        self.mode.remove(&elevator);
109    }
110
111    fn builtin_id(&self) -> Option<super::BuiltinStrategy> {
112        Some(super::BuiltinStrategy::Scan)
113    }
114
115    fn snapshot_config(&self) -> Option<String> {
116        ron::to_string(self).ok()
117    }
118
119    fn restore_config(&mut self, serialized: &str) -> Result<(), String> {
120        let restored: Self = ron::from_str(serialized).map_err(|e| e.to_string())?;
121        *self = restored;
122        Ok(())
123    }
124}
elevator_core/dispatch/scan.rs

elevator_core/dispatch/
scan.rs
