refactor: move graph traversal code into traversal.rs

src/entity/traversal.rs

@@ -0,0 +1,205 @@
+use super::direction::Direction;
+use super::graph::{Edge, Graph, NodeId};
+
+/// Represents the current position of an entity traversing the graph.
+///
+/// This enum allows for precise tracking of whether an entity is exactly at a node
+/// or moving along an edge between two nodes.
+#[derive(Debug, PartialEq, Clone, Copy)]
+pub enum Position {
+    /// The traverser is located exactly at a node.
+    AtNode(NodeId),
+    /// The traverser is on an edge between two nodes.
+    BetweenNodes {
+        from: NodeId,
+        to: NodeId,
+        /// The floating-point distance traversed along the edge from the `from` node.
+        traversed: f32,
+    },
+}
+
+#[allow(dead_code)]
+impl Position {
+    /// Returns `true` if the position is exactly at a node.
+    pub fn is_at_node(&self) -> bool {
+        matches!(self, Position::AtNode(_))
+    }
+
+    /// Returns the `NodeId` of the current or most recently departed node.
+    #[allow(clippy::wrong_self_convention)]
+    pub fn from_node_id(&self) -> NodeId {
+        match self {
+            Position::AtNode(id) => *id,
+            Position::BetweenNodes { from, .. } => *from,
+        }
+    }
+
+    /// Returns the `NodeId` of the destination node, if currently on an edge.
+    #[allow(clippy::wrong_self_convention)]
+    pub fn to_node_id(&self) -> Option<NodeId> {
+        match self {
+            Position::AtNode(_) => None,
+            Position::BetweenNodes { to, .. } => Some(*to),
+        }
+    }
+
+    /// Returns `true` if the traverser is stopped at a node.
+    pub fn is_stopped(&self) -> bool {
+        matches!(self, Position::AtNode(_))
+    }
+}
+
+/// Manages an entity's movement through the graph.
+///
+/// A `Traverser` encapsulates the state of an entity's position and direction,
+/// providing a way to advance along the graph's paths based on a given distance.
+/// It also handles direction changes, buffering the next intended direction.
+pub struct Traverser {
+    /// The current position of the traverser in the graph.
+    pub position: Position,
+    /// The current direction of movement.
+    pub direction: Direction,
+    /// Buffered direction change with remaining frame count for timing.
+    ///
+    /// The `u8` value represents the number of frames remaining before
+    /// the buffered direction expires. This allows for responsive controls
+    /// by storing direction changes for a limited time.
+    pub next_direction: Option<(Direction, u8)>,
+}
+
+impl Traverser {
+    /// Creates a new traverser starting at the given node ID.
+    ///
+    /// The traverser will immediately attempt to start moving in the initial direction.
+    pub fn new<F>(graph: &Graph, start_node: NodeId, initial_direction: Direction, can_traverse: &F) -> Self
+    where
+        F: Fn(Edge) -> bool,
+    {
+        let mut traverser = Traverser {
+            position: Position::AtNode(start_node),
+            direction: initial_direction,
+            next_direction: Some((initial_direction, 1)),
+        };
+
+        // This will kickstart the traverser into motion
+        traverser.advance(graph, 0.0, can_traverse);
+
+        traverser
+    }
+
+    /// Sets the next direction for the traverser to take.
+    ///
+    /// The direction is buffered and will be applied at the next opportunity,
+    /// typically when the traverser reaches a new node. This allows for responsive
+    /// controls, as the new direction is stored for a limited time.
+    pub fn set_next_direction(&mut self, new_direction: Direction) {
+        if self.direction != new_direction {
+            self.next_direction = Some((new_direction, 30));
+        }
+    }
+
+    /// Advances the traverser along the graph by a specified distance.
+    ///
+    /// This method updates the traverser's position based on its current state
+    /// and the distance to travel.
+    ///
+    /// - If at a node, it checks for a buffered direction to start moving.
+    /// - If between nodes, it moves along the current edge.
+    /// - If it reaches a node, it attempts to transition to a new edge based on
+    ///   the buffered direction or by continuing straight.
+    /// - If no valid move is possible, it stops at the node.
+    pub fn advance<F>(&mut self, graph: &Graph, distance: f32, can_traverse: &F)
+    where
+        F: Fn(Edge) -> bool,
+    {
+        // Decrement the remaining frames for the next direction
+        if let Some((direction, remaining)) = self.next_direction {
+            if remaining > 0 {
+                self.next_direction = Some((direction, remaining - 1));
+            } else {
+                self.next_direction = None;
+            }
+        }
+
+        match self.position {
+            Position::AtNode(node_id) => {
+                // We're not moving, but a buffered direction is available.
+                if let Some((next_direction, _)) = self.next_direction {
+                    if let Some(edge) = graph.find_edge_in_direction(node_id, next_direction) {
+                        if can_traverse(edge) {
+                            // Start moving in that direction
+                            self.position = Position::BetweenNodes {
+                                from: node_id,
+                                to: edge.target,
+                                traversed: distance.max(0.0),
+                            };
+                            self.direction = next_direction;
+                        }
+                    }
+
+                    self.next_direction = None; // Consume the buffered direction regardless of whether we started moving with it
+                }
+            }
+            Position::BetweenNodes { from, to, traversed } => {
+                // There is no point in any of the next logic if we don't travel at all
+                if distance <= 0.0 {
+                    return;
+                }
+
+                let edge = graph
+                    .find_edge(from, to)
+                    .expect("Inconsistent state: Traverser is on a non-existent edge.");
+
+                let new_traversed = traversed + distance;
+
+                if new_traversed < edge.distance {
+                    // Still on the same edge, just update the distance.
+                    self.position = Position::BetweenNodes {
+                        from,
+                        to,
+                        traversed: new_traversed,
+                    };
+                } else {
+                    let overflow = new_traversed - edge.distance;
+                    let mut moved = false;
+
+                    // If we buffered a direction, try to find an edge in that direction
+                    if let Some((next_dir, _)) = self.next_direction {
+                        if let Some(edge) = graph.find_edge_in_direction(to, next_dir) {
+                            if can_traverse(edge) {
+                                self.position = Position::BetweenNodes {
+                                    from: to,
+                                    to: edge.target,
+                                    traversed: overflow,
+                                };
+
+                                self.direction = next_dir; // Remember our new direction
+                                self.next_direction = None; // Consume the buffered direction
+                                moved = true;
+                            }
+                        }
+                    }
+
+                    // If we didn't move, try to continue in the current direction
+                    if !moved {
+                        if let Some(edge) = graph.find_edge_in_direction(to, self.direction) {
+                            if can_traverse(edge) {
+                                self.position = Position::BetweenNodes {
+                                    from: to,
+                                    to: edge.target,
+                                    traversed: overflow,
+                                };
+                            } else {
+                                self.position = Position::AtNode(to);
+                                self.next_direction = None;
+                            }
+                        } else {
+                            self.position = Position::AtNode(to);
+                            self.next_direction = None;
+                        }
+                    }
+                }
+            }
+        }
+    }
+}