Add spawn points (#71)

Reviewed-on: #71
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/stage/manifest.rs

@@ -1,4 +1,4 @@
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet};
 
 use bevy_asset_loader::asset_collection::AssetCollection;
 use ron_asset_manager::prelude::RonAsset;
@@ -88,6 +88,10 @@ pub struct StageManifest {
     pub blast_zone_top_left: FGVec2,
     #[reflect(ignore)]
     pub blast_zone_bottom_right: FGVec2,
+
+    #[reflect(ignore)]
+    pub spawn_points: Vec<FGVec2>,
+    pub neutral_spawn_point_sets: Vec<Vec<usize>>
 }
 
 #[derive(Resource)]
@@ -163,6 +167,13 @@ pub fn prepare_stage_manifests(
             );
             return;
         }
+        if let Err(error) = manifest.validate_spawn_points() {
+            fail(
+                &mut next_state,
+                &format!("{:?} has invalid spawn points: {error}", manifest.id),
+            );
+            return;
+        }
         if registry.insert(manifest.id, handle.clone()).is_some() {
             fail(
                 &mut next_state,
@@ -187,6 +198,69 @@ pub fn prepare_stage_manifests(
 }
 
 impl StageManifest {
+    pub fn neutral_spawn_positions(&self, player_count: usize) -> Result<Vec<FGVec2>, String> {
+        if player_count == 0 {
+            return Err("a match must contain at least one player".to_owned());
+        }
+
+        let layout = self
+            .neutral_spawn_point_sets
+            .get(player_count - 1)
+            .ok_or_else(|| format!("no spawn layout for {player_count} players"))?;
+
+        if layout.len() != player_count {
+            return Err(format!(
+                "the {player_count}-player layout contains {} positions",
+                layout.len()
+            ));
+        }
+
+        layout
+            .iter()
+            .map(|&index| {
+                self.spawn_points.get(index).copied().ok_or_else(|| {
+                    format!("the {player_count}-player layout references spawn point {index}")
+                })
+            })
+            .collect()
+    }
+
+    fn validate_spawn_points(&self) -> Result<(), String> {
+        if self.spawn_points.is_empty() {
+            return Err("no spawn points are configured".to_owned());
+        }
+
+        if self.neutral_spawn_point_sets.is_empty() {
+            return Err("no neutral spawn layouts are configured".to_owned());
+        }
+
+        for (layout_index, layout) in self.neutral_spawn_point_sets.iter().enumerate() {
+            let player_count = layout_index + 1;
+            if layout.len() != player_count {
+                return Err(format!(
+                    "the {player_count}-player layout contains {} positions",
+                    layout.len()
+                ));
+            }
+
+            let mut unique_indices = HashSet::with_capacity(layout.len());
+            for &spawn_point_index in layout {
+                if spawn_point_index >= self.spawn_points.len() {
+                    return Err(format!(
+                        "the {player_count}-player layout references spawn point {spawn_point_index}"
+                    ));
+                }
+                if !unique_indices.insert(spawn_point_index) {
+                    return Err(format!(
+                        "the {player_count}-player layout uses spawn point {spawn_point_index} more than once"
+                    ));
+                }
+            }
+        }
+
+        Ok(())
+    }
+
     pub fn to_collision(&self) -> StageCollision {
         let stage_polys: Vec<StagePoly> = self
             .polygons