the ECS is now using a more data oriented approach.

By defining archetypes using SOAs (Zig has this data structure in `std`,
called std.MultiArrayList), the engine can iterate faster over commonly
defined entities avoiding cache misses since each component is aligned
with other components of the same entity.

4 files changed, 34 insertions, 107 deletions

diff --git a/src/ecs/components.zig b/src/ecs/components.zig
index 5c35cbf..1309e70 100644
--- a/src/ecs/components.zig
+++ b/src/ecs/components.zig
@@ -1,8 +1,6 @@
 const std = @import("std");
 const Allocator = std.mem.Allocator;
 
-const COMPONENT_NUMBER = 2;
-
 pub const Position = packed struct {
     x: f32,
     y: f32,
@@ -13,6 +11,4 @@ pub const Position = packed struct {
 
 pub const Speed = packed struct {
     speed: f32,
-
-    pub const id: usize = 1;
 };
diff --git a/src/ecs/ecs.zig b/src/ecs/ecs.zig
index 9efc044..e389223 100644
--- a/src/ecs/ecs.zig
+++ b/src/ecs/ecs.zig
@@ -1,5 +1,5 @@
 pub const components = @import("components.zig");
-const entities = @import("entities.zig");
+pub const entities = @import("entities.zig");
 
 pub const Pool = entities.Pool;
 pub const Resources = entities.Resources;
diff --git a/src/ecs/entities.zig b/src/ecs/entities.zig
index 9337d6b..0f1c58d 100644
--- a/src/ecs/entities.zig
+++ b/src/ecs/entities.zig
@@ -12,32 +12,29 @@ pub const Resources = struct {
     renderer: Renderer,
 };
 
+pub const Human = struct {
+    position: components.Position,
+    speed: components.Speed,
+};
+
 pub const Pool = struct {
-    // Components
-    position: sparse.SparseSet(components.Position),
-    speed: sparse.SparseSet(components.Speed),
+    humans: std.MultiArrayList(Human),
     resources: Resources,
+    allocator: Allocator,
     system_groups: std.ArrayList(SystemGroup),
     thread_pool: *std.Thread.Pool,
     wait_group: std.Thread.WaitGroup,
     mutex: std.Thread.Mutex,
-    last_entity: usize,
-    free_ids: std.ArrayList(usize),
-
-    component_flags: std.AutoHashMap(usize, usize),
 
     pub fn init(allocator: Allocator, resources: Resources) !@This() {
         var pool = @This(){
-            .position = sparse.SparseSet(components.Position).init(allocator),
-            .speed = sparse.SparseSet(components.Speed).init(allocator),
+            .humans = .{},
             .resources = resources,
             .system_groups = std.ArrayList(SystemGroup).init(allocator),
             .thread_pool = try allocator.create(std.Thread.Pool),
             .wait_group = .{},
             .mutex = .{},
-            .last_entity = 0,
-            .free_ids = std.ArrayList(usize).init(allocator),
-            .component_flags = std.AutoHashMap(usize, usize).init(allocator),
+            .allocator = allocator,
         };
 
         try pool.thread_pool.init(.{
@@ -48,63 +45,16 @@ pub const Pool = struct {
         return pool;
     }
 
-    pub fn getQuery(self: *@This(), comptime T: type) []T {
-        const set = switch (T) {
-            components.Speed => &self.speed,
-            components.Position => &self.position,
-            else => unreachable,
-        };
-
-        return set.components.items;
-    }
-
-    pub fn getEntity(self: *@This(), component: usize, comptime T: type) usize {
-        const set = switch (T) {
-            components.Speed => &self.speed,
-            components.Position => &self.position,
-            else => unreachable,
-        };
-
-        return set.dense.items[component];
-    }
-
-    pub fn getComponent(self: *@This(), entity: usize, comptime T: type) ?T {
-        const set = switch (T) {
-            components.Speed => &self.speed,
-            components.Position => &self.position,
-            else => unreachable,
-        };
-
-        if (self.hasComponent(entity, T)) {
-            return set.components.items[set.sparse.items[entity]];
-        } else {
-            return null;
-        }
-    }
-
-    pub fn hasComponent(self: *@This(), entity: usize, component: type) bool {
-        const set = switch (component) {
-            components.Speed => &self.speed,
-            components.Position => &self.position,
-            else => unreachable,
-        };
-
-        return set.dense.items[set.sparse.items[entity]] == entity;
-    }
-
     pub fn addSystemGroup(self: *@This(), group: SystemGroup) !void {
         try self.system_groups.append(group);
     }
 
-    pub fn deinit(self: *@This(), allocator: Allocator) void {
-        self.position.deinit();
-        self.speed.deinit();
+    pub fn deinit(self: *@This()) void {
+        self.humans.deinit(self.allocator);
 
         self.system_groups.deinit();
         self.thread_pool.deinit();
-        allocator.destroy(self.thread_pool);
-        self.free_ids.deinit();
-        self.component_flags.deinit();
+        self.allocator.destroy(self.thread_pool);
     }
 
     pub fn tick(self: *@This()) void {
@@ -120,43 +70,21 @@ pub const Pool = struct {
         }
     }
 
-    pub fn createEntity(self: *@This()) !usize {
-        const id = self.free_ids.pop() orelse self.last_entity;
-        self.last_entity += 1;
-        try self.component_flags.put(id, 0x0);
-
-        return id;
-    }
-
-    pub fn destroyEntity(self: *@This(), entity: usize) void {
-        self.free_ids.append(entity);
-
-        const flags = self.component_flags.get(entity);
-        for (0..components.COMPONENT_NUMBER) |i| {
-            if (((flags >> i) & 0x1) != 0x0) {
-                self.removeComponent(entity, i);
-            }
-        }
-    }
-
-    pub fn addComponent(self: *@This(), entity: usize, component: anytype) !void {
-        var set = switch (@TypeOf(component)) {
-            components.Speed => &self.speed,
-            components.Position => &self.position,
+    fn getEntities(self: *@This(), T: type) *std.MultiArrayList(T) {
+        return switch (T) {
+            Human => &self.humans,
             else => unreachable,
         };
-
-        try self.component_flags.put(entity, self.component_flags.get(entity).? | (0x1 << @TypeOf(component).id));
-        try set.addEntity(entity, component);
     }
 
-    pub fn removeComponent(self: *@This(), entity: usize, component_id: usize) void {
-        const set = switch (component_id) {
-            components.Speed.id => self.speed,
-            components.Position.id => self.position,
-        };
+    pub fn createEntity(self: *@This(), entity: anytype) !usize {
+        var list = self.getEntities(@TypeOf(entity));
+        const index = list.len;
+        try list.append(self.allocator, entity);
+        return index;
+    }
 
-        self.component_flags.put(entity, self.component_flags.get(entity) & ~(0x1 << component_id));
-        set.removeEntity(entity);
+    pub fn destroyEntity(self: *@This(), comptime T: type, entity: usize) void {
+        self.getEntities(T).swapRemove(entity);
     }
 };
diff --git a/src/main.zig b/src/main.zig
index 4bbe084..5bcff43 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -1,5 +1,4 @@
 const std = @import("std");
-
 const config = @import("config");
 const math = @import("sideros").math;
 const mods = @import("mods");
@@ -7,11 +6,10 @@ const ecs = @import("ecs/ecs.zig");
 pub const Renderer = @import("renderer");
 
 fn testSystem2(pool: *ecs.Pool) void {
-    for (pool.getQuery(ecs.components.Position), 0..) |position, i| {
-        const entity = pool.getEntity(i, ecs.components.Position);
-        if (pool.getComponent(entity, ecs.components.Speed)) |speed| {
-            std.debug.print("entity{d}: {any},{any},{any} {any}\n", .{ i, position.x, position.y, position.z, speed.speed });
-        }
+    const slice = pool.humans.slice();
+
+    for (slice.items(.position), slice.items(.speed)) |position, speed| {
+        std.debug.print("entity: {any} {any} {any}: {any}\n", .{ position.x, position.y, position.z, speed.speed });
     }
 }
 
@@ -53,7 +51,12 @@ pub fn main() !void {
     };
 
     var pool = try ecs.Pool.init(allocator, resources);
-    defer pool.deinit(allocator);
+    defer pool.deinit();
+
+    _ = try pool.createEntity(ecs.entities.Human{
+        .position = .{ .x = 0.0, .y = 1.0, .z = 0.0 },
+        .speed = .{ .speed = 5.0 },
+    });
 
     try pool.addSystemGroup(&[_]ecs.System{
         testSystem2,