From 74e4abd8fdae8324dc68581abb6ca34562b10356 Mon Sep 17 00:00:00 2001
From: cosmonaut <evan@moonside.games>
Date: Sat, 5 Dec 2020 01:10:29 -0800
Subject: [PATCH] entity and ID manager

---
 build.zig              |  8 ++---
 src/entity.zig         | 10 ++-----
 src/entity_manager.zig | 60 ++++++++++++++++++++++++++++++++++++++
 src/id_manager.zig     | 66 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 133 insertions(+), 11 deletions(-)
 create mode 100644 src/entity_manager.zig
 create mode 100644 src/id_manager.zig

diff --git a/build.zig b/build.zig
index 677886c..e871dfc 100644
--- a/build.zig
+++ b/build.zig
@@ -6,13 +6,13 @@ pub fn build(b: *Builder) void {
     lib.setBuildMode(mode);
     lib.install();
 
-    var entity_test = b.addTest("src/entity.zig");
-    entity_test.setBuildMode(mode);
-
     var component_store_test = b.addTest("src/component_store.zig");
     component_store_test.setBuildMode(mode);
 
+    var entity_manager_test = b.addTest("src/entity_manager.zig");
+    entity_manager_test.setBuildMode(mode);
+
     const test_step = b.step("test", "Run library tests");
-    test_step.dependOn(&entity_test.step);
     test_step.dependOn(&component_store_test.step);
+    test_step.dependOn(&entity_manager_test.step);
 }
diff --git a/src/entity.zig b/src/entity.zig
index 6011fde..5279bbe 100644
--- a/src/entity.zig
+++ b/src/entity.zig
@@ -2,16 +2,12 @@ const std = @import("std");
 const testing = std.testing;
 
 pub const Entity = struct {
-    ID: u32,
-
-    pub fn init(id: u32) Entity {
-        return .{ .ID = id };
-    }
+    ID: usize
 };
 
 test "entity init" {
-    const e1 = Entity.init(12);
-    const e2 = Entity.init(21);
+    const e1 = Entity { .ID = 12 };
+    const e2 = Entity { .ID = 21 };
 
     testing.expect(e1.ID == 12);
     testing.expect(e2.ID == 21);
diff --git a/src/entity_manager.zig b/src/entity_manager.zig
new file mode 100644
index 0000000..dade305
--- /dev/null
+++ b/src/entity_manager.zig
@@ -0,0 +1,60 @@
+const IDManager = @import("id_manager.zig").IDManager;
+const Entity = @import("entity.zig").Entity;
+
+const std = @import("std");
+const mem = std.mem;
+const testing = std.testing;
+const Allocator = mem.Allocator;
+
+pub const EntityManager = struct {
+    const Self = @This();
+
+    allocator: *Allocator,
+    idManager: IDManager,
+    ids: std.AutoHashMap(usize, bool),
+
+    pub fn init(allocator: *Allocator) Self {
+        return Self {
+            .allocator = allocator,
+            .idManager = IDManager.init(allocator),
+            .ids = std.AutoHashMap(usize, bool).init(allocator)
+        };
+    }
+
+    pub fn create(self: *Self) !Entity {
+        const id = self.idManager.next();
+        var entity = Entity { .ID = id };
+        self.ids.put(id, true) catch |err| return err;
+        return entity;
+    }
+
+    pub fn exists(self: Self, entityID: usize) bool {
+        return self.ids.contains(entityID);
+    }
+};
+
+test "create" {
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+
+    const allocator = &arena.allocator;
+
+    var entity_manager = EntityManager.init(allocator);
+
+    var entity = try entity_manager.create();
+
+    testing.expect(entity.ID == 0);
+}
+
+test "exists" {
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+
+    const allocator = &arena.allocator;
+
+    var entity_manager = EntityManager.init(allocator);
+
+    var entity = try entity_manager.create();
+
+    testing.expect(entity_manager.exists(0));
+}
diff --git a/src/id_manager.zig b/src/id_manager.zig
new file mode 100644
index 0000000..0c7c160
--- /dev/null
+++ b/src/id_manager.zig
@@ -0,0 +1,66 @@
+const std = @import("std");
+const testing = std.testing;
+const mem = std.mem;
+const Allocator = mem.Allocator;
+
+pub const IDManager = struct {
+    const Self = @This();
+
+    allocator: *Allocator,
+    nextID: usize = 0,
+    availableIDs: std.ArrayList(usize),
+
+    pub fn init(allocator: *Allocator) Self {
+        return Self {
+            .allocator = allocator,
+            .availableIDs = std.ArrayList(usize).init(allocator),
+        };
+    }
+
+    pub fn next(self: *Self) usize {
+        if (self.availableIDs.items.len > 0)
+        {
+            return self.availableIDs.pop();
+        }
+        else
+        {
+            var result = self.nextID;
+            self.nextID += 1;
+            return result;
+        }
+    }
+
+    pub fn free(self: *Self, id: usize) !void {
+        self.availableIDs.append(id) catch |err| return err;
+    }
+};
+
+test "next" {
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+
+    const allocator = &arena.allocator;
+
+    var id_manager = IDManager.init(allocator);
+
+    testing.expect(id_manager.next() == 0);
+    testing.expect(id_manager.next() == 1);
+}
+
+test "free" {
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+
+    const allocator = &arena.allocator;
+
+    var id_manager = IDManager.init(allocator);
+
+    var id_zero = id_manager.next();
+    var id_one = id_manager.next();
+    var id_two = id_manager.next();
+
+    try id_manager.free(id_one);
+
+    testing.expect(id_manager.next() == id_one);
+    testing.expect(id_manager.next() == 3);
+}