From 4e21b3b6f584eeac946357ed41cffc6553259ca8 Mon Sep 17 00:00:00 2001
From: "Dylan R. E. Moonfire" <d.moonfire@mfgames.com>
Date: Sat, 9 Jul 2022 16:59:15 -0500
Subject: [PATCH] feat: introduced SelectComponent and SelectComponentOrDefault

---
 src/Gallium/SelectComponentExtensions.cs      | 49 +++++++++++++++++
 .../SelectComponentOrDefaultExtensions.cs     | 52 +++++++++++++++++++
 2 files changed, 101 insertions(+)
 create mode 100644 src/Gallium/SelectComponentExtensions.cs
 create mode 100644 src/Gallium/SelectComponentOrDefaultExtensions.cs

diff --git a/src/Gallium/SelectComponentExtensions.cs b/src/Gallium/SelectComponentExtensions.cs
new file mode 100644
index 0000000..a6fbe19
--- /dev/null
+++ b/src/Gallium/SelectComponentExtensions.cs
@@ -0,0 +1,49 @@
+using System;
+using System.Collections.Generic;
+
+namespace Gallium;
+
+/// <summary>
+/// Extension methods for selecting components from a list.
+/// </summary>
+public static class SelectComponentExtensions
+{
+    /// <summary>
+    /// Retrieves a component from an entity and return it. If the entity does not have
+    /// the component, it will be
+    /// filtered out.
+    /// </summary>
+    /// <param name="entities">The entities to process.</param>
+    /// <typeparam name="T1">The component type being searched.</typeparam>
+    /// <returns>A sequence of T1.</returns>
+    public static IEnumerable<T1> SelectComponent<T1>(this IEnumerable<Entity> entities)
+    {
+        foreach (Entity entity in entities)
+        {
+            if (entity.TryGet(out T1 v1))
+            {
+                yield return v1;
+            }
+        }
+    }
+
+    /// <summary>
+    /// Retrieves a component from an entity and return it. If the entity does not have
+    /// the component, it will be filtered out.
+    /// </summary>
+    /// <param name="entities">The entities to process.</param>
+    /// <param name="t1">The component type being searched.</param>
+    /// <returns>A sequence of T1.</returns>
+    public static IEnumerable<object> SelectComponent(
+        IEnumerable<Entity> entities,
+        Type t1)
+    {
+        foreach (Entity entity in entities)
+        {
+            if (entity.Has(t1))
+            {
+                yield return entity.Get<object>(t1);
+            }
+        }
+    }
+}
diff --git a/src/Gallium/SelectComponentOrDefaultExtensions.cs b/src/Gallium/SelectComponentOrDefaultExtensions.cs
new file mode 100644
index 0000000..6e40b42
--- /dev/null
+++ b/src/Gallium/SelectComponentOrDefaultExtensions.cs
@@ -0,0 +1,52 @@
+using System;
+using System.Collections.Generic;
+
+namespace Gallium;
+
+/// <summary>
+/// Extension methods for selecting components from a list.
+/// </summary>
+public static class SelectComponentOrDefaultExtensions
+{
+    /// <summary>
+    /// Retrieves a component from an entity and return it. If the entity does not have
+    /// the component, then null will be returned.
+    /// </summary>
+    /// <param name="entities">The entities to process.</param>
+    /// <param name="t1">The component type being searched.</param>
+    /// <returns>A sequence of T1 or nulls.</returns>
+    public static IEnumerable<object?> SelectComponent(
+        IEnumerable<Entity> entities,
+        Type t1)
+    {
+        foreach (Entity entity in entities)
+        {
+            if (entity.Has(t1))
+            {
+                yield return entity.Get<object>(t1);
+            }
+
+            yield return null;
+        }
+    }
+
+    /// <summary>
+    /// Retrieves a component from an entity and return it. If the entity does not have
+    /// the component, then the default value will be returned.
+    /// </summary>
+    /// <param name="entities">The entities to process.</param>
+    /// <typeparam name="T1">The component type being searched.</typeparam>
+    /// <returns>A sequence of T1.</returns>
+    public static IEnumerable<T1?> SelectComponentOrDefault<T1>(this IEnumerable<Entity> entities)
+    {
+        foreach (Entity entity in entities)
+        {
+            if (entity.TryGet(out T1 v1))
+            {
+                yield return v1;
+            }
+
+            yield return default;
+        }
+    }
+}