From 8dade8c4b2ce04986a7c10ca00d59e0202d2e884 Mon Sep 17 00:00:00 2001
From: Jon Kazama <kontakt@hellion-media.de>
Date: Wed, 20 May 2026 08:57:33 +0200
Subject: [PATCH] feat(themes): add ThemeAbgrCacheLerp pure-helper for
 crossfade

Per-slot ABGR byte-lerp between two cache value-records, stack-allocated
output, t clamped. Pattern anchor: imgui.cpp ImAlphaBlendColors.
---
 HellionChat/Themes/ThemeAbgrCacheLerp.cs | 61 ++++++++++++++++++++++++
 1 file changed, 61 insertions(+)
 create mode 100644 HellionChat/Themes/ThemeAbgrCacheLerp.cs

diff --git a/HellionChat/Themes/ThemeAbgrCacheLerp.cs b/HellionChat/Themes/ThemeAbgrCacheLerp.cs
new file mode 100644
index 0000000..e2e750d
--- /dev/null
+++ b/HellionChat/Themes/ThemeAbgrCacheLerp.cs
@@ -0,0 +1,61 @@
+namespace HellionChat.Themes;
+
+// Per-slot ABGR byte-lerp between two ThemeAbgrCache value-records.
+// Pattern anchor: imgui.cpp:2820-2828 ImAlphaBlendColors -- decompose
+// each byte, lerp via Math.Round, recompose. Stack-allocated output
+// (readonly record struct), no heap pressure inside the crossfade
+// window. t is clamped to [0, 1] so float drift cannot overshoot.
+internal static class ThemeAbgrCacheLerp
+{
+    public static ThemeAbgrCache Lerp(ThemeAbgrCache from, ThemeAbgrCache to, float t)
+    {
+        t = Math.Clamp(t, 0f, 1f);
+
+        return new ThemeAbgrCache(
+            PrimaryDark: LerpAbgr(from.PrimaryDark, to.PrimaryDark, t),
+            Primary: LerpAbgr(from.Primary, to.Primary, t),
+            PrimaryLight: LerpAbgr(from.PrimaryLight, to.PrimaryLight, t),
+            PrimaryGlow: LerpAbgr(from.PrimaryGlow, to.PrimaryGlow, t),
+            AccentDark: LerpAbgr(from.AccentDark, to.AccentDark, t),
+            Accent: LerpAbgr(from.Accent, to.Accent, t),
+            AccentLight: LerpAbgr(from.AccentLight, to.AccentLight, t),
+            Identity: LerpAbgr(from.Identity, to.Identity, t),
+            WindowBg: LerpAbgr(from.WindowBg, to.WindowBg, t),
+            ChildBg: LerpAbgr(from.ChildBg, to.ChildBg, t),
+            FrameBg: LerpAbgr(from.FrameBg, to.FrameBg, t),
+            Surface: LerpAbgr(from.Surface, to.Surface, t),
+            SurfaceHover: LerpAbgr(from.SurfaceHover, to.SurfaceHover, t),
+            Border: LerpAbgr(from.Border, to.Border, t),
+            TextPrimary: LerpAbgr(from.TextPrimary, to.TextPrimary, t),
+            TextMuted: LerpAbgr(from.TextMuted, to.TextMuted, t),
+            TextDim: LerpAbgr(from.TextDim, to.TextDim, t),
+            StatusSuccess: LerpAbgr(from.StatusSuccess, to.StatusSuccess, t),
+            StatusDanger: LerpAbgr(from.StatusDanger, to.StatusDanger, t),
+            StatusWarning: LerpAbgr(from.StatusWarning, to.StatusWarning, t),
+            StatusInfo: LerpAbgr(from.StatusInfo, to.StatusInfo, t)
+        );
+    }
+
+    private static uint LerpAbgr(uint from, uint to, float t)
+    {
+        var ra = (byte)(from & 0xFFu);
+        var ga = (byte)((from >> 8) & 0xFFu);
+        var ba = (byte)((from >> 16) & 0xFFu);
+        var aa = (byte)((from >> 24) & 0xFFu);
+
+        var rb = (byte)(to & 0xFFu);
+        var gb = (byte)((to >> 8) & 0xFFu);
+        var bb = (byte)((to >> 16) & 0xFFu);
+        var ab = (byte)((to >> 24) & 0xFFu);
+
+        // Math.Round (default ToEven) matches ColourUtil.ApplyAlpha so a
+        // crossfade-into-hover transition does not produce a one-byte
+        // jump at the midpoint between the two paths.
+        var r = (byte)Math.Round(ra + (rb - ra) * t);
+        var g = (byte)Math.Round(ga + (gb - ga) * t);
+        var b = (byte)Math.Round(ba + (bb - ba) * t);
+        var a = (byte)Math.Round(aa + (ab - aa) * t);
+
+        return ((uint)a << 24) | ((uint)b << 16) | ((uint)g << 8) | r;
+    }
+}