From a2786e6266ad9bd36d2c9bef871a901292c014fb Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Feb 2023 21:14:20 -0500
Subject: [PATCH] Invest Colour with its own logic, including potential
 emptiness.

---
 Components/9918/Implementation/9918.cpp       | 12 +------
 Components/9918/Implementation/9918Base.hpp   |  8 ++---
 .../9918/Implementation/YamahaCommands.hpp    | 31 +++++++++++++++----
 3 files changed, 30 insertions(+), 21 deletions(-)

diff --git a/Components/9918/Implementation/9918.cpp b/Components/9918/Implementation/9918.cpp
index 80c67c833..1eb761350 100644
--- a/Components/9918/Implementation/9918.cpp
+++ b/Components/9918/Implementation/9918.cpp
@@ -829,17 +829,7 @@ void Base<personality>::commit_register(int reg, uint8_t value) {
 			case 43:	Storage<personality>::command_context_.size.template set<1, true>(value);			break;
 
 			case 44:
-				Storage<personality>::command_context_.colour = value;
-				Storage<personality>::command_context_.colour4bpp = uint8_t(
-					(value & 0xf) |
-					(value << 4)
-				);
-				Storage<personality>::command_context_.colour2bpp = uint8_t(
-					(value & 0x3) |
-					((value & 0x3) << 2) |
-					((value & 0x3) << 4) |
-					((value & 0x3) << 6)
-				);
+				Storage<personality>::command_context_.colour.set(value);
 
 				// Check whether a command was blocked on this.
 				if(
diff --git a/Components/9918/Implementation/9918Base.hpp b/Components/9918/Implementation/9918Base.hpp
index 663a3cd83..ba6071335 100644
--- a/Components/9918/Implementation/9918Base.hpp
+++ b/Components/9918/Implementation/9918Base.hpp
@@ -683,21 +683,21 @@ template <Personality personality> struct Base: public Storage<personality> {
 					return
 						std::make_pair(
 							0xf0 >> ((location.v[0] & 1) << 2),
-							Storage<personality>::command_context_.colour4bpp
+							Storage<personality>::command_context_.colour.colour4bpp
 						);
 
 				case ScreenMode::YamahaGraphics5:	// 512 pixels @ 2bpp
 					return
 						std::make_pair(
 							0xc0 >> ((location.v[0] & 3) << 1),
-							Storage<personality>::command_context_.colour2bpp
+							Storage<personality>::command_context_.colour.colour2bpp
 						);
 
 				case ScreenMode::YamahaGraphics7:	// 256 pixels @ 8bpp
 					return
 						std::make_pair(
 							0xff,
-							Storage<personality>::command_context_.colour
+							Storage<personality>::command_context_.colour.colour
 						);
 			}
 		} else {
@@ -786,7 +786,7 @@ template <Personality personality> struct Base: public Storage<personality> {
 					break;
 
 					case CommandStep::WriteByte:
-						ram_[command_address(context.destination)] = context.colour;
+						ram_[command_address(context.destination)] = context.colour.colour;
 						Storage<personality>::command_->advance(pixels_per_byte(this->underlying_mode_));
 						Storage<personality>::update_command_step(access_column);
 					break;
diff --git a/Components/9918/Implementation/YamahaCommands.hpp b/Components/9918/Implementation/YamahaCommands.hpp
index 9d844698e..415338b0b 100644
--- a/Components/9918/Implementation/YamahaCommands.hpp
+++ b/Components/9918/Implementation/YamahaCommands.hpp
@@ -42,18 +42,37 @@ struct Vector {
 	}
 };
 
+struct Colour {
+	void set(uint8_t value) {
+		colour = value;
+		colour4bpp = uint8_t((value & 0xf) | (value << 4));
+		colour2bpp = uint8_t((colour4bpp & 0x33) | ((colour4bpp & 0x33) << 2));
+	}
+
+	void reset() {
+		colour = 0x00;
+		colour4bpp = 0xff;
+	}
+
+	bool has_value() {
+		return (colour & 0x3) == (colour4bpp & 0x3);
+	}
+
+	/// Colour as written by the CPU.
+	uint8_t colour = 0x00;
+	/// The low four bits of the CPU-written colour, repeated twice.
+	uint8_t colour4bpp = 0xff;
+	/// The low two bits of the CPU-written colour, repeated four times.
+	uint8_t colour2bpp = 0xff;
+};
+
 struct CommandContext {
 	Vector source;
 	Vector destination;
 	Vector size;
 
 	uint8_t arguments = 0;
-	/// Colour as written by the CPU.
-	uint8_t colour = 0;
-	/// The low four bits of the CPU-written colour, repeated twice.
-	uint8_t colour4bpp = 0;
-	/// The low two bits of the CPU-written colour, repeated four times.
-	uint8_t colour2bpp = 0;
+	Colour colour;
 
 	enum class LogicalOperation {
 		Copy			= 0b0000,