Make hex string conversion constexpr and add test stubs

wpilibc/src/main/native/include/frc/util/Color.h

@@ -8,6 +8,7 @@
 #include <stdexcept>
 #include <string>
 #include <string_view>
+#include <type_traits>
 
 #include <fmt/core.h>
 #include <wpi/StringExtras.h>
@@ -846,7 +847,26 @@ class Color {
    *
    * @return a string of the format <tt>\#RRGGBB</tt>
    */
-  std::string HexString() const;
+  constexpr std::string HexString() const {
+    int r = static_cast<int>(255.0 * red);
+    int g = static_cast<int>(255.0 * green);
+    int b = static_cast<int>(255.0 * blue);
+
+    if (std::is_constant_evaluated()) {
+      std::string str = "#";
+
+      str += wpi::hexdigit(r / 16);
+      str += wpi::hexdigit(r % 16);
+      str += wpi::hexdigit(g / 16);
+      str += wpi::hexdigit(g % 16);
+      str += wpi::hexdigit(b / 16);
+      str += wpi::hexdigit(b % 16);
+
+      return str;
+    } else {
+      return fmt::format("#{:02X}{:02X}{:02X}", r, g, b);
+    }
+  }
 
   double red = 0.0;
   double green = 0.0;

wpilibc/src/main/native/include/frc/util/Color8Bit.h

@@ -8,6 +8,7 @@
 #include <stdexcept>
 #include <string>
 #include <string_view>
+#include <type_traits>
 
 #include <fmt/core.h>
 #include <wpi/StringExtras.h>
@@ -45,6 +46,8 @@ class Color8Bit {
         green(color.green * 255),
         blue(color.blue * 255) {}
 
+  constexpr bool operator==(const Color8Bit&) const = default;
+
   constexpr operator Color() const {  // NOLINT
     return Color(red / 255.0, green / 255.0, blue / 255.0);
   }
@@ -74,14 +77,27 @@ class Color8Bit {
     return Color8Bit{r, g, b};
   }
 
-  constexpr bool operator==(const Color8Bit&) const = default;
-
   /**
    * Return this color represented as a hex string.
    *
    * @return a string of the format <tt>\#RRGGBB</tt>
    */
-  std::string HexString() const;
+  constexpr std::string HexString() const {
+    if (std::is_constant_evaluated()) {
+      std::string str = "#";
+
+      str += wpi::hexdigit(red / 16);
+      str += wpi::hexdigit(red % 16);
+      str += wpi::hexdigit(green / 16);
+      str += wpi::hexdigit(green % 16);
+      str += wpi::hexdigit(blue / 16);
+      str += wpi::hexdigit(blue % 16);
+
+      return str;
+    } else {
+      return fmt::format("#{:02X}{:02X}{:02X}", red, green, blue);
+    }
+  }
 
   int red = 0;
   int green = 0;

wpilibc/src/test/native/cpp/util/Color8BitTest.cpp

@@ -0,0 +1,19 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include <gtest/gtest.h>
+
+#include "frc/util/Color8Bit.h"
+
+TEST(Color8BitTest, ConstructDefault) {}
+
+TEST(Color8BitTest, ConstructFromInts) {}
+
+TEST(Color8BitTest, ConstructFromColor) {}
+
+TEST(Color8BitTest, ImplicitConversionToColor) {}
+
+TEST(Color8BitTest, FromHexString) {}
+
+TEST(Color8BitTest, HexString) {}

wpilibc/src/test/native/cpp/util/ColorTest.cpp

@@ -0,0 +1,19 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include <gtest/gtest.h>
+
+#include "frc/util/Color.h"
+
+TEST(ColorTest, ConstructDefault) {}
+
+TEST(ColorTest, ConstructFromDoubles) {}
+
+TEST(ColorTest, ConstructFromInts) {}
+
+TEST(ColorTest, FromHSV) {}
+
+TEST(ColorTest, FromHexString) {}
+
+TEST(ColorTest, HexString) {}

wpilibj/src/test/java/edu/wpi/first/wpilibj/util/Color8BitTest.java

@@ -0,0 +1,24 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+package edu.wpi.first.wpilibj;
+
+import org.junit.jupiter.api.Test;
+
+class Color8BitTest {
+  @Test
+  void testConstructDefault() {}
+
+  @Test
+  void testConstructFromInts() {}
+
+  @Test
+  void testConstructFromColor() {}
+
+  @Test
+  void testFromHexString() {}
+
+  @Test
+  void testHexString() {}
+}

wpilibj/src/test/java/edu/wpi/first/wpilibj/util/ColorTest.java

@@ -0,0 +1,27 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+package edu.wpi.first.wpilibj;
+
+import org.junit.jupiter.api.Test;
+
+class ColorTest {
+  @Test
+  void testConstructDefault() {}
+
+  @Test
+  void testConstructFromDoubles() {}
+
+  @Test
+  void testConstructFromInts() {}
+
+  @Test
+  void testFromHSV() {}
+
+  @Test
+  void testFromHexString() {}
+
+  @Test
+  void testHexString() {}
+}