Add support for short hex color codes like #CCC (#2658)

This adds a few lines to support shorthand color hex codes like #ABC. They are treated as equivalent of #AABBCC.

Fixes #2639.

src/cascadia/ut_app/JsonTests.cpp

@@ -79,7 +79,7 @@ namespace TerminalAppUnitTests
                                           "\"name\" : \"Campbell\","
                                           "\"purple\" : \"#881798\","
                                           "\"red\" : \"#C50F1F\","
-                                          "\"white\" : \"#CCCCCC\","
+                                          "\"white\" : \"#CCC\","
                                           "\"yellow\" : \"#C19C00\""
                                           "}" };
 

src/types/utils.cpp

@@ -83,20 +83,33 @@ std::string Utils::ColorToHexString(const COLORREF color)
 }
 
 // Function Description:
-// - Parses a color from a string. The string should be in the format "#RRGGBB"
+// - Parses a color from a string. The string should be in the format "#RRGGBB" or "#RGB"
 // Arguments:
 // - str: a string representation of the COLORREF to parse
 // Return Value:
 // - A COLORREF if the string could successfully be parsed. If the string is not
 //      the correct format, throws E_INVALIDARG
 COLORREF Utils::ColorFromHexString(const std::string str)
 {
-    THROW_HR_IF(E_INVALIDARG, str.size() < 7 || str.size() >= 8);
+    THROW_HR_IF(E_INVALIDARG, str.size() != 7 && str.size() != 4);
     THROW_HR_IF(E_INVALIDARG, str[0] != '#');
 
-    std::string rStr{ &str[1], 2 };
-    std::string gStr{ &str[3], 2 };
-    std::string bStr{ &str[5], 2 };
+    std::string rStr;
+    std::string gStr;
+    std::string bStr;
+
+    if (str.size() == 4)
+    {
+        rStr = std::string(2, str[1]);
+        gStr = std::string(2, str[2]);
+        bStr = std::string(2, str[3]);
+    }
+    else
+    {
+        rStr = std::string(&str[1], 2);
+        gStr = std::string(&str[3], 2);
+        bStr = std::string(&str[5], 2);
+    }
 
     BYTE r = static_cast<BYTE>(std::stoul(rStr, nullptr, 16));
     BYTE g = static_cast<BYTE>(std::stoul(gStr, nullptr, 16));