Deal with CSI 38:2 which turns out it has a color type and color space and...

Deal with CSI 38:2 which turns out it has a color type and color space and everyone did it wrong. Issue 6377

sources/VT100Terminal.m

@@ -685,30 +685,40 @@ static const int kMaxScreenRows = 4096;
                     graphicRendition_.bgColorMode = ColorModeAlternate;
                     break;
                 case VT100CHARATTR_FG_256:
-                    // First subparam means:   # additional subparams:  Accepts optional params:
-                    // 1: transparent          0                        NO
-                    // 2: RGB                  3                        YES
-                    // 3: CMY                  3                        YES
-                    // 4: CMYK                 4                        YES
-                    // 5: Indexed color        1                        NO
+                    // The actual spec for this is called ITU T.416-199303
+                    // You can download it for free! If you prefer to spend money, ISO/IEC 8613-6
+                    // is supposedly the same thing.
                     //
-                    // Optional paramters go at position 7 and 8, and indicate toleranace as an
-                    // integer; and color space (0=CIELUV, 1=CIELAB). Example:
+                    // Here's a sad story about CSI 38:2, which is used to do 24-bit color.
                     //
-                    // CSI 38:2:255:128:64:0:5:1 m
+                    // Lots of terminal emulators, iTerm2 included, misunderstood the spec. That's
+                    // easy to understand if you read it, which I can't recommend doing unless
+                    // you're looking for inspiration for your next Bulwer-Lytton Fiction Contest
+                    // entry.
                     //
-                    // Also accepted for xterm compatibility, but never with optional parameters:
-                    // CSI 38;2;255;128;64 m
+                    // See issue 6377 for more context.
                     //
-                    // Set the foreground color to red=255, green=128, blue=64 with a tolerance of
-                    // 5 in the CIELAB color space. The 0 at the 6th position has no meaning and
-                    // is just a filler.
-                    // 
-                    // For 256-color mode (indexed) use this for the foreground:
-                    // CSI 38;5;N m
-                    // where N is a value between 0 and 255. See the colors described in screen_char_t
-                    // in the comments for fgColorCode.
-
+                    // Ignoring color types we don't support like CMYK, the spec says to do this:
+                    // CSI 38:2:[color space]:[red]:[green]:[blue]:[unused]:[tolerance]:[tolerance colorspace]
+                    //
+                    // Everything after [blue] is optional. Values are decimal numbers in 0...255.
+                    //
+                    // Unfortunately, what was implemented for a long time was this:
+                    // CSI 38:2:[red]:[green]:[blue]:[unused]:[tolerance]:[tolerance colorspace]
+                    //
+                    // And for xterm compatibility, the following was also accepted:
+                    // CSI 38;2;[red];[green];[blue]
+                    //
+                    // The New Order
+                    // -------------
+                    // Tolerance never did anything, so we'll accept this non-standards compliant
+                    // code, which people use:
+                    // CSI 38:2:[red]:[green]:[blue]
+                    //
+                    // As well as the following forms:
+                    // CSI 38:2:[colorspace]:[red]:[green]:[blue]
+                    // CSI 38:2:[colorspace]:[red]:[green]:[blue]:<one or more additional colon-delimited arguments, all ignored>
+                    // CSI 38;2;[red];[green];[blue]   // Notice semicolons in place of colons here
                     if (token.csi->subCount[i] > 0) {
                         // Preferred syntax using colons to delimit subparameters
                         if (token.csi->subCount[i] >= 2 && token.csi->sub[i][0] == 5) {
@@ -718,15 +728,30 @@ static const int kMaxScreenRows = 4096;
                             graphicRendition_.fgBlue = 0;
                             graphicRendition_.fgColorMode = ColorModeNormal;
                         } else if (token.csi->subCount[i] >= 4 && token.csi->sub[i][0] == 2) {
-                            // CSI 38:2:R:G:B m
                             // 24-bit color
-                            graphicRendition_.fgColorCode = token.csi->sub[i][1];
-                            graphicRendition_.fgGreen = token.csi->sub[i][2];
-                            graphicRendition_.fgBlue = token.csi->sub[i][3];
-                            graphicRendition_.fgColorMode = ColorMode24bit;
+                            if (token.csi->subCount[i] >= 5) {
+                                // Spec-compliant. Likely rarely used in 2017.
+                                // CSI 38:2:colorspace:R:G:B m
+                                // TODO: Respect the color space argument. See ITU-T Rec. T.414,
+                                // but good luck actually finding the colour space IDs.
+                                graphicRendition_.fgColorCode = token.csi->sub[i][2];
+                                graphicRendition_.fgGreen = token.csi->sub[i][3];
+                                graphicRendition_.fgBlue = token.csi->sub[i][4];
+                                graphicRendition_.fgColorMode = ColorMode24bit;
+                            } else {
+                                // Misinterpration compliant.
+                                // CSI 38:2:R:G:B m  <- misinterpration compliant
+                                graphicRendition_.fgColorCode = token.csi->sub[i][1];
+                                graphicRendition_.fgGreen = token.csi->sub[i][2];
+                                graphicRendition_.fgBlue = token.csi->sub[i][3];
+                                graphicRendition_.fgColorMode = ColorMode24bit;
+                            }
                         }
                     } else if (token.csi->count - i >= 3 && token.csi->p[i + 1] == 5) {
-                        // CSI 38;5;P m
+                        // For 256-color mode (indexed) use this for the foreground:
+                        // CSI 38;5;N m
+                        // where N is a value between 0 and 255. See the colors described in screen_char_t
+                        // in the comments for fgColorCode.
                         graphicRendition_.fgColorCode = token.csi->p[i + 2];
                         graphicRendition_.fgGreen = 0;
                         graphicRendition_.fgBlue = 0;
@@ -734,6 +759,7 @@ static const int kMaxScreenRows = 4096;
                         i += 2;
                     } else if (token.csi->count - i >= 5 && token.csi->p[i + 1] == 2) {
                         // CSI 38;2;R;G;B m
+                        // Hack for xterm compatibility
                         // 24-bit color support
                         graphicRendition_.fgColorCode = token.csi->p[i + 2];
                         graphicRendition_.fgGreen = token.csi->p[i + 3];