osc: use STRLEN() macro to make parse_rgb() more self-documenting

The empty literals in the macro are to ensure the argument itself
is a string literal, so it can't be used on anything else.

macros.h

@@ -2,6 +2,7 @@
 
 #define PASTE(a, b) a##b
 #define XPASTE(a, b) PASTE(a, b)
+#define STRLEN(str) (sizeof("" str "") - 1)
 #define DO_PRAGMA(x) _Pragma(#x)
 #define VERCMP(x, y, cx, cy) ((cx > x) || ((cx == x) && (cy >= y)))
 

meson.build

@@ -158,6 +158,7 @@ vtlib = static_library(
   'composed.c', 'composed.h',
   'csi.c', 'csi.h',
   'dcs.c', 'dcs.h',
+  'macros.h',
   'osc.c', 'osc.h',
   'sixel.c', 'sixel.h',
   'vt.c', 'vt.h',

osc.c

@@ -11,6 +11,7 @@
 #include "base64.h"
 #include "config.h"
 #include "grid.h"
+#include "macros.h"
 #include "notify.h"
 #include "render.h"
 #include "selection.h"
@@ -341,12 +342,10 @@ parse_rgb(const char *string, uint32_t *color, bool *_have_alpha,
 
     /* Verify we have the minimum required length (for "") */
     if (have_alpha) {
-        /* rgba:x/x/x/x */
-        if (len < 4 /* 'rgba' */ + 1 /* ':' */ + 3 /* '/' */ + 4 * 1 /* 4 * 'x' */)
+        if (len < STRLEN("rgba:x/x/x/x"))
             return false;
     } else {
-        /* rgb:x/x/x */
-        if (len < 3 /* 'rgb' */ + 1 /* ':' */ + 2 /* '/' */ + 3 * 1 /* 3 * 'x' */)
+        if (len < STRLEN("rgb:x/x/x"))
             return false;
     }