Add math expansion variables.

  This adds new expansion variables $[math.add.<x>.<y>],
  $[math.subtract.<x>.<y>], $[math.multiply.<x>.<y>],
  $[math.divide.<x>.<y>], and $[math.mod.<x>.<y>], which can be
  used to do basic integer mathematics on the integers <x> and <y>.
  This also updates the default-config to use these expansion
  variables instead of a shell to do the mathematics in its size
  computation.

default-config/config

@@ -637,10 +637,10 @@ DestroyModuleConfig RightPanel:*
 Test (x stalonetray) *RightPanel: (120x20, Swallow(NoClose,UseOld) \
     stalonetray 'Exec exec stalonetray --config \
     "$[FVWM_DATADIR]/default-config/stalonetrayrc"', Frame 0)
-Test (x stalonetray) PipeRead 'echo "*RightPanel: (120x$(($[monitor.$[monitor.primary].height]-225)), \
-    Top, Swallow FvwmIconMan \'Module FvwmIconMan\', Frame 0)"'
-Test (!x stalonetray) PipeRead 'echo "*RightPanel: (120x$(($[monitor.$[monitor.primary].height]-205)),\
-    Top, Swallow FvwmIconMan \'Module FvwmIconMan\', Frame 0)"'
+Test (x stalonetray) *RightPanel: (120x$[math.-.$[monitor.$[monitor.primary].height].225], \
+    Top, Swallow FvwmIconMan \'Module FvwmIconMan\', Frame 0)
+Test (!x stalonetray) *RightPanel: (120x$[math.-.$[monitor.$[monitor.primary].height].205], \
+    Top, Swallow FvwmIconMan \'Module FvwmIconMan\', Frame 0)
 *RightPanel: (120x45, Swallow DateTime 'Module FvwmScript FvwmScript-DateTime',\
     Frame 0)
 *RightPanel: (120x5, Frame 0)

doc/fvwm3_manpage_source.adoc

@@ -1616,7 +1616,6 @@ $[screen.count]::
 +
 This is deprecated; use $[monitor.count] instead.
 
-
 $[fg.cs<n>] $[bg.cs<n>] $[hilight.cs<n>] $[shadow.cs<n>] $[fgsh.cs<n>]::
 	These parameters are replaced with the name of the foreground (fg),
 	background (bg), hilight (hilight), shadow (shadow), or the font shadow
@@ -1636,6 +1635,25 @@ $[bg.cs3.lighten15.hash].
 +
 Please refer to the *Colorsets* section for details about colorsets.
 
+$[math.+.<x>.<y>] $[math.-.<x>.<y>] $[math.*.<x>.<y>] $[math./.<x>.<y>] $[math.%.<x>.<y>] $[math.^.<x>.<y>]::
+	The math expansion variables can be used to do some basic arithmetic
+	on the integers <x> and <y>. These expressions can be used to add
+	({plus}), subtract (-), multiply ({asterisk}), divide (/), modulus
+	(%), and exponentiation ({caret}). This can be useful when computing
+	the size of panels or locations of windows relative to a monitor, for
+	example $[math.-.$[monitor.$[monitor.primary].height].200] will
+	subtract 200 pixels from the height of the primary monitor. These can
+	also be nested for more complex operations,
+	$[math.{plus}.$[math.{asterisk}.3.5].$[math.-.4.-2]]
+	is replaced with 21.
++
+All operations use C integer mathematics. The result of integer
+division is the quotient. The modulus can be used to get the remainder
+of the integer division. Note that the C modulus returns negative
+remainders for negative values. Finally the exponentiation operation
+only makes sense for positive values of <y>. If <y> is zero or
+negative, exponentiation will return 1.
+
 $[schedule.last]::
 	This is replaced by the id of the last command that was scheduled with
 	the *Schedule* command, even if this command was already executed.

fvwm/expand.c

@@ -73,6 +73,7 @@ static char *partial_function_vars[] =
 	"infostore.",
 	"monitor.",
 	"shadow.cs",
+	"math.",
 	NULL
 };
 
@@ -164,7 +165,8 @@ enum
 	VAR_HILIGHT_CS,
 	VAR_INFOSTORE_,
 	VAR_MONITOR_,
-	VAR_SHADOW_CS
+	VAR_SHADOW_CS,
+	VAR_MATH_,
 } partial_extended_vars;
 
 enum
@@ -672,6 +674,56 @@ static signed int expand_vars_extended(
 		}
 		break;
 	}
+	case VAR_MATH_:
+		if (rest == NULL || rest[1] != '.')
+		{
+			/* parsing error */
+			return -1;
+		}
+		l = rest[0];
+		rest += 2;
+		if (sscanf(rest, "%d.%d%n", &x, &y, &n) < 2)
+		{
+			/* parsing error */
+			return -1;
+		}
+		if (*(rest + n) != 0)
+		{
+			/* trailing characters */
+			return -1;
+		}
+		switch (l) {
+			case '+':
+				val = x + y;
+				break;
+			case '-':
+				val = x - y;
+				break;
+			case '*':
+				val = x * y;
+				break;
+			case '/':
+				val = x / y;
+				break;
+			case '%':
+				val = x % y;
+				break;
+			case '^':
+				val = 1;
+				/* Should we limit the value of y here? */
+				for (i = 0; i < y; i++)
+				{
+					val *= x;
+				}
+				break;
+			default:
+				/* undefined operation */
+				return -1;
+		}
+
+		is_numeric = True;
+		goto GOT_STRING;
+		break;
 	default:
 		break;
 	}