randompercentage, randomelement (#4189)

'randompercentage X' sets VAR_RESULT to TRUE X% of the time, or FALSE
100-X% of the time.

'randomelement X, Y, ...' sets VAR_RESULT to one of X, Y, ... with equal
probability.

asm/macros/event.inc

@@ -2133,3 +2133,34 @@
 	callnative ToggleGigantamaxFactor
 	.2byte \slot
 	.endm
+
+	@ Sets VAR_RESULT to one of the arguments (via setorcopyvar).
+	.macro randomelement element:req, elements:vararg
+	.set _randomelement_n, 0
+	.irp el, \element, \elements
+	.set _randomelement_n, _randomelement_n + 1
+	.endr
+		random _randomelement_n
+
+	.set _randomelement_n, 0
+	.irp el, \element, \elements
+		goto_if_ne VAR_RESULT, _randomelement_n, 1f
+		setorcopyvar VAR_RESULT, \el
+		goto 2f
+	1:
+	.set _randomelement_n, _randomelement_n + 1
+	.endr
+	2:
+	.endm
+
+	@ Sets VAR_RESULT to TRUE with probability 'percent', and FALSE
+	@ with probability '100% - percent'.
+	.macro randompercentage percent:req
+		random 100
+		goto_if_lt VAR_RESULT, \percent, 1f
+		setvar VAR_RESULT, FALSE
+		goto 2f
+	1:
+		setvar VAR_RESULT, TRUE
+	2:
+	.endm