Implement dice and percent as global lua functions

Intended to simplify many of the math.random calls currently in use, and
make them more semantic and thus more readable.

The dice function d() takes either a two-argument form which is the same
as in the C source (number of dice, faces per die) or a one-argument
form that rolls a single die.

The percent(N) function returns true N% of the time.

dat/nhlib.lua

@@ -21,3 +21,23 @@ end
 
 align = { "law", "neutral", "chaos" };
 shuffle(align);
+
+-- d(2,6) = 2d6
+-- d(20) = 1d20 (single argument = implicit 1 die)
+function d(dice, faces)
+   if (faces == nil) then
+      -- 1-arg form: argument "dice" is actually the number of faces
+      return math.random(1, dice)
+   else
+      local sum = 0
+      for i=1,dice do
+         sum = sum + math.random(1, faces)
+      end
+      return sum
+   end
+end
+
+-- percent(20) returns true 20% of the time
+function percent(threshold)
+   return math.random(0,99) < threshold
+end