Add natural log to level-calc formula parsing

Relates to #274

src/main/java/world/bentobox/level/calculators/IslandLevelCalculator.java

@@ -135,6 +135,9 @@ public class IslandLevelCalculator {
                     case "tan":
                         x = Math.tan(Math.toRadians(x));
                         break;
+                    case "log":
+                        x = Math.log(x);
+                        break;
                     default:
                         throw new RuntimeException("Unknown function: " + func);
                     }

src/main/java/world/bentobox/level/config/ConfigSettings.java

@@ -90,7 +90,7 @@ public class ConfigSettings implements ConfigObject {
     @ConfigComment("Island level calculation formula")
     @ConfigComment("blocks - the sum total of all block values, less any death penalty")
     @ConfigComment("level_cost - in a linear equation, the value of one level")
-    @ConfigComment("This formula can include +,=,*,/,sqrt,^,sin,cos,tan. Result will always be rounded to a long integer")
+    @ConfigComment("This formula can include +,=,*,/,sqrt,^,sin,cos,tan,log (natural log). Result will always be rounded to a long integer")
     @ConfigComment("for example, an alternative non-linear option could be: 3 * sqrt(blocks / level_cost)")
     @ConfigEntry(path = "level-calc")
     private String levelCalc = "blocks / level_cost";

src/main/resources/config.yml

@@ -53,7 +53,7 @@ levelcost: 100
 # Island level calculation formula
 # blocks - the sum total of all block values, less any death penalty
 # level_cost - in a linear equation, the value of one level
-# This formula can include +,=,*,/,sqrt,^,sin,cos,tan. Result will always be rounded to a long integer
+# This formula can include +,=,*,/,sqrt,^,sin,cos,tan,log (natural log). Result will always be rounded to a long integer
 # for example, an alternative non-linear option could be: 3 * sqrt(blocks / level_cost)
 level-calc: blocks / level_cost
 #