bahaghari/lib: update percentage

Also reordered the arguments to make it more "functional".

subprojects/bahaghari/lib/math.nix

@@ -93,14 +93,22 @@ rec {
      Type: percentage :: Number -> Number -> Number
 
      Example:
-       percentage 4 100.0
+       percentage 100.0 4
        => 4
 
-       percentage 5 200.0
+       percentage 200.0 5
        => 10
+
+       percentage 55.4 58
+       => 32.132
+
+       percentage 0 24654
+       => 0
   */
-  percentage = number: value:
+  percentage = value: number:
-    number / (100.0 / value);
+    if value == 0
+    then 0
+    else number / (100.0 / value);
 
   /* Given a number, round up (or down) its number to the nearest integer.
 

subprojects/bahaghari/tests/lib/math.nix

@@ -12,7 +12,7 @@ pkgs.lib.runTests {
   };
 
   testMathPowZero = {
-    expr = lib.math.pow 34 0;
+    expr = lib.math.pow 31 0;
     expected = 1;
   };
 
@@ -27,15 +27,35 @@ pkgs.lib.runTests {
   };
 
   testMathPercentage = {
-    expr = lib.math.percentage 100 50;
+    expr = lib.math.percentage 50 100;
     expected = 50;
   };
 
   testMathPercentage2 = {
-    expr = lib.math.percentage 453 13;
+    expr = lib.math.percentage 13 453;
     expected = 58.89;
   };
 
+  testMathPercentageNegative = {
+    expr = lib.math.percentage (-20) 500;
+    expected = -100;
+  };
+
+  testMathPercentageNegative2 = {
+    expr = lib.math.percentage (-64) 843;
+    expected = -539.52;
+  };
+
+  testMathPercentageZero = {
+    expr = lib.math.percentage 0 45723;
+    expected = 0;
+  };
+
+  testMathPercentageZero2 = {
+    expr = lib.math.percentage 0 (-3423);
+    expected = 0;
+  };
+
   testMathGrow = {
     expr = lib.math.grow 500 12;
     expected = 72;