bahaghari/lib: add math.mod

Now, it's the real modulo operation.

subprojects/bahaghari/lib/math.nix

@@ -252,6 +252,23 @@ rec {
     in
       floor (difference + 0.5) * nearest;
 
+  /* Given a base and a modulus, returns the value of a modulo operation.
+
+     Type: mod :: Number -> Number -> Number
+
+     Example:
+       mod 5 4
+       => 1
+
+       mod 1245 4.5
+       => 3
+
+       mod 19 (-12)
+       => -5
+  */
+  mod = base: modulus:
+    remainder ((remainder base modulus) + modulus) modulus;
+
   /* Similar to the nixpkgs' `trivial.mod` but retain the decimal values. This
      is just an approximation from ECMAScript's implementation of the remainder
      operator.

subprojects/bahaghari/tests/lib/math.nix

@@ -227,6 +227,36 @@ lib.runTests {
     expected = 1.4142135624;
   };
 
+  testMathMod = {
+    expr = self.math.mod 5 4;
+    expected = 1;
+  };
+
+  testMathMod2 = {
+    expr = self.math.mod 1245 4.5;
+    expected = 3;
+  };
+
+  testMathModPositiveOperands = {
+    expr = self.math.mod 19 12;
+    expected = 7;
+  };
+
+  testMathModNegativeDividend = {
+    expr = self.math.mod (-19) 12;
+    expected = 5;
+  };
+
+  testMathModNegativeDivisor = {
+    expr = self.math.mod 19 (-12);
+    expected = -5;
+  };
+
+  testMathModNegativeOperands = {
+    expr = self.math.mod (-19) (-12);
+    expected = -7;
+  };
+
   testMathRemainder = {
     expr = self.math.remainder 65.5 3;
     expected = 2.5;