fix: Implemented fix by @Oxean (#14)

* fix: Implemented fix by @Oxean

This fixed edge cases when substracting with from/with zero

Author: shinuza

lib/decimal.js

@@ -126,38 +126,34 @@ var as_integer = function (number) {
 };
 
 // Helpers
-var neg_exp = function (str, position) {
+// Helpers
+var neg_exp = function (str, position, sign) {
   position = Math.abs(position);
 
   var offset = position - str.length;
   var sep = DECIMAL_SEPARATOR;
 
   if (offset >= 0) {
     str = zero(offset) + str;
-    sep = '0' + DECIMAL_SEPARATOR;
+    sep = '0.';
   }
 
   var length = str.length;
   var head = str.substr(0, length - position);
   var tail = str.substring(length - position, length);
-
-  if (tail.charAt(0) === '-') {
-    head = '-' + head;
-    tail = tail.substr(1);
-  }
-
-  return (head + sep + tail).replace('-' + DECIMAL_SEPARATOR, '-0' + DECIMAL_SEPARATOR);
+  return sign + head + sep + tail;
 };
 
-var pos_exp = function (str, exp) {
+var pos_exp = function (str, exp, sign) {
   var zeros = zero(exp);
-  return String(str + zeros);
+  return String(sign + str + zeros);
 };
 
 var format = function (num, exp) {
-  num = String(num);
+  var lt0 = num < 0;
+  num = String(lt0 ? -num : num);
   var func = exp >= 0 ? pos_exp : neg_exp;
-  return func(num, exp);
+  return func(num, exp, lt0 ? '-' : '');
 };
 
 var zero = function (exp) {

tests/subtraction.js

@@ -15,3 +15,15 @@ test('subtraction toString conversion', function (t) {
   assert.equal(Decimal('123.456').sub('123000'), Decimal.sub('123.456', '123000').toString());
   assert.equal(Decimal('100.2').sub('1203.12'), Decimal.sub('100.2', '1203.12').toString());
 });
+
+test('subtraction zero', function (t) {
+  assert.equal(Decimal('0').sub('123000'), '-123000');
+  assert.equal(Decimal('123000').sub('0'), '123000');
+  assert.equal(Decimal(0.92).sub(1), '-0.08');
+  assert.equal(Decimal(1).sub(0.92), '0.08');
+  assert.equal(Decimal(1).sub(1), '0');
+
+  assert.equal(Decimal(0).sub(0.01), '-0.01');
+  assert.equal(Decimal(0).sub(0.09), '-0.09');
+  assert.equal(Decimal(0.01).sub(0), '0.01');
+});