fix: incorrect reduction of subtraction from zero (postcss#88) (postcss#93)

Author: jasminexie

src/__tests__/index.js

@@ -118,6 +118,13 @@ test(
   'calc((100vw - 50em)/2)',
 );
 
+test(
+  'should reduce additions and subtractions (5)',
+  testValue,
+  'calc(10px - (100vw - 50em) / 2)',
+  'calc(10px - (100vw - 50em)/2)',
+);
+
 test(
   'should ignore value surrounding calc function (1)',
   testValue,
@@ -360,6 +367,34 @@ test(
   'calc(-1px + -1em)',
 );
 
+test(
+  'should reduce substracted expression from zero (1)',
+  testValue,
+  'calc( 0 - (100vw - 10px) / 2 )',
+  'calc((-100vw - -10px)/2)',
+);
+
+test(
+  'should reduce substracted expression from zero (2)',
+  testValue,
+  'calc( 0px - (100vw - 10px))',
+  'calc(-100vw - -10px)',
+);
+
+test(
+  'should reduce substracted expression from zero (3)',
+  testValue,
+  'calc( 0px - (100vw - 10px) * 2px )',
+  'calc((-100vw - -10px)*2px)',
+);
+
+test(
+  'should reduce substracted expression from zero (4)',
+  testValue,
+  'calc( 0px - (100vw + 10px))',
+  'calc(-100vw + -10px)',
+);
+
 test(
   'should reduce nested expression',
   testValue,

src/lib/reducer.js

@@ -34,8 +34,12 @@ function flipValue(node) {
   if (isValueType(node.type)) {
     node.value = -node.value;
   } else if (node.type === 'MathExpression') {
-    node.left = flipValue(node.left);
-    node.right = flipValue(node.right);
+    if (node.operator === '*' || node.operator === '/') {
+      node.left = flipValue(node.left);
+    } else {
+      node.left = flipValue(node.left);
+      node.right = flipValue(node.right);
+    }
   }
 
   return node;