Add detect() method for the tab-size.js

src/options/tab-size.js

@@ -22,5 +22,55 @@ module.exports = {
     ast.traverseByType('space', function(space) {
       space.content = space.content.replace(/\t/, value);
     });
+  },
+
+  /**
+  * Detects the value of this option in ast.
+  * @param {Node} ast
+  * @return {Array?} List of detected values
+  */
+  detect(ast) {
+    var detected = [];
+
+    ast.forEach(node => {
+      if (node.is('ruleset') || node.is('atrule')) {
+        var spaces = 0;
+        var tabs = 0;
+        var tabsize;
+
+        node.forEach(block => {
+          if (block && block.is('block')) {
+            block.forEach(blockContent => {
+              // The indent before the very first property declaration in the
+              // block usually has a length of one tab or the number of
+              // whitespaces set for a tab. Because there might be nested
+              // blocks having incremented indent, we continue to the next root
+              // block in the each ruleset where tabsize is not detected yet.
+              if (tabsize !== 'detected') {
+                if (blockContent.is('space')) {
+                  spaces = blockContent.content.replace(/\n/g, '');
+                  tabs = spaces.match(/\t/g);
+                  spaces = spaces.match(/( )/g);
+                  if (tabs) {
+                    // It is a very rare case when tabs are used in a file
+                    // instead of whitespaces and it requires a lot of efforts
+                    // to find the tab size in a particular environment. So we
+                    // imply that tab size = 2.
+                    tabsize = 2;
+                  } else if (spaces) {
+                    tabsize = spaces.length;
+                  }
+                } else if (tabsize && blockContent.is('declaration')) {
+                  detected.push(tabsize);
+                  tabsize = 'detected';
+                }
+              }
+            });
+          }
+        });
+      }
+    });
+
+    return detected;
   }
 };

test/options/integral/detect/test.js

@@ -9,6 +9,8 @@ describe('Option `integral`, detect', function() {
       let expected = JSON.parse(JSON.stringify(test.Comb.getConfig('csscomb')));
       delete expected['sort-order'];
       delete expected.exclude;
+      // The default csscomb.json has no tab-size option.
+      expected['tab-size'] = 4;
       test.shouldDetect(undefined, input, expected);
     });
   });