Add removeDocument to TFIDF (#749)

* Add removeDocument to TFIDF

* Bug

* Typo

lib/natural/tfidf/tfidf.js

@@ -132,6 +132,25 @@ class TfIdf {
     }
   }
 
+  // Remove a document from the corpus
+  // Returns true if the document was found
+  // Returns false if the document was not found
+  removeDocument (key) {
+    // Find the document
+    const index = this.documents.findIndex(function (document) {
+      return document.__key === key
+    })
+    // If found, remove it
+    if (index > -1) {
+      this.documents.splice(index, 1)
+      // Invalidate the cache
+      this._idfCache = Object.create(null)
+      return true
+    }
+
+    return false
+  }
+
   // If restoreCache is set to true, all terms idf scores currently cached will be recomputed.
   // Otherwise, the cache will just be wiped clean
   addFileSync (path, encoding, key, restoreCache) {

spec/tfidf_spec.ts

@@ -283,4 +283,18 @@ describe('tfidf', function () {
       expect(tfidf.setStopwords(stopwords)).toEqual(false)
     })
   })
+
+  describe('Remove documents', function () {
+    it('should remove a document', function () {
+      tfidf = new TfIdf()
+
+      tfidf.addDocument('this document is about node.', 0)
+      tfidf.addDocument('this document isn\'t about node.', 1)
+
+      const result1 = tfidf.removeDocument(0)
+      expect(result1).toEqual(true)
+      const result2 = tfidf.removeDocument(0)
+      expect(result2).toEqual(false)
+    })
+  })
 })