JS: introduce RegExpSequence::nextElement and previousElement

Author: esbena

javascript/ql/src/semmle/javascript/Regexp.qll

@@ -76,40 +76,22 @@ class RegExpTerm extends Locatable, @regexpterm {
   /** Gets the regular expression term that is matched (textually) before this one, if any. */
   RegExpTerm getPredecessor() {
     exists(RegExpTerm parent | parent = getParent() |
-      if parent instanceof RegExpSequence
-      then
-        exists(RegExpSequence seq, int i |
-          seq = parent and
-          seq.getChild(i) = this
-        |
-          seq.getChild(i - 1) = result
-          or
-          i = 0 and result = seq.getPredecessor()
-        )
-      else (
-        not parent instanceof RegExpSubPattern and
-        result = parent.getPredecessor()
-      )
+      result = parent.(RegExpSequence).previousElement(this)
+      or
+      not exists(parent.(RegExpSequence).previousElement(this)) and
+      not parent instanceof RegExpSubPattern and
+      result = parent.getPredecessor()
     )
   }
 
   /** Gets the regular expression term that is matched (textually) after this one, if any. */
   RegExpTerm getSuccessor() {
     exists(RegExpTerm parent | parent = getParent() |
-      if parent instanceof RegExpSequence
-      then
-        exists(RegExpSequence seq, int i |
-          seq = parent and
-          seq.getChild(i) = this
-        |
-          seq.getChild(i + 1) = result
-          or
-          i = seq.getNumChild() - 1 and result = seq.getSuccessor()
-        )
-      else (
-        not parent instanceof RegExpSubPattern and
-        result = parent.getSuccessor()
-      )
+      result = parent.(RegExpSequence).nextElement(this)
+      or
+      not exists(parent.(RegExpSequence).nextElement(this)) and
+      not parent instanceof RegExpSubPattern and
+      result = parent.getSuccessor()
     )
   }
 
@@ -328,6 +310,19 @@ class RegExpSequence extends RegExpTerm, @regexp_seq {
     or
     result = getChild(i).getConstantValue() + getConstantValue(i+1)
   }
+
+    /** Gets the element preceding `element` in this sequence. */
+  RegExpTerm previousElement(RegExpTerm element) {
+    element = nextElement(result)
+  }
+
+  /** Gets the element following `element` in this sequence. */
+  RegExpTerm nextElement(RegExpTerm element) {
+    exists(int i |
+      element = this.getChild(i) and
+      result = this.getChild(i + 1)
+    )
+  }
 }
 
 /**