Binary search can returns insertion index

src/main/scala/Search/BinarySearch.scala

@@ -15,23 +15,33 @@ object BinarySearch {
     */
 
   def binarySearch(arr: List[Int], elem: Int): Int = {
-    binarySearch(arr,elem,0,arr.length)
+    binarySearch(arr, elem, 0, arr.length)
   }
 
   /**
     * @param arr   - a sequence of integers
     * @param elem - a integer to search for in the @args
     * @param fromIndex - the index of the first element (inclusive) to be searched
     * @param toIndex - toIndex the index of the last element (exclusive) to be searched
-    * @return - index of the @elem otherwise -1
+    * @param returnInsertIdx - if `true`, returns info about the insertion index
+    * @return - index of the @elem otherwise -1. If `returnInsertIdx` is true, returns
+    *         (-insertion_idx - 1) where insertion_idx is the 1st index where `elem` can be
+    *         inserted into `arr` and `arr` is still sorted.
     */
 
-  def binarySearch(arr: List[Int], elem: Int, fromIndex: Int, toIndex: Int): Int = {
+  def binarySearch(
+    arr: List[Int],
+    elem: Int,
+    fromIndex: Int,
+    toIndex: Int,
+    returnInsertIdx: Boolean = false
+  ): Int = {
 
     @tailrec
     def SearchImpl(lo: Int, hi: Int): Int = {
-      if (lo > hi)
-        -1
+      if (lo > hi) {
+        if(returnInsertIdx) -lo - 1 else -1
+      }
       else {
         val mid: Int = lo + (hi - lo) / 2
         arr(mid) match {

src/test/scala/Search/BinarySearchSpec.scala

@@ -32,4 +32,17 @@ class BinarySearchSpec extends FlatSpec {
     assert(BinarySearch.binarySearch(l,7,0,4) === -1)
     assert(BinarySearch.binarySearch(l,7,1,3) === -1)
   }
+
+  it should "return insertion index if the element is not found" in {
+    def search(l: List[Int], elem: Int) = {
+      val rs = BinarySearch.binarySearch(l, elem, 0, l.length, returnInsertIdx = true)
+      -rs - 1
+    }
+
+    val l = List(-5, 10, 15)
+    assert(search(l, 0) === 1)
+    assert(search(l, 1) === 1)
+    assert(search(l, 12) === 2)
+    assert(search(l, 22) === 3)
+  }
 }