Adds binary search algorithm in Erlang

erlang/binary_search.erl

@@ -0,0 +1,57 @@
+-include_lib("eunit/include/eunit.hrl").
+
+-module(binary_search).
+-export([start/0]).
+
+% Binary search implementation in Erlang
+% https://en.wikipedia.org/wiki/Binary_search_algorithm
+
+
+% Entrypoint that calls the function with an additional accumulator
+% to save the index of the found element
+binary_search(Haystack, Needle) ->
+  binary_search(Haystack, Needle, 0).
+
+binary_search(Haystack, Needle, PrevIndex) ->
+  % Get the middle element
+  HaystackLength = length(Haystack),
+  Middle = (HaystackLength + 1) div 2,
+  case Middle of
+    % If there is no middle, the list is empty, so no element is found
+    0 -> -1;
+    _ ->
+      % In any other case, get the element at the middle and compare it
+      % with the element looked for
+      Item = lists:nth(Middle,Haystack),
+      case Item of
+        % If it is the element looked for,
+        % We return the sum of indexes and substract 1
+        % Since it is a zero-based index search
+        Item when Item =:= Needle ->
+          PrevIndex + Middle - 1;
+        % When the item is either greater or smaller than the
+        % value looked for, search in the left or right lists
+        % and also send the current index of the search
+        Item when Item > Needle ->
+          Left = lists:sublist(Haystack, HaystackLength - Middle),
+          binary_search(Left, Needle, PrevIndex + Middle);
+        Item when Item < Needle ->
+          Right = lists:nthtail(Middle, Haystack),
+          binary_search(Right, Needle, PrevIndex + Middle)
+        end
+  end.
+
+start() ->
+  Test1 = [1, 2, 3, 4, 6, 7, 9],
+  Test2 = [-1, 0, 1, 5, 6, 9, 15, 15],
+  Test3 = [-1, 0, 0, 0, 0, 1],
+  Test4 = [],
+
+  ?assert(binary_search(Test1,5) =:= -1),
+  ?assert(binary_search(Test2,15) =:= 6),
+
+  % Finding the left-most occurrence is not implemented
+  % but it would find the element in the first iteration
+  ?assert(binary_search(Test3,0) =/= -1),
+
+  ?assert(binary_search(Test4, 10) =:= -1).