test binarySearch, fix for search in empty. (#7769)

lib/pure/algorithm.nim

@@ -69,7 +69,7 @@ proc reversed*[T](a: openArray[T]): seq[T] =
 
 proc binarySearch*[T](a: openArray[T], key: T): int =
   ## binary search for `key` in `a`. Returns -1 if not found.
-  if ((a.len - 1) and a.len) == 0:
+  if ((a.len - 1) and a.len) == 0 and a.len > 0:
     # when `a.len` is a power of 2, a faster div can be used.
     var step = a.len div 2
     while step > 0:
@@ -367,10 +367,11 @@ when isMainModule:
   var srt1 = [1,2,3,4,4,4,4,5]
   var srt2 = ["iello","hello"]
   var srt3 = [1.0,1.0,1.0]
-  var srt4: seq[int] = @[]
+  var srt4: seq[int]
   assert srt1.isSorted(cmp) == true
   assert srt2.isSorted(cmp) == false
   assert srt3.isSorted(cmp) == true
+  assert srt4.isSorted(cmp) == true
   var srtseq = newSeq[int]()
   assert srtseq.isSorted(cmp) == true
   # Tests for reversed
@@ -518,4 +519,24 @@ when isMainModule:
 
   block fillEmptySeq:
     var s = newSeq[int]()
-    s.fill(0)
+    s.fill(0)
+
+  block testBinarySearch:
+    var noData: seq[int]
+    doAssert binarySearch(noData, 7)    == -1
+    let oneData = @[1]
+    doAssert binarySearch(oneData, 1)   == 0
+    doAssert binarySearch(onedata, 7)   == -1
+    let someData = @[1,3,4,7]
+    doAssert binarySearch(someData, 1)  == 0
+    doAssert binarySearch(somedata, 7)  == 3
+    doAssert binarySearch(someData, -1) == -1
+    doAssert binarySearch(someData, 5)  == -1
+    doAssert binarySearch(someData, 13) == -1
+    let moreData = @[1,3,5,7,4711]
+    doAssert binarySearch(moreData, -1) == -1
+    doAssert binarySearch(moreData,  1) == 0
+    doAssert binarySearch(moreData,  5) == 2
+    doAssert binarySearch(moreData,  6) == -1
+    doAssert binarySearch(moreData,  4711) == 4
+    doAssert binarySearch(moreData,  4712) == -1