Merge pull request #4957 from RazvanN7/Issue_16724

Fix Issue 16724 - RandomCover.popFront is a no-op for the first call

Author: andralex

std/random.d

@@ -2160,16 +2160,21 @@ struct RandomCover(Range, UniformRNG = void)
     private bool[] _chosen;
     private size_t _current;
     private size_t _alreadyChosen = 0;
+    private bool _isEmpty = false;
 
     static if (is(UniformRNG == void))
     {
         this(Range input)
         {
             _input = input;
             _chosen.length = _input.length;
-            if (_chosen.length == 0)
+            if (_input.empty)
             {
-                _alreadyChosen = 1;
+                _isEmpty = true;
+            }
+            else
+            {
+                _current = uniform(0, _chosen.length);
             }
         }
     }
@@ -2182,9 +2187,13 @@ struct RandomCover(Range, UniformRNG = void)
             _input = input;
             _rng = rng;
             _chosen.length = _input.length;
-            if (_chosen.length == 0)
+            if (_input.empty)
+            {
+                _isEmpty = true;
+            }
+            else
             {
-                _alreadyChosen = 1;
+                _current = uniform(0, _chosen.length, rng);
             }
         }
 
@@ -2198,39 +2207,32 @@ struct RandomCover(Range, UniformRNG = void)
     {
         @property size_t length()
         {
-            if (_alreadyChosen == 0)
-            {
-                return _input.length;
-            }
-            else
-            {
-                return (1 + _input.length) - _alreadyChosen;
-            }
+            return _input.length - _alreadyChosen;
         }
     }
 
     @property auto ref front()
     {
-        if (_alreadyChosen == 0)
-        {
-            popFront();
-        }
+        assert(!_isEmpty);
         return _input[_current];
     }
 
     void popFront()
     {
-        if (_alreadyChosen >= _input.length)
+        assert(!_isEmpty);
+
+        size_t k = _input.length - _alreadyChosen - 1;
+        if (k == 0)
         {
-            // No more elements
-            ++_alreadyChosen; // means we're done
+            _isEmpty = true;
+            ++_alreadyChosen;
             return;
         }
-        size_t k = _input.length - _alreadyChosen;
+
         size_t i;
         foreach (e; _input)
         {
-            if (_chosen[i]) { ++i; continue; }
+            if (_chosen[i] || i == _current) { ++i; continue; }
             // Roll a dice with k faces
             static if (is(UniformRNG == void))
             {
@@ -2243,7 +2245,7 @@ struct RandomCover(Range, UniformRNG = void)
             assert(k > 1 || chooseMe);
             if (chooseMe)
             {
-                _chosen[i] = true;
+                _chosen[_current] = true;
                 _current = i;
                 ++_alreadyChosen;
                 return;
@@ -2264,7 +2266,7 @@ struct RandomCover(Range, UniformRNG = void)
         }
     }
 
-    @property bool empty() { return _alreadyChosen > _input.length; }
+    @property bool empty() { return _isEmpty; }
 }
 
 /// Ditto
@@ -2286,6 +2288,7 @@ auto randomCover(Range)(Range r)
     import std.algorithm;
     import std.conv;
     int[] a = [ 0, 1, 2, 3, 4, 5, 6, 7, 8 ];
+    int[] c;
     foreach (UniformRNG; std.meta.AliasSeq!(void, PseudoRngTypes))
     {
         static if (is(UniformRNG == void))
@@ -2302,6 +2305,8 @@ auto randomCover(Range)(Range r)
             // check for constructor passed a value-type RNG
             auto rc2 = RandomCover!(int[], UniformRNG)(a, UniformRNG(unpredictableSeed));
             static assert(isForwardRange!(typeof(rc2)));
+            auto rcEmpty = randomCover(c, rng);
+            assert(rcEmpty.length == 0);
         }
 
         int[] b = new int[9];
@@ -2323,6 +2328,17 @@ auto randomCover(Range)(Range r)
     auto rc = randomCover(r);
     assert(rc.length == 0);
     assert(rc.empty);
+
+    // Bugzilla 16724
+    import std.range : iota;
+    auto range = iota(10);
+    auto randy = range.randomCover;
+
+    for (int i=1; i<=range.length; i++)
+    {
+        randy.popFront;
+        assert(randy.length == range.length - i);
+    }
 }
 
 // RandomSample