putil: Fix SparseArray get_lowest_on_bit and get_lowest_off_bit

Cherry-pick of d173086a6f

panda/src/putil/sparseArray.cxx

@@ -89,12 +89,16 @@ get_num_off_bits() const {
 
 /**
  * Returns the index of the lowest 1 bit in the array.  Returns -1 if there
- * are no 1 bits or if there are an infinite number of 1 bits.
+ * are no 1 bits and 0 if there are an infinite number of 1 bits.
  */
 int SparseArray::
 get_lowest_on_bit() const {
   if (_inverse) {
-    return -1;
+    if (_subranges.empty() || _subranges[0]._begin > 0) {
+      return 0;
+    } else {
+      return _subranges[0]._end;
+    }
   }
 
   if (_subranges.empty()) {
@@ -111,7 +115,11 @@ get_lowest_on_bit() const {
 int SparseArray::
 get_lowest_off_bit() const {
   if (!_inverse) {
-    return -1;
+    if (_subranges.empty() || _subranges[0]._begin > 0) {
+      return 0;
+    } else {
+      return _subranges[0]._end;
+    }
   }
 
   if (_subranges.empty()) {

tests/putil/test_sparsearray.py

@@ -235,6 +235,35 @@ def test_sparse_array_augm_assignment():
     assert s is t
 
 
+def test_sparse_array_lowest_bit():
+    sa = core.SparseArray()
+    assert sa.get_lowest_off_bit() == 0
+    assert sa.get_lowest_on_bit() == -1
+
+    sa.invert_in_place()
+    assert sa.get_lowest_off_bit() == -1
+    assert sa.get_lowest_on_bit() == 0
+
+    sa = core.SparseArray()
+    sa.set_bit(0)
+    sa.set_bit(1)
+    assert sa.get_lowest_off_bit() == 2
+    assert sa.get_lowest_on_bit() == 0
+
+    sa.invert_in_place()
+    assert sa.get_lowest_off_bit() == 0
+    assert sa.get_lowest_on_bit() == 2
+
+    sa = core.SparseArray()
+    sa.set_bit(2)
+    assert sa.get_lowest_off_bit() == 0
+    assert sa.get_lowest_on_bit() == 2
+
+    sa.invert_in_place()
+    assert sa.get_lowest_off_bit() == 2
+    assert sa.get_lowest_on_bit() == 0
+
+
 def test_sparse_array_getstate():
     sa = core.SparseArray()
     assert sa.__getstate__() == ()