--- a/spec/util_indexedbheap_spec.lua	Tue Sep 29 15:30:48 2020 +0100
+++ b/spec/util_indexedbheap_spec.lua	Wed Sep 30 07:25:32 2020 +0100
@@ -1,15 +1,33 @@
 local ibh = require"util.indexedbheap";
+
+local function verify_heap_property(priorities)
+	for k in ipairs(priorities) do
+		local parent = priorities[k];
+		local childA = priorities[2*k];
+		local childB = priorities[2*k+1];
+		-- print("-", parent, childA, childB)
+		assert(childA == nil or childA > parent, "heap property violated");
+		assert(childB == nil or childB > parent, "heap property violated");
+	end
+end
+
 local h
 setup(function ()
 	h = ibh.create();
 end)
 describe("util.indexedbheap", function ()
-	pending("item can be moved from end to top", function ()
+	it("item can be moved from end to top", function ()
+		verify_heap_property(h);
 		h:insert("a", 1);
+		verify_heap_property(h);
 		h:insert("b", 2);
+		verify_heap_property(h);
 		h:insert("c", 3);
+		verify_heap_property(h);
 		local id = h:insert("*", 10);
+		verify_heap_property(h);
 		h:reprioritize(id, 0);
+		verify_heap_property(h);
 		assert.same({ 0, "*", id }, { h:pop() });
 	end)
 end);

--- a/util/indexedbheap.lua	Tue Sep 29 15:30:48 2020 +0100
+++ b/util/indexedbheap.lua	Wed Sep 30 07:25:32 2020 +0100
@@ -23,7 +23,7 @@
 	local tmp_sync = sync[k];
 	while k ~= 1 do
 		local parent = math_floor(k/2);
-		if tmp < self[parent] then break; end
+		if tmp >= self[parent] then break; end
 		self[k] = self[parent];
 		sync[k] = sync[parent];
 		index[sync[k]] = k;