From 86e7a96d76ab04d32f92beace38ab0593e8d1b02 Mon Sep 17 00:00:00 2001
From: Waqas Hussain <waqas20@gmail.com>
Date: Wed, 30 Oct 2013 17:30:35 -0400
Subject: util.indexedbheap: A priority queue implementation with a reverse
 index with no per-entry memory allocation.

---
 util/indexedbheap.lua | 153 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 153 insertions(+)
 create mode 100644 util/indexedbheap.lua

diff --git a/util/indexedbheap.lua b/util/indexedbheap.lua
new file mode 100644
index 00000000..3cb03037
--- /dev/null
+++ b/util/indexedbheap.lua
@@ -0,0 +1,153 @@
+
+local setmetatable = setmetatable;
+local math_floor = math.floor;
+local t_remove = table.remove;
+
+local function _heap_insert(self, item, sync, item2, index)
+	local pos = #self + 1;
+	while true do
+		local half_pos = math_floor(pos / 2);
+		if half_pos == 0 or item > self[half_pos] then break; end
+		self[pos] = self[half_pos];
+		sync[pos] = sync[half_pos];
+		index[sync[pos]] = pos;
+		pos = half_pos;
+	end
+	self[pos] = item;
+	sync[pos] = item2;
+	index[item2] = pos;
+end
+
+local function _percolate_up(self, k, sync, index)
+	local tmp = self[k];
+	local tmp_sync = sync[k];
+	while k ~= 1 do
+		local parent = math_floor(k/2);
+		if tmp < self[parent] then break; end
+		self[k] = self[parent];
+		sync[k] = sync[parent];
+		index[sync[k]] = k;
+		k = parent;
+	end
+	self[k] = tmp;
+	sync[k] = tmp_sync;
+	index[tmp_sync] = k;
+	return k;
+end
+
+local function _percolate_down(self, k, sync, index)
+	local tmp = self[k];
+	local tmp_sync = sync[k];
+	local size = #self;
+	local child = 2*k;
+	while 2*k <= size do
+		if child ~= size and self[child] > self[child + 1] then
+			child = child + 1;
+		end
+		if tmp > self[child] then
+			self[k] = self[child];
+			sync[k] = sync[child];
+			index[sync[k]] = k;
+		else
+			break;
+		end
+
+		k = child;
+		child = 2*k;
+	end
+	self[k] = tmp;
+	sync[k] = tmp_sync;
+	index[tmp_sync] = k;
+	return k;
+end
+
+local function _heap_pop(self, sync, index)
+	local size = #self;
+	if size == 0 then return nil; end
+
+	local result = self[1];
+	local result_sync = sync[1];
+	index[result_sync] = nil;
+	if size == 1 then
+		self[1] = nil;
+		sync[1] = nil;
+		return result, result_sync;
+	end
+	self[1] = t_remove(self);
+	sync[1] = t_remove(sync);
+	index[sync[1]] = 1;
+
+	_percolate_down(self, 1, sync, index);
+
+	return result, result_sync;
+end
+
+local indexed_heap = {};
+
+function indexed_heap:insert(item, priority, id)
+	if id == nil then
+		id = self.current_id;
+		self.current_id = id + 1;
+	end
+	self.items[id] = item;
+	_heap_insert(self.priorities, priority, self.ids, id, self.index);
+	return id;
+end
+function indexed_heap:pop()
+	local priority, id = _heap_pop(self.priorities, self.ids, self.index);
+	if id then
+		local item = self.items[id];
+		self.items[id] = nil;
+		return priority, item, id;
+	end
+end
+function indexed_heap:peek()
+	return self.priorities[1];
+end
+function indexed_heap:reprioritize(id, priority)
+	local k = self.index[id];
+	if k == nil then return; end
+	self.priorities[k] = priority;
+
+	k = _percolate_up(self.priorities, k, self.ids, self.index);
+	k = _percolate_down(self.priorities, k, self.ids, self.index);
+end
+function indexed_heap:remove_index(k)
+	local size = #self.priorities;
+
+	local result = self.priorities[k];
+	local result_sync = self.ids[k];
+	local item = self.items[result_sync];
+	if result == nil then return; end
+	self.index[result_sync] = nil;
+	self.items[result_sync] = nil;
+
+	self.priorities[k] = self.priorities[size];
+	self.ids[k] = self.ids[size];
+	self.index[self.ids[k]] = k;
+	t_remove(self.priorities);
+	t_remove(self.ids);
+
+	k = _percolate_up(self.priorities, k, self.ids, self.index);
+	k = _percolate_down(self.priorities, k, self.ids, self.index);
+
+	return result, item, result_sync;
+end
+function indexed_heap:remove(id)
+	return self:remove_index(self.index[id]);
+end
+
+local mt = { __index = indexed_heap };
+
+local _M = {
+	create = function()
+		return setmetatable({
+			ids = {}; -- heap of ids, sync'd with priorities
+			items = {}; -- map id->items
+			priorities = {}; -- heap of priorities
+			index = {}; -- map of id->index of id in ids
+			current_id = 1.5
+		}, mt);
+	end
+};
+return _M;
-- 
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