feat(pathfinding): add a-star search and line of sight

src/astar.lua

@@ -0,0 +1,88 @@
+-- A* on the 4-connected grid with a binary min-heap open set and Manhattan
+-- heuristic. Returns the path start..goal, or nil when unreachable.
+
+local Astar = {}
+
+local function heapPush(heap, node, priority)
+  heap[#heap + 1] = { node = node, priority = priority }
+  local i = #heap
+  while i > 1 do
+    local parent = i // 2
+    if heap[parent].priority <= heap[i].priority then break end
+    heap[parent], heap[i] = heap[i], heap[parent]
+    i = parent
+  end
+end
+
+local function heapPop(heap)
+  local top = heap[1]
+  local last = heap[#heap]
+  heap[#heap] = nil
+  local size = #heap
+  if size > 0 then
+    heap[1] = last
+    local i = 1
+    while true do
+      local left, right = i * 2, i * 2 + 1
+      local smallest = i
+      if left <= size and heap[left].priority < heap[smallest].priority then smallest = left end
+      if right <= size and heap[right].priority < heap[smallest].priority then smallest = right end
+      if smallest == i then break end
+      heap[i], heap[smallest] = heap[smallest], heap[i]
+      i = smallest
+    end
+  end
+  return top.node
+end
+
+local DIRS = { { 1, 0 }, { -1, 0 }, { 0, 1 }, { 0, -1 } }
+
+local function key(x, y)
+  return y * 100000 + x
+end
+
+function Astar.find(map, startX, startY, goalX, goalY, isBlocked)
+  isBlocked = isBlocked or function(x, y) return not map:isWalkable(x, y) end
+  if isBlocked(goalX, goalY) then return nil end
+
+  local heap = {}
+  local cameFrom = { [key(startX, startY)] = false }
+  local costSoFar = { [key(startX, startY)] = 0 }
+  heapPush(heap, { x = startX, y = startY }, 0)
+
+  while #heap > 0 do
+    local current = heapPop(heap)
+    if current.x == goalX and current.y == goalY then
+      local reversed = {}
+      local cx, cy = goalX, goalY
+      while true do
+        reversed[#reversed + 1] = { x = cx, y = cy }
+        local prev = cameFrom[key(cx, cy)]
+        if not prev then break end
+        cx, cy = prev.x, prev.y
+      end
+      local path = {}
+      for i = #reversed, 1, -1 do
+        path[#path + 1] = reversed[i]
+      end
+      return path
+    end
+    local currentCost = costSoFar[key(current.x, current.y)]
+    for _, dir in ipairs(DIRS) do
+      local nx, ny = current.x + dir[1], current.y + dir[2]
+      if map:inBounds(nx, ny) and not isBlocked(nx, ny) then
+        local nKey = key(nx, ny)
+        local newCost = currentCost + 1
+        if costSoFar[nKey] == nil or newCost < costSoFar[nKey] then
+          costSoFar[nKey] = newCost
+          local heuristic = math.abs(nx - goalX) + math.abs(ny - goalY)
+          heapPush(heap, { x = nx, y = ny }, newCost + heuristic)
+          cameFrom[nKey] = { x = current.x, y = current.y }
+        end
+      end
+    end
+  end
+  return nil
+end
+
+return Astar