Lua九宫格AOI：视野计算的网格切分

-- 错误：每帧全图两两比距离
for _, a in pairs(allUnits) do
    for _, b in pairs(allUnits) do
        if dist(a, b) < VIEW then
            markVisible(a, b)
        end
    end
end

local CELL = 400
local grid = {}
local function reg(u, x, y)
    local k = math.floor(x / CELL) .. "_"
        .. math.floor(y / CELL)
    grid[k] = grid[k] or {}
    table.insert(grid[k], u)
end
local function neighbors(x, y)
    local out, cx, cy = {},
        math.floor(x / CELL), math.floor(y / CELL)
    for dx = -1, 1 do
        for dy = -1, 1 do
            local cell = grid[(cx + dx) .. "_" .. (cy + dy)]
            if cell then
                for _, u in ipairs(cell) do
                    out[#out + 1] = u
                end
            end
        end
    end
    return out
end