get_node_active_selection_box.lua

-- Gets the active `selection_box` for the specified `node` which
-- has abs `paramtype2` of `facedir`, based on its `param2` value.
local function get_node_active_selection_box(node)
    local def = minetest.registered_nodes[node.name]
    local box = def.selection_box and def.selection_box.fixed
    if     not box then box = DEFAULT_SELECTION_BOX
    elseif box[1]  then box = box[1] end
    box = table_copy(box) -- Don't modify the original.

    local param2 = node.param2
    if param2 == 0 then return box end

    local axis = ({ 5, 6, 3, 4, 1, 2 })[math_floor(param2 / 4) + 1]
    local other_axis_1, other_axis_2 = (axis % 3) + 1, ((axis + 1) % 3) + 1
    local rotation = (param2 % 4) / 2 * math_pi
    local flip = axis > 3
    if flip then axis = axis - 3; rotation = -rotation end
    local sin, cos = math_sin(rotation), math_cos(rotation)
    if axis == 2 then sin = -sin end

    for _, off in ipairs({ 0, 3 }) do
        local  axis_1,  axis_2 = other_axis_1 + off, other_axis_2 + off
        local value_1, value_2 = box[axis_1], box[axis_2]
        box[axis_1] = value_1 * cos - value_2 * sin
        box[axis_2] = value_1 * sin + value_2 * cos
    end
    if not flip then box[axis], box[axis + 3] = -box[axis + 3], -box[axis] end
    for _, coord in ipairs({ other_axis_1, other_axis_2 }) do
        if box[coord] > box[coord + 3] then
            box[coord], box[coord + 3] = box[coord + 3], box[coord]
        end
    end

    return box
end