local math, ipairs
    = math, ipairs

local minetest, vector
    = minetest, vector

local rotate = {}

-- Axis lookup table based on how Minetest's `facedir` operates.
local AXIS_LOOKUP = {
    vector.new( 0,  1,  0), -- +Y
    vector.new( 0,  0,  1), -- +Z
    vector.new( 0,  0, -1), -- -Z
    vector.new( 1,  0,  0), -- +X
    vector.new(-1,  0,  0), -- -X
    vector.new( 0, -1,  0), -- -Y
}

-- Takes an axis vector and returns its index in the `AXIS_LOOKUP` table.
-- Returns `nil` for any vector that is not a valid axis vector.
local function axis_vector_to_index(vec)
    if     vec.y ==  1 then if vec.x == 0 and vec.z == 0 then return 1 end
    elseif vec.z ==  1 then if vec.x == 0 and vec.y == 0 then return 2 end
    elseif vec.z == -1 then if vec.x == 0 and vec.y == 0 then return 3 end
    elseif vec.x ==  1 then if vec.y == 0 and vec.z == 0 then return 4 end
    elseif vec.x == -1 then if vec.y == 0 and vec.z == 0 then return 5 end
    elseif vec.y == -1 then if vec.x == 0 and vec.z == 0 then return 6 end
    else return nil end
end

local FACEDIR_LOOKUP = {}
for up_index, up in ipairs(AXIS_LOOKUP) do
    FACEDIR_LOOKUP[up_index] = {}
    for rot = 0, 3 do
        local facedir = (up_index - 1) * 4 + rot
        local back = minetest.facedir_to_dir(facedir)
        local back_index = axis_vector_to_index(back)
        FACEDIR_LOOKUP[up_index][back_index] = facedir
    end
end

-- Returns whether the specified `node` is rotatable by this mod.
-- TODO: Register such that only certain nodes may be rotated.
local function is_rotatable(node)
    local def = minetest.registered_nodes[node.name]
    return def.paramtype2 == "facedir"
end
rotate.is_rotatable = is_rotatable

-- Rotates `node` at the specified `pos` around `axis` by `degrees` counter-clockwise.
local function rotate_node(pos, node, axis, degrees)
    if degrees % 90 ~= 0 then error("degrees must be divisible by 90") end
    if axis_vector_to_index(axis) == nil then error("axis must be an axis vector") end
    if not is_rotatable(node) then error("node is not rotatable") end

    local up   = AXIS_LOOKUP[1 + math.floor(node.param2 / 4)]
    local back = minetest.facedir_to_dir(node.param2)

    up   = up  :rotate_around_axis(axis, math.rad(degrees)):round()
    back = back:rotate_around_axis(axis, math.rad(degrees)):round()

    local up_index   = axis_vector_to_index(up)
    local back_index = axis_vector_to_index(back)

    node.param2 = FACEDIR_LOOKUP[up_index][back_index]
    minetest.set_node(pos, node)
end
rotate.rotate_node = rotate_node

return rotate