local math, pairs, ipairs, setmetatable, table_insert, print
    = math, pairs, ipairs, setmetatable, table.insert, print

local minetest, vector, nodecore
    = minetest, vector, nodecore

local mod_name    = minetest.get_current_modname()
local mod_storage = minetest.get_mod_storage()

-- This is the object that is returned by this script.
local contraption = {}

-- Some constants. Should probably be removed.
local AIR = { name = "air" }
local PLANK = { name = "nc_woodwork:plank" }
local CONTRAPTION_WOOD = { name = "nc_contraptions:contraption_wood" }

-- Counts up to infinity as contraptions get created.
-- Each contraption gets assigned a unique id for its lifetime.
local id_counter = 0

-- Table containing the currently active contraptions keyed by their id.
local active_contraptions = {}

-- TODO: Push along items.
-- TODO: Push along players.
-- TODO: Friction depending on ground.
-- TODO: Gravity.
-- TODO: Water physics!
-- TODO: Rotation.

-- TODO: Unload and save contraptions when they're in unloaded chunks.
-- TODO: Use meta field `infotext` for displaying debug info about a contraption?

local function error(...)
    local str = "[" .. mod_name .. "] "
    for _, v in ipairs({...}) do str = str + tostring(v) end
    print(str)
end

--------------------
-- EVENT HANDLING --
--------------------

function contraption.on_global_step(delta_time)
    for _, contrap in pairs(active_contraptions) do
        contrap:update(delta_time)
    end
end

function contraption.on_startup()
    local version = mod_storage:get_int("version")

    if version == 0 then
        -- Assume that no mod data has been saved yet.
        return
    elseif version ~= 1 then
        error("Unknown mod storage version " .. version)
        return
    end

    id_counter = mod_storage:get_int("id_counter")

    local contraptions_str = mod_storage:get("contraptions")
    local contraptions = minetest.deserialize(contraptions_str)

    for _, id in ipairs(contraptions) do
        local str = mod_storage:get("contraption_" .. id)
        contraption.load_from_string(id, str)
    end
end

function contraption.on_shutdown()
    -- TODO: Only modify mod storage where contraptions have changed / been destroyed.
    --       For now we're just going to wipe the storage and re-create it.
    mod_storage:from_table(nil)

    mod_storage:set_int("version", 1)
    mod_storage:set_int("id_counter", id_counter)

    local contraptions = {}

    for id, contrap in pairs(active_contraptions) do
        local str = contrap:save_to_string()
        mod_storage:set_string("contraption_" .. id, str)

        table_insert(contraptions, id)
    end

    mod_storage:set_string("contraptions", minetest.serialize(contraptions));
end

----------------------
-- PUBLIC FUNCTIONS --
----------------------

function contraption.find(pos)
    -- First check if the node at `pos` is a contraption block.
    for _, c in pairs(active_contraptions) do
        if  pos.x >= c.region.min.x and pos.x <= c.region.max.x
        and pos.y >= c.region.min.y and pos.y <= c.region.max.y
        and pos.z >= c.region.min.z and pos.z <= c.region.max.z
        then
            local rel_pos = pos - c.region.min
            local rel_pos_hash = minetest.hash_node_position(rel_pos)
            if c.nodes[rel_pos_hash] then return c end
        end
    end
    return nil
end

-------------------------------------
-- CONTRAPTION TYPE IMPLEMENTATION --
-------------------------------------

local metatable = { __index = contraption }
contraption.metatable = metatable

function contraption.create(region)
    local nodes = {}
    local num_nodes = 0
    for x = region.min.x, region.max.x do
        for y = region.min.y, region.max.y do
            for z = region.min.z, region.max.z do
                local pos = vector.new(x, y, z)
                local node = minetest.get_node(pos)
                if nodecore.match(node, PLANK) then
                    local offset = pos - region.min
                    local pos_hash = minetest.hash_node_position(offset)
                    nodes[pos_hash] = CONTRAPTION_WOOD
                    num_nodes = num_nodes + 1
                end
            end
        end
    end

    -- Less than 4 nodes don't make a contraption.
    if num_nodes < 4 then return nil end

    -- TODO: Shrink region to the minimum required cuboid.
    --       In the above code we could just use pos as table key.

    -- TODO: Flood fill algorithm to make sure all nodes are connected.

    id_counter = id_counter + 1
    local result = setmetatable({
        id = id_counter,
        region = region,
        nodes = nodes,
        num_nodes = num_nodes,
    }, metatable)

    -- Change nodes from base nodes to their contraption version.
    for pos_hash, node in pairs(nodes) do
        local rel_pos = minetest.get_position_from_hash(pos_hash)
        local abs_pos = vector.add(region.min, rel_pos)
        minetest.set_node(abs_pos, node)
    end

    active_contraptions[result.id] = result
    return result
end

function contraption.load_from_string(id, str)
    local obj = minetest.deserialize(str)

    if obj.version ~= 1 then
        error("Unknown contraption version " .. obj.version)
        return nil
    end

    local result = setmetatable({
        id = id,

        region    = obj.region,
        nodes     = obj.nodes,
        num_nodes = obj.num_nodes,

        motion  = obj.motion  and vector.copy(obj.motion ) or nil,
        partial = obj.partial and vector.copy(obj.partial) or nil,
    }, metatable)

    active_contraptions[id] = result
    return obj
end

function contraption:save_to_string()
    local to_save = {
        version = 1,

        region    = self.region,
        nodes     = self.nodes,
        num_nodes = self.num_nodes,

        motion  = self.motion,
        partial = self.partial,
    }
    return minetest.serialize(to_save)
end

function contraption:destroy()
    active_contraptions[self.id] = nil

    for pos_hash, node in pairs(self.nodes) do
        local rel_pos = minetest.get_position_from_hash(pos_hash)
        local abs_pos = vector.add(self.region.min, rel_pos)
        local def = minetest.registered_nodes[node.name]
        minetest.set_node(abs_pos, def.drop_in_place)
    end
end

function contraption:push(pusher, dir)
    -- See if the same player has already pushed this contraption recently.
    -- If so, do not apply the full amount of motion from the push.
    local pusher_name = pusher.get_player_name and pusher:get_player_name() or ""
    self.recent_pusher = self.recent_pusher or {}
    local delay = self.recent_pusher[pusher_name]
    if delay then dir = dir * (1 - math.min(1, delay)) end
    self.recent_pusher[pusher_name] = 1.0 -- seconds

    self.motion = self.motion or vector.zero()
    self.motion = vector.add(self.motion, dir)
end

function contraption:update(delta_time)
    if not self.motion then return end

    -- Partial motion that accumulates over time,
    -- since we can only move in increments of 1.
    self.partial = self.partial or vector.zero()
    self.partial = self.partial + self.motion * delta_time

    -- If partial motion has increased to more than 1 on
    -- one axis, find out which axis has the largest motion ..
    local max = 0
    local which = 0
    for i = 1, 3 do
        local abs = math.abs(self.partial[i])
        if abs >= 1 and abs > max then
            max = abs
            which = i
        end
    end
    -- .. and move it 1 step into that direction.
    if which ~= 0 then
        local step = vector.zero()
        step[which] = math.sign(self.partial[which])
        if self:move(step) then
            self.partial = self.partial - step
        else
            -- Reset motion into the direction that we bumped into.
            -- Allows for "sliding" along walls instead of stopping outright.
            self.motion[which] = 0
            self.partial[which] = 0
        end
    end

    -- Apply friction.
    local FRICTION = 0.25
    self.motion = self.motion * (1 - FRICTION * delta_time)

    -- If motion is small enough, unset it for performance.
    if self.motion:length() < 0.1 then
        self.motion = nil
        self.partial = nil
    end

    if self.recent_pusher then
        local has_pushers = false
        for pusher, delay in pairs(self.recent_pusher) do
            if delay > 0 then
                self.recent_pusher[pusher] = math.max(0, delay - delta_time)
                has_pushers = true
            end
        end
        -- If there are no active pushers, delete the `recent_pushers` table.
        if not has_pushers then self.recent_pusher = nil end
    end
end

function contraption:move(offset)
    local to_set   = {} -- Nodes that need to be modified.
    local to_clear = {} -- Nodes to be cleared (to AIR).

    -- Assume that every node this contraption occupies needs to be cleared.
    -- We'll remove entries from `to_clear` when we know a node is moved there.
    for pos_hash in pairs(self.nodes) do to_clear[pos_hash] = AIR end

    for old_rel_pos_hash, node in pairs(self.nodes) do
        local old_rel_pos = minetest.get_position_from_hash(old_rel_pos_hash)
        local old_abs_pos = vector.add(self.region.min, old_rel_pos)

        local new_abs_pos = vector.add(old_abs_pos, offset)
        local new_rel_pos = vector.add(old_rel_pos, offset)
        local new_rel_pos_hash = minetest.hash_node_position(new_rel_pos)

        -- Node is being moved to this position, no need to clear here.
        to_clear[new_rel_pos_hash] = nil

        local self_overlap = self.nodes[new_rel_pos_hash]
        if self_overlap then
            if not nodecore.match(node, self_overlap) then
                to_set[new_rel_pos_hash] = node
            end
        else
            local new_node = minetest.get_node(new_abs_pos)
            if nodecore.buildable_to(new_node) then
                to_set[new_rel_pos_hash] = node
            else
                -- We bumped into a wall or something.
                return false
            end
        end
    end

    -- Set nodes that need to be changed.
    for pos_hash, node in pairs(to_set) do
        local rel_pos = minetest.get_position_from_hash(pos_hash)
        local abs_pos = vector.add(self.region.min, rel_pos)
        minetest.set_node(abs_pos, node)
    end

    -- Clear nodes that need to be cleared (to AIR).
    for pos_hash, node in pairs(to_clear) do
        local rel_pos = minetest.get_position_from_hash(pos_hash)
        local abs_pos = vector.add(self.region.min, rel_pos)
        minetest.set_node(abs_pos, node)
    end

    -- Move the contraption in the world.
    self.region.min = self.region.min + offset
    self.region.max = self.region.max + offset
    return true
end

return contraption