-- Convert 3d relative coordinates to an index on a flat array
local function from_3d_to_flat(dx, dy, dz, nx, ny)
    return (nx * ny * dz) + (nx * dy) + dx + 1
end

-- Convert an index on a flat array to 3d relative coordinates
local function from_flat_to_3d(i, nx, ny)
    return {
        dx = (i - 1) % nx,
        dy = (i - 1) // nx % ny,
        dz = (i - 1) // nx // ny
    }
end

-- Iterate over a 3d area, both indexing by the index and the ansolute position
local function iter_3d_area(minp, maxp, callback)
    local nx = maxp.x - minp.x + 1
    local ny = maxp.y - minp.y + 1
    local nz = maxp.z - minp.z + 1

    for i = 1, nx * ny * nz do
        local dpos = from_flat_to_3d(i, nx, ny)
        local pos = {
            x = minp.x + dpos.x,
            y = minp.y + dpos.y,
            z = minp.z + dpos.z,
        }

        callback(i, pos)
    end
end

-- Get an enhanced function from the def function that warns us when the
-- function does not behave properly
local function enhanced_func(def)
    if type(def.name) ~= "string" then
        error("Invalid nameless shape definition")
    elseif type(def.func) ~= "function" then
        error(
            "Invalid shape definition misses an adjustment function"
        )
    else
        return function(pos, n)
            local out = def.func(pos, n)

            if type(out) == "number" then
                return out
            elseif n == nil then
                error(
                    "Shape " .. def.name .. " function must return a number. The input `n` was nil. Perhaps your `noise_params` field is invalid?"
                )
            else
                error("Shape " .. def.name .. " function must return a number.")
            end
        end
    end
end

-- Get a flat array of cave shape noise values from a given cave shape def
local function get_flat_from_shape_def(def, minp, maxp)
    local f = enhanced_func(def)

    local nx = maxp.x - minp.x + 1
    local ny = maxp.y - minp.y + 1
    local nz = maxp.z - minp.z + 1

    local noise_flat_map = {}

    -- If noise parameters have been defined, fill the table with noise
    -- If not, all values remain nil
    if def.noise_params ~= nil then
        local p = PerlinNoiseMap(def.noise_params, { x = nx, y = ny, z = nz })
        
        if nz == 1 then
            p:get_2d_map_flat(minp, noise_flat_map)
        else
            p:get_3d_map_flat(minp, noise_flat_map)
        end
    end
    
    iter_3d_area(minp, maxp, function(i, pos)
        noise_flat_map[i] = f(pos, noise_flat_map[i])
    end)

    return noise_flat_map
end

local function get_flat_from_noise_params(minp, maxp, noise_params)
    local nx = maxp.x - minp.x + 1
    local ny = maxp.y - minp.y + 1
    local nz = maxp.z - minp.z + 1

    local buffer = {}

    local p = PerlinNoiseMap(noise_params, { x = nx, y = ny, z = nz })

    if nz == 1 then
        p:get_2d_map_flat(minp, buffer)
    else
        p:get_3d_map_flat(minp, buffer)
    end

    return buffer
end

-- Based on the number of cave shapes, calculate how quickly connectivity is
-- meant to change
local function get_connectivity_noise_params(shape_size)
    local factor = math.max(math.abs(shape_size) ^ 0.5, 1)

    return {
        offset = 50,
        scale = 50,
        spread = { x = factor * 250, y = factor * 100, z = factor * 250 },
        seed = 297948,
        octaves = 2,
        persistence = 0.2,
        lacunarity = 2.0,
        flags = "eased"
    }
end

-- Based on the number of cave shapes, calculate how quickly verticality is
-- meant to change
local function get_verticality_noise_params(shape_size)
    local factor = math.max(math.abs(shape_size) ^ 0.5, 1)

    return {
        offset = 50,
        scale = 50,
        spread = { x = factor * 100, y = factor * 250, z = factor * 100 },
        seed = 35644,
        octaves = 2,
        persistence = 0.2,
        lacunarity = 2.0,
        flags = "eased"
    }
end

-- Get the distance of each cave shape to calculate their weight.
local function shape_distance_weight(shape_size, dx, dy)
    local factor = math.max(math.abs(shape_size) ^ 0.5, 1)
    local max_distance = 100 / factor

    if dx^2 + dy^2 > max_distance^2 then
        return 0
    else
        return 1 / (dx^5 + dy^5)
    end
end

-- Get a flat map containing all flat threshold values
local function get_threshold_flat(minp, maxp)
    local connectivity = get_flat_from_noise_params(
        minp, maxp,
        get_connectivity_noise_params(#noordstar_caves.registered_shapes)
    )
    local verticality = get_flat_from_noise_params(
        minp, maxp,
        get_verticality_noise_params(#noordstar_caves.registered_shapes)
    )

    local noise = {}

    -- Get noise for all cave shapes
    for key, def in pairs(noordstar_caves.registered_shapes) do
        noise[key] = {
            key = key,
            noise = get_flat_from_shape_def(def, minp, maxp),
            def = def
        }
    end

    local thresholds = {}

    -- Fill the table
    iter_3d_area(minp, maxp, function(i, pos)
        local total = 0
        local count = 0

        local x = connectivity[i]
        local y = verticality[i]

        for _, n in pairs(noise) do
            local v = n.noise[i]

            local dx = math.abs(x - n.def.connectivity)
            local dy = math.abs(y - n.def.verticality)

            local w = math.abs(shape_distance_weight(
                #noordstar_caves.registered_shapes, dx, dy
            ))

            total = total + v * w
            count = count + w
        end

        if count <= 0 then
            thresholds[i] = -1000
        else
            thresholds[i] = total / count
        end
    end)

    return thresholds
end


-- Old minimum height for the overworld
local old_overworld_min = mcl_vars.mg_overworld_min

-- If another mod doesn't override the maximum world depth, we will assume that
-- the world depth is the following value.
local world_depth = -800

-- Otherwise, this variable can be changed using the following function,
-- which will also update all the other necessary variables
function noordstar_caves.set_world_depth(h)
    -- Set world depth variable
    world_depth = h
end
noordstar_caves.set_world_depth(world_depth)

-- The `cave_vastness` is a variable that determines the size of caves.
-- The function takes in an x, y and z variable, and returns a number between
-- 0 and 1.
-- Low numbers mean very rare and tiny caves, while high numbers mean massive
-- caves, with massive open spaces.
-- If you wish to overwrite this function, it is good to keep in mind to:
--  - Make sure that the output changes VERY SLOWLY over time
--  - This function will be run a LOT so it is very performance sensitive
noordstar_caves.cave_vastness = function(pos)
    return math.abs(pos.y) / world_depth
end

-- Secretly, we're using an internal function that also adds a safe layer for
-- when we're approaching bedrock levels
local function cave_vastness(pos)
    if world_depth + 20 < pos.y then
        return noordstar_caves.cave_vastness(pos)
    elseif world_depth + 5 < pos.y then
        return noordstar_caves.cave_vastness(pos) * math.abs(y - world_depth - 5) / 15
    else
        return -1000
    end
end

minetest.register_on_generated(function(minp, maxp, blockseed)
    -- Get voxelmanip
    local vm = minetest.get_mapgen_object("voxelmanip")
    local data = vm:get_data()
    
    -- Get threshold values
    local thresholds = get_threshold_flat(minp, maxp)
    local air = minetest.get_content_id("air")

    iter_3d_area(minp, maxp, function(i, pos)
        if thresholds[i] >= cave_vastness(pos) then
            data[i] = air
        end
    end)

    -- Write all changes to the Minetest world
    vm:set_data(data)
    vm:write_to_map()
end)