nc_contraptions/search.lua

local util = {}

local AXES = {
    x = vector.new(1, 0, 0),
    y = vector.new(0, 1, 0),
    z = vector.new(0, 0, 1),
}
util.AXES = AXES;

-- Gets an array of vectors relative to `pos` of neighbors that have the specified `match`.
local function find_neighbors_with_name(pos, match)
    local result = {}
    for _, v in pairs(AXES) do
        if minetest.get_node(pos - v).name == match then
            table.insert(result, -v)
        end
    end
    for _, v in pairs(AXES) do
        if minetest.get_node(pos + v).name == match then
            table.insert(result, v)
        end
    end
    return result
end
util.find_neighbors_with_name = find_neighbors_with_name;

-- Moves `pos` towards `dir` while node is still `match`.
-- returns (vector, array) or nil
local function move_until_corner(pos, match, dir)
    repeat pos = pos + dir
    until minetest.get_node(pos + dir).name ~= match
    local neighbors = find_neighbors_with_name(pos, match)
    if #neighbors == 2
        then return pos, neighbors
        else return nil
    end
end

-- Moves `pos` towards `dir`, checking that each node has exactly two neighbors
-- that of type `match`, until a corner turning towards `expected_corner_dir` is hit.
-- returns vector or nil
local function move_until_corner_with_checks(pos, match, dir, expected_corner_dir)
    while true do
        pos = pos + dir

        local neighbors = find_neighbors_with_name(pos, match)
        if #neighbors ~= 2 then return nil end

        -- Get the "next direction" the piece is going.
        local next_dir = neighbors[1]
        if next_dir == -dir then next_dir = neighbors[2] end

        if next_dir ~= dir then
            if next_dir == expected_corner_dir
                then return pos
                else return nil
            end
        end
    end
end

-- Finds a rectangle structure made from the specified node `match`.
-- returns { min = vector, max = vector } or nil
local function find_rectangle(pos, match)
    local neighbors = find_neighbors_with_name(pos, match)
    if #neighbors ~= 2 then return nil end

    -- If we found a line piece (neighbors are opposite) then move in the
    -- direction of the smallest xyz coordinate.
    if neighbors[1] == -neighbors[2] then
        -- NOTE: Due to how `get_neighbors_with_name` is written, the first element
        -- is guaranteed to contain the vector pointing towards negative.
        pos, neighbors = move_until_corner(pos, match, neighbors[1])
        if not pos then return nil end
    end

    -- We're now guaranteed to be at a corner piece.
    -- Move to the corner piece with the smallest xyz coordinate.
    while neighbors[1].x + neighbors[1].y + neighbors[1].z < 1 do
        pos, neighbors = move_until_corner(pos, match, neighbors[1])
        if not pos then return nil end
    end

    -- Find the neighboring corners of the starting corner.
    local first_corner = move_until_corner_with_checks(pos, match, neighbors[1], neighbors[2])
    if not first_corner then return nil end
    local second_corner = move_until_corner_with_checks(pos, match, neighbors[2], neighbors[1])
    if not second_corner then return nil end

    -- Continue towards the final corner from the new corners.
    first_corner = move_until_corner_with_checks(first_corner, match, neighbors[2], -neighbors[1])
    if not first_corner then return nil end
    second_corner = move_until_corner_with_checks(second_corner, match, neighbors[1], -neighbors[2])
    if not second_corner then return nil end

    -- Ensure that they have met up at the same location.
    if first_corner ~= second_corner then return nil end

    return { min = pos, max = second_corner }
end
util.find_rectangle = find_rectangle;

return util