memoize.lua

local memoize = {
  _VERSION     = 'memoize v2.0',
  _DESCRIPTION = 'Memoized functions in Lua',
  _URL         = 'https://github.com/kikito/memoize.lua',
  _LICENSE     = [[
    MIT LICENSE

    Copyright (c) 2018 Enrique García Cota

    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the
    "Software"), to deal in the Software without restriction, including
    without limitation the rights to use, copy, modify, merge, publish,
    distribute, sublicense, and/or sell copies of the Software, and to
    permit persons to whom the Software is furnished to do so, subject to
    the following conditions:

    The above copyright notice and this permission notice shall be included
    in all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
    OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
    MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
    IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
    CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
    TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
    SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  ]]
}
-- Inspired by http://stackoverflow.com/questions/129877/how-do-i-write-a-generic-memoize-function

-- Lua 5.3 compatibility
local unpack = unpack or table.unpack

-- private stuff

local function is_callable(f)
  local tf = type(f)
  if tf == 'function' then return true end
  if tf == 'table' then
    local mt = getmetatable(f)
    return type(mt) == 'table' and is_callable(mt.__call)
  end
  return false
end

local function cache_get(cache, params)
  local node = cache
  for i=1, #params do
    node = node.children and node.children[params[i]]
    if not node then return nil end
  end
  return node.results
end

local function cache_put(cache, params, results)
  local node = cache
  local param
  for i=1, #params do
    param = params[i]
    node.children = node.children or {}
    node.children[param] = node.children[param] or {}
    node = node.children[param]
  end
  node.results = results
end

-- public function

function memoize.memoize(f, cache)
  cache = cache or {}

  if not is_callable(f) then
    error(string.format(
            "Only functions and callable tables are memoizable. Received %s (a %s)",
             tostring(f), type(f)))
  end

  return function (...)
    local params = {...}

    local results = cache_get(cache, params)
    if not results then
      results = { f(...) }
      cache_put(cache, params, results)
    end

    return unpack(results)
  end
end

setmetatable(memoize, { __call = function(_, ...) return memoize.memoize(...) end })

return memoize