--- A set of basic functional utilities
local fun = {}

local unpack = table.unpack or unpack

function fun.concat(xs, ys)
   local rs = {}
   local n = #xs
   for i = 1, n do
      rs[i] = xs[i]
   end
   for i = 1, #ys do
      rs[i + n] = ys[i]
   end
   return rs
end

function fun.contains(xs, v)
   for _, x in ipairs(xs) do
      if v == x then
         return true
      end
   end
   return false
end

function fun.map(xs, f)
   local rs = {}
   for i = 1, #xs do
      rs[i] = f(xs[i])
   end
   return rs
end

function fun.filter(xs, f)
   local rs = {}
   for i = 1, #xs do
      local v = xs[i]
      if f(v) then
         rs[#rs+1] = v
      end
   end
   return rs
end

function fun.traverse(t, f)
   return fun.map(t, function(x)
      return type(x) == "table" and fun.traverse(x, f) or f(x)
   end)
end

function fun.reverse_in(t)
   for i = 1, math.floor(#t/2) do
      local m, n = i, #t - i + 1
      local a, b = t[m], t[n]
      t[m] = b
      t[n] = a
   end
   return t
end

function fun.sort_in(t, f)
   table.sort(t, f)
   return t
end

function fun.flip(f)
   return function(a, b)
      return f(b, a)
   end
end

function fun.find(xs, f)
   if type(xs) == "function" then
      for v in xs do
         local x = f(v)
         if x then
            return x
         end
      end
   elseif type(xs) == "table" then
      for _, v in ipairs(xs) do
         local x = f(v)
         if x then
            return x
         end
      end
   end
end

function fun.partial(f, ...)
   local n = select("#", ...)
   if n == 1 then
      local a = ...
      return function(...)
         return f(a, ...)
      end
   elseif n == 2 then
      local a, b = ...
      return function(...)
         return f(a, b, ...)
      end
   else
      local pargs = { n = n, ... }
      return function(...)
         local m = select("#", ...)
         local fargs = { ... }
         local args = {}
         for i = 1, n do
            args[i] = pargs[i]
         end
         for i = 1, m do
            args[i+n] = fargs[i]
         end
         local unpack = unpack or table.unpack
         return f(unpack(args, 1, n+m))
      end
   end
end

function fun.memoize(fn)
   local memory = setmetatable({}, { __mode = "k" })
   local errors = setmetatable({}, { __mode = "k" })
   local NIL = {}
   return function(arg)
      if memory[arg] then
         if memory[arg] == NIL then
            return nil, errors[arg]
         end
         return memory[arg]
      end
      local ret1, ret2 = fn(arg)
      if ret1 ~= nil then
         memory[arg] = ret1
      else
         memory[arg] = NIL
         errors[arg] = ret2
      end
      return ret1, ret2
   end
end

return fun