-- stack.lua

local type, select, error, coyield, cowrap, corunning =
type, select, error, coroutine.yield, coroutine.wrap, coroutine.running

local dummy = {} -- simple dummy

local func = {}

function func.push(f, op, val, self, modcount, ...)
  local nop, nv = coyield()
  return f(nop, nv, self, modcount + 1, val, ...)
end

function func.pop(f, op, val, self, modcount, ...)
  local nop, nv = coyield((...))
  return f(nop, nv, self, modcount + 1, select(2, ...))
end

function func.peek(f, op, val, self, modcount, ...)
  local nop, nv = coyield((...))
  return f(nop, nv, self, modcount + 1, ...)
end

function func.iter(f, op, val, self, modcount, ...)
  local nop, nv = coyield(self, modcount, 0)
  return f(nop, nv, self, modcount, ...)
end

local function stack(op, val, self, modcount, ...)
  if op == dummy and val == dummy then
    local nop, nv = coyield()
    return stack(nop, nv, self, modcount, ...)
  end
  if type(op) == "number" and type(val) == "number" then -- iterator
    if op ~= modcount then error("modified while iterating") end
    local val = val + 1
    if val > select("#", ...) then return nil end
    local v = select(val, ...)
    local nop, nv = coyield(val, v)
    return stack(nop, nv, self, modcount, ...)
  end
  return func[op](stack, op, val, self, modcount, ...)
end


local function newstack(...)
  local s = cowrap(stack)
  s(dummy, dummy, s, 0, ...)
  return s
end

return {newstack = newstack}