local ffi = require"ffi"
-- We write our own machine code, which is currently only done for x86.
assert(ffi.arch == "x86")
-- The definitions we want to use.
ffi.cdef[[
  typedef void* HWND;
  typedef bool (*WNDENUMPROC)(HWND, long);
  bool EnumChildWindows(HWND hWndParent, WNDENUMPROC lpEnumFunc, long lParam);
  int GetWindowTextA(HWND hWnd, char* lpString, int nMaxCount);
]]
-- Extra definitions we need for performing contortions with fibers.
ffi.cdef[[
  void* ConvertThreadToFiber(void* lpParameter);
  void SwitchToFiber(void* lpFiber);
  typedef void (*LPFIBER_START_ROUTINE)(void*);
  void* CreateFiber(size_t dwStackSize, LPFIBER_START_ROUTINE lpStartAddress, void* lpParameter);
  uint32_t GetLastError(void);
  void* VirtualAlloc(void* lpAddress, size_t dwSize, uint32_t flAllocationType, uint32_t flProtect);
]]

local EnumChildWindows
do
  local GetLastError = ffi.C.GetLastError
  local contortion_fiber
  local procs
  local transfer_slot
  local init_callbacks
  init_callbacks = function()
    -- Ensure that the thread is a fiber, converting if required.
    local our_fiber = ffi.C.ConvertThreadToFiber(nil)
    if our_fiber == nil and GetLastError() ~= 1280 then
      error("Unable to convert thread to fiber")
    end
    transfer_slot = ffi.new("void*[2]")
    -- fiber_proc: for(;;) {
    --               EnumChildWindows(transfer_slot[0], enum_proc, 0);
    --               transfer_slot[1] = 0; // to mark end of iteration
    --               SwitchToFiber(our_fiber);
    --             }
    local asm = "\x6A\x00" -- push 0
             .. "\x68????" -- push ????
             .. "\xA1????\x50" -- mov eax, dword ptr [????], push eax
             .. "\xE8????" -- call ????
             .. "\xC7\x05????\x00\x00\x00\x00" -- mov dword ptr [????], 0
             .. "\x68????" -- push ????
             .. "\xE8????" -- call ????
             .. "\xEB\xD8" -- jmp $-40
    -- enum_proc: transfer_slot[0] = *(esp+4); // the HWND
    --            SwitchToFiber(our_fiber);
    --            return TRUE;
             .. "\x8B\x44\x24\x04" -- mov eax, dword ptr [esp+4]
             .. "\x3E\xA3????" -- mov dword ptr [????], eax
             .. "\x68????" -- push ????
             .. "\xE8????" -- call ????
             .. "\x33\xC0\x40" -- mov eax, 1
             .. "\xC2\x08" -- retn 8 (*)
    procs = ffi.C.VirtualAlloc(nil, #asm + 1, 0x3000, 0x40)
    if our_fiber == nil then
      -- GetCurrentFiber()
      ffi.copy(procs, "\x64\xA1\x10\x00\x00\x00\xC3") -- return __readfsdword(0x10)
      our_fiber = ffi.cast("void*(*)(void)", procs)()
    end
    ffi.copy(procs, asm)
    local function fixup(offset, ptr, isrelative)
      local dst = ffi.cast("char*", procs) + offset
      ptr = ffi.cast("char*", ptr)
      if isrelative then
        ptr = ffi.cast("char*", ptr - (dst + 4))
      end
      ffi.cast("char**", dst)[0] = ptr
    end
    fixup( 3, ffi.cast("char*", procs) + 40)
    fixup( 8, transfer_slot)
    fixup(14, ffi.C.EnumChildWindows, true)
    fixup(20, transfer_slot + 1)
    fixup(29, our_fiber)
    fixup(34, ffi.C.SwitchToFiber, true)
    fixup(46, transfer_slot)
    fixup(51, our_fiber)
    fixup(56, ffi.C.SwitchToFiber, true)
    contortion_fiber = ffi.C.CreateFiber(1024, ffi.cast("LPFIBER_START_ROUTINE", procs), nil)
    init_callbacks = function() end
  end
  EnumChildWindows = function(wnd)
    init_callbacks()
    transfer_slot[0] = wnd
    transfer_slot[1] = ffi.cast("void*", 1)
    local results = {}
    while true do
      ffi.C.SwitchToFiber(contortion_fiber)
      if transfer_slot[1] == nil then
        return results
      else
        results[#results + 1] = transfer_slot[0]
      end
    end
  end
end

-- Example of using EnumChildWindows()
-- it takes a HWND and returns a table of HWNDs
local namebuf = ffi.new("char[300]")
for _, wnd in ipairs(EnumChildWindows(nil)) do
  local len = ffi.C.GetWindowTextA(wnd, namebuf, 300)
  if len ~= 0 then
    print(ffi.string(namebuf, len))
  end
end