#include <stdio.h>
#include <windows.h>
#include <ntdef.h>

int wmain(int argc, wchar_t *argv[]) {
  static BYTE reparse_data_buffer[MAXIMUM_REPARSE_DATA_BUFFER_SIZE];
  REPARSE_DATA_BUFFER *reparse_buffer = (REPARSE_DATA_BUFFER *)reparse_data_buffer;
  DWORD reparse_buffer_length;
  TOKEN_PRIVILEGES privileges;
  DWORD target_length;
  wchar_t *target;
  HANDLE handle;
  HANDLE token;
  BOOL success;
  DWORD i;

  if (argc != 3) {
    printf("Usage: %ls new_file target\n", argv[0]);
    return 1;
  }

  target = argv[2];
  for (i = 0, target_length = 0; target[i]; i++, target_length++) {
    if (target[i] == L'\\') {
      if (target[i+1] == L'\\') {
        target[target_length] = L'\\';
        i++;
        continue;
      } else if (target[i+1] == L'x' && target[i+2] >= L'0' && target[i+2] <= L'9' && target[i+3] >= L'0' && target[i+3] <= L'9') {
        target[target_length] = ((target[i+2]-L'0') << 4) | (target[i+3]-L'0');
        i += 3;
        continue;
      }
    }
    target[target_length] = target[i];
  }
  target_length *= sizeof(target[0]);

  reparse_buffer_length = FIELD_OFFSET(REPARSE_DATA_BUFFER, SymbolicLinkReparseBuffer.PathBuffer) + target_length*2;
  if (reparse_buffer_length > MAXIMUM_REPARSE_DATA_BUFFER_SIZE) {
    printf("Target path is too long\n");
    return 1;
  }

  reparse_buffer->ReparseTag = IO_REPARSE_TAG_SYMLINK;
  reparse_buffer->ReparseDataLength = reparse_buffer_length - REPARSE_DATA_BUFFER_HEADER_SIZE;
  reparse_buffer->SymbolicLinkReparseBuffer.SubstituteNameOffset = 0;
  reparse_buffer->SymbolicLinkReparseBuffer.SubstituteNameLength = target_length;
  reparse_buffer->SymbolicLinkReparseBuffer.PrintNameOffset = target_length;
  reparse_buffer->SymbolicLinkReparseBuffer.PrintNameLength = target_length;
  reparse_buffer->SymbolicLinkReparseBuffer.Flags = SYMLINK_FLAG_RELATIVE;
  memcpy(reparse_buffer->SymbolicLinkReparseBuffer.PathBuffer, target, target_length);
  memcpy(((BYTE*)reparse_buffer->SymbolicLinkReparseBuffer.PathBuffer)+target_length, target, target_length);

  if (OpenProcessToken(GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES, &token)) {
    if (LookupPrivilegeValue(NULL, SE_CREATE_SYMBOLIC_LINK_NAME, &privileges.Privileges[0].Luid)) {
      privileges.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;
      privileges.PrivilegeCount = 1;
      AdjustTokenPrivileges(token, FALSE, &privileges, sizeof(privileges), NULL, NULL);
    }
    CloseHandle(token);
  }

  handle = CreateFileW(argv[1], FILE_WRITE_DATA, FILE_SHARE_VALID_FLAGS, NULL, OPEN_ALWAYS, FILE_FLAG_OPEN_REPARSE_POINT, NULL);
  if (handle == INVALID_HANDLE_VALUE && GetLastError() == ERROR_ACCESS_DENIED)
    handle = CreateFileW(argv[1], FILE_WRITE_DATA, FILE_SHARE_VALID_FLAGS, NULL, OPEN_ALWAYS, FILE_FLAG_OPEN_REPARSE_POINT | FILE_FLAG_BACKUP_SEMANTICS, NULL);
  if (handle == INVALID_HANDLE_VALUE) {
    printf("CreateFileW failed: %lu\n", GetLastError());
    return 1;
  }

  success = DeviceIoControl(handle, FSCTL_SET_REPARSE_POINT, reparse_buffer, reparse_buffer_length, NULL, 0, NULL, NULL);
  CloseHandle(handle);
  if (!success) {
    printf("FSCTL_SET_REPARSE_POINT failed: %lu\n", GetLastError());
    return 1;
  }

  return 0;
}