hex.pointers.read_byte_from_inners_ptrs

hex.pointers.read_byte[:2] = *ptr — read one byte through the currently-set flip/jump pointers. Use after hex.pointers.set_flip_and_jump_pointers.

Signature

def read_byte_from_inners_ptrs @ read_ptr_and_flip_back, cleanup < hex.pointers.ret_after_read_byte, hex.pointers.read_byte, hex.pointers.to_jump, hex.pointers.to_flip { ... }

Defined in hex/pointers/xor_from_pointer.fj — lines 29–54 (view on GitHub).

Complexity

  • Time: 5@+13

  • Space: 5@+24

See the complexity glossary for what @, w, dw, dbit, n mean.

Source

Click to view the macro body

def read_byte_from_inners_ptrs @ read_ptr_and_flip_back, cleanup \
        < hex.pointers.ret_after_read_byte, hex.pointers.read_byte, hex.pointers.to_jump, hex.pointers.to_flip {
    // 1.  setup:
    //       zero read_byte.
    //       to_flip   = ptr + dbit+8.
    //       to_jump+w = ptr.
    hex.zero 2, hex.pointers.read_byte
    wflip hex.pointers.to_flip, dbit+8

    // 2.  *(ptr+w)^=256, so that now *(ptr+w) == 256 + original_value.
    wflip hex.pointers.to_flip+w, read_ptr_and_flip_back, hex.pointers.to_flip

    pad 4
  read_ptr_and_flip_back:
    // 3.  Jump to *(ptr+w). It will xor the pointed original_value byte into hex.pointers.read_byte.
    // 4.  Then jump to hex.pointers.to_flip to make *(ptr+w)==original_value back again.
    // 5.  Then return to cleanup.
    wflip hex.pointers.to_flip+w, read_ptr_and_flip_back^cleanup
    wflip hex.pointers.ret_after_read_byte+w, hex.pointers.to_flip, hex.pointers.to_jump

  cleanup:
    // 6.  to_flip = ptr, clean jump-back addresses.
    wflip hex.pointers.ret_after_read_byte+w, hex.pointers.to_flip
    wflip hex.pointers.to_flip, dbit+8
    wflip hex.pointers.to_flip+w, cleanup
}

Depends on

Used by

Example uses

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