[REBOL] Re: On mutability and sameness
From: lmecir:mbox:vol:cz at: 7-Jun-2001 11:02
The problem with the proof of DATE! mutability is, that it doesn't work. The
same "proof" could e.g. prove, that integers are mutable:
a: 11
bm [a/1] ; == 1
bm [a/1: 0] ; == 0
a ; == 9
block1: [11]
block2: :block1
bm [block1/1/1] ; == 1
bm [block1/1/1: 0] ; == 0
block1/1 ; == 9
same? block2 block1 ; == true
The bit-manipulation is described below:
bit: func [
i [integer!]
bitno [integer!]
] [
(to integer! i / (2 ** bitno)) // 2
]
bm: function [
block [block!]
] [path part-path integer bitno old-bit new-bit to-add len block-path] [
(path: copy first block)
(block-path: to block! :path)
len: (length? :block-path) - 1
(part-path: to path! copy/part block-path len)
(integer: part-path)
(bitno: last block-path)
(old-bit: bit integer bitno)
either path? :path [
return old-bit
] [
(part-path: load mold to set-path! copy/part block-path len)
(part-path: first bind reduce [:part-path] first :path)
new-bit: second block
if old-bit <> new-bit [
to-add: to integer! 2 ** bitno
if new-bit < old-bit [to-add: - to-add]
part-path integer + to-add
]
new-bit
]
]
