World: r4wp

>> head rle/into "ddeeee" rle/into "aaabbc" make block! 10
== [3 #"a" 2 #"b" 1 #"c" 2 #"d" 4 #"e"]

Functions with /into work like INSERT when chained together. This 
works with other block types too, though OFFSET? will be slow with 
the list! type.

now that's a pretty nice RLE encoder.     :-)

R3 version, same /into option:


rle: funct ["Run length encode" s [series!] /into output [any-block!]] 
[
	unless into [output: make block! 2] x: none

 r: either any-block? :s [qr: copy [quote 1] [(qr/2: :x) any qr]] 
 [[any x]]
	parse/case :s [any [pos1: set x skip r pos2: (

  output: reduce/into [subtract index? :pos2 index? :pos1 :x] :output
	)]]
	either into [:output] [head :output]
]

Sorry, same unset problems, have to use POKE:


rle: funct ["Run length encode" s [series!] /into output [any-block!]] 
[
	unless into [output: make block! 2] x: none

 r: either any-block? :s [qr: copy [quote 1] [(poke qr 2 :x) any qr]] 
 [[any x]]
	parse/case :s [any [pos1: set x skip r pos2: (

  output: reduce/into [subtract index? :pos2 index? :pos1 :x] :output
	)]]
	either into [:output] [head :output]
]

>> mold/all rle reduce [() () 'a () 1]
== "[2 #[unset!] 1 a 1 #[unset!] 1 1]"

It is what I like about this community :)

I knew that when I write a RLE function, BrianH will come up a much 
better version. Doc and others joined as well and now we have a very 
good function. Just like the CSV tools. Thanks.

Ehm.. what about the decoder? how do I decode unset! values?
I was using somthing like:

decode-rle: func [b /local r] [r: copy [] foreach [x y] b [loop x 
[append r y]]]

decode-rle: func [b /local r i] [

 i: 0  r: make block! foreach [x y] b [i: i + x] ;better for big blocks?
 foreach [x y] b [loop x [append r y]]
]

In mezzanine style:

decode-rle: func [
	"Decode a run length encoded block"
	rle [any-block!] "Block of [integer value]"

 /into "Insert into a buffer instead (returns position after insert)"
	output [series!] "The output buffer (modified)"
	/local x
] [
	unless into [
		x: 0  foreach [i v] :rle [x: x + :i]  output: make block! x
	]

 foreach [i v] :rle [output: insert/only/dup :output get/any 'v :i]
	either into [:output] [head :output]
]


Instead of testing for strict format compliance of the input block, 
it uses get-words to keep people from sneaking in functions and then 
passes the length value to + and INSERT/dup, counting on the type 
tests of those functions to do the screening for us.

You're right, having the make block! take the foreach expression 
as a parameter is safe; I forgot that make block! can take none as 
a parameter.

That should work in R3 as well. Though FORSKIP might be faster than 
FOREACH in R3, the simplicity of the code might be worth it.

I like the detail of using  :i   to prevent function hacking.   I 
should use it more often.

Trick I picked up when securing the mezzanines. It's slightly faster 
to evaluate too since it does less work.

The reason I use :output there isn't to prevent function hacking, 
it's to prevent converting lit-path! values to the path! type.

Sometimes you want to allow someone to pass in functions and then 
let them evaluate, as long as you have a good semantic model for 
what is supposed to happen and are careful about how you call them. 
The ARRAY, EXTRACT and REPLACE functions in R3 and R2 2.7.7+ are 
a good example of this.

Slightly more optimal version for R3, taking advantage of how get-words 
and get-paths mean GET/any, and how FORSKIP is faster than FOREACH:

decode-rle: func [
	"Decode a run length encoded block"
	rle [any-block!] "Block of [integer value]"

 /into "Insert into a buffer instead (returns position after insert)"
	output [series!] "The output buffer (modified)"
	/local x
] [
	unless into [
		x: 0  output: make block! forskip rle 2 [x: x + :rle/1]
	]
	forskip rle 2 [output: insert/only/dup :output :rle/2 :rle/1]
	either into [:output] [head :output]
]

these would be nice funcs to add to mezz in R3 and R2-forwards

Darn, just found a bug in ARRAY for R2 and R3. Litwords are converted 
to words and litpaths are converted to paths.

This is so obscure that I doubt it has affected any existing code 
though.

Discovered that d: 1.1.1 then d/1/2: 0 and d:/1/3: 0 and then d/1/2: 
d/1/3: 1 results in d == 1.1.0 ??

This keeps me just inches away from releasing my script before my 
holiday.

Only thing to add is 1 small function to reduce the moves when the 
king is under attack. I discovered some weird VID behaviour too where 
alert boxes have strange formats they inherited from earlier defines 
fields.

An alternative for R3 (strings and blocks)
rle: func [s /local p e o][
	o: copy []
	parse/case s [
		any [

   p: skip any [e: if (p/1 == e/1) skip] (repend o [offset? p e p/1])
		]
	]
	o
]

Steeve, that's basically the same as my R2 RLE's block rule, but 
with the IF workaround replaced with IF. It has a few gotchas:
- Executes function values in block data
- Doesn't handle unset! or error! values

- Converts lit-paths to paths and lit-words to words before comparison 
and again before putting in the output.
- Lots of intermediate block creation overhead

- Considers bindings of words when comparing them, not just case-sensitive 
spelling


The first 3 can be handled by using :p/1 and :e/1 instead of p/1 
and e/1, and the fourth by using REDUCE/into instead of REPEND. The 
last one can't be handled by any built-in function or operator in 
R3 (see http://issue.cc/r3/1834for details) but you could do a combination 
of functions and operators to get case-sensitive comparison without 
considering bindings. PARSE/case's QUOTE operation is the fastest 
method for doing that at the moment.


Nice job on neatly bypassing the relaxed bounds checking of R3 blocks. 
Though the if (p/1 == e/1) would succeed if p/1 is none and e is 
at the end of the block, the skip would still fail. That trick saves 
one e: operation.

R3's == operator handles unset and error values better than R2's 
though, which is why the explicit unset! testing in the rule can 
be removed.

Having a strict line of progression for the R3 equalities turned 
out to be a bad idea, since the binding check seems to be tripping 
up case checks.

Unfortunately, R3 development was put on hold before that could be 
fixed.

If you *want* to consider binding when doing your comparisons, perhaps 
for more lossless in-memory compression, then Steeve's IF == method 
is the way to go. If you want true lossless compression then you 
could even use =? to make sure that only runs of exact references 
compress.

The advantages of == or =? comparison over PARSE QUOTE would be lost 
if you serialize the data and save it to a file or send it over a 
network. REBOL syntax doesn't keep track of those distinctions.

The PARSE IF method does let you add a /compare function option though, 
so you can be as specific as you want. Instead of if (:p/1 == :e/1) 
you would do if (apply :f [:p/1 :e/1]) then pass :== or :strict-equal? 
as a parameter..

Here's a version of my last one above, but with Steeve's trick adapted 
to make a /compare option. It defaults to its old case-sensitive 
behavior.

rle: func [
	"Run length encode to series of [length value]"
	s [series!] "The series to encode"

 /into {Insert into a buffer instead (returns position after insert)}
	output [any-block!] "The output buffer (modified)"
	/compare "Comparator function for equvilance"
	comparator [any-function!]
	/local x r qr b e
] [
	unless into [output: make block! 2] x: none
	r: case [
		compare [[any [e: if (apply :comparator [:x :e/1]) skip]]]
		any-string? :s [[any x]]
		'else [qr: copy [quote 1] [(poke qr 2 :x) any qr]
	]
	parse/case :s [any [b: set x skip r e: (
		output: reduce/into [offset? :b :e :x] :output
	)]]
	either into [:output] [head :output]
]

Whoops, forgot a bracket:

rle: func [
	"Run length encode to series of [length value]"
	s [series!] "The series to encode"

 /into {Insert into a buffer instead (returns position after insert)}
	output [any-block!] "The output buffer (modified)"
	/compare "Comparator function for equvilance"
	comparator [any-function!]
	/local x r qr b e
] [
	unless into [output: make block! 2] x: none
	r: case [
		compare [[any [e: if (apply :comparator [:x :e/1]) skip]]]
		any-string? :s [[any x]]
		'else [qr: copy [quote 1] [(poke qr 2 :x) any qr]]
	]
	parse/case :s [any [b: set x skip r e: (
		output: reduce/into [offset? :b :e :x] :output
	)]]
	either into [:output] [head :output]
]

There's no need to store lengths of 1 in the output.
Length of 1 can be infered during decoding.
The compression ratio woulb be better.

Length of 1 can't be inferred in the decoding, not for blocks.
>> rle [1 2 2 3 3 3 4 4 4 4 5 5 5 5 5 6 6 6 6 6 6]
== [1 1 2 2 3 3 4 4 5 5 6 6]

Unless you make it so it treats integers specially. This would slow 
down the encoder and decoder, but reduce the compressed size.

and... you can't compress numbers anymore... forget it

You could reduce the compressed size of a string-specific RLE by 
putting runs of singletons into strings, like this:
>> rle "Hello World!"
== ["He" 2 #"l" "o World!"]

LZ77 could replace RLE. It would do RLE + patterns compression

I don't think it would be hard to code with parse

Agreed. Not that many repetive runs of characters in string data, 
so a better compression method would be preferable.

RLE is better for image data. Any takers?

I've some code when I studied 8bit computer data crunchers

But I didn't use parse at that time

RLE might help for binary data too, including that reduced encoding 
I mentioned for strings above.

Thanks for that Ladislav (median calculation)

>> do http://reb4.me/r/altjson.r
connecting to: reb4.me
Script: "REBOL <-> JSON" (15-Jul-2011)
>> j: to-json make object! [ b: none ]
== {{"b":null}}

but when you submit this as a JSON parameter ... it fails

Javascript produces the exact same output:

> JSON.stringify({b:null});
'{"b":null}'

so, the problem has to be somewhere else.