! Copyright (C) 2023 CapitalEx.
! See https://factorcode.org/license.txt for BSD license.
USING: accessors combinators io.encodings.binary io.files kernel
math ranges sequences ;
IN: ulz

TUPLE: decoder data ip out ;

: <decoder> ( file -- decoder )
    binary file-contents 0 SBUF"" clone \ decoder boa ;

: copy-forward ( src to from -- )
    [a..b) dupd [ swap nth swap push ] 2with each ;

: next-by ( decoder length -- decoder ) '[ _ + ] change-ip ; inline
: next    ( decoder        -- decoder ) 1 next-by ; inline
: current ( decoder        -- byte    ) [ ip>> ] [ data>> ] bi ?nth ;

: current2 ( decoder -- dbyte )
    [ current 8 shift ] [ next current ] bi + ;

: lit ( decoder length -- )
    { [ drop out>> ] 
      [ drop ip>> ] 
      [ swap ip>> + ] 
      [ drop data>> ] } 2cleave subseq append! drop ;

: cpy-start ( decoder offset -- i ) neg swap out>> length + ;

: cpy ( decoder length offset -- )
    { [ 2drop out>> ] 
      [ nip cpy-start ] 
      [ swap [ cpy-start ] dip + ] } 3cleave copy-forward ;

GENERIC: interpret ( decoder -- decoder )

M: predicate{ decoder [ current 0xc0 bitand 0x00 = ] } interpret
    [ current ] [ next ] bi swap 1 + [ lit ] [ next-by ] 2bi ;

M: predicate{ decoder [ current 0xc0 bitand 0x80 = ] } interpret
    [ current 0x3F bitand 4 + ] 
    [ next current 1 + ] [ next ] tri -rot [ cpy ] keepdd ;

M: predicate{ decoder [ current 0xc0 bitand 0xc0 = ] } interpret
    [ current2 0x3FFF bitand 4 + ]
    [ next current 1 + ] [ next ] tri -rot [ cpy ] keepdd ;

: decode ( file -- string )
    <decoder> 
        [ dup current ] 
        [ interpret   ] 
    while out>> ;