How to resolve the algorithm UTF-8 encode and decode step by step in the Forth programming language

Published on 12 May 2024 09:40 PM
#Forth

How to resolve the algorithm UTF-8 encode and decode step by step in the Forth programming language

Table of Contents

Problem Statement

As described in UTF-8 and in Wikipedia, UTF-8 is a popular encoding of (multi-byte) Unicode code-points into eight-bit octets. The goal of this task is to write a encoder that takes a unicode code-point (an integer representing a unicode character) and returns a sequence of 1–4 bytes representing that character in the UTF-8 encoding. Then you have to write the corresponding decoder that takes a sequence of 1–4 UTF-8 encoded bytes and return the corresponding unicode character. Demonstrate the functionality of your encoder and decoder on the following five characters: Provided below is a reference implementation in Common Lisp.

Let's start with the solution:

Step by Step solution about How to resolve the algorithm UTF-8 encode and decode step by step in the Forth programming language

Source code in the forth programming language

: showbytes ( c-addr u -- )
    over + swap ?do
	i c@ 3 .r loop ;

: test {: xc -- :}
    xc xemit xc 6 .r xc pad xc!+ pad tuck - ( c-addr u )
    2dup showbytes drop xc@+ xc <> abort" test failed" drop cr ;

hex
$41 test $f6 test $416 test $20ac test $1d11e test
\ can also be written as
\ 'A' test 'ö' test 'Ж' test '€' test '𝄞' test


-77 Constant UTF-8-err

$80 Constant max-single-byte

: u8@+ ( u8addr -- u8addr' u )
    count  dup max-single-byte u< ?EXIT  \ special case ASCII
    dup $C2 u< IF  UTF-8-err throw  THEN  \ malformed character
    $7F and  $40 >r
    BEGIN  dup r@ and  WHILE  r@ xor
	    6 lshift r> 5 lshift >r >r count
	    dup $C0 and $80 <> IF   UTF-8-err throw  THEN
	    $3F and r> or
    REPEAT  rdrop ;

: u8!+ ( u u8addr -- u8addr' )
    over max-single-byte u< IF  tuck c! 1+  EXIT  THEN \ special case ASCII
    >r 0 swap  $3F
    BEGIN  2dup u>  WHILE
	    2/ >r  dup $3F and $80 or swap 6 rshift r>
    REPEAT  $7F xor 2* or  r>
    BEGIN   over $80 u>= WHILE  tuck c! 1+  REPEAT  nip ;

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