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--- en:pfw:string_handling [2023-09-04 18:22] – gelöscht - Externe Bearbeitung (Unbekanntes Datum) 127.0.0.1
+++ en:pfw:string_handling [2024-04-17 15:59] (current) – [Generic Forth] willem
@@ Line 1: / Line 1: @@
+{{pfw:banner.png}}
+====== String handling ======
+===== The idea =====
+Character strings are mostly associated with dynamic memory and garbage collection.
+That is an overkill with the string handling that is used in most Forth programs.
+In particular we can get by with buffers that are statically allocated using CREATE.
+It is still useful to lift manipulating single characters to manipulating strings as a whole.
+(We define) A few words that make string manipulation in forth a little smoother.\\
+Original idea Albert Nijhof & [[https://home.hccnet.nl/a.w.m.van.der.horst/index.html|Albert van der Horst]]. Examples are:
+  * Manipulate files
+  * Start programs
+  * Add, delete and use folders/directories
+  * Etc.
+===== Construction of strings =====
+Strings in Forth are of the type address & length. The length is stored in front of the string.
+There are two views possible. The classic view is to store the length in a byte.
+The so called counted strings, as is shown in the picture:
+{{https://user-images.githubusercontent.com/11397265/142727480-4cb13037-c118-4d05-9eec-529aeaf23cad.jpg| string usage example}}
+===== Pseudo code =====
+<code>
+Function: $VARIABLE
+    reserve a buffer for the count-byte + 'maxlen' characters
+    Alternatively" reserve a buffer for the count-cell + 'maxlen' characters
+    Define: ( maxlen "name" -- )
+          Save maxlen & buffer-address
+    Action: ( -- s )
+          Leave address of string variable
+Function: $@   ( s -- c )
+  Read counted string from address
+Function: $+!  ( c s -- )
+  Extend counted string at address
+Function: $!   ( c s -- )
+  Store counted string at address
+Function: $.   ( c -- )
+  Print counted string
+Function: $C+! ( char s -- )
+  Add one character to counted string at address
+</code>
+The original idea also contains : <code> $^ $? $/ $\ </code>
+See the reference in the introduction.
+Two tools, idea Albert Nijhof:
+<code>
+Function: -HEAD ( adr len i -- adr' len' ) cut first 'i' characters from string
+Function: -TAIL ( adr len i -- adr len' )  cut last  'i' characters from string
+</code>
+However that flies in the face of the goals mentionned in the introduction.
+We promised to get rid of characters, never count characters, only concern ourselves with strings.
+A better example in this context is:
+<code>
+Function: -TRAILING ( c -- c' ) remove trailing blanks space from string.
+Function: -LEADING  ( c -- c' ) remove leading  blanks space from string.
+</code>
+===== Generic Forth =====
+The idea of strings is that a character string (s) is in fact a counted string <nowiki>(c)</nowiki>  that has been stored. s (c-addr) is the string, c (c-addr u) is constant string
+<code forth>
+: $VARIABLE     \ Reserve space for a string buffer
+    here  swap 1+ allot  align  \ Reserve RAM buffer
+    create  ( here) ,       ( +n "name" -- )
+    does>  @ ;              ( -- s )
+: C+!   ( n a -- )      >r  r@ c@ +  r> c! ;    \ Incr. byte with n at a
+: $@    ( s -- c )      count ;                 \ Fetch string
+: $+!   ( c s -- )      >r  tuck  r@ $@ +  swap cmove  r> c+! ; \ Extend string
+: $!    ( c s -- )      0 over c!  $+! ;        \ Store string
+: $.    ( c -- )        type ;                  \ Print string
+: $C+!  ( char s -- )   dup >r  $@ + c!  1 r> c+! ; \ Add char to string
+</code>
+Here is a version where the count is stored in a cell, it is hardly different.
+Note that it uses the non Generic Forth word ''%%@+%%'' you can find an implementation example in
+the [[https://project-forth-works.github.io/well-known-words.txt|well known words]] list.
+<code forth>
+: $VARIABLE     \ Reserve space for a string buffer
+    here  swap CELL+ allot  align  \ Reserve RAM buffer
+    create  ( here) ,       ( +n "name" -- )
+    does>  @ ;              ( -- s )
+: $@    ( s -- c )      @+  ;                  \ Fetch string
+: $+!   ( c s -- )      >r  tuck  r@ $@ +  swap cmove  r> +! ; \ Extend string
+: $!    ( c s -- )      0 over !  $+! ;        \ Store string
+: $.    ( c -- )        type ;                 \ Print string
+: $C+!  ( char s -- )   dup >r  $@ + c!  1 r> +! ; \ Add char to string
+</code>
+===== Implementations =====
+Have a look at the sub directories for implementations for different systems.
+  * String word sets
+    * [[https://github.com/project-forth-works/project-forth-works/blob/main/Algorithms/String%20handling/Primitive-string-word-set.f|Primitive string word set]], Simple string word set e.g. for file and OS interfacing
+    * [[https://github.com/project-forth-works/project-forth-works/blob/main/Algorithms/String%20handling/Safe-string-word-set-pr.f|Safe primitive string word set]], Version with string overflow warning!
+    * [[https://github.com/project-forth-works/project-forth-works/blob/main/Algorithms/String%20handling/Safe-string-word-set.f|Safe string word set v1]], Version with string limiting
+    * [[https://github.com/project-forth-works/project-forth-works/blob/main/Algorithms/String%20handling/building-strings-an.f|Building strings]], A different approach, author Albert Nijhof
+    * Etc.
+Note that Albert Nijhof's string version puts the address of the structure of the ''%%$VARIABLE%%'' on the stack. The original example puts the address of the string on the stack. Functionally they are equivalent.
+^      Name      ^    Alt-name    ^Function                       ^
+|   ''%%S@%%''   |  ''%%GET$%%''  |Read string variable           |
+|  ''%%$+!%%''   |  ''%%ADD$%%''  |Add string to string variable  |
+|   ''%%$!%%''   |  ''%%SET$%%''  |Store string in string variable|
+|   ''%%$.%%''   |  ''%%TYPE%%''  |Type string                    |
+|  ''%%@C+!%%''  |  ''%%INC$%%''  |Add char to string variable    |
+===== String tools =====
+Two string tools as implemented by Albert Nijhof.\\
+- ''%%-HEAD%%'' cuts the first 'i' characters from the given string.\\
+- ''%%-TAIL%%'' cuts the last 'i' characters from the given string.
+<code forth>
+\ Extra: cut i characters from a string, with underflow protection
+: -TAIL ( adr len i -- adr len' )   0 max  over min - ;
+: -HEAD ( adr len i -- adr' len' )  0 max  over min  tuck - >r + r> ;
+\ -HEAD and -TAIL do not store anything.
+</code>