strings: add functions to encode/decode strings into byte sequences

API copied from sbcl.

src/doc/help.lsp

@@ -2301,6 +2301,15 @@ Evaluates FORMs in order from left to right.  If any FORM evaluates to non-
 NIL, quits and returns that (single) value.  If the last FORM is reached,
 returns whatever values it returns.")
 
+(docfun ext::octets-to-string function (input &key
+                                              (external-format :default)
+                                              (start 0)
+                                              (end nil)) "
+Decode a sequence of octets into a string according to the given
+external format. The bounding index designators start and end optionally
+denote a subsequence to be decoded.
+")
+
 (docfun output-stream-p function (stream) "
 Returns T if STREAM can handle output operations; NIL otherwise.")
 
@@ -3014,6 +3023,17 @@ Similar to STRING>=, but ignores cases.")
 Returns a copy of STRING with the specified characters removed from the right
 end.  CHAR-SPEC must be a sequence of characters.")
 
+(docfun ext::string-to-octets function (input &key
+                                              (external-format :default)
+                                              (start 0)
+                                              (end nil)
+                                              (null-terminate nil)) "
+Encode a string into a sequence of octets according to the given
+external format. The bounding index designators start and end
+optionally denote a subsequence to be encoded. If null-terminate is
+true, add a terminating null byte.
+")
+
 (docfun si::string-to-object function (string) "
 ECL specific.
 Equivalent to (READ-FROM-STRING STRING), but is much faster.")

src/doc/manual/standards/strings.txi

@@ -67,6 +67,29 @@ The counterpart of the previous function is @coderef{ecl_char_set}, which implem
 Both functions check the type of their arguments and verify that the indices do not exceed the string boundaries. Otherwise they signal a @code{serious-condition}.
 @end deftypefun
 
+@subsubsection Converting Unicode strings
+Converting between different encodings. See @ref{Streams - External formats} for a list of supported encodings (external formats).
+
+@subsubheading Functions
+@cppdef si_octets_to_string
+@lspdef ext:octets-to-string
+@defun ext:octets-to-string octets &key (external-format :default) (start 0) (end nil)
+Decode a sequence of octets (i.e. 8-bit bytes) into a string according
+to the given external format. @var{octets} must be a vector whose
+elements have a size of 8-bit. The bounding index designators
+@var{start} and @var{end} optionally denote a subsequence to be decoded.
+Signals an @coderef{ext:character-decoding-error} if the decoding fails.
+@end defun
+
+@cppdef si_string_to_octets
+@lspdef ext:string-to-octets
+@defun ext:string-to-octets string &key (external-format :default) (start 0) (end nil) (null-terminate nil)
+Encode a string into a sequence of octets according to the given
+external format. The bounding index designators @var{start} and
+@var{end} optionally denote a subsequence to be encoded. If
+@var{null-terminate} is true, add a terminating null byte. Signals an
+@coderef{ext:character-encoding-error} if the encoding fails.
+@end defun
 @subsubsection ANSI dictionary
 Common Lisp and C equivalence