calc: Allow strings with character codes above Latin-1

The current behavior of the functions 'calc-display-strings',
'strings', and 'bstrings' is to skip any vector containing
integers outside the Latin-1 range (0x00-0xFF).  We introduce a
custom variable 'calc-string-maximum-character' to replace this
hard-coded maximum, and to allow vectors containing higher
character codes to be displayed as strings.  The default value
of 0xFF preserves the existing behavior.

* lisp/calc/calc.el (calc-string-maximum-character): Add custom
variable 'calc-string-maximum-character'.
* lisp/calc/calccomp.el (math-vector-is-string): Replace hard-coded
maximum with 'calc-string-maximum-character', and the 'natnump'
assertion with 'characterp'.  The latter guards against the
maximum being larger than '(max-char)', but not on invalid types of
the maximum such as strings.

* test/lisp/calc/calc-tests.el (calc-math-vector-is-string): Add
tests for 'math-vector-is-string' using different values of
'calc-string-maximum-character'.

* doc/misc/calc.texi (Quick Calculator, Strings, Customizing Calc):
Add variable definition for 'calc-string-maximum-character' and
reference thereof when discussing 'calc-display-strings'.
Generalize a comment about string display and availability of 8-bit
fonts.
(Bug#78528)

5 files changed, 164 insertions, 17 deletions

diff --git a/doc/misc/calc.texi b/doc/misc/calc.texi
index 61466b55201..eda442ecb38 100644
--- a/doc/misc/calc.texi
+++ b/doc/misc/calc.texi
@@ -10179,7 +10179,7 @@ result @samp{[120]} (because 120 is the ASCII code of the lower-case
 is displayed only according to the current mode settings.  But
 running Quick Calc again and entering @samp{120} will produce the
 result @samp{120 (16#78, 8#170, x)} which shows the number in its
-decimal, hexadecimal, octal, and ASCII forms.
+decimal, hexadecimal, octal, and character forms.
 
 Please note that the Quick Calculator is not any faster at loading
 or computing the answer than the full Calculator; the name ``quick''
@@ -10836,11 +10836,11 @@ from 1 to @samp{n}.
 @cindex Strings
 @cindex Character strings
 Character strings are not a special data type in the Calculator.
-Rather, a string is represented simply as a vector all of whose
-elements are integers in the range 0 to 255 (ASCII codes).  You can
-enter a string at any time by pressing the @kbd{"} key.  Quotation
-marks and backslashes are written @samp{\"} and @samp{\\}, respectively,
-inside strings.  Other notations introduced by backslashes are:
+Rather, a string is represented simply as a vector all of whose elements
+are integers in the Latin-1 range 0 to 255.  You can enter a string at
+any time by pressing the @kbd{"} key.  Quotation marks and backslashes
+are written @samp{\"} and @samp{\\}, respectively, inside strings.
+Other notations introduced by backslashes are:
 
 @example
 @group
@@ -10857,21 +10857,24 @@ inside strings.  Other notations introduced by backslashes are:
 
 @noindent
 Finally, a backslash followed by three octal digits produces any
-character from its ASCII code.
+character from its code.
 
 @kindex d "
 @pindex calc-display-strings
 Strings are normally displayed in vector-of-integers form.  The
 @w{@kbd{d "}} (@code{calc-display-strings}) command toggles a mode in
 which any vectors of small integers are displayed as quoted strings
-instead.
+instead.  The display of strings containing higher character codes can
+be enabled by increasing the custom variable
+@code{calc-string-maximum-character} (@pxref{Customizing Calc}).
 
 The backslash notations shown above are also used for displaying
-strings.  Characters 128 and above are not translated by Calc; unless
-you have an Emacs modified for 8-bit fonts, these will show up in
-backslash-octal-digits notation.  For characters below 32, and
-for character 127, Calc uses the backslash-letter combination if
-there is one, or otherwise uses a @samp{\^} sequence.
+strings.  For ASCII control characters (below 32), and for the
+@code{DEL} character (127), Calc uses the backslash-letter combination
+if there is one, or otherwise uses a @samp{\^} sequence.  Control
+characters above 127 are not translated by Calc, and will show up in
+backslash-octal-digits notation.  The display of higher character codes
+will depend on your display settings and system font coverage.
 
 The only Calc feature that uses strings is @dfn{compositions};
 @pxref{Compositions}.  Strings also provide a convenient
@@ -35684,6 +35687,33 @@ choose from, or the user can enter their own date.
 The default value of @code{calc-gregorian-switch} is @code{nil}.
 @end defvar
 
+@defvar calc-string-maximum-character
+@xref{Strings}.@*
+
+The variable @code{calc-string-maximum-character} is the maximum value
+of a vector's elements for @code{calc-display-strings}, @code{string},
+and @code{bstring} to display the vector as a string.  This maximum
+@emph{must} represent a character, i.e. it's a non-negative integer less
+than or equal to @code{(max-char)} or @code{0x3FFFFF}.  Any negative
+value effectively disables the display of strings, and for values larger
+than @code{0x3FFFFF} the display acts as if the maximum were
+@code{0x3FFFFF}.  Some natural choices (and their resulting ranges) are:
+
+@itemize
+@item
+@code{0x7F} or 127 (ASCII),
+@item
+@code{0xFF} or 255 (Latin-1, the default),
+@item
+@code{0x10FFFF} (Unicode),
+@item
+@code{0x3FFFFF} (Emacs).
+@end itemize
+
+The default value of @code{calc-string-maximum-character} is @code{0xFF}
+or 255.
+@end defvar
+
 @node Reporting Bugs
 @appendix Reporting Bugs
 
diff --git a/etc/NEWS b/etc/NEWS
index 34f9725d992..d643723945f 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2182,7 +2182,19 @@ modifier, it scrolls by year.
 The month and year navigation key bindings 'M-}', 'M-{', 'C-x ]' and
 'C-x [' now have the alternative keys '}', '{', ']' and '['.
 
+** Calc
+
+*** New user option 'calc-string-maximum-character'.
+
+Previously, the 'calc-display-strings', 'string', and 'bstring'
+functions only considered integer vectors whose elements are all in the
+Latin-1 range 0-255.  This hard-coded maximum is replaced by
+'calc-string-maximum-character', and setting it to a higher value allows
+the display of matching vectors as Unicode strings.  The default value
+is '0xFF' or '255' to preserve the existing behavior.
+
 
+
 * New Modes and Packages in Emacs 31.1
 
 ** New minor mode 'delete-trailing-whitespace-mode'.
diff --git a/lisp/calc/calc.el b/lisp/calc/calc.el
index a7bd671998e..a350419b320 100644
--- a/lisp/calc/calc.el
+++ b/lisp/calc/calc.el
@@ -628,6 +628,37 @@ Otherwise, 1 / 0 is changed to uinf (undirected infinity).")
 (defcalcmodevar calc-display-strings nil
   "If non-nil, display vectors of byte-sized integers as strings.")
 
+(defcustom calc-string-maximum-character #xFF
+  "Maximum value of vector contents to be displayed as a string.
+
+If a vector consists of characters up to this maximum value, the
+function `calc-display-strings' will toggle displaying the vector as a
+string.  This maximum value must represent a character (see `characterp').
+Some natural choices (and their resulting ranges) are:
+
+- `0x7F'     (`ASCII'),
+- `0xFF'     (`Latin-1', the default),
+- `0x10FFFF' (`Unicode'),
+- `0x3FFFFF' (`Emacs').
+
+Characters for low control codes are either caret or backslash escaped,
+while others without a glyph are displayed in backslash-octal notation.
+The display of strings containing higher character codes will depend on
+your display settings and system font coverage.
+
+See the following for further information:
+
+- info node `(calc)Strings',
+- info node `(elisp)Text Representations',
+- info node `(emacs)Text Display'."
+  :version "31.1"
+  :type '(choice (restricted-sexp :tag "Character Code"
+                                  :match-alternatives (characterp))
+                 (const :tag "ASCII"   #x7F)
+                 (const :tag "Latin-1" #xFF)
+                 (const :tag "Unicode" #x10FFFF)
+                 (const :tag "Emacs"   #x3FFFFF)))
+
 (defcalcmodevar calc-matrix-just 'center
   "If nil, vector elements are left-justified.
 If `right', vector elements are right-justified.
diff --git a/lisp/calc/calccomp.el b/lisp/calc/calccomp.el
index cc27e6c2025..7e1f8378d80 100644
--- a/lisp/calc/calccomp.el
+++ b/lisp/calc/calccomp.el
@@ -907,13 +907,20 @@
                 (concat " " math-comp-right-bracket)))))
 
 (defun math-vector-is-string (a)
+  "Return t if A can be displayed as a string, and nil otherwise.
+
+Elements of A must either be a character (see `characterp') or a complex
+number with only a real character part, each with a value less than or
+equal to the custom variable `calc-string-maximum-character'."
   (while (and (setq a (cdr a))
-              (or (and (natnump (car a))
-                       (<= (car a) 255))
+              (or (and (characterp (car a))
+                       (<= (car a)
+                           calc-string-maximum-character))
                   (and (eq (car-safe (car a)) 'cplx)
-                       (natnump (nth 1 (car a)))
+                       (characterp (nth 1 (car a)))
                        (eq (nth 2 (car a)) 0)
-                       (<= (nth 1 (car a)) 255)))))
+                       (<= (nth 1 (car a))
+                           calc-string-maximum-character)))))
   (null a))
 
 (defconst math-vector-to-string-chars '( ( ?\" . "\\\"" )
diff --git a/test/lisp/calc/calc-tests.el b/test/lisp/calc/calc-tests.el
index 42eb6077b04..2fd6a6be45e 100644
--- a/test/lisp/calc/calc-tests.el
+++ b/test/lisp/calc/calc-tests.el
@@ -879,5 +879,72 @@ An existing calc stack is reused, otherwise a new one is created."
   (should-error (math-read-preprocess-string nil))
   (should-error (math-read-preprocess-string 42)))
 
+(ert-deftest calc-math-vector-is-string ()
+  "Test `math-vector-is-string' with varying `calc-string-maximum-character'.
+
+All tests operate on both an integer vector and the corresponding
+complex vector.  The sets covered are:
+
+1. `calc-string-maximum-character' is a valid character. The last case
+with `0x3FFFFF' is borderline, as integers above it will not make it
+past the `characterp' test.
+2. `calc-string-maximum-character' is negative, so the test always fails.
+3. `calc-string-maximum-character' is above `(max-char)', so only the
+first `characterp' test is active.
+4. `calc-string-maximum-character' has an invalid type, which triggers
+an error in the comparison."
+  (cl-flet* ((make-vec (lambda (contents) (append (list 'vec) contents)))
+             (make-cplx (lambda (x) (list 'cplx x 0)))
+             (make-cplx-vec (lambda (contents)
+                              (make-vec (mapcar #'make-cplx contents)))))
+    ;; 1: calc-string-maximum-character is a valid character
+    (dolist (maxchar '(#x7F #xFF #x10FFFF #x3FFFFD #x3FFFFF))
+      (let* ((calc-string-maximum-character maxchar)
+             (small-chars (number-sequence (- maxchar 2) maxchar))
+             (large-chars (number-sequence maxchar (+ maxchar 2)))
+             (small-real-vec (make-vec small-chars))
+             (large-real-vec (make-vec large-chars))
+             (small-cplx-vec (make-cplx-vec small-chars))
+             (large-cplx-vec (make-cplx-vec large-chars)))
+        (should (math-vector-is-string small-real-vec))
+        (should-not (math-vector-is-string large-real-vec))
+        (should (math-vector-is-string small-cplx-vec))
+        (should-not (math-vector-is-string large-cplx-vec))))
+    ;; 2: calc-string-maximum-character is negative
+    (let* ((maxchar -1)
+           (calc-string-maximum-character maxchar)
+           (valid-contents (number-sequence 0 2))
+           (invalid-contents (number-sequence (- maxchar 2) maxchar))
+           (valid-real-vec (make-vec valid-contents))
+           (invalid-real-vec (make-vec invalid-contents))
+           (valid-cplx-vec (make-cplx-vec valid-contents))
+           (invalid-cplx-vec (make-cplx-vec invalid-contents)))
+      (should-not (math-vector-is-string valid-real-vec))
+      (should-not (math-vector-is-string invalid-real-vec))
+      (should-not (math-vector-is-string valid-cplx-vec))
+      (should-not (math-vector-is-string invalid-cplx-vec)))
+    ;; 3: calc-string-maximum-character is larger than (max-char)
+    (let* ((maxchar (+ (max-char) 3))
+           (calc-string-maximum-character maxchar)
+           (valid-chars (number-sequence (- (max-char) 2) (max-char)))
+           (invalid-chars (number-sequence (1+ (max-char)) maxchar))
+           (valid-real-vec (make-vec valid-chars))
+           (invalid-real-vec (make-vec invalid-chars))
+           (valid-cplx-vec (make-cplx-vec valid-chars))
+           (invalid-cplx-vec (make-cplx-vec invalid-chars)))
+      (should (math-vector-is-string valid-real-vec))
+      (should-not (math-vector-is-string invalid-real-vec))
+      (should (math-vector-is-string valid-cplx-vec))
+      (should-not (math-vector-is-string invalid-cplx-vec)))
+    ;; 4: calc-string-maximum-character has the wrong type
+    (let* ((calc-string-maximum-character "wrong type")
+           (contents (number-sequence 0 2))
+           (real-vec (make-vec contents))
+           (cplx-vec (make-cplx-vec contents)))
+      (should-error (math-vector-is-string real-vec)
+                    :type 'wrong-type-argument)
+      (should-error (math-vector-is-string cplx-vec)
+                    :type 'wrong-type-argument))))
+
 (provide 'calc-tests)
 ;;; calc-tests.el ends here