Improve replace-buffer-contents/replace-region-contents

* src/editfns.c (Freplace_buffer_contents): Add two optional arguments
  for mitigating performance issues.
* lisp/emacs-lisp/subr-x.el (replace-region-contents): Move from
  subr.el.  Add the same two arguments as for replace-buffer-contents.
* lisp/json.el (json-pretty-print-max-secs): New variable holding the
  default MAX-SECS value json-pretty-print passes to
  replace-buffer-contents.
  (json-pretty-print): Use it.
* doc/lispref/text.texi (Replacing): Add documentation for
  replace-buffer-contents two new optional arguments.  Document
  replace-region-contents.

6 files changed, 174 insertions, 56 deletions

diff --git a/doc/lispref/text.texi b/doc/lispref/text.texi
index 6dfd211d1a0..88843c3764f 100644
--- a/doc/lispref/text.texi
+++ b/doc/lispref/text.texi
@@ -4436,20 +4436,57 @@ all markers unrelocated.
   You can use the following function to replace the text of one buffer
 with the text of another buffer:
 
-@deffn Command replace-buffer-contents source
+@deffn Command replace-buffer-contents source &optional max-secs max-costs
 This function replaces the accessible portion of the current buffer
 with the accessible portion of the buffer @var{source}.  @var{source}
 may either be a buffer object or the name of a buffer.  When
 @code{replace-buffer-contents} succeeds, the text of the accessible
 portion of the current buffer will be equal to the text of the
-accessible portion of the @var{source} buffer.  This function attempts
-to keep point, markers, text properties, and overlays in the current
-buffer intact.  One potential case where this behavior is useful is
-external code formatting programs: they typically write the
-reformatted text into a temporary buffer or file, and using
-@code{delete-region} and @code{insert-buffer-substring} would destroy
-these properties.  However, the latter combination is typically
-faster.  @xref{Deletion}, and @ref{Insertion}.
+accessible portion of the @var{source} buffer.
+
+This function attempts to keep point, markers, text properties, and
+overlays in the current buffer intact.  One potential case where this
+behavior is useful is external code formatting programs: they
+typically write the reformatted text into a temporary buffer or file,
+and using @code{delete-region} and @code{insert-buffer-substring}
+would destroy these properties.  However, the latter combination is
+typically faster (@xref{Deletion}, and @ref{Insertion}).
+
+For its working, @code{replace-buffer-contents} needs to compare the
+contents of the original buffer with that of @code{source} which is a
+costly operation if the buffers are huge and there is a high number of
+differences between them.  In order to keep
+@code{replace-buffer-contents}'s runtime in bounds, it has two
+optional arguments.
+
+@code{max-secs} defines a hard boundary in terms of seconds.  If given
+and exceeded, it will fall back to @code{delete-region} and
+@code{insert-buffer-substring}.
+
+@code{max-costs} defines the quality of the difference computation.
+If the actual costs exceed this limit, heuristics are used to provide
+a faster but suboptimal solution.  The default value is 1000000.
+
+@code{replace-buffer-contents} returns t if a non-destructive
+replacement could be performed.  Otherwise, i.e., if MAX-SECS was
+exceeded, it returns nil.
+@end deffn
+
+@defun Command replace-region-contents beg end replace-fn &optional max-secs max-costs
+This function replaces the region between @code{beg} and @code{end}
+using the given @code{replace-fn}.  The function @code{replace-fn} is
+run in the current buffer narrowed to the specified region and it
+should return either a string or a buffer replacing the region.
+
+The replacement is performed using @code{replace-buffer-contents}
+which also describes the @code{max-secs} and @code{max-costs}
+arguments and the return value.
+
+Note: If the replacement is a string, it will be placed in a temporary
+buffer so that @code{replace-buffer-contents} can operate on it.
+Therefore, if you already have the replacement in a buffer, it makes
+no sense to convert it to a string using @code{buffer-substring} or
+similar.
 @end deffn
 
 @node Decompression
diff --git a/etc/NEWS b/etc/NEWS
index 3c5fb24b0e4..67e376d9b38 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -335,6 +335,16 @@ the node "(emacs) Directory Variables" of the user manual.
 'make-network-process' now uses the correct loopback address when
 asked to use :host 'local and :family 'ipv6.
 
++++
+** The new function `replace-region-contents' replaces the current
+region using a given replacement-function in a non-destructive manner
+(in terms of `replace-buffer-contents').
+
++++
+** The command `replace-buffer-contents' now has two optional
+arguments mitigating performance issues when operating on huge
+buffers.
+
 
 * Changes in Specialized Modes and Packages in Emacs 27.1
 
diff --git a/lisp/emacs-lisp/subr-x.el b/lisp/emacs-lisp/subr-x.el
index 7d9f0bba4c7..b9ffe6a6fc6 100644
--- a/lisp/emacs-lisp/subr-x.el
+++ b/lisp/emacs-lisp/subr-x.el
@@ -250,6 +250,35 @@ TRIM-LEFT and TRIM-RIGHT default to \"[ \\t\\n\\r]+\"."
       (substring string 0 (- (length string) (length suffix)))
     string))
 
+(defun replace-region-contents (beg end replace-fn
+                                    &optional max-secs max-costs)
+  "Replace the region between BEG and END using REPLACE-FN.
+REPLACE-FN runs on the current buffer narrowed to the region.  It
+should return either a string or a buffer replacing the region.
+
+The replacement is performed using `replace-buffer-contents'
+which also describes the MAX-SECS and MAX-COSTS arguments and the
+return value.
+
+Note: If the replacement is a string, it'll be placed in a
+temporary buffer so that `replace-buffer-contents' can operate on
+it.  Therefore, if you already have the replacement in a buffer,
+it makes no sense to convert it to a string using
+`buffer-substring' or similar."
+  (save-excursion
+    (save-restriction
+      (narrow-to-region beg end)
+      (goto-char (point-min))
+      (let ((repl (funcall replace-fn)))
+        (if (bufferp repl)
+            (replace-buffer-contents repl max-secs max-costs)
+          (let ((source-buffer (current-buffer)))
+            (with-temp-buffer
+              (insert repl)
+              (let ((tmp-buffer (current-buffer)))
+                (set-buffer source-buffer)
+                (replace-buffer-contents tmp-buffer max-secs max-costs)))))))))
+
 (provide 'subr-x)
 
 ;;; subr-x.el ends here
diff --git a/lisp/json.el b/lisp/json.el
index 19b8f09dcda..44b3c33df7c 100644
--- a/lisp/json.el
+++ b/lisp/json.el
@@ -49,10 +49,13 @@
 ;; 2008-02-21 - Installed in GNU Emacs.
 ;; 2011-10-17 - Patch `json-alist-p' and `json-plist-p' to avoid recursion -tzz
 ;; 2012-10-25 - Added pretty-printed reformatting -Ryan Crum (ryan@ryancrum.org)
+;; 2019-02-02 - Pretty-printing now uses replace-region-contents and support for
+;;              minimization -tsdh
 
 ;;; Code:
 
 (require 'map)
+(require 'subr-x)
 
 ;; Parameters
 
@@ -738,6 +741,12 @@ With prefix argument MINIMIZE, minimize it instead."
   (interactive "P")
   (json-pretty-print (point-min) (point-max) minimize))
 
+(defvar json-pretty-print-max-secs 2.0
+  "Maximum time for `json-pretty-print's comparison.
+The function `json-pretty-print' uses `replace-region-contents'
+(which see) passing the value of this variable as argument
+MAX-SECS.")
+
 (defun json-pretty-print (begin end &optional minimize)
   "Pretty-print selected region.
 With prefix argument MINIMIZE, minimize it instead."
@@ -749,7 +758,11 @@ With prefix argument MINIMIZE, minimize it instead."
         (json-object-type 'alist))
     (replace-region-contents
      begin end
-     (lambda () (json-encode (json-read))))))
+     (lambda () (json-encode (json-read)))
+     json-pretty-print-max-secs
+     ;; FIXME: What's a good value here?  Can we use something better,
+     ;; e.g., by deriving a value from the size of the region?
+     64)))
 
 (defun json-pretty-print-buffer-ordered (&optional minimize)
   "Pretty-print current buffer with object keys ordered.
diff --git a/lisp/subr.el b/lisp/subr.el
index 69ae804e200..5c8b84b8e9c 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -5476,30 +5476,4 @@ returned list are in the same order as in TREE.
 ;; for discoverability:
 (defalias 'flatten-list 'flatten-tree)
 
-(defun replace-region-contents (beg end replace-fn)
-  "Replace the region between BEG and END using REPLACE-FN.
-REPLACE-FN runs on the current buffer narrowed to the region.  It
-should return either a string or a buffer replacing the region.
-
-The replacement is performed using `replace-buffer-contents'.
-
-Note: If the replacement is a string, it'll be placed in a
-temporary buffer so that `replace-buffer-contents' can operate on
-it.  Therefore, if you already have the replacement in a buffer,
-it makes no sense to convert it to a string using
-`buffer-substring' or similar."
-  (save-excursion
-    (save-restriction
-      (narrow-to-region beg end)
-      (goto-char (point-min))
-      (let ((repl (funcall replace-fn)))
-        (if (bufferp repl)
-            (replace-buffer-contents repl)
-          (let ((source-buffer (current-buffer)))
-            (with-temp-buffer
-              (insert repl)
-              (let ((tmp-buffer (current-buffer)))
-                (set-buffer source-buffer)
-                (replace-buffer-contents tmp-buffer)))))))))
-
 ;;; subr.el ends here
diff --git a/src/editfns.c b/src/editfns.c
index 7a600bacf18..8f21f8a677e 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -20,6 +20,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 
 #include <config.h>
 #include <sys/types.h>
+#include <sys/time.h>
 #include <stdio.h>
 
 #ifdef HAVE_PWD_H
@@ -1912,10 +1913,6 @@ determines whether case is significant or ignored.  */)
 #undef EQUAL
 #define USE_HEURISTIC
 
-#ifdef USE_HEURISTIC
-#define DIFFSEQ_HEURISTIC
-#endif
-
 /* Counter used to rarely_quit in replace-buffer-contents.  */
 static unsigned short rbc_quitcounter;
 
@@ -1937,30 +1934,54 @@ static unsigned short rbc_quitcounter;
   /* Bit vectors recording for each character whether it was deleted
      or inserted.  */                           \
   unsigned char *deletions;                     \
-  unsigned char *insertions;
+  unsigned char *insertions;                    \
+  struct timeval start;                         \
+  double max_secs;                              \
+  unsigned int early_abort_tests;
 
 #define NOTE_DELETE(ctx, xoff) set_bit ((ctx)->deletions, (xoff))
 #define NOTE_INSERT(ctx, yoff) set_bit ((ctx)->insertions, (yoff))
+#define EARLY_ABORT(ctx) compareseq_early_abort (ctx)
 
 struct context;
 static void set_bit (unsigned char *, OFFSET);
 static bool bit_is_set (const unsigned char *, OFFSET);
 static bool buffer_chars_equal (struct context *, OFFSET, OFFSET);
+static bool compareseq_early_abort (struct context *);
 
 #include "minmax.h"
 #include "diffseq.h"
 
 DEFUN ("replace-buffer-contents", Freplace_buffer_contents,
-       Sreplace_buffer_contents, 1, 1, "bSource buffer: ",
+       Sreplace_buffer_contents, 1, 3, "bSource buffer: ",
        doc: /* Replace accessible portion of current buffer with that of SOURCE.
 SOURCE can be a buffer or a string that names a buffer.
 Interactively, prompt for SOURCE.
+
 As far as possible the replacement is non-destructive, i.e. existing
 buffer contents, markers, properties, and overlays in the current
 buffer stay intact.
-Warning: this function can be slow if there's a large number of small
-differences between the two buffers.  */)
-  (Lisp_Object source)
+
+Because this function can be very slow if there is a large number of
+differences between the two buffers, there are two optional arguments
+mitigating this issue.
+
+The MAX-SECS argument, if given, defines a hard limit on the time used
+for comparing the buffers.  If it takes longer than MAX-SECS, the
+function falls back to a plain `delete-region' and
+`insert-buffer-substring'.  (Note that the checks are not performed
+too evenly over time, so in some cases it may run a bit longer than
+allowed).
+
+The optional argument MAX-COSTS defines the quality of the difference
+computation.  If the actual costs exceed this limit, heuristics are
+used to provide a faster but suboptimal solution.  The default value
+is 1000000.
+
+This function returns t if a non-destructive replacement could be
+performed.  Otherwise, i.e., if MAX-SECS was exceeded, it returns
+nil.  */)
+  (Lisp_Object source, Lisp_Object max_secs, Lisp_Object max_costs)
 {
   struct buffer *a = current_buffer;
   Lisp_Object source_buffer = Fget_buffer (source);
@@ -1985,15 +2006,18 @@ differences between the two buffers.  */)
      empty.  */
 
   if (a_empty && b_empty)
-    return Qnil;
+    return Qt;
 
   if (a_empty)
-    return Finsert_buffer_substring (source, Qnil, Qnil);
+    {
+      Finsert_buffer_substring (source, Qnil, Qnil);
+      return Qt;
+    }
 
   if (b_empty)
     {
       del_range_both (BEGV, BEGV_BYTE, ZV, ZV_BYTE, true);
-      return Qnil;
+      return Qt;
     }
 
   ptrdiff_t count = SPECPDL_INDEX ();
@@ -2007,6 +2031,12 @@ differences between the two buffers.  */)
   ptrdiff_t *buffer;
   USE_SAFE_ALLOCA;
   SAFE_NALLOCA (buffer, 2, diags);
+
+  if (NILP (max_costs))
+    XSETFASTINT (max_costs, 1000000);
+  else
+    CHECK_FIXNUM (max_costs);
+
   /* Micro-optimization: Casting to size_t generates much better
      code.  */
   ptrdiff_t del_bytes = (size_t) size_a / CHAR_BIT + 1;
@@ -2022,20 +2052,26 @@ differences between the two buffers.  */)
     .insertions = SAFE_ALLOCA (ins_bytes),
     .fdiag = buffer + size_b + 1,
     .bdiag = buffer + diags + size_b + 1,
-#ifdef DIFFSEQ_HEURISTIC
     .heuristic = true,
-#endif
-    /* FIXME: Find a good number for .too_expensive.  */
-    .too_expensive = 64,
+    .too_expensive = XFIXNUM (max_costs),
+    .max_secs = FLOATP (max_secs) ? XFLOAT_DATA (max_secs) : -1.0,
+    .early_abort_tests = 0
   };
   memclear (ctx.deletions, del_bytes);
   memclear (ctx.insertions, ins_bytes);
+
+  gettimeofday (&ctx.start, NULL);
   /* compareseq requires indices to be zero-based.  We add BEGV back
      later.  */
   bool early_abort = compareseq (0, size_a, 0, size_b, false, &ctx);
-  /* Since we didn’t define EARLY_ABORT, we should never abort
-     early.  */
-  eassert (! early_abort);
+
+  if (early_abort)
+    {
+      del_range (min_a, ZV);
+      Finsert_buffer_substring (source, Qnil,Qnil);
+      SAFE_FREE_UNBIND_TO (count, Qnil);
+      return Qnil;
+    }
 
   rbc_quitcounter = 0;
 
@@ -2097,6 +2133,7 @@ differences between the two buffers.  */)
       --i;
       --j;
     }
+
   SAFE_FREE_UNBIND_TO (count, Qnil);
   rbc_quitcounter = 0;
 
@@ -2106,7 +2143,7 @@ differences between the two buffers.  */)
       update_compositions (BEGV, ZV, CHECK_INSIDE);
     }
 
-  return Qnil;
+  return Qt;
 }
 
 static void
@@ -2173,6 +2210,18 @@ buffer_chars_equal (struct context *ctx,
     == BUF_FETCH_MULTIBYTE_CHAR (ctx->buffer_b, bpos_b);
 }
 
+static bool
+compareseq_early_abort (struct context *ctx)
+{
+  if (ctx->max_secs < 0.0)
+    return false;
+
+  struct timeval now, diff;
+  gettimeofday (&now, NULL);
+  timersub (&now, &ctx->start, &diff);
+  return diff.tv_sec + diff.tv_usec / 1000000.0 > ctx->max_secs;
+}
+
 
 static void
 subst_char_in_region_unwind (Lisp_Object arg)
@@ -4441,6 +4490,12 @@ it to be non-nil.  */);
   binary_as_unsigned = true;
 #endif
 
+  DEFVAR_LISP ("replace-buffer-contents-max-secs",
+               Vreplace_buffer_contents_max_secs,
+               doc: /* If differencing the two buffers takes longer than this,
+`replace-buffer-contents' falls back to a plain delete and insert.  */);
+  Vreplace_buffer_contents_max_secs = Qnil;
+
   defsubr (&Spropertize);
   defsubr (&Schar_equal);
   defsubr (&Sgoto_char);