animate: Add -c option for variable-width frames

Not all scripts produce fixed-width frames.  Variable-width frames leave
behind artifacts caused by characters from the previous frame not being
overwritten.

* regex/animate (usage): Document new `-c' flag.
  (main): Handle new `-c' flag.
    [refresh]: New variable.
* regex/base10-mul.sed: Remove extra whitespace used for frame clearing for
    animate script (just use `-c' now).

regex/animate

@@ -23,13 +23,19 @@ set -euo pipefail
 usage()
 {
   cat <<EOF
-Usage: $0 script frame [speed]
+Usage: $0 [-c] script frame [speed]
 Recursively render FRAME using SCRIPT
 
 The current frame is stored in the file \`last-frame' in the working
 directory.  It may be used to continue the animation after quitting,
 or save the state of the system.
 
+To prevent flickering, each frame is rendered atop of the previous without
+clearing the screen (your terminal must support this).  This is desirable
+for fixed-width frames, but not for scripts with variable-width output,
+since that will leave behind artifacts.  Provide the \`-c' flag to clear the
+screen before rendering each frame.
+
 SPEED should be a value recognized by the \`sleep' command.  It defaults
 to 0.25 to give some level of appreciation for each frame while still
 looking animated.  It looks fairly smooth at 0.05 but can go much faster on
@@ -37,6 +43,7 @@ modern systems.
 
 Example:
   $0 fall.sed fall/frame-lalaloop
+  $0 -c base10-mul.sed <( echo 010 005 ) 0.1
 EOF
 
   exit 64  # EX_USAGE
@@ -56,6 +63,15 @@ render()
 
 main()
 {
+  # Do not clear the screen on each frame by default (we'll be using tput
+  # to avoid flickering; see `render').  Certain scripts do not produce
+  # fixed-width frames, and so we must clear to avoid artifacts.
+  declare refresh=true
+  test "${1:-}" == -c && {
+    refresh=clear
+    shift
+  }
+
   test $# -ge 2 -a $# -le 3 || usage
 
   local -r script=${1?Missing script}
@@ -67,7 +83,10 @@ main()
   clear
   render "$speed"
 
-  while sed -f "$script" -i last-frame; do render "$speed"; done
+  while sed -f "$script" -i last-frame; do
+    $refresh
+    render "$speed";
+  done
 }
 
 main "$@"

regex/base10-mul.sed

@@ -94,10 +94,8 @@ s/^\(...\) 000/000 001/
 # register two is `001'), then we are done!  If that's the case, clean up
 # our output to display only the final result.  The script will
 # automatically exit with a non-zero status the next iteration because of
-# the guard at the top of this script.  The trailing spaces ensure that
-# leftover output from previous iterations is erased in the animation
-# script.
-s/^\(...\) 001.*/\1            /
+# the guard at the top of this script.
+s/^\(...\) 001.*/\1/
 /^... \+$/b