maint: refactor common expand(1) and unexpand(1) code

* src/expand.c, src/unexpand.c: Move global variables from here...
* src/expand-common.h, src/expand-common.c: ... to here.
* src/expand.c, src/unexpand.c: (parse_tab_stops, validate_tab_stops,
next_file): Move identical functions to new module.
(add_tab_stop): Move to new module, including additional code from
'unexpand' (keeping max_column_width) which will have no effect in when
used in 'expand'. Refactor common next-column calculation code
into a new function 'get_next_tab_column'.
* src/local.mk: (src_expand_SOURCES, src_unexpand_SOURCES): Add
'expand-common.c'; (noinst_HEADERS): Add 'expand-common.h'.
* po/POTFILES.in: Add 'expand-common.c'.
* tests/misc/expand.pl: Add more tests.
* tests/misc/unexpand.pl: Likewise.
* TODO: Move conclusions to above test after investigation.

9 files changed, 529 insertions, 424 deletions

diff --git a/TODO b/TODO
index de95e5a64..dc1a9e2a2 100644
--- a/TODO
+++ b/TODO
@@ -67,10 +67,6 @@ lib/strftime.c: Since %N is the only format that we need but that
   would expand /%(-_)?\d*N/ to the desired string and then pass the
   resulting string to glibc's strftime.
 
-unexpand: [http://www.opengroup.org/onlinepubs/007908799/xcu/unexpand.html]
-  printf 'x\t \t y\n'|unexpand -t 8,9 should print its input, unmodified.
-  printf 'x\t \t y\n'|unexpand -t 5,8 should print "x\ty\n"
-
 sort: Investigate better sorting algorithms; see Knuth vol. 3.
 
   We tried list merge sort, but it was about 50% slower than the
diff --git a/po/POTFILES.in b/po/POTFILES.in
index 7583f635d..b8a28bdee 100644
--- a/po/POTFILES.in
+++ b/po/POTFILES.in
@@ -57,6 +57,7 @@ src/dirname.c
 src/du.c
 src/echo.c
 src/env.c
+src/expand-common.c
 src/expand.c
 src/expr.c
 src/factor.c
diff --git a/src/expand-common.c b/src/expand-common.c
new file mode 100644
index 000000000..41d7003a4
--- /dev/null
+++ b/src/expand-common.c
@@ -0,0 +1,274 @@
+/* expand-common - common functionality for expand/unexapnd
+   Copyright (C) 1989-2016 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+#include <stdio.h>
+#include <sys/types.h>
+#include "system.h"
+#include "error.h"
+#include "fadvise.h"
+#include "quote.h"
+#include "xstrndup.h"
+
+#include "expand-common.h"
+
+/* If true, convert blanks even after nonblank characters have been
+   read on the line.  */
+bool convert_entire_line = false;
+
+/* If nonzero, the size of all tab stops.  If zero, use 'tab_list' instead.  */
+static uintmax_t tab_size = 0;
+
+/* The maximum distance between tab stops.  */
+size_t max_column_width;
+
+/* Array of the explicit column numbers of the tab stops;
+   after 'tab_list' is exhausted, each additional tab is replaced
+   by a space.  The first column is column 0.  */
+static uintmax_t *tab_list = NULL;
+
+/* The number of allocated entries in 'tab_list'.  */
+static size_t n_tabs_allocated = 0;
+
+/* The index of the first invalid element of 'tab_list',
+   where the next element can be added.  */
+static size_t first_free_tab = 0;
+
+/* Null-terminated array of input filenames.  */
+static char **file_list = NULL;
+
+/* Default for 'file_list' if no files are given on the command line.  */
+static char *stdin_argv[] =
+{
+  (char *) "-", NULL
+};
+
+/* True if we have ever read standard input.  */
+static bool have_read_stdin = false;
+
+/* The desired exit status.  */
+int exit_status = EXIT_SUCCESS;
+
+
+
+/* Add tab stop TABVAL to the end of 'tab_list'.  */
+extern void
+add_tab_stop (uintmax_t tabval)
+{
+  uintmax_t prev_column = first_free_tab ? tab_list[first_free_tab - 1] : 0;
+  uintmax_t column_width = prev_column <= tabval ? tabval - prev_column : 0;
+
+  if (first_free_tab == n_tabs_allocated)
+    tab_list = X2NREALLOC (tab_list, &n_tabs_allocated);
+  tab_list[first_free_tab++] = tabval;
+
+  if (max_column_width < column_width)
+    {
+      if (SIZE_MAX < column_width)
+        error (EXIT_FAILURE, 0, _("tabs are too far apart"));
+      max_column_width = column_width;
+    }
+}
+
+/* Add the comma or blank separated list of tab stops STOPS
+   to the list of tab stops.  */
+extern void
+parse_tab_stops (char const *stops)
+{
+  bool have_tabval = false;
+  uintmax_t tabval IF_LINT ( = 0);
+  char const *num_start IF_LINT ( = NULL);
+  bool ok = true;
+
+  for (; *stops; stops++)
+    {
+      if (*stops == ',' || isblank (to_uchar (*stops)))
+        {
+          if (have_tabval)
+            add_tab_stop (tabval);
+          have_tabval = false;
+        }
+      else if (ISDIGIT (*stops))
+        {
+          if (!have_tabval)
+            {
+              tabval = 0;
+              have_tabval = true;
+              num_start = stops;
+            }
+
+          /* Detect overflow.  */
+          if (!DECIMAL_DIGIT_ACCUMULATE (tabval, *stops - '0', uintmax_t))
+            {
+              size_t len = strspn (num_start, "0123456789");
+              char *bad_num = xstrndup (num_start, len);
+              error (0, 0, _("tab stop is too large %s"), quote (bad_num));
+              free (bad_num);
+              ok = false;
+              stops = num_start + len - 1;
+            }
+        }
+      else
+        {
+          error (0, 0, _("tab size contains invalid character(s): %s"),
+                 quote (stops));
+          ok = false;
+          break;
+        }
+    }
+
+  if (!ok)
+    exit (EXIT_FAILURE);
+
+  if (have_tabval)
+    add_tab_stop (tabval);
+}
+
+/* Check that the list of tab stops TABS, with ENTRIES entries,
+   contains only nonzero, ascending values.  */
+
+static void
+validate_tab_stops (uintmax_t const *tabs, size_t entries)
+{
+  uintmax_t prev_tab = 0;
+  size_t i;
+
+  for (i = 0; i < entries; i++)
+    {
+      if (tabs[i] == 0)
+        error (EXIT_FAILURE, 0, _("tab size cannot be 0"));
+      if (tabs[i] <= prev_tab)
+        error (EXIT_FAILURE, 0, _("tab sizes must be ascending"));
+      prev_tab = tabs[i];
+    }
+}
+
+/* Called after all command-line options have been parsed,
+   and add_tab_stop/parse_tab_stops have been called.
+   Will validate the tab-stop values,
+   and set the final values to:
+   tab-stops = 8 (if no tab-stops given on command line)
+   tab-stops = N (if value N specified as the only value).
+   tab-stops = distinct values given on command line (if multiple values given).
+*/
+extern void
+finalize_tab_stops (void)
+{
+  validate_tab_stops (tab_list, first_free_tab);
+
+  if (first_free_tab == 0)
+    tab_size = max_column_width = 8;
+  else if (first_free_tab == 1)
+    tab_size = tab_list[0];
+  else
+    tab_size = 0;
+}
+
+
+extern uintmax_t
+get_next_tab_column (const uintmax_t column, size_t* tab_index,
+                     bool* last_tab)
+{
+  *last_tab = false;
+
+  /* single tab-size - return multiples of it */
+  if (tab_size)
+    return column + (tab_size - column % tab_size);
+
+  /* multiple tab-sizes - iterate them until the tab position is beyond
+     the current input column. */
+  for ( ; *tab_index < first_free_tab ; (*tab_index)++ )
+    {
+        uintmax_t tab = tab_list[*tab_index];
+        if (column < tab)
+            return tab;
+    }
+
+  *last_tab = true;
+  return 0;
+}
+
+
+
+
+/* Sets new file-list */
+extern void
+set_file_list (char **list)
+{
+  have_read_stdin = false;
+
+  if (!list)
+    file_list = stdin_argv;
+  else
+    file_list = list;
+}
+
+/* Close the old stream pointer FP if it is non-NULL,
+   and return a new one opened to read the next input file.
+   Open a filename of '-' as the standard input.
+   Return NULL if there are no more input files.  */
+
+extern FILE *
+next_file (FILE *fp)
+{
+  static char *prev_file;
+  char *file;
+
+  if (fp)
+    {
+      if (ferror (fp))
+        {
+          error (0, errno, "%s", quotef (prev_file));
+          exit_status = EXIT_FAILURE;
+        }
+      if (STREQ (prev_file, "-"))
+        clearerr (fp);		/* Also clear EOF.  */
+      else if (fclose (fp) != 0)
+        {
+          error (0, errno, "%s", quotef (prev_file));
+          exit_status = EXIT_FAILURE;
+        }
+    }
+
+  while ((file = *file_list++) != NULL)
+    {
+      if (STREQ (file, "-"))
+        {
+          have_read_stdin = true;
+          fp = stdin;
+        }
+      else
+        fp = fopen (file, "r");
+      if (fp)
+        {
+          prev_file = file;
+          fadvise (fp, FADVISE_SEQUENTIAL);
+          return fp;
+        }
+      error (0, errno, "%s", quotef (file));
+      exit_status = EXIT_FAILURE;
+    }
+  return NULL;
+}
+
+/* */
+extern void
+cleanup_file_list_stdin (void)
+{
+    if (have_read_stdin && fclose (stdin) != 0)
+      error (EXIT_FAILURE, errno, "-");
+}
diff --git a/src/expand-common.h b/src/expand-common.h
new file mode 100644
index 000000000..8cb207997
--- /dev/null
+++ b/src/expand-common.h
@@ -0,0 +1,72 @@
+/* expand-common - common functionality for expand/unexapnd
+
+   Copyright (C) 1989-2016 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* If true, convert blanks even after nonblank characters have been
+   read on the line.  */
+extern bool convert_entire_line;
+
+/* The maximum distance between tab stops.  */
+extern size_t max_column_width;
+
+/* Null-terminated array of input filenames.  */
+//extern char **file_list;
+
+/* Default for 'file_list' if no files are given on the command line.  */
+//extern char *stdin_argv[];
+
+/* True if we have ever read standard input.  */
+//extern bool have_read_stdin;
+
+/* The desired exit status.  */
+extern int exit_status;
+
+/* Add tab stop TABVAL to the end of 'tab_list'.  */
+extern void
+add_tab_stop (uintmax_t tabval);
+
+/* Add the comma or blank separated list of tab stops STOPS
+   to the list of tab stops.  */
+extern void
+parse_tab_stops (char const *stops);
+
+/* TODO: Document */
+extern uintmax_t
+get_next_tab_column (const uintmax_t column, size_t* tab_index,
+                     bool* last_tab);
+
+/* Called after all command-line options have been parsed,
+   sets the final tab-stops values */
+extern void
+finalize_tab_stops (void);
+
+
+
+
+/* Sets new file-list */
+extern void
+set_file_list (char **file_list);
+
+/* Close the old stream pointer FP if it is non-NULL,
+   and return a new one opened to read the next input file.
+   Open a filename of '-' as the standard input.
+   Return NULL if there are no more input files.  */
+extern FILE *
+next_file (FILE *fp);
+
+/* */
+extern void
+cleanup_file_list_stdin (void);
diff --git a/src/expand.c b/src/expand.c
index e11f1afc7..9f84de825 100644
--- a/src/expand.c
+++ b/src/expand.c
@@ -39,49 +39,15 @@
 #include <sys/types.h>
 #include "system.h"
 #include "error.h"
-#include "fadvise.h"
-#include "quote.h"
 #include "xstrndup.h"
 
+#include "expand-common.h"
+
 /* The official name of this program (e.g., no 'g' prefix).  */
 #define PROGRAM_NAME "expand"
 
 #define AUTHORS proper_name ("David MacKenzie")
 
-/* If true, convert blanks even after nonblank characters have been
-   read on the line.  */
-static bool convert_entire_line;
-
-/* If nonzero, the size of all tab stops.  If zero, use 'tab_list' instead.  */
-static uintmax_t tab_size;
-
-/* Array of the explicit column numbers of the tab stops;
-   after 'tab_list' is exhausted, each additional tab is replaced
-   by a space.  The first column is column 0.  */
-static uintmax_t *tab_list;
-
-/* The number of allocated entries in 'tab_list'.  */
-static size_t n_tabs_allocated;
-
-/* The index of the first invalid element of 'tab_list',
-   where the next element can be added.  */
-static size_t first_free_tab;
-
-/* Null-terminated array of input filenames.  */
-static char **file_list;
-
-/* Default for 'file_list' if no files are given on the command line.  */
-static char *stdin_argv[] =
-{
-  (char *) "-", NULL
-};
-
-/* True if we have ever read standard input.  */
-static bool have_read_stdin;
-
-/* The desired exit status.  */
-static int exit_status;
-
 static char const shortopts[] = "it:0::1::2::3::4::5::6::7::8::9::";
 
 static struct option const longopts[] =
@@ -125,137 +91,6 @@ Convert tabs in each FILE to spaces, writing to standard output.\n\
   exit (status);
 }
 
-/* Add tab stop TABVAL to the end of 'tab_list'.  */
-
-static void
-add_tab_stop (uintmax_t tabval)
-{
-  if (first_free_tab == n_tabs_allocated)
-    tab_list = X2NREALLOC (tab_list, &n_tabs_allocated);
-  tab_list[first_free_tab++] = tabval;
-}
-
-/* Add the comma or blank separated list of tab stops STOPS
-   to the list of tab stops.  */
-
-static void
-parse_tab_stops (char const *stops)
-{
-  bool have_tabval = false;
-  uintmax_t tabval IF_LINT ( = 0);
-  char const *num_start IF_LINT ( = NULL);
-  bool ok = true;
-
-  for (; *stops; stops++)
-    {
-      if (*stops == ',' || isblank (to_uchar (*stops)))
-        {
-          if (have_tabval)
-            add_tab_stop (tabval);
-          have_tabval = false;
-        }
-      else if (ISDIGIT (*stops))
-        {
-          if (!have_tabval)
-            {
-              tabval = 0;
-              have_tabval = true;
-              num_start = stops;
-            }
-
-          /* Detect overflow.  */
-          if (!DECIMAL_DIGIT_ACCUMULATE (tabval, *stops - '0', uintmax_t))
-            {
-              size_t len = strspn (num_start, "0123456789");
-              char *bad_num = xstrndup (num_start, len);
-              error (0, 0, _("tab stop is too large %s"), quote (bad_num));
-              free (bad_num);
-              ok = false;
-              stops = num_start + len - 1;
-            }
-        }
-      else
-        {
-          error (0, 0, _("tab size contains invalid character(s): %s"),
-                 quote (stops));
-          ok = false;
-          break;
-        }
-    }
-
-  if (!ok)
-    exit (EXIT_FAILURE);
-
-  if (have_tabval)
-    add_tab_stop (tabval);
-}
-
-/* Check that the list of tab stops TABS, with ENTRIES entries,
-   contains only nonzero, ascending values.  */
-
-static void
-validate_tab_stops (uintmax_t const *tabs, size_t entries)
-{
-  uintmax_t prev_tab = 0;
-  size_t i;
-
-  for (i = 0; i < entries; i++)
-    {
-      if (tabs[i] == 0)
-        error (EXIT_FAILURE, 0, _("tab size cannot be 0"));
-      if (tabs[i] <= prev_tab)
-        error (EXIT_FAILURE, 0, _("tab sizes must be ascending"));
-      prev_tab = tabs[i];
-    }
-}
-
-/* Close the old stream pointer FP if it is non-NULL,
-   and return a new one opened to read the next input file.
-   Open a filename of '-' as the standard input.
-   Return NULL if there are no more input files.  */
-
-static FILE *
-next_file (FILE *fp)
-{
-  static char *prev_file;
-  char *file;
-
-  if (fp)
-    {
-      if (ferror (fp))
-        {
-          error (0, errno, "%s", quotef (prev_file));
-          exit_status = EXIT_FAILURE;
-        }
-      if (STREQ (prev_file, "-"))
-        clearerr (fp);		/* Also clear EOF.  */
-      else if (fclose (fp) != 0)
-        {
-          error (0, errno, "%s", quotef (prev_file));
-          exit_status = EXIT_FAILURE;
-        }
-    }
-
-  while ((file = *file_list++) != NULL)
-    {
-      if (STREQ (file, "-"))
-        {
-          have_read_stdin = true;
-          fp = stdin;
-        }
-      else
-        fp = fopen (file, "r");
-      if (fp)
-        {
-          prev_file = file;
-          fadvise (fp, FADVISE_SEQUENTIAL);
-          return fp;
-        }
-      error (0, errno, "%s", quotef (file));
-      exit_status = EXIT_FAILURE;
-    }
-  return NULL;
-}
 
 /* Change tabs to spaces, writing to stdout.
    Read each file in 'file_list', in order.  */
@@ -301,25 +136,13 @@ expand (void)
                 {
                   /* Column the next input tab stop is on.  */
                   uintmax_t next_tab_column;
+                  bool last_tab IF_LINT (=0);
 
-                  if (tab_size)
-                    next_tab_column = column + (tab_size - column % tab_size);
-                  else
-                    while (true)
-                      if (tab_index == first_free_tab)
-                        {
-                          next_tab_column = column + 1;
-                          break;
-                        }
-                      else
-                        {
-                          uintmax_t tab = tab_list[tab_index++];
-                          if (column < tab)
-                            {
-                              next_tab_column = tab;
-                              break;
-                            }
-                        }
+                  next_tab_column = get_next_tab_column (column, &tab_index,
+                                                         &last_tab);
+
+                  if (last_tab)
+                    next_tab_column = column + 1;
 
                   if (next_tab_column < column)
                     error (EXIT_FAILURE, 0, _("input line is too long"));
@@ -369,12 +192,7 @@ main (int argc, char **argv)
   textdomain (PACKAGE);
 
   atexit (close_stdout);
-
-  have_read_stdin = false;
-  exit_status = EXIT_SUCCESS;
   convert_entire_line = true;
-  tab_list = NULL;
-  first_free_tab = 0;
 
   while ((c = getopt_long (argc, argv, shortopts, longopts, NULL)) != -1)
     {
@@ -410,21 +228,13 @@ main (int argc, char **argv)
         }
     }
 
-  validate_tab_stops (tab_list, first_free_tab);
-
-  if (first_free_tab == 0)
-    tab_size = 8;
-  else if (first_free_tab == 1)
-    tab_size = tab_list[0];
-  else
-    tab_size = 0;
+  finalize_tab_stops ();
 
-  file_list = (optind < argc ? &argv[optind] : stdin_argv);
+  set_file_list ( (optind < argc) ? &argv[optind] : NULL);
 
   expand ();
 
-  if (have_read_stdin && fclose (stdin) != 0)
-    error (EXIT_FAILURE, errno, "-");
+  cleanup_file_list_stdin ();
 
   return exit_status;
 }
diff --git a/src/local.mk b/src/local.mk
index 12a0d55d4..6afaf836a 100644
--- a/src/local.mk
+++ b/src/local.mk
@@ -44,6 +44,7 @@ noinst_HEADERS =		\
   src/copy.h			\
   src/cp-hash.h			\
   src/dircolors.h		\
+  src/expand-common.h		\
   src/fiemap.h			\
   src/find-mount-point.h	\
   src/fs.h			\
@@ -408,6 +409,9 @@ src_base32_CPPFLAGS = -DBASE_TYPE=32 $(AM_CPPFLAGS)
 
 src_ginstall_CPPFLAGS = -DENABLE_MATCHPATHCON=1 $(AM_CPPFLAGS)
 
+src_expand_SOURCES = src/expand.c src/expand-common.c
+src_unexpand_SOURCES = src/unexpand.c src/expand-common.c
+
 # Ensure we don't link against libcoreutils.a as that lib is
 # not compiled with -fPIC which causes issues on 64 bit at least
 src_libstdbuf_so_LDADD = $(LIBINTL)
diff --git a/src/unexpand.c b/src/unexpand.c
index a75875659..a1317b173 100644
--- a/src/unexpand.c
+++ b/src/unexpand.c
@@ -40,51 +40,16 @@
 #include <sys/types.h>
 #include "system.h"
 #include "error.h"
-#include "fadvise.h"
-#include "quote.h"
 #include "xstrndup.h"
 
+#include "expand-common.h"
+
 /* The official name of this program (e.g., no 'g' prefix).  */
 #define PROGRAM_NAME "unexpand"
 
 #define AUTHORS proper_name ("David MacKenzie")
 
-/* If true, convert blanks even after nonblank characters have been
-   read on the line.  */
-static bool convert_entire_line;
-
-/* If nonzero, the size of all tab stops.  If zero, use 'tab_list' instead.  */
-static size_t tab_size;
-
-/* The maximum distance between tab stops.  */
-static size_t max_column_width;
-
-/* Array of the explicit column numbers of the tab stops;
-   after 'tab_list' is exhausted, the rest of the line is printed
-   unchanged.  The first column is column 0.  */
-static uintmax_t *tab_list;
-
-/* The number of allocated entries in 'tab_list'.  */
-static size_t n_tabs_allocated;
-
-/* The index of the first invalid element of 'tab_list',
-   where the next element can be added.  */
-static size_t first_free_tab;
 
-/* Null-terminated array of input filenames.  */
-static char **file_list;
-
-/* Default for 'file_list' if no files are given on the command line.  */
-static char *stdin_argv[] =
-{
-  (char *) "-", NULL
-};
-
-/* True if we have ever read standard input.  */
-static bool have_read_stdin;
-
-/* The desired exit status.  */
-static int exit_status;
 
 /* For long options that have no equivalent short option, use a
    non-character as a pseudo short option, starting with CHAR_MAX + 1.  */
@@ -134,148 +99,6 @@ Convert blanks in each FILE to tabs, writing to standard output.\n\
   exit (status);
 }
 
-/* Add tab stop TABVAL to the end of 'tab_list'.  */
-
-static void
-add_tab_stop (uintmax_t tabval)
-{
-  uintmax_t prev_column = first_free_tab ? tab_list[first_free_tab - 1] : 0;
-  uintmax_t column_width = prev_column <= tabval ? tabval - prev_column : 0;
-
-  if (first_free_tab == n_tabs_allocated)
-    tab_list = X2NREALLOC (tab_list, &n_tabs_allocated);
-  tab_list[first_free_tab++] = tabval;
-
-  if (max_column_width < column_width)
-    {
-      if (SIZE_MAX < column_width)
-        error (EXIT_FAILURE, 0, _("tabs are too far apart"));
-      max_column_width = column_width;
-    }
-}
-
-/* Add the comma or blank separated list of tab stops STOPS
-   to the list of tab stops.  */
-
-static void
-parse_tab_stops (char const *stops)
-{
-  bool have_tabval = false;
-  uintmax_t tabval IF_LINT ( = 0);
-  char const *num_start IF_LINT ( = NULL);
-  bool ok = true;
-
-  for (; *stops; stops++)
-    {
-      if (*stops == ',' || isblank (to_uchar (*stops)))
-        {
-          if (have_tabval)
-            add_tab_stop (tabval);
-          have_tabval = false;
-        }
-      else if (ISDIGIT (*stops))
-        {
-          if (!have_tabval)
-            {
-              tabval = 0;
-              have_tabval = true;
-              num_start = stops;
-            }
-
-          /* Detect overflow.  */
-          if (!DECIMAL_DIGIT_ACCUMULATE (tabval, *stops - '0', uintmax_t))
-            {
-              size_t len = strspn (num_start, "0123456789");
-              char *bad_num = xstrndup (num_start, len);
-              error (0, 0, _("tab stop is too large %s"), quote (bad_num));
-              free (bad_num);
-              ok = false;
-              stops = num_start + len - 1;
-            }
-        }
-      else
-        {
-          error (0, 0, _("tab size contains invalid character(s): %s"),
-                 quote (stops));
-          ok = false;
-          break;
-        }
-    }
-
-  if (!ok)
-    exit (EXIT_FAILURE);
-
-  if (have_tabval)
-    add_tab_stop (tabval);
-}
-
-/* Check that the list of tab stops TABS, with ENTRIES entries,
-   contains only nonzero, ascending values.  */
-
-static void
-validate_tab_stops (uintmax_t const *tabs, size_t entries)
-{
-  uintmax_t prev_tab = 0;
-  size_t i;
-
-  for (i = 0; i < entries; i++)
-    {
-      if (tabs[i] == 0)
-        error (EXIT_FAILURE, 0, _("tab size cannot be 0"));
-      if (tabs[i] <= prev_tab)
-        error (EXIT_FAILURE, 0, _("tab sizes must be ascending"));
-      prev_tab = tabs[i];
-    }
-}
-
-/* Close the old stream pointer FP if it is non-NULL,
-   and return a new one opened to read the next input file.
-   Open a filename of '-' as the standard input.
-   Return NULL if there are no more input files.  */
-
-static FILE *
-next_file (FILE *fp)
-{
-  static char *prev_file;
-  char *file;
-
-  if (fp)
-    {
-      if (ferror (fp))
-        {
-          error (0, errno, "%s", quotef (prev_file));
-          exit_status = EXIT_FAILURE;
-        }
-      if (STREQ (prev_file, "-"))
-        clearerr (fp);		/* Also clear EOF.  */
-      else if (fclose (fp) != 0)
-        {
-          error (0, errno, "%s", quotef (prev_file));
-          exit_status = EXIT_FAILURE;
-        }
-    }
-
-  while ((file = *file_list++) != NULL)
-    {
-      if (STREQ (file, "-"))
-        {
-          have_read_stdin = true;
-          fp = stdin;
-        }
-      else
-        fp = fopen (file, "r");
-      if (fp)
-        {
-          prev_file = file;
-          fadvise (fp, FADVISE_SEQUENTIAL);
-          return fp;
-        }
-      error (0, errno, "%s", quotef (file));
-      exit_status = EXIT_FAILURE;
-    }
-  return NULL;
-}
-
 /* Change blanks to tabs, writing to stdout.
    Read each file in 'file_list', in order.  */
 
@@ -344,28 +167,13 @@ unexpand (void)
 
               if (blank)
                 {
-                  if (next_tab_column <= column)
-                    {
-                      if (tab_size)
-                        next_tab_column =
-                          column + (tab_size - column % tab_size);
-                      else
-                        while (true)
-                          if (tab_index == first_free_tab)
-                            {
-                              convert = false;
-                              break;
-                            }
-                          else
-                            {
-                              uintmax_t tab = tab_list[tab_index++];
-                              if (column < tab)
-                                {
-                                  next_tab_column = tab;
-                                  break;
-                                }
-                            }
-                    }
+                  bool last_tab IF_LINT (=0);
+
+                  next_tab_column = get_next_tab_column (column, &tab_index,
+                                                         &last_tab);
+
+                  if (last_tab)
+                    convert = false;
 
                   if (convert)
                     {
@@ -464,12 +272,6 @@ main (int argc, char **argv)
 
   atexit (close_stdout);
 
-  have_read_stdin = false;
-  exit_status = EXIT_SUCCESS;
-  convert_entire_line = false;
-  tab_list = NULL;
-  first_free_tab = 0;
-
   while ((c = getopt_long (argc, argv, ",0123456789at:", longopts, NULL))
          != -1)
     {
@@ -512,21 +314,13 @@ main (int argc, char **argv)
   if (have_tabval)
     add_tab_stop (tabval);
 
-  validate_tab_stops (tab_list, first_free_tab);
-
-  if (first_free_tab == 0)
-    tab_size = max_column_width = 8;
-  else if (first_free_tab == 1)
-    tab_size = tab_list[0];
-  else
-    tab_size = 0;
+  finalize_tab_stops ();
 
-  file_list = (optind < argc ? &argv[optind] : stdin_argv);
+  set_file_list ( (optind < argc) ? &argv[optind] : NULL);
 
   unexpand ();
 
-  if (have_read_stdin && fclose (stdin) != 0)
-    error (EXIT_FAILURE, errno, "-");
+  cleanup_file_list_stdin ();
 
   return exit_status;
 }
diff --git a/tests/misc/expand.pl b/tests/misc/expand.pl
index a6b4a4d61..8a9cad144 100755
--- a/tests/misc/expand.pl
+++ b/tests/misc/expand.pl
@@ -18,7 +18,11 @@
 
 use strict;
 
+my $limits = getlimits ();
+my $UINTMAX_OFLOW = $limits->{UINTMAX_OFLOW};
+
 (my $program_name = $0) =~ s|.*/||;
+my $prog = 'expand';
 
 # Turn off localization of executable's output.
 @ENV{qw(LANGUAGE LANG LC_ALL)} = ('C') x 3;
@@ -27,13 +31,127 @@ my @Tests =
   (
    ['t1', '--tabs=3',     {IN=>"a\tb"}, {OUT=>"a  b"}],
    ['t2', '--tabs=3,6,9', {IN=>"a\tb\tc\td\te"}, {OUT=>"a  b  c  d e"}],
+   ['t3', '--tabs="3 6 9"',   {IN=>"a\tb\tc\td\te"}, {OUT=>"a  b  c  d e"}],
+   # Leading space/commas are silently ignored; Mixing space/commas is allowed.
+   # (a side-effect of allowing direct "-3,9" parameter).
+   ['t4', '--tabs=", 3,6 9"', {IN=>"a\tb\tc\td\te"}, {OUT=>"a  b  c  d e"}],
+   # tab stops parameter without values
+   ['t5', '--tabs=""',        {IN=>"a\tb\tc"}, {OUT=>"a       b       c"}],
+   ['t6', '--tabs=","',       {IN=>"a\tb\tc"}, {OUT=>"a       b       c"}],
+   ['t7', '--tabs=" "',       {IN=>"a\tb\tc"}, {OUT=>"a       b       c"}],
+
+   # Input field wider than the specified tab list
+   ['t8', '--tabs=6,9', {IN=>"a\tbbbbbbbbbbbbb\tc"},
+    {OUT=>"a     bbbbbbbbbbbbb c"}],
+
    ['i1', '--tabs=3 -i', {IN=>"\ta\tb"}, {OUT=>"   a\tb"}],
    ['i2', '--tabs=3 -i', {IN=>" \ta\tb"}, {OUT=>"   a\tb"}],
+
+   # Undocumented feature:
+   #   treat "expand -7"  as "expand --tabs 7" ,
+   #   and   "expand -90" as "expand --tabs 90",
+   ['u1', '-3',    {IN=>"a\tb\tc"}, {OUT=>"a  b  c"}],
+   ['u2', '-4 -9', {IN=>"a\tb\tc"}, {OUT=>"a   b    c"}],
+   ['u3', '-11',   {IN=>"a\tb\tc"}, {OUT=>"a          b          c"}],
+   # Test all digits (for full code coverage)
+   ['u4', '-2 -6', {IN=>"a\tb\tc"}, {OUT=>"a b   c"}],
+   ['u5', '-7',    {IN=>"a\tb"},    {OUT=>"a      b"}],
+   ['u6', '-8',    {IN=>"a\tb"},    {OUT=>"a       b"}],
+   # This syntax is handled internally as "-3, -9"
+   ['u7', '-3,9',  {IN=>"a\tb\tc"}, {OUT=>"a  b     c"}],
+
+   # Multiple non-empty files
+   ['f1', '--tabs=4',
+    {IN=>{"in1" => "a\tb\n"}}, {IN=>{"in2" => "c\td\n"}},
+    {OUT=>"a   b\nc   d\n"}],
+   # Multiple files, first file is empty
+   ['f2', '--tabs=4',
+    {IN=>{"in1" => ""}}, {IN=>{"in2" => "c\td\n"}},
+    {OUT=>"c   d\n"}],
+   # Multiple files, second file is empty
+   ['f3', '--tabs=4',
+    {IN=>{"in1" => "a\tb\n"}}, {IN=>{"in2" => ""}},
+    {OUT=>"a   b\n"}],
+
+
+   # Test '\b' (back-space) - subtract one column.
+   #
+   # Note:
+   # In a terminal window, 'expand' will appear to erase the 'a' characters
+   # due to overwriting them with spaces:
+   #
+   #    $ printf 'aaa\b\b\bc\td\n'
+   #    caa     d
+   #    $ printf 'aaa\b\b\bc\td\n' | expand
+   #    c       d
+   #
+   # However the characters are all printed:
+   #
+   #    $ printf 'aaa\b\b\bc\td\n' | expand | od -An -ta
+   #      a   a   a  bs  bs  bs   c  sp  sp  sp  sp  sp  sp  sp   d  nl
+   #
+   # If users ever report a problem with these tests and just
+   # copy&paste from the terminal, their report will be confusing
+   # (the 'a' will not appear).
+   #
+   # To see an example, enable the 'b-confusing' test, and examine the
+   # reported log:
+   #
+   #     expand.pl: test b-confusing: stdout mismatch
+   #     *** b-confusing.2       Fri Jun 24 15:43:21 2016
+   #     --- b-confusing.O       Fri Jun 24 15:43:21 2016
+   #     ***************
+   #     *** 1 ****
+   #     ! c       d
+   #     --- 1 ----
+   #     ! c       d
+   #
+   # ['b-confusing','', {IN=>"aaa\b\b\bc\td\n"}, {OUT=>"c       d\n"}],
+
+   ['b1','', {IN=>"aaa\b\b\bc\td\n"}, {OUT=>"aaa\b\b\bc       d\n"}],
+
+   # \b as first character, when column is zero
+   ['b2','', {IN=>"\bc\td"}, {OUT=>"\bc       d"}],
+
+   # Testing tab list adjusted due to backspaces
+   # ('b3' is the baseline without backspaces).
+   ['b3','--tabs 2,4,6,10',
+    {IN=>"1\t2\t3\t4\t5\n" .
+         "a\tb\tc\td\te\n"},
+    {OUT=>"1 2 3 4   5\n" .
+          "a b c d   e\n"}],
+
+   # On screen this will appear the same as 'b3'
+   ['b4','--tabs 2,4,6,10',
+    {IN=>"1\t2\t3\t4\t5\n" .
+         "a\tbHELLO\b\b\b\b\b\tc\td\te\n"},
+    {OUT=>"1 2 3 4   5\n" .
+          "a bHELLO\b\b\b\b\b c d   e\n"}],
+
+   # On screen on 'bHE' will appear (LLO overwritten by spaces),
+   # 'c' should align with 4, 'd' with 5:
+   #   1 2 3 4   5
+   #   a bHE c   d e
+   ['b5','--tabs 2,4,6,10',
+    {IN=>"1\t2\t3\t4\t5\n" .
+         "a\tbHELLO\b\b\b\tc\td\te\n"},
+    {OUT=>"1 2 3 4   5\n" .
+          "a bHELLO\b\b\b c   d e\n"}],
+
+
+   # Test errors
+   ['e1', '--tabs="a"', {IN=>''}, {OUT=>''}, {EXIT=>1},
+    {ERR => "$prog: tab size contains invalid character(s): 'a'\n"}],
+   ['e2', "-t $UINTMAX_OFLOW", {IN=>''}, {OUT=>''}, {EXIT=>1},
+    {ERR => "$prog: tab stop is too large '$UINTMAX_OFLOW'\n"}],
+   ['e3', '--tabs=0',   {IN=>''}, {OUT=>''}, {EXIT=>1},
+    {ERR => "$prog: tab size cannot be 0\n"}],
+   ['e4', '--tabs=3,3', {IN=>''}, {OUT=>''}, {EXIT=>1},
+    {ERR => "$prog: tab sizes must be ascending\n"}],
   );
 
 my $save_temps = $ENV{DEBUG};
 my $verbose = $ENV{VERBOSE};
 
-my $prog = 'expand';
 my $fail = run_tests ($program_name, $prog, \@Tests, $save_temps, $verbose);
 exit $fail;
diff --git a/tests/misc/unexpand.pl b/tests/misc/unexpand.pl
index c592c5a52..6ba6d405c 100755
--- a/tests/misc/unexpand.pl
+++ b/tests/misc/unexpand.pl
@@ -90,6 +90,42 @@ my @Tests =
      # setting of e.g., _POSIX2_VERSION=1.
      ['obs-ovflo', "-$limits->{UINTMAX_OFLOW}", {IN=>''}, {OUT=>''},
       {EXIT => 1}, {ERR => "$prog: tab stop value is too large\n"}],
+
+
+     # Test input with backspaces '\b' ('bs1' is the baseline, without \b)
+     # Note: If users report errors in these tests, copy&pasting results from
+     # their terminate output might be confusing due to '\b' overriding
+     # characters. For details see '\b' tests in 'expand.pl'.
+     ['bs1', '-a -t4', {IN=>"aa  c\n"},    {OUT=>"aa\tc\n"}],
+     ['bs2', '-a -t4', {IN=>"aa\b  c\n"},  {OUT=>"aa\b  c\n"}],
+     ['bs3', '-a -t4', {IN=>"aa\b   c\n"}, {OUT=>"aa\b\tc\n"}],
+     ['bs4', '-a -t3', {IN=>"aa\b  c\n"},  {OUT=>"aa\b\tc\n"}],
+
+     # Undocumented feature:
+     #   treat "unexpand -7"  as "unexpand --first-only --tabs 7" ,
+     #   and   "unexpand -90" as "unexpand --first-only --tabs 90",
+     ['u1', '-a -3',    {IN=>"a  b  c"}, {OUT=>"a\tb\tc"}],
+     ['u2', '-a -4,9',  {IN=>"a   b    c"}, {OUT=>"a\tb\tc"}],
+     ['u3', '-a -11',   {IN=>"a          b"}, {OUT=>"a\tb"}],
+     # Test all digits (for full code coverage)
+     ['u4', '-a -2,6',  {IN=>"a b   c"}, {OUT=>"a b\tc"}],
+     ['u5', '-a -7',    {IN=>"a      b"},    {OUT=>"a\tb"}],
+     ['u6', '-a -8',    {IN=>"a       b"},    {OUT=>"a\tb"}],
+     # This syntax is handled internally as "-3, -9"
+     ['u7', '-a -3,9',  {IN=>"a  b     c"}, {OUT=>"a\tb\tc"}],
+     # Default (without -a) is --first-only:
+     ['u8', '-3',  {IN=>"   a   b"}, {OUT=>"\ta   b"}],
+
+     # Arguably this should minimize translation as is done on Solaris.
+     # I.e., not modify the input.  But since the result is equivalent,
+     # and to be consistent in output with older versions, we output
+     # a '\t' rather than a space for the second tab position.
+     # For more detailed comparison with other implementations see:
+     # http://lists.gnu.org/archive/html/coreutils/2016-06/msg00015.html
+     # http://lists.gnu.org/archive/html/coreutils/2016-07/msg00011.html
+     ['ts1', '-t8,9', {IN=>"x\t \t y\n"},    {OUT=>"x\t\t\t y\n"}],
+     # There is no ambiguity here. This should always be the output.
+     ['ts2', '-t5,8', {IN=>"x\t \t y\n"},    {OUT=>"x\t\t y\n"}],
     );
 
 my $save_temps = $ENV{DEBUG};