WIP: new m4sugar function m4_join_uniq.

This is intended to be a pure version of m4_append_uniq,
i.e. it does all its work by expansion and not by defining macros.
It will be used by the code in the next commit.

WIP: currently has severe bugs, foreach.m4 version still needed.

* lib/m4sugar/m4sugar.m4 (m4_join_uniq, _m4_join_uniq): New macros.
* doc/autoconf.texi: Document m4_join_uniq.
* tests/m4sugar.at: Test m4_join_uniq.

doc/autoconf.texi

@@ -13299,13 +13299,16 @@ pairs, and replace all remaining newlines with a space.  The result is
 still a quoted string.
 @end defmac
 
-@defmac m4_join (@ovar{separator}, @var{args}@dots{})
-@defmacx m4_joinall (@ovar{separator}, @var{args}@dots{})
+@defmac m4_join (@var{separator}, @var{args}@dots{})
+@defmacx m4_joinall (@var{separator}, @var{args}@dots{})
+@defmacx m4_join_uniq (@var{separator}, @var{args}@dots{})
 @msindex{join}
 @msindex{joinall}
 Concatenate each @var{arg}, separated by @var{separator}.
 @code{joinall} uses every argument, while @code{join} omits empty
 arguments so that there are no back-to-back separators in the output.
+@code{join_uniq} omits empty arguments and also omits each argument
+that is already included in the string so far.
 The result is a quoted string.
 @example
 m4_define([active], [ACTIVE])dnl
@@ -13313,11 +13316,20 @@ m4_join([|], [one], [], [active], [two])
 @result{}one|active|two
 m4_joinall([|], [one], [], [active], [two])
 @result{}one||active|two
+m4_join([|], [onethree], [one], [], [two])
+@result{}onethree|one|two
+m4_join_uniq([|], [onethree], [one], [], [two])
+@result{}onethree|two
 @end example
 
 Note that if all you intend to do is join @var{args} with commas between
 them, to form a quoted list suitable for @code{m4_foreach}, it is more
 efficient to use @code{m4_dquote}.
+
+@code{m4_join_uniq} was introduced in Autoconf 2.73.
+Like @code{m4_append_uniq} it is inherently quadratic in the total
+length of the @var{args}; when this is large it is more efficient to use
+the @samp{m4_set_} functions instead (@pxref{Set manipulation Macros}).
 @end defmac
 
 @defmac m4_newline (@ovar{text})

lib/m4sugar/m4sugar.m4

@@ -2467,6 +2467,23 @@ m4_define([m4_joinall], [[$2]_$0([$1], m4_shift($@))])
 m4_define([_m4_joinall],
 [m4_if([$#], [2], [], [[$1$3]$0([$1], m4_shift2($@))])])
 
+# m4_join_uniq(SEP, ARG1, ARG2...)
+# ------------------------------------------
+# Same as m4_join, but also if any argument is already a substring of the
+# concatenation-so-far it is dropped.  No expansion is performed on SEP
+# or ARGS.  See m4_join for implementation notes.
+m4_define([m4_join_uniq],
+[m4_if([$#], [1], [],
+       [$#], [2], [[$2]],
+       [m4_if([$2], [], [], m4_index([$2], [$3]), [-1], [_], []
+        )$0([$1], [[$2]], m4_shift3($@))])])
+m4_define([_m4_join_uniq],
+[m4_if([$#$2], [2], [],
+       
+       [m4_if([$2], [], [], m4_index([$2], [$3]), [-1], [[$1$2]],
+              [])$0([$1], m4_shift2($@))])])
+
+
 # m4_combine([SEPARATOR], PREFIX-LIST, [INFIX], SUFFIX...)
 # --------------------------------------------------------
 # Produce the pairwise combination of every element in the quoted,

tests/m4sugar.at

@@ -1157,7 +1157,7 @@ AT_CLEANUP
 
 AT_SETUP([m4@&t@_join])
 
-AT_KEYWORDS([m4@&t@_joinall])
+AT_KEYWORDS([m4@&t@_joinall m4@&t@_join_uniq])
 
 AT_CHECK_M4SUGAR_TEXT(
 [[m4_define([active], [ACTIVE])dnl
@@ -1202,6 +1202,27 @@ m4_joinall([-], [one],,,)
 m4_joinall([], ,,,[two])
 m4_joinall([], [two],,,)
 m4_joinall([-], [], [], [three], [], [])
+----
+m4_join_uniq
+m4_join_uniq([-])
+m4_join_uniq([/], [one])
+m4_join_uniq([/], [one], [two])
+m4_join_uniq([/], [one], [], [two])
+m4_join_uniq([], [one], [two])
+m4_join_uniq([], [one], [], [two])
+m4_join_uniq([/], [one], [one], [two])
+m4_join_uniq([/], [one], [two], [one])
+m4_join_uniq([/], [one], [on], [two])
+m4_join_uniq([, ], [one], [two])
+m4_dquote(m4_join_uniq([, ], [one], [two]))
+m4_join_uniq([/], [active], [active])
+m4_join_uniq([/], [active], [], [active])
+m4_join_uniq([ active ], [one], , [two])
+m4_join_uniq([/], ,,,[one])
+m4_join_uniq([/], [one],,,)
+m4_join_uniq([], ,,,[two])
+m4_join_uniq([], [two],,,)
+m4_join_uniq([/], [], [], [three], [], [])dnl
 ]],
 [[
 
@@ -1244,6 +1265,27 @@ one---
 two
 two
 --three--
+----
+
+
+one
+one/two
+one/two
+onetwo
+onetwo
+one/two
+one/two
+one/two
+one, two
+[one, two]
+active/active
+active/active
+one active two
+one
+one
+two
+two
+three
 ]])
 
 AT_CLEANUP