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+==============
+Option parsing
+==============
+
+libHX uses a table-based approach like libpopt[#f3]. It provides for both long
+and short options and the different styles associated with them, such as
+absence or presence of an equals sign for long options (``--foo=bar`` and ``--foo
+bar``), bundling (writing ``-abc`` for non-argument taking options ``-a -b -c``),
+squashing (writing ``-fbar`` for an argument-requiring option ``-f bar``). The “lone
+dash” that is often used to indicate standard input or standard output, is
+correctly handled[#f4], as in ``-f -``.
+
+.. [#f3] The alternative would be an iterative, open-coded approach like
+         ``getopt``(3) requires.
+
+.. [#f4] popt failed to do this for a long time.
+
+A table-based approach allows for the parser to run as one unit, quite unlike
+the open-coded ``getopt``(3) loop where the function returns for every argument
+it parsed and needs to be called repeatedly.
+
+
+Synopsis
+========
+
+.. code-block:: c
+
+	#include <libHX/option.h>
+
+	struct HXoption {
+		const char *ln;
+		char sh;
+		unsigned int type;
+		void *ptr, *uptr;
+		void (*cb)(const struct HXoptcb *);
+		int val;
+		const char *help, *htyp;
+	};
+
+	int HX_getopt(const struct HXoption *options_table, int *argc, const char ***argv, unsigned int flags);
+
+The various fields of ``struct HXoption`` are:
+
+``ln``
+	The long option name, if any. May be ``NULL`` if none is to be assigned
+	for this entry.
+
+``sh``
+	The short option name/character, if any. May be '``\0``' if none is to
+	be assigned for this entry.
+
+``type``
+	The type of the entry, essentially denoting the type of the target
+	variable.
+
+``val``
+	An integer value to be stored into ``*(int *)ptr`` when the option type
+	is ``HXTYPE_IVAL``.
+
+``ptr``
+	A pointer to the variable so that the option parser can store the
+	requested data in it. The pointer may be ``NULL``, in which case no
+	data is stored (but ``cb`` is still called if defined, with the data).
+
+``uptr``
+	A user-supplied pointer. Its value is passed verbatim to the callback,
+	and may be used for any purpose the user wishes. If the option type is
+	``HXTYPE_SVAL``, it is the value in uptr that will be used to populate
+	``*(const char **)ptr``.
+
+``cb``
+	If not ``NULL``, call out to the referenced function after the option
+	has been parsed (and the results possibly be stored in ``ptr``).
+
+``help``
+	A help string that is shown for the option when the option table is
+	dumped by request (e.g. ``yourprgram --help``).
+
+``htyp``
+	String containing a keyword to aid the user in understanding the
+	available options during dump. See examples.
+
+Due to the amount of fields, it is advised to use C99 named initializers to
+populate a struct, as they allow to omit unspecified fields, and assume no
+specific order of the members:
+
+.. code-block:: c
+
+	struct HXoption e = {.sh = 'f', .help = "Force"};
+
+It is a sad fact that C++ has not gotten around to implement named initializers
+as of C++17. It is possible to put the option parsing code into a separate C
+source file that can then be compiled in C99 rather than C++ mode.
+
+
+Type map
+========
+
+``HXTYPE_NONE``
+	The option does not take any argument, but the presence of the option
+	may be record by setting ``*(int *)ptr`` to ``1``. Other rules apply
+	when ``HXOPT_INC`` or ``HXOPT_DEC`` are specified as flags.
+
+``HXTYPE_VAL``
+	Use the integer value specified by ``ival`` and store it in
+	``*(int *)ptr``.
+
+``HXTYPE_SVAL``
+	Use the memory location specified by ``sval`` and store it in ``*(const
+	char **)ptr``. OUTDATED.
+
+``HXTYPE_BOOL``
+	Interpret the supplied argument as a boolean descriptive (must be
+	``yes``, ``no``, ``on``, ``off``, ``true``, ``false``, ``0`` or ``1``)
+	and store the result in ``*(int *)ptr``.
+
+``HXTYPE_STRING``
+	The argument string is duplicated to a new memory region and the
+	resulting pointer stored into ``*(char **)ptr``. This incurs an
+	allocation so that subsequently modifying the original argument string
+	in any way will not falsely propagate.
+
+``HXTYPE_STRDQ``
+	The argument string is duplicated to a new memory region and the
+	resulting pointer is added to the given HXdeque. Note that you often
+	need to use deferred initialization of the options table to avoid
+	putting ``NULL`` into the entry. See section about pitfalls.
+
+The following overview lists the types that map to the common integral and
+floating-point types. Signed and unsigned integeral types are processed using
+``strtol`` and ``strtoul``, respectively. ``strtol`` and ``strtoul`` will be
+called with automatic base detection. This usually means that a leading ``0``
+indicates the string is given in octal base, a leading ``0x`` indicates
+hexadecimal base, and decimal otherwise. ``HXTYPE_LLONG``, ``HXTYPE_ULLONG``,
+``HXTYPE_INT64`` and ``HXTYPE_UINT64`` use ``strtoll`` and/or ``strtoull``,
+which may not be available on all platforms.
+
+``HXTYPE_CHAR``
+	maps to ``char``
+
+``HXTYPE_UCHAR``
+	maps to ``unsigned char``
+
+``HXTYPE_SHORT``
+	maps to ``short``
+
+``HXTYPE_USHORT``
+	maps to ``unsigned short``
+
+``HXTYPE_INT``
+	maps to ``int``
+
+``HXTYPE_UINT``
+	maps to ``unsigned int``
+
+``HXTYPE_LONG``
+	maps to ``long``
+
+``HXTYPE_ULONG``
+	maps to ``unsigned long``
+
+``HXTYPE_LLONG``
+	maps to ``long long``
+
+``HXTYPE_ULLONG``
+	maps to ``unsigned long long``
+
+``HXTYPE_SIZE_T``
+	maps to ``size_t``
+
+``HXTYPE_FLOAT``
+	maps to ``float``
+
+``HXTYPE_DOUBLE``
+	maps to ``double``
+
+``HXTYPE_INT8``
+	maps to ``int8_t``
+
+``HXTYPE_UINT8``
+	maps to ``uint8_t``
+
+``HXTYPE_INT16``
+	maps to ``int16_t``
+
+``HXTYPE_UINT16``
+	maps to ``uint16_t``
+
+``HXTYPE_INT32``
+	maps to ``int32_t``
+
+``HXTYPE_UINT32``
+	maps to ``uint32_t``
+
+``HXTYPE_INT64``
+	maps to ``int64_t``
+
+``HXTYPE_UINT64``
+	maps to ``uint64_t``
+
+``HXTYPE_FLOAT`` and ``HXTYPE_DOUBLE`` make use of ``strtod`` (``strtof`` is
+not used). A corresponding type for the ``long double`` format is not
+specified, but may be implemented on behalf of the user via a callback.
+
+
+Flags
+=====
+
+Flags can be combined into the type parameter by OR'ing them. It is valid to
+not specify any flags at all, but most flags collide with one another.
+
+``HXOPT_INC``
+	Perform an increment on the memory location specified by the
+	``*(int *)ptr`` pointer. Make sure the referenced variable is
+	initialized beforehand!
+
+``HXOPT_DEC``
+	Perform a decrement on the pointee. Same requirements as ``HXOPT_INC``.
+
+Only one of ``HXOPT_INC`` and ``HXOPT_DEC`` may be specified at a time,
+and they require that the base type is ``HXTYPE_NONE``, or they will
+have no effect. An example may be found below.
+
+``HXOPT_NOT``
+	Binary negation of the argument directly after reading it from the
+	command line into memory. Any of the three following operations are
+	executed with the already-negated value.
+
+``HXOPT_OR``
+	Apply bitwise OR on the pointee with the specified/transformed value.
+
+``HXOPT_AND``
+	Apply bitwise AND on the pointee with the specified/transformed value.
+
+``HXOPT_XOR``
+	Apply bitwise XOR on the pointee with the specified/transformed value.
+
+Only one of ``HXOPT_OR``, ``HXOPT_AND`` and ``HXOPT_XOR`` may be specified at
+a time, but they can be used with any integral type (``HXTYPE_UINT``,
+``HXTYPE_ULONG``, etc.). An example can be found below.
+
+``HXOPT_OPTIONAL``
+	This flag allows for an option to take zero or one argument. Needless
+	to say that this can be confusing to the user. iptables's ``-L`` option
+	for example is one of this kind (though it does not use the libHX
+	option parser). When this flag is used, ``-f -b`` is interpreted as
+	``-f`` without an argument, as is ``-f --bar`` — things that look like
+	an option take precedence over an option with an optional argument.
+	``-f -`` of course denotes an option with an argument, as ``-`` is
+	often used to indicate standard input/output.
+
+
+Special entries
+===============
+
+HXopt provides two special entries via macros:
+
+``HXOPT_AUTOHELP``
+	Adds entries to recognize ``-?`` and ``--help`` that will display the
+	(long-format) help screen, and ``--usage`` that will display the short
+	option syntax overview. All three options will exit the program
+	afterwards.
+
+``HXOPT_TABLEEND``
+	This sentinel marks the end of the table and is required on all tables.
+	(See examples for details.)
+
+
+Invoking the parser
+===================
+
+.. code-block:: c
+
+	int HX_getopt(const struct HXoption *options_table, int *argc, const char ***argv, unsigned int flags);HX_getopt
+
+``HX_getopt`` is the actual parsing function. It takes the option table, and a
+pointer to your argc and argv variables that you get from the main function.
+The parser will, by default, consume all options and their arguments, similar
+to Perl's ``Getopt::Long`` module. ``*argv`` is then updated to point to a new
+array of strings (to be deallocated with ``HX_zvecfree``) and ``*argc`` is
+updated accordingly. Additional flags can control the exact behavior of
+``HX_getopt``:
+
+``HXOPT_PTHRU``
+	“Passthrough mode”. Any unknown options are not “eaten” and are instead
+	passed back into the resulting argv array.
+
+``HXOPT_QUIET``
+	Do not print any diagnostics when encountering errors in the user's
+	input.
+
+``HXOPT_HELPONERR``
+	Display the (long-format) help when an error, such as an unknown option
+	or a violation of syntax, is encountered.
+
+``HXOPT_USAGEONERR``
+	Display the short-format usage syntax when an error is encountered.
+
+``HXOPT_RQ_ORDER``
+	Specifying this option terminates option processing when the first
+	non-option argument in argv is encountered. This behavior is also
+	implicit when the environment variable ``POSIXLY_CORRECT`` is set.
+
+``HXOPT_KEEP_ARGV``
+	Do not modify ``argc`` and ``argv`` at all.
+
+The return value can be one of the following:
+
+``HXOPT_ERR_SUCCESS``
+	Parsing was successful.
+
+``HXOPT_ERR_UNKN``
+	An unknown option was encountered.
+
+``HXOPT_ERR_VOID``
+	An argument was given for an option which does not allow one. In
+	practice this only happens with ``--foo=bar`` when ``--foo`` is of type
+	``HXTYPE_NONE``, ``HXTYPE_VAL`` or ``HXTYPE_SVAL``. This does not
+	affect ``--foo bar``, because this can be unambiguously interpreted as
+	``bar`` being a remaining argument to the program.
+
+``HXOPT_ERR_MIS``
+	Missing argument for an option that requires one.
+
+``HXOPT_ERR_AMBIG``
+	An abbreviation of a long option was ambiguous.
+
+negative non-zero
+	Failure on behalf of lower-level calls; errno.
+
+
+Pitfalls
+========
+
+Staticness of tables
+--------------------
+
+The following is an example of a possible pitfall regarding ``HXTYPE_STRDQ``:
+
+.. code-block:: c
+
+	static struct HXdeque *dq;
+
+	static bool get_options(int *argc, const char ***argv)
+	{
+		static const struct HXoption options_table[] = {
+			{.sh = 'N', .type = HXTYPE_STRDQ, .ptr = dq,
+			 .help = "Add name"},
+			HXOPT_TABLEEND,
+		};
+		return HX_getopt(options_table, argc, argv, HXOPT_USAGEONERR) ==
+		       HXOPT_ERR_SUCCESS;
+	}
+
+	int main(int argc, const char **argv)
+	{
+		dq = HXdeque_init();
+		get_options(&argc, &argv);
+		return 0;
+	}
+
+The problem here is that ``options_table`` is, due to the static keyword,
+initialized at compile-time when ``dq`` is still ``NULL``. To counter this
+problem and have it doing the right thing, you must remove the static qualifier
+on the options table when used with ``HXTYPE_STRDQ``, so that it will be
+evaluated when it is first executed.
+
+It was not deemed worthwhile to have ``HXTYPE_STRDQ`` take an indirect
+``HXdeque`` (``struct HXdeque **``) instead just to bypass this issue. (Live
+with it.)
+
+Limitations
+-----------
+
+The HX option parser has been influenced by both popt and Getopt::Long, but
+eventually, there are differences:
+
+* Long options with a single dash (``-foo bar``). This unsupported
+  syntax clashes easily with support for option bundling or squashing. In case
+  of bundling, ``-foo`` might actually be ``-f -o -o``, or ``-f oo`` in case of
+  squashing. It also introduces redundant ways to specify options, which is not
+  in the spirit of the author.
+
+* Options using a ``+`` as a prefix, as in ``+foo``. Xterm for
+  example uses it as a way to negate an option. In the author's opinion, using
+  one character to specify options is enough — by GNU standards, a negator is
+  named ``--no-foo``.
+
+* Table nesting like implemented in popt. HXopt has no provision for nested
+  tables, as the need has not come up yet. It does however support chained
+  processing. You cannot do nested tables even with callbacks, as the new argv
+  array is only put in place shortly before ``HX_getopt`` returns.
+
+
+Examples
+========
+
+Basic example
+-------------
+
+The following code snippet should provide an equivalent of the
+GNU getopt sample.[#f5]
+
+.. [#f5] http://www.gnu.org/software/libtool/manual/libc/Example-of-Getopt.html\#Example-of-Getopt
+
+.. code-block:: c
+
+	#include <stdio.h>
+	#include <stdilb.h>
+	#include <libHX/option.h>
+
+	int main(int argc, const char **argv)
+	{
+		int aflag = 0;
+		int bflag = 0;
+		char *cflag = NULL;
+
+		struct HXoption options_table[] = {
+			{.sh = 'a', .type = HXTYPE_NONE, .ptr = &aflag},
+			{.sh = 'b', .type = HXTYPE_NONE, .ptr = &bflag},
+			{.sh = 'c', .type = HXTYPE_STRING, .ptr = &cflag},
+			HXOPT_AUTOHELP,
+			HXOPT_TABLEEND,
+		};
+
+		if (HX_getopt(options_table, &argc, &argv, HXOPT_USAGEONERR) !=
+		    HXOPT_ERR_SUCCESS)
+
+			return EXIT_FAILURE;
+
+		printf("aflag = %d, bflag = %d, cvalue = %s\n",
+		       aflag, bflag, cvalue);
+
+		while (*++argv != NULL)
+			printf("Non-option argument %s\n", *argv);
+
+		return EXIT_SUCCESS;
+	}
+
+Verbosity levels
+----------------
+
+.. code-block:: c
+
+	static int verbosity = 1; /* somewhat silent by default */
+	static const struct HXoption options_table[] = {
+		{.sh = 'q', .type = HXTYPE_NONE | HXOPT_DEC, .ptr = &verbosity,
+		 .help = "Reduce verbosity"},
+		{.sh = 'v', .type = HXTYPE_NONE | HXOPT_INC, .ptr = &verbosity,
+		 .help = "Increase verbosity"},
+		HXOPT_TABLEEND,
+	};
+
+This sample option table makes it possible to turn the verbosity of the program
+up or down, depending on whether the user specified ``-q`` or ``-v``. By passing
+multiple ``-v`` flags, the verbosity can be turned up even more. The range depends
+on the ``int`` data type for your particular platform and compiler; if you want
+to have the verbosity capped at a specific level, you will need to use an extra
+callback:
+
+.. code-block:: c
+
+	static int verbosity = 1;
+
+	static void v_check(const struct HXoptcb *cbi)
+	{
+		if (verbosity < 0)
+			verbosity = 0;
+		else if (verbosity > 4)
+			verbosity = 4;
+	}
+
+	static const struct HXoption options_table[] = {
+		{.sh = 'q', .type = HXTYPE_NONE | HXOPT_DEC, .ptr = &verbosity,
+		 .cb = v_check, .help = "Lower verbosity"},
+		{.sh = 'v', .type = HXTYPE_NONE | HXOPT_INC, .ptr = &verbosity,
+		 .cb = v_check, .help = "Raise verbosity"},
+		HXOPT_TABLEEND,
+	};
+
+Mask operations
+---------------
+
+.. code-block:: c
+
+	/* run on all CPU cores by default */
+	static unsigned int cpu_mask = ~0U;
+	/* use no network connections by default */
+	static unsigned int net_mask = 0;
+	static struct HXoption options_table[] = {
+		{.sh = 'c', .type = HXTYPE_UINT | HXOPT_NOT | HXOPT_AND, .ptr = &cpu_mask,
+		 .help = "Mask of cores to exclude", .htyp = "cpu_mask"},
+		{.sh = 'n', .type = HXTYPE_UINT | HXOPT_OR, .ptr = &net_mask,
+
+		 .help = "Mask of network channels to additionally use",
+		 .htyp = "channel_mask"},
+		HXOPT_TABLEEND,
+	};
+
+What this options table does is ``cpu_mask &= ~x`` and ``net_mask |= y``, the
+classic operations of clearing and setting bits.
+
+Support for non-standard actions
+--------------------------------
+
+Supporting additional types or custom storage formats is easy, by simply using
+``HXTYPE_STRING``, ``NULL`` as the data pointer (usually by not specifying it
+at all), the pointer to your data in the user-specified pointer ``uptr``, and
+the callback function in ``cb``.
+
+.. code-block:: c
+
+	struct fixed_point {
+		int integral;
+		unsigned int fraction;
+	};
+
+	static struct fixed_point number;
+
+	static void fixed_point_parse(const struct HXoptcb *cbi)
+	{
+		char *end;
+
+		number.integral = strtol(cbi->data, &end, 0);
+		if (*end == '\0')
+			number.fraction = 0;
+		else if (*end == '.')
+			number.fraction = strtoul(end + 1, NULL, 0);
+		else
+			fprintf(stderr, "Illegal input.\n");
+	}
+
+	static const struct HXoption options_table[] = {
+		{.sh = 'n', .type = HXTYPE_STRING, .cb = fixed_point_parse,
+		 .uptr = &number, .help = "Do this or that",
+		HXOPT_TABLEEND,
+
+	};
+
+Chained argument processing
+---------------------------
+
+On the first run, only ``--cake`` and ``--fruit`` is considered, which is then
+used to select the next set of accepted options. Note that
+``HXOPT_DESTROY_OLD`` is used here, which causes the ``argv`` that is produced
+by the first invocation of ``HX_getopt`` in the ``get_options`` function to be
+freed as it gets replaced by a new argv again by ``HX_getopt`` in
+``get_cakes``/``get_fruit``. ``HXOPT_DESTROY_OLD`` is however not specified in
+the first invocation, because the initial argv resides on the stack and cannot
+be freed.
+
+.. code-block:: c
+
+	static bool get_cakes(int *argc, const char ***argv)
+	{
+		struct HXoption option_table[] = {
+			...
+		};
+		return HX_getopt(cake_table, argc, argv,
+		       HXOPT_USAGEONERR | HXOPT_DESTROY_OLD) == HXOPT_ERR_SUCCESS;
+	}
+
+	static bool get_fruit(int *argc, const char ***argv)
+	{
+		struct HXoption fruit_table[] = {
+			...
+		};
+		return HX_getopt(fruit_table, argc, argv,
+		       HXOPT_USAGEONERR | HXOPT_DESTROY_OLD) == HXOPT_ERR_SUCCESS;
+	}
+
+	static bool get_options(int *argc, const char ***argv)
+	{
+		int cake = 0, fruit = 0;
+		struct HXoption option_table[] = {
+			{.ln = "cake",  .type = HXTYPE_NONE, .ptr = &cake},
+			{.ln = "fruit", .type = HXTYPE_NONE, .ptr = &fruit},
+			HXOPT_TABLEEND,
+		};
+		if (HX_getopt(option_table, argc, argv, HXOPT_PTHRU) != HXOPT_ERR_SUCCESS)
+			return false;
+		if (cake)
+			return get_cakes(argc, argv);
+		else if (fruit)
+			return get_fruit(argc, argv);
+		return false;
+	}