# LV2 State Extension
# Copyright 2010-2012 David Robillard <d@drobilla.net>
# Copyright 2010 Leonard Ritter <paniq@paniq.org>
#
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@prefix dcs:   <http://ontologi.es/doap-changeset#> .
@prefix doap:  <http://usefulinc.com/ns/doap#> .
@prefix foaf:  <http://xmlns.com/foaf/0.1/> .
@prefix lv2:   <http://lv2plug.in/ns/lv2core#> .
@prefix rdf:   <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs:  <http://www.w3.org/2000/01/rdf-schema#> .
@prefix state: <http://lv2plug.in/ns/ext/state#> .

<http://drobilla.net/drobilla#me>
	a foaf:Person ;
	foaf:name "David Robillard" ;
	foaf:homepage <http://drobilla.net/> ;
	foaf:mbox <mailto:d@drobilla.net> ;
	rdfs:seeAlso <http://drobilla.net/drobilla> .

<http://lv2plug.in/ns/ext/state>
	a lv2:Specification ;
	doap:name "LV2 State" ;
	doap:shortdesc "An interface for LV2 plugins to save and restore state." ;
	doap:license <http://opensource.org/licenses/isc> ;
	doap:release [
		doap:revision "0.6" ;
		doap:created "2012-03-03" ;
		dcs:blame <http://drobilla.net/drobilla#me>
	] ;
	doap:developer [
		a foaf:Person ;
		foaf:name "Leonard Ritter" ;
		foaf:homepage <http://paniq.org>
	] ;
	doap:maintainer <http://drobilla.net/drobilla#me> ;
	lv2:documentation """

<p>This extension defines a simple mechanism which allows hosts to save and
restore a plugin instance's state.  The goal is for an instance's state to be
<em>completely</em> described by port values (as with all LV2 plugins) and a
simple dictionary.</p>

<p>The <q>state</q> defined here is conceptually a key:value dictionary, with
URI keys and values of any type.  For performance reasons the key and value
type are actually a <q>URID</q>, a URI mapped to an integer.  A single
key:value pair is called a <q>property</q>.</p>

<p>This state model is simple yet has many benefits:</p>
<ul>
  <li>Both fast and extensible thanks to URID keys.</li>
  <li>No limitations on possible value types.</li>
  <li>Easy to serialise in almost any format.</li>
  <li>Easy to store in a typical <q>map</q> or <q>dictionary</q> data
  structure.</li>
  <li>Elegantly described in Turtle, so state can be described in LV2 data
  files (including presets).</li>
  <li>Does not impose any file formats, data structures, or file system
  requirements.</li>
  <li>Suitable for portable persistent state as well as fast in-memory
  snapshots.</li>
  <li>Keys <em>may</em> be well-defined and used meaningfully across several
  implementations.</li>
  <li>State <em>may</em> be dynamic, but plugins are not required to have a
  dynamic dictionary data structure available.</li>
</ul>

<p>To implement state, the plugin provides a state:Interface to the host.  To
save or restore, the host calls LV2_State_Interface::save() or
LV2_State_Interface::restore(), passing a callback to be used for handling a
single property.  The host is free to implement property storage and retrieval
in any way.</p>

<p>Since value types are defined by URI, any type is possible.  However, a set
of standard types is defined by the <a href="http://lv2plug.in/ns/ext/atom">LV2
Atom</a> extension.  Use of these types is recommended.  Hosts MUST implement
at least <a href="http://lv2plug.in/ns/ext/atom#String">atom:String</a>, which
is simply a C string.</p>

<h3>Referring to Files</h3>

<p>Plugins may need to refer to existing files (e.g. loaded samples) in their
state.  This is done by storing the file's path as a property just like any
other value.  However, there are some rules which MUST be followed when storing
paths, see <a href="#mapPath">state:mapPath</a> for details.  Plugins MUST use
the type <a href="http://lv2plug.in/ns/ext/atom#Path">atom:Path</a> for all
paths in their state.</p>

<p>Plugins are strongly encouraged to avoid creating files, instead storing all
state as properties.  However, occasionally the ability to create files is
necessary.  To make this possible, the host can provide the feature <a
href="#makePath">state:makePath</a> which allocates paths for plugin-created
files.  Plugins MUST NOT create files in any other locations.</p>

<h3>Plugin Code Example</h3>

<pre class="c-code">

/* Namespace for this plugin's keys.  This SHOULD be something that could be
   published as a document, even if that document does not exist right now.
*/
#define NS_MY "http://example.org/myplugin/schema#"

LV2_Handle my_instantiate(...)
{
    MyPlugin* plugin = ...;
    plugin->uris.atom_String = map_uri(LV2_ATOM__String);
    plugin->uris.my_greeting = map_uri(NS_MY "greeting");
    plugin->state.greeting   = strdup("Hello");  // Default value
    return plugin;
}

void my_save(LV2_Handle                 instance,
             LV2_State_Store_Function   store,
             LV2_State_Handle           handle,
             uint32_t                   flags,
             const LV2_Feature *const * features)
{
    MyPlugin*   plugin   = (MyPlugin*)instance;
    const char* greeting = plugin->state.greeting;

    store(handle,
          plugin->uris.my_greeting,
          greeting,
          strlen(greeting) + 1,  // Careful!  Need space for terminator
          plugin->uris.atom_String,
          LV2_STATE_IS_POD | LV2_STATE_IS_PORTABLE);
}

void my_restore(LV2_Handle                  instance,
                LV2_State_Retrieve_Function retrieve,
                LV2_State_Handle            handle,
                uint32_t                    flags,
                const LV2_Feature *const *  features)
{
    MyPlugin* plugin = (MyPlugin*)instance;

    size_t      size;
    uint32_t    type;
    uint32_t    flags;
    const char* greeting = retrieve(
        handle, plugin->uris.my_greeting, &amp;size, &amp;type, &amp;flags);

    if (greeting) {
        free(plugin->state->greeting);
        plugin->state->greeting = strdup(greeting);
    } else {
        plugin->state->greeting = strdup("Hello");
    }
}

const void* my_extension_data(const char* uri)
{
    static const LV2_State_Interface state_iface = { my_save, my_restore };
    if (!strcmp(uri, LV2_STATE_INTERFACE_URI)) {
        return &amp;state_iface;
    }
}
</pre>

<h3>Host Code Example</h3>

<pre class="c-code">
int store_callback(LV2_State_Handle handle,
                   uint32_t         key,
                   const void*      value,
                   size_t           size,
                   uint32_t         type,
                   uint32_t         flags)
{
    if ((flags &amp; LV2_STATE_IS_POD)) {
        /* We only care about POD since we're keeping state in memory only.
           For disk or network use, LV2_STATE_IS_PORTABLE must also be checked.
        */
        Map* state_map = (Map*)handle;
        state_map->insert(key, Value(copy(value), size, type));
        return 0;
    } else {
        return 1; /* Non-POD events are unsupported. */
    }
}

Map get_plugin_state(LV2_Handle instance)
{
    LV2_State* state = instance.extension_data(LV2_STATE__Interface);
    Map state_map;
    /** Request a fast/native/POD save, since we're just copying in memory */
    state.save(instance, store_callback, &amp;state_map,
               LV2_STATE_IS_POD|LV2_STATE_IS_NATIVE);
    return state_map;
}
</pre>

<h3>Extensions to this Specification</h3>

<p>It is likely that other interfaces for working with plugin state will be
developed as needed.  This is encouraged, however everything SHOULD work within
the state <em>model</em> defined here.  That is, <strong>do not complicate the
state model</strong>.  Implementations can assume the following:</p>

<ul>
<li>The current port values and state dictionary completely describe a plugin
instance, at least well enough that saving and restoring will yield an
<q>identical</q> instance from the user's perspective.</li>
<li>Hosts are not expected to save and/or restore any other attributes of a
plugin instance.</li>
</ul>

<p>The recommended way to add new functionality that may affect state is to
simply define it in terms of properties.  For example, instead of defining a
new set of commands, define properties whose values can be set appropriately.
This provides persistence <q>for free</q>, since rather than having to define a
set of commands <em>and</em> a set of properties for storing their effects,
only the properties need to be defined.</p>

<p>This <q>property principle</q> is summed up in the phrase: <q>Don't
stop; set playing to false</q>.</p>
""" .

state:Interface
	a rdfs:Class ;
	rdfs:subClassOf lv2:ExtensionData ;
	lv2:documentation """
<p>A structure (LV2_State_Interface) which contains functions to be called by
the host to save and restore state.  In order to support this extension, the
plugin must return a valid LV2_State_Interface from
LV2_Descriptor::extension_data() when it is called with URI
LV2_STATE__Interface.</p>

<p>The plugin data file should describe this like so:</p>
<pre class="turtle-code">
@prefix state: &lt;http://lv2plug.in/ns/ext/state#&gt; .

&lt;plugin&gt;
    a lv2:Plugin ;
    lv2:extensionData state:Interface .
</pre>
""" .

state:State
	a rdfs:Class ;
	rdfs:label "Plugin Instance State" ;
	lv2:documentation """
<p>A state dictionary.  This type should be used wherever instance state is
described in RDF, or any other context that uses type URIs.  The properties of
a resource with this type correspond directly to the properties of the state
dictionary (except the property that states it has this type).</p>
""" .

state:state
	a rdf:Property ;
	rdfs:label "State" ;
	rdfs:range state:State ;
	lv2:documentation """
<p>The state of this plugin instance, or similar resource.  This property may
be used anywhere a state needs to be described, for example:</p>

<pre class="turtle-code">
@prefix eg: &lt;http://example.org/&gt; .

&lt;plugininstance&gt;
    state:state [
        eg:somekey "some value" ;
        eg:someotherkey "some other value" ;
        eg:favourite-number 2
    ] .
</pre>
""" .

state:mapPath
	a lv2:Feature ;
	rdfs:label "Support for storing paths in state" ;
	lv2:documentation """
<p>This feature maps absolute paths to/from <q>abstract paths</q> which are
stored in state.  To support this feature a host must pass an LV2_Feature with
URI LV2_STATE__mapPath and data pointed to an LV2_State_Map_Path to the
plugin's LV2_State_Interface methods.</p>

<p>The plugin MUST map <em>all</em> paths stored in its state (including those
inside any files in its state).  This is necessary to enable host to handle
file system references correctly, e.g. for distribution or archival.</p>

<p>For example, a plugin may write a path to a state file like so:</p>

<pre class="c-code">
void write_path(LV2_State_Map_Path* map_path, FILE* myfile, const char* path)
{
    char* abstract_path = map_path->abstract_path(map_path->handle, path);
    fprintf(myfile, "%s", abstract_path);
    free(abstract_path);
}
</pre>

<p>Then, later reload the path like so:</p>

<pre class="c-code">
char* read_path(LV2_State_Map_Path* map_path, FILE* myfile)
{
    /* Obviously this is not production quality code! */
    char abstract_path[1024];
    fscanf(myfile, "%s", abstract_path);
    return map_path->absolute_path(map_path->handle, abstract_path);
}
</pre>
""" .

state:makePath
	a lv2:Feature ;
	rdfs:label "Support for creating new files and directories" ;
	lv2:documentation """
<p>This feature allows plugins to create new files and/or directories.  To
support this feature the host passes an LV2_Feature with URI
LV2_STATE__makePath and data pointed to an LV2_State_Make_Path to the
plugin.  The host may make this feature available only during save by passing
it to LV2_State_Interface::save(), or available any time by passing it to
LV2_Descriptor::instantiate().  If passed to LV2_State_Interface::save(), the
feature MUST NOT be used beyond the scope of that call.</p>

<p>The plugin is guaranteed a hierarchial namespace unique to that plugin
instance, and may expect the returned path to have the requested path as a
suffix.  There is <em>one</em> such namespace, even if the feature is passed to
both LV2_Descriptor::instantiate() <em>and</em>
LV2_State_Interface::save().  Beyond this, the plugin MUST NOT make any
assumptions about the returned paths.</p>

<p>Like any other paths, the plugin MUST map these paths using <a
href="#mapPath">state:mapPath</a> before storing them in state.  The plugin
MUST NOT assume these paths will be available across a save/restore otherwise,
i.e.  only mapped paths saved to state are persistent, any other created paths
are temporary.</p>

<p>For example, a plugin may create a file in a subdirectory like so:</p>

<pre class="c-code">
char* save_myfile(LV2_State_Make_Path* make_path)
{
    char* path   = make_path->path(make_path->handle, "foo/bar/myfile.txt");
    FILE* myfile = fopen(path, 'w');
    fprintf(myfile, "Hello");
    fclose(myfile);
    return path;
}
</pre>
""" .