Move documentation to metadata files and convert it to Markdown

1 files changed, 380 insertions, 1 deletions

diff --git a/lv2/state/state.meta.ttl b/lv2/state/state.meta.ttl
index 784946d..ac3f2d6 100644
--- a/lv2/state/state.meta.ttl
+++ b/lv2/state/state.meta.ttl
@@ -1,7 +1,9 @@
 @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 rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
+@prefix state: <http://lv2plug.in/ns/ext/state#> .
 
 <http://lv2plug.in/ns/ext/state>
 	a doap:Project ;
@@ -73,5 +75,382 @@
 				rdfs:label "Initial release."
 			]
 		]
-	] .
+	] ;
+	lv2:documentation """
+
+This extension defines a simple mechanism that allows hosts to save and restore
+a plugin instance's state.  The goal is for an instance's state to be
+completely described by port values and a simple dictionary.
+
+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 "URID", a URI mapped to an integer. A single key:value pair is
+called a "property".
+
+This state model is simple yet has many benefits:
+
+  * Both fast and extensible thanks to URID keys.
+
+  * No limitations on possible value types.
+
+  * Easy to serialise in almost any format.
+
+  * Easy to store in a typical "map" or "dictionary" data structure.
+
+  * Elegantly described in Turtle, so state can be described in LV2 data files
+    (including presets).
+
+  * Does not impose any file formats, data structures, or file system
+    requirements.
+
+  * Suitable for portable persistent state as well as fast in-memory snapshots.
+
+  * Keys _may_ be well-defined and used meaningfully across several
+    implementations.
+
+  * State _may_ be dynamic, but plugins are not required to have a dynamic
+    dictionary data structure available.
+
+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.
+
+Since value types are defined by URI, any type is possible. However, a set of
+standard types is defined by the [LV2 Atom](atom.html) extension. Use of these
+types is recommended. Hosts MUST implement at least
+[atom:String](atom.html#String), which is simply a C string.
+
+### Referring to Files
+
+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 state:mapPath for details. Plugins MUST use the type
+[atom:Path](atom.html#Path) for all paths in their state.
+
+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
+state:makePath which allocates paths for plugin-created files. Plugins MUST
+NOT create files in any other locations.
+
+### Plugin Code Example
+
+    :::c
+
+    /* 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#"
+
+    #define DEFAULT_GREETING "Hello"
+
+    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(DEFAULT_GREETING);
+        return plugin;
+    }
+
+    LV2_State_Status
+    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);
+
+        return LV2_STATE_SUCCESS;
+    }
+
+    LV2_State_Status
+    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, &size, &type, &flags);
+
+        if (greeting) {
+            free(plugin->state->greeting);
+            plugin->state->greeting = strdup(greeting);
+        } else {
+            plugin->state->greeting = strdup(DEFAULT_GREETING);
+        }
+
+        return LV2_STATE_SUCCESS;
+    }
+
+    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)) {
+            return &state_iface;
+        }
+    }
+
+### Host Code Example
+
+    :::c
+    LV2_State_Status
+    store_callback(LV2_State_Handle handle,
+                   uint32_t         key,
+                   const void*      value,
+                   size_t           size,
+                   uint32_t         type,
+                   uint32_t         flags)
+    {
+        if ((flags & LV2_STATE_IS_POD)) {
+            // We only care about POD since we're keeping state in memory only.
+            // Disk or network use would also require LV2_STATE_IS_PORTABLE.
+            Map* state_map = (Map*)handle;
+            state_map->insert(key, Value(copy(value), size, type));
+            return LV2_STATE_SUCCESS;;
+        } else {
+            return LV2_STATE_ERR_BAD_FLAGS; // Non-POD events are unsupported
+        }
+    }
+
+    Map
+    get_plugin_state(LV2_Handle instance)
+    {
+        LV2_State* state = instance.extension_data(LV2_STATE__interface);
+
+        // Request a fast/native/POD save, since we're just copying in memory
+        Map state_map;
+        state.save(instance, store_callback, &state_map,
+                   LV2_STATE_IS_POD|LV2_STATE_IS_NATIVE);
+
+        return state_map;
+    }
+
+### Extensions to this Specification
+
+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 _model_ defined here. That is, **do not complicate the state
+model**. Implementations can assume the following:
+
+  * The current port values and state dictionary completely describe a plugin
+    instance, at least well enough that saving and restoring will yield an
+    "identical" instance from the user's perspective.
+
+  * Hosts are not expected to save and/or restore any other attributes of a
+    plugin instance.
+
+### The "Property Principle"
+
+The main benefit of this meaningful state model is that it can double as a
+plugin control/query mechanism. For plugins that require more advanced control
+than simple control ports, instead of defining a set of commands, define
+properties whose values can be set appropriately. This provides both a way to
+control and save that state "for free", since there is no need to define
+commands _and_ a set of properties for storing their effects. In particular,
+this is a good way for UIs to achieve more advanced control of plugins.
+
+This "property principle" is summed up in the phrase: "Don't stop; set playing
+to false".
+
+This extension does not define a dynamic mechanism for state access and
+manipulation. The [LV2 Patch](patch.html) extension defines a generic set of
+messages which can be used to access or manipulate properties, and the [LV2
+Atom](atom.html) extension defines a port type and data container capable of
+transmitting those messages.
+
+"""^^lv2:Markdown .
+
+state:interface
+	lv2:documentation """
+
+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.
+
+The plugin data file should describe this like so:
+
+    :::turtle
+    @prefix state: &lt;http://lv2plug.in/ns/ext/state#&gt; .
+
+    &lt;plugin&gt;
+        a lv2:Plugin ;
+        lv2:extensionData state:interface .
+
+"""^^lv2:Markdown .
+
+state:State
+	lv2:documentation """
+
+This type should be used wherever instance state is described.  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).
+
+"""^^lv2:Markdown .
+
+state:loadDefaultState
+	lv2:documentation """
+
+This feature indicates that the plugin has default state listed with the
+state:state property which should be loaded by the host before running the
+plugin.  Requiring this feature allows plugins to implement a single state
+loading mechanism which works for initialisation as well as restoration,
+without having to hard-code default state.
+
+To support this feature, the host MUST restore the default state after
+instantiating the plugin but before calling run().
+
+"""^^lv2:Markdown .
+
+state:state
+	lv2:documentation """
+
+This property may be used anywhere a state needs to be described, for example:
+
+    :::turtle
+    @prefix eg: &lt;http://example.org/&gt; .
+
+    &lt;plugin-instance&gt;
+        state:state [
+            eg:somekey "some value" ;
+            eg:someotherkey "some other value" ;
+            eg:favourite-number 2
+        ] .
+
+"""^^lv2:Markdown .
+
+state:mapPath
+	lv2:documentation """
+
+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.
+
+The plugin MUST map _all_ paths stored in its state (including those inside any
+files).  This is necessary so that hosts can handle file system references
+correctly, for example to share common files, or bundle state for distribution
+or archival.
+
+For example, a plugin may write a path to a state file like so:
+
+    :::c
+    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);
+    }
+
+Then, later reload the path like so:
+
+    :::c
+    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);
+    }
+
+"""^^lv2:Markdown .
+
+state:makePath
+	lv2:documentation """
+
+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.
+
+The plugin is guaranteed a hierarchical namespace unique to that plugin
+instance, and may expect the returned path to have the requested path as a
+suffix.  There is one such namespace, even if the feature is passed to both
+LV2_Descriptor::instantiate() and LV2_State_Interface::save().  Beyond this,
+the plugin MUST NOT make any assumptions about the returned paths.
+
+Like any other paths, the plugin MUST map these paths using state:mapPath
+before storing them in state.  The plugin MUST NOT assume these paths will be
+available across a save/restore otherwise, that is, only mapped paths saved to
+state are persistent, any other created paths are temporary.
+
+For example, a plugin may create a file in a subdirectory like so:
+
+    :::c
+    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, "I am some data");
+        fclose(myfile);
+        return path;
+    }
+
+"""^^lv2:Markdown .
+
+state:threadSafeRestore
+	lv2:documentation """
+
+If a plugin supports this feature, its LV2_State_Interface::restore method is
+thread-safe and may be called concurrently with audio class functions.
+
+To support this feature, the host MUST pass a
+[work:schedule](worker.html#schedule) feature to the restore method, which will
+be used to complete the state restoration.  The usual mechanics of the worker
+apply: the host will call the plugin's work method, which emits a response
+which is later applied in the audio thread.
+
+The host is not required to block audio processing while restore() and work()
+load the state, so this feature allows state to be restored without dropouts.
+
+"""^^lv2:Markdown .
+
+state:freePath
+	lv2:documentation """
+
+This feature provides a function that can be used by plugins to free paths that
+were allocated by the host via other state features (state:mapPath and
+state:makePath).
+
+"""^^lv2:Markdown .
+
+state:Changed
+	lv2:documentation """
+
+A notification that the internal state of the plugin has been changed in a way
+that the host can not otherwise know about.
+
+This is a one-way notification, intended to be used as the type of an
+[Object](atom.html#Object) sent from plugins when necessary.
+
+Plugins SHOULD emit such an event whenever a change has occurred that would
+result in a different state being saved, but not when the host explicity makes
+a change which it knows is likely to have that effect, such as changing a
+parameter.
+
+"""^^lv2:Markdown .