hwcontext: Make it easier to work with device types

Adds functions to convert to/from strings and a function to iterate
over all supported device types.  Also adds a new invalid type
AV_HWDEVICE_TYPE_NONE, which acts as a sentinel value.

(cherry picked from commit b7487f4f3c)

doc/APIchanges

@@ -15,6 +15,10 @@ libavutil:     2015-08-28
 
 API changes, most recent first:
 
+2017-06-14 - xxxxxxx - lavu 55.65.100 - hwcontext.h
+  Add AV_HWDEVICE_TYPE_NONE, av_hwdevice_find_type_by_name(),
+  av_hwdevice_get_type_name() and av_hwdevice_iterate_types().
+
 2017-06-14 - xxxxxxx - lavu 55.64.100 - hwcontext.h
   Add av_hwdevice_ctx_create_derived().
 

libavutil/hwcontext.c

@@ -28,7 +28,7 @@
 #include "pixdesc.h"
 #include "pixfmt.h"
 
-static const HWContextType *hw_table[] = {
+static const HWContextType *const hw_table[] = {
 #if CONFIG_CUDA
     &ff_hwcontext_type_cuda,
 #endif
@@ -50,6 +50,48 @@ static const HWContextType *hw_table[] = {
     NULL,
 };
 
+static const char *const hw_type_names[] = {
+    [AV_HWDEVICE_TYPE_CUDA]   = "cuda",
+    [AV_HWDEVICE_TYPE_DXVA2]  = "dxva2",
+    [AV_HWDEVICE_TYPE_QSV]    = "qsv",
+    [AV_HWDEVICE_TYPE_VAAPI]  = "vaapi",
+    [AV_HWDEVICE_TYPE_VDPAU]  = "vdpau",
+    [AV_HWDEVICE_TYPE_VIDEOTOOLBOX] = "videotoolbox",
+};
+
+enum AVHWDeviceType av_hwdevice_find_type_by_name(const char *name)
+{
+    int type;
+    for (type = 0; type < FF_ARRAY_ELEMS(hw_type_names); type++) {
+        if (hw_type_names[type] && !strcmp(hw_type_names[type], name))
+            return type;
+    }
+    return AV_HWDEVICE_TYPE_NONE;
+}
+
+const char *av_hwdevice_get_type_name(enum AVHWDeviceType type)
+{
+    if (type >= 0 && type < FF_ARRAY_ELEMS(hw_type_names))
+        return hw_type_names[type];
+    else
+        return NULL;
+}
+
+enum AVHWDeviceType av_hwdevice_iterate_types(enum AVHWDeviceType prev)
+{
+    enum AVHWDeviceType next;
+    int i, set = 0;
+    for (i = 0; hw_table[i]; i++) {
+        if (prev != AV_HWDEVICE_TYPE_NONE && hw_table[i]->type <= prev)
+            continue;
+        if (!set || hw_table[i]->type < next) {
+            next = hw_table[i]->type;
+            set = 1;
+        }
+    }
+    return set ? next : AV_HWDEVICE_TYPE_NONE;
+}
+
 static const AVClass hwdevice_ctx_class = {
     .class_name = "AVHWDeviceContext",
     .item_name  = av_default_item_name,

libavutil/hwcontext.h

@@ -31,6 +31,7 @@ enum AVHWDeviceType {
     AV_HWDEVICE_TYPE_DXVA2,
     AV_HWDEVICE_TYPE_QSV,
     AV_HWDEVICE_TYPE_VIDEOTOOLBOX,
+    AV_HWDEVICE_TYPE_NONE,
 };
 
 typedef struct AVHWDeviceInternal AVHWDeviceInternal;
@@ -223,6 +224,33 @@ typedef struct AVHWFramesContext {
     int width, height;
 } AVHWFramesContext;
 
+/**
+ * Look up an AVHWDeviceType by name.
+ *
+ * @param name String name of the device type (case-insensitive).
+ * @return The type from enum AVHWDeviceType, or AV_HWDEVICE_TYPE_NONE if
+ *         not found.
+ */
+enum AVHWDeviceType av_hwdevice_find_type_by_name(const char *name);
+
+/** Get the string name of an AVHWDeviceType.
+ *
+ * @param type Type from enum AVHWDeviceType.
+ * @return Pointer to a static string containing the name, or NULL if the type
+ *         is not valid.
+ */
+const char *av_hwdevice_get_type_name(enum AVHWDeviceType type);
+
+/**
+ * Iterate over supported device types.
+ *
+ * @param type AV_HWDEVICE_TYPE_NONE initially, then the previous type
+ *             returned by this function in subsequent iterations.
+ * @return The next usable device type from enum AVHWDeviceType, or
+ *         AV_HWDEVICE_TYPE_NONE if there are no more.
+ */
+enum AVHWDeviceType av_hwdevice_iterate_types(enum AVHWDeviceType prev);
+
 /**
  * Allocate an AVHWDeviceContext for a given hardware type.
  *

libavutil/version.h

@@ -80,7 +80,7 @@
 
 
 #define LIBAVUTIL_VERSION_MAJOR  55
-#define LIBAVUTIL_VERSION_MINOR  64
+#define LIBAVUTIL_VERSION_MINOR  65
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \