avfilter/asubprocess: keep track of sp_can_read (performance)

Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

libavfilter/af_asubprocess.c

@@ -42,6 +42,7 @@ typedef struct SP
     SPB    *unused_buffers;
     int     pid;
     size_t  input_count;
+    size_t  can_read;
     bool    done;
 } SP;
 
@@ -209,17 +210,15 @@ sp_new(AVFilterContext *ctx)
 static size_t
 sp_can_read(SP *sp)
 {
-    int can_read = 0;
-    for (SPB *spb = sp->out_buffers; spb; spb = spb->next)
-        can_read += spb->count - spb->offset;
-
-    return can_read;
+    return sp->can_read;
 }
 
 static void
-sp_read(SP *sp, char *buffer, int count)
+sp_read(SP *sp, char *buffer, size_t count)
 {
     SPB *spb = sp->out_buffers;
+    av_assert0(count <= sp->can_read); // caller should check this before calling sp_read
+    sp->can_read -= count;
     while (spb) {
         int n = FFMIN(spb->count - spb->offset, count);
 
@@ -389,6 +388,7 @@ sp_write(AVFilterContext *ctx, char *buffer, int count)
             if (rc > 0) {
                 last->count += rc;
                 sp->input_count += rc;
+                sp->can_read += rc;
                 ret = update_state(ctx);
                 if (ret != 0)
                     return ret;