avutil/fifo: add function av_fifo_generic_peek_at()

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/fifo.c

@@ -148,6 +148,44 @@ int av_fifo_generic_write(AVFifoBuffer *f, void *src, int size,
     return total - size;
 }
 
+int av_fifo_generic_peek_at(AVFifoBuffer *f, void *dest, int offset, int buf_size, void (*func)(void*, void*, int))
+{
+    uint8_t *rptr = f->rptr;
+
+    av_assert2(offset >= 0);
+
+    /*
+     * *ndx are indexes modulo 2^32, they are intended to overflow,
+     * to handle *ndx greater than 4gb.
+     */
+    av_assert2(buf_size + (unsigned)offset <= f->wndx - f->rndx);
+
+    if (offset >= f->end - rptr)
+        rptr += offset - (f->end - f->buffer);
+    else
+        rptr += offset;
+
+    while (buf_size > 0) {
+        int len;
+
+        if (rptr >= f->end)
+            rptr -= f->end - f->buffer;
+
+        len = FFMIN(f->end - rptr, buf_size);
+        if (func)
+            func(dest, rptr, len);
+        else {
+            memcpy(dest, rptr, len);
+            dest = (uint8_t *)dest + len;
+        }
+
+        buf_size -= len;
+        rptr     += len;
+    }
+
+    return 0;
+}
+
 int av_fifo_generic_peek(AVFifoBuffer *f, void *dest, int buf_size,
                          void (*func)(void *, void *, int))
 {
@@ -221,13 +259,36 @@ int main(void)
     }
     printf("\n");
 
+    /* peek_at at FIFO */
+    n = av_fifo_size(fifo) / sizeof(int);
+    for (i = 0; i < n; i++) {
+        av_fifo_generic_peek_at(fifo, &j, i * sizeof(int), sizeof(j), NULL);
+        printf("%d: %d\n", i, j);
+    }
+    printf("\n");
+
     /* read data */
     for (i = 0; av_fifo_size(fifo) >= sizeof(int); i++) {
         av_fifo_generic_read(fifo, &j, sizeof(int), NULL);
         printf("%d ", j);
     }
     printf("\n");
 
+    /* test *ndx overflow */
+    av_fifo_reset(fifo);
+    fifo->rndx = fifo->wndx = ~(uint32_t)0 - 5;
+
+    /* fill data */
+    for (i = 0; av_fifo_space(fifo) >= sizeof(int); i++)
+        av_fifo_generic_write(fifo, &i, sizeof(int), NULL);
+
+    /* peek_at at FIFO */
+    n = av_fifo_size(fifo) / sizeof(int);
+    for (i = 0; i < n; i++) {
+        av_fifo_generic_peek_at(fifo, &j, i * sizeof(int), sizeof(j), NULL);
+        printf("%d: %d\n", i, j);
+    }
+
     av_fifo_free(fifo);
 
     return 0;

libavutil/fifo.h

@@ -83,6 +83,17 @@ int av_fifo_size(const AVFifoBuffer *f);
  */
 int av_fifo_space(const AVFifoBuffer *f);
 
+/**
+ * Feed data at specific position from an AVFifoBuffer to a user-supplied callback.
+ * Similar as av_fifo_gereric_read but without discarding data.
+ * @param f AVFifoBuffer to read from
+ * @param offset offset from current read position
+ * @param buf_size number of bytes to read
+ * @param func generic read function
+ * @param dest data destination
+ */
+int av_fifo_generic_peek_at(AVFifoBuffer *f, void *dest, int offset, int buf_size, void (*func)(void*, void*, int));
+
 /**
  * Feed data from an AVFifoBuffer to a user-supplied callback.
  * Similar as av_fifo_gereric_read but without discarding data.

tests/ref/fate/fifo

@@ -24,4 +24,31 @@
 11: 11
 12: 12
 
+0: 0
+1: 1
+2: 2
+3: 3
+4: 4
+5: 5
+6: 6
+7: 7
+8: 8
+9: 9
+10: 10
+11: 11
+12: 12
+
 0 1 2 3 4 5 6 7 8 9 10 11 12
+0: 0
+1: 1
+2: 2
+3: 3
+4: 4
+5: 5
+6: 6
+7: 7
+8: 8
+9: 9
+10: 10
+11: 11
+12: 12