apedec: add ability to check CRC

The CRC flag is only signalled once every few minutes but CRC is still
always present so the patch uses the file version instead.
CRC on 24-bit files wants non-padded samples so skip such files.
Some corrupt samples may have been output before the final check
depending on the -max_samples setting.

libavcodec/apedec.c

@@ -24,6 +24,7 @@
 
 #include "libavutil/avassert.h"
 #include "libavutil/channel_layout.h"
+#include "libavutil/crc.h"
 #include "libavutil/opt.h"
 #include "lossless_audiodsp.h"
 #include "avcodec.h"
@@ -147,7 +148,8 @@ typedef struct APEContext {
     int fset;                                ///< which filter set to use (calculated from compression level)
     int flags;                               ///< global decoder flags
 
-    uint32_t CRC;                            ///< frame CRC
+    uint32_t CRC;                            ///< signalled frame CRC
+    uint32_t CRC_state;                      ///< accumulated CRC
     int frameflags;                          ///< frame flags
     APEPredictor predictor;                  ///< predictor used for final reconstruction
 
@@ -750,6 +752,7 @@ static int init_entropy_decoder(APEContext *ctx)
 
     /* Read the frame flags if they exist */
     ctx->frameflags = 0;
+    ctx->CRC_state = UINT32_MAX;
     if ((ctx->fileversion > 3820) && (ctx->CRC & 0x80000000)) {
         ctx->CRC &= ~0x80000000;
 
@@ -1577,6 +1580,27 @@ static int ape_decode_frame(AVCodecContext *avctx, void *data,
 
     s->samples -= blockstodecode;
 
+    if (avctx->err_recognition & AV_EF_CRCCHECK &&
+        s->fileversion >= 3900 && s->bps < 24) {
+        uint32_t crc = s->CRC_state;
+        const AVCRC *crc_tab = av_crc_get_table(AV_CRC_32_IEEE_LE);
+        for (i = 0; i < blockstodecode; i++) {
+            for (ch = 0; ch < s->channels; ch++) {
+                uint8_t *smp = frame->data[ch] + (i*(s->bps >> 3));
+                crc = av_crc(crc_tab, crc, smp, s->bps >> 3);
+            }
+        }
+
+        if (!s->samples && (~crc >> 1) ^ s->CRC) {
+            av_log(avctx, AV_LOG_ERROR, "CRC mismatch! Previously decoded "
+                   "frames may have been affected as well.\n");
+            if (avctx->err_recognition & AV_EF_EXPLODE)
+                return AVERROR_INVALIDDATA;
+        }
+
+        s->CRC_state = crc;
+    }
+
     *got_frame_ptr = 1;
 
     return !s->samples ? avpkt->size : 0;