From 02dd37a2a3263dee4fa6490e4895824f6439c8d8 Mon Sep 17 00:00:00 2001
From: James Zern <jzern@google.com>
Date: Thu, 14 Jun 2012 16:59:58 -0700
Subject: [PATCH] demux: add {Next,Prev}Frame

Replaces WebPDemuxSetFrame().

Change-Id: I38cef0ebeabb5c2f164322502abe1780f7a65e06
---
 src/mux/demux.c | 10 ++++++++--
 src/webp/mux.h  |  6 ++++--
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/mux/demux.c b/src/mux/demux.c
index 411befd6..b578ac52 100644
--- a/src/mux/demux.c
+++ b/src/mux/demux.c
@@ -768,9 +768,15 @@ int WebPDemuxGetFrame(const WebPDemuxer* const dmux,
   return SetFrame(frame, iter);
 }
 
-int WebPDemuxSetFrame(WebPIterator* const iter, int frame) {
+int WebPDemuxNextFrame(WebPIterator* const iter) {
   if (iter == NULL) return 0;
-  return SetFrame(frame, iter);
+  return SetFrame(iter->frame_num_ + 1, iter);
+}
+
+int WebPDemuxPrevFrame(WebPIterator* const iter) {
+  if (iter == NULL) return 0;
+  if (iter->frame_num_ <= 1) return 0;
+  return SetFrame(iter->frame_num_ - 1, iter);
 }
 
 int WebPDemuxSetTile(WebPIterator* const iter, int tile) {
diff --git a/src/webp/mux.h b/src/webp/mux.h
index ac2acf81..d21efa3a 100644
--- a/src/webp/mux.h
+++ b/src/webp/mux.h
@@ -546,9 +546,11 @@ typedef struct {
 WEBP_EXTERN(int) WebPDemuxGetFrame(
     const WebPDemuxer* const dmux, int frame_number, WebPIterator* const iter);
 
-// Sets 'iter->tile_' to point to frame number 'frame_number'.
+// Sets 'iter->tile_' to point to the next ('iter->frame_num_' + 1) or previous
+// ('iter->frame_num_' - 1) frame. These functions do not loop.
 // Returns true on success, false otherwise.
-WEBP_EXTERN(int) WebPDemuxSetFrame(WebPIterator* const iter, int frame_number);
+WEBP_EXTERN(int) WebPDemuxNextFrame(WebPIterator* const iter);
+WEBP_EXTERN(int) WebPDemuxPrevFrame(WebPIterator* const iter);
 
 // Sets 'iter->tile_' to reflect tile number 'tile_number'.
 // Returns true if tile 'tile_number' is present, false otherwise.