From 18e96d8c9dd6b0346514121facf8137f13ccd6dc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?P=C5=99emysl=20Eric=20Janouch?= <p@janouch.name>
Date: Fri, 26 Nov 2021 20:54:08 +0100
Subject: [PATCH] Allow frame iteration in both directions

---
 fastiv-io.c   | 8 ++++++++
 fastiv-io.h   | 4 ++++
 fastiv-view.c | 8 +++++++-
 3 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/fastiv-io.c b/fastiv-io.c
index a2ac166..35e640d 100644
--- a/fastiv-io.c
+++ b/fastiv-io.c
@@ -317,6 +317,8 @@ load_wuffs_frame(struct load_wuffs_frame_context *ctx, GError **error)
 		(void *) (intptr_t) (wuffs_base__frame_config__duration(&fc) /
 			WUFFS_BASE__FLICKS_PER_MILLISECOND), NULL);
 
+	cairo_surface_set_user_data(surface, &fastiv_io_key_frame_previous,
+		ctx->result_tail, NULL);
 	if (ctx->result_tail)
 		cairo_surface_set_user_data(ctx->result_tail, &fastiv_io_key_frame_next,
 			surface, (cairo_destroy_func_t) cairo_surface_destroy);
@@ -466,6 +468,11 @@ open_wuffs(
 	while (load_wuffs_frame(&ctx, error))
 		;
 
+	// Wrap the chain around, since our caller receives only one pointer.
+	if (ctx.result)
+		cairo_surface_set_user_data(ctx.result, &fastiv_io_key_frame_previous,
+			ctx.result_tail, NULL);
+
 fail:
 	free(ctx.workbuf.ptr);
 	g_clear_pointer(&ctx.meta_exif, g_bytes_unref);
@@ -1024,6 +1031,7 @@ cairo_user_data_key_t fastiv_io_key_orientation;
 cairo_user_data_key_t fastiv_io_key_icc;
 
 cairo_user_data_key_t fastiv_io_key_frame_next;
+cairo_user_data_key_t fastiv_io_key_frame_previous;
 cairo_user_data_key_t fastiv_io_key_frame_duration;
 cairo_user_data_key_t fastiv_io_key_loops;
 
diff --git a/fastiv-io.h b/fastiv-io.h
index 7bce925..20fa4f6 100644
--- a/fastiv-io.h
+++ b/fastiv-io.h
@@ -37,6 +37,10 @@ extern cairo_user_data_key_t fastiv_io_key_icc;
 /// The next frame in a sequence, as a surface, in a chain, pre-composited.
 /// There is no wrap-around.
 extern cairo_user_data_key_t fastiv_io_key_frame_next;
+/// The previous frame in a sequence, as a surface, in a chain, pre-composited.
+/// This is a weak pointer that wraps around, and needn't be present
+/// for static images.
+extern cairo_user_data_key_t fastiv_io_key_frame_previous;
 /// Frame duration in milliseconds as an intptr_t.
 extern cairo_user_data_key_t fastiv_io_key_frame_duration;
 /// How many times to repeat the animation, or zero for +inf, as a uintptr_t.
diff --git a/fastiv-view.c b/fastiv-view.c
index 1cbb837..1573ff9 100644
--- a/fastiv-view.c
+++ b/fastiv-view.c
@@ -452,9 +452,15 @@ fastiv_view_key_press_event(GtkWidget *widget, GdkEventKey *event)
 		gtk_widget_queue_resize(widget);
 		return TRUE;
 
+	case GDK_KEY_bracketleft:
+		if (!(self->frame = cairo_surface_get_user_data(
+				self->frame, &fastiv_io_key_frame_previous)))
+			self->frame = self->surface;
+		gtk_widget_queue_draw(widget);
+		return TRUE;
 	case GDK_KEY_bracketright:
 		if (!(self->frame = cairo_surface_get_user_data(
-				  self->frame, &fastiv_io_key_frame_next)))
+				self->frame, &fastiv_io_key_frame_next)))
 			self->frame = self->surface;
 		gtk_widget_queue_draw(widget);
 		return TRUE;