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diff --git a/src/lib/font_id_allocator.h b/src/lib/font_id_allocator.h
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+++ b/src/lib/font_id_allocator.h
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+/*
+    Copyright (C) 2023 Carl Hetherington <cth@carlh.net>
+
+    This file is part of DCP-o-matic.
+
+    DCP-o-matic is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    DCP-o-matic is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with DCP-o-matic.  If not, see <http://www.gnu.org/licenses/>.
+
+*/
+
+
+#ifndef DCPOMATIC_FONT_ID_ALLOCATOR_H
+#define DCPOMATIC_FONT_ID_ALLOCATOR_H
+
+
+#include <map>
+#include <memory>
+#include <string>
+#include <vector>
+
+
+namespace dcp {
+	class Reel;
+	class SubtitleAsset;
+}
+
+
+/** A class which, given some pairs of (asset-id, font-id) can return a font ID
+ *  which is unique in a piece of content.
+ *
+ *  When we examine a 2-reel DCP we may have a pair of subtitle assets that
+ *  each have a font with ID "foo".  We want to store these in
+ *  TextContent::_fonts in such a way that they are distinguishable.
+ *
+ *  If TextContent is to carry on storing a list of dcpomatic::Font, we can
+ *  only do this by making each dcpomatic::Font have a different ID (i.e. not
+ *  both "foo").
+ *
+ *  Hence when we add the fonts to the TextContent we re-write them to have
+ *  unique IDs.
+ *
+ *  When we do this, we must do it in a repeatable way so that when the
+ *  DCPDecoder receives the "foo" font IDs it can obtain the same "new" ID given
+ *  "foo" and the asset ID that it came from.
+ *
+ *  FontIDAllocator can help with that: call add_fonts_from_reels() or add_font(),
+ *  then allocate(), then it will return repeatable unique "new" font IDs from
+ *  an asset map and "old" ID.
+ */
+class FontIDAllocator
+{
+public:
+	void add_fonts_from_reels(std::vector<std::shared_ptr<dcp::Reel>> const& reels);
+	void add_font(int reel_index, std::string asset_id, std::string font_id);
+
+	void allocate();
+
+	std::string font_id(int reel_index, std::string asset_id, std::string font_id) const;
+
+private:
+	void add_fonts_from_asset(int reel_index, std::shared_ptr<const dcp::SubtitleAsset> asset);
+
+	struct Font
+	{
+		Font(int reel_index_, std::string asset_id_, std::string font_id_)
+			: reel_index(reel_index_)
+			, asset_id(asset_id_)
+			, font_id(font_id_)
+		{}
+
+		bool operator<(Font const& other) const {
+			if (reel_index != other.reel_index) {
+				return reel_index < other.reel_index;
+			}
+
+			if (asset_id != other.asset_id) {
+				return asset_id < other.asset_id;
+			}
+
+			return font_id < other.font_id;
+		}
+
+		int reel_index;
+		std::string asset_id;
+		std::string font_id;
+	};
+
+	std::map<Font, int> _map;
+};
+
+
+#endif