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diff --git a/third_party/jpeg-xl/lib/jxl/dec_context_map.h b/third_party/jpeg-xl/lib/jxl/dec_context_map.h
new file mode 100644
index 0000000000..95b8a0ca92
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+++ b/third_party/jpeg-xl/lib/jxl/dec_context_map.h
@@ -0,0 +1,30 @@
+// Copyright (c) the JPEG XL Project Authors. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#ifndef LIB_JXL_DEC_CONTEXT_MAP_H_
+#define LIB_JXL_DEC_CONTEXT_MAP_H_
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <vector>
+
+#include "lib/jxl/dec_bit_reader.h"
+
+namespace jxl {
+
+// Context map uses uint8_t.
+constexpr size_t kMaxClusters = 256;
+
+// Reads the context map from the bit stream. On calling this function,
+// context_map->size() must be the number of possible context ids.
+// Sets *num_htrees to the number of different histogram ids in
+// *context_map.
+Status DecodeContextMap(std::vector<uint8_t>* context_map, size_t* num_htrees,
+                        BitReader* input);
+
+}  // namespace jxl
+
+#endif  // LIB_JXL_DEC_CONTEXT_MAP_H_