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diff --git a/lib/jxl/enc_linalg.h b/lib/jxl/enc_linalg.h
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+// 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_LINALG_H_
+#define LIB_JXL_LINALG_H_
+
+// Linear algebra.
+
+#include "lib/jxl/base/compiler_specific.h"
+#include "lib/jxl/image.h"
+
+namespace jxl {
+
+using ImageD = Plane<double>;
+
+// A is symmetric, U is orthogonal, and A = U * Diagonal(diag) * Transpose(U).
+void ConvertToDiagonal(const ImageD& A, ImageD* JXL_RESTRICT diag,
+                       ImageD* JXL_RESTRICT U);
+
+}  // namespace jxl
+
+#endif  // LIB_JXL_LINALG_H_