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diff --git a/third_party/jpeg-xl/lib/jxl/dct_for_test.h b/third_party/jpeg-xl/lib/jxl/dct_for_test.h
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+++ b/third_party/jpeg-xl/lib/jxl/dct_for_test.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_DCT_FOR_TEST_H_
+#define LIB_JXL_DCT_FOR_TEST_H_
+
+// Unoptimized DCT only for use in tests.
+
+#include <string.h>  // memcpy
+
+#include <cmath>
+#include <vector>
+
+#include "lib/jxl/common.h"  // Pi
+
+namespace jxl {
+
+namespace test {
+static inline double alpha(int u) { return u == 0 ? 0.7071067811865475 : 1.0; }
+
+// N-DCT on M columns, divided by sqrt(N). Matches the definition in the spec.
+template <size_t N, size_t M>
+void DCT1D(double block[N * M], double out[N * M]) {
+  std::vector<double> matrix(N * N);
+  const double scale = std::sqrt(2.0) / N;
+  for (size_t y = 0; y < N; y++) {
+    for (size_t u = 0; u < N; u++) {
+      matrix[N * u + y] = alpha(u) * cos((y + 0.5) * u * Pi(1.0 / N)) * scale;
+    }
+  }
+  for (size_t x = 0; x < M; x++) {
+    for (size_t u = 0; u < N; u++) {
+      out[M * u + x] = 0;
+      for (size_t y = 0; y < N; y++) {
+        out[M * u + x] += matrix[N * u + y] * block[M * y + x];
+      }
+    }
+  }
+}
+
+// N-IDCT on M columns, multiplied by sqrt(N). Matches the definition in the
+// spec.
+template <size_t N, size_t M>
+void IDCT1D(double block[N * M], double out[N * M]) {
+  std::vector<double> matrix(N * N);
+  const double scale = std::sqrt(2.0);
+  for (size_t y = 0; y < N; y++) {
+    for (size_t u = 0; u < N; u++) {
+      // Transpose of DCT matrix.
+      matrix[N * y + u] = alpha(u) * cos((y + 0.5) * u * Pi(1.0 / N)) * scale;
+    }
+  }
+  for (size_t x = 0; x < M; x++) {
+    for (size_t u = 0; u < N; u++) {
+      out[M * u + x] = 0;
+      for (size_t y = 0; y < N; y++) {
+        out[M * u + x] += matrix[N * u + y] * block[M * y + x];
+      }
+    }
+  }
+}
+
+template <size_t N, size_t M>
+void TransposeBlock(double in[N * M], double out[M * N]) {
+  for (size_t x = 0; x < N; x++) {
+    for (size_t y = 0; y < M; y++) {
+      out[y * N + x] = in[x * M + y];
+    }
+  }
+}
+}  // namespace test
+
+// Untransposed DCT.
+template <size_t N>
+void DCTSlow(double block[N * N]) {
+  constexpr size_t kBlockSize = N * N;
+  std::vector<double> g(kBlockSize);
+  test::DCT1D<N, N>(block, g.data());
+  test::TransposeBlock<N, N>(g.data(), block);
+  test::DCT1D<N, N>(block, g.data());
+  test::TransposeBlock<N, N>(g.data(), block);
+}
+
+// Untransposed IDCT.
+template <size_t N>
+void IDCTSlow(double block[N * N]) {
+  constexpr size_t kBlockSize = N * N;
+  std::vector<double> g(kBlockSize);
+  test::IDCT1D<N, N>(block, g.data());
+  test::TransposeBlock<N, N>(g.data(), block);
+  test::IDCT1D<N, N>(block, g.data());
+  test::TransposeBlock<N, N>(g.data(), block);
+}
+
+}  // namespace jxl
+
+#endif  // LIB_JXL_DCT_FOR_TEST_H_