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diff --git a/third_party/jpeg-xl/lib/jxl/field_encodings.h b/third_party/jpeg-xl/lib/jxl/field_encodings.h
new file mode 100644
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+++ b/third_party/jpeg-xl/lib/jxl/field_encodings.h
@@ -0,0 +1,134 @@
+// 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_FIELD_ENCODINGS_H_
+#define LIB_JXL_FIELD_ENCODINGS_H_
+
+// Constants needed to encode/decode fields; avoids including the full fields.h.
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <hwy/base.h>
+#include <vector>
+
+#include "lib/jxl/base/bits.h"
+#include "lib/jxl/base/status.h"
+
+namespace jxl {
+
+// Macro to define the Fields' derived class Name when compiling with debug
+// names.
+#if JXL_IS_DEBUG_BUILD
+#define JXL_FIELDS_NAME(X) \
+  const char* Name() const override { return #X; }
+#else
+#define JXL_FIELDS_NAME(X)
+#endif  // JXL_IS_DEBUG_BUILD
+
+class Visitor;
+class Fields {
+ public:
+  virtual ~Fields() = default;
+#if JXL_IS_DEBUG_BUILD
+  virtual const char* Name() const = 0;
+#endif  // JXL_IS_DEBUG_BUILD
+  virtual Status VisitFields(Visitor* JXL_RESTRICT visitor) = 0;
+};
+
+// Distribution of U32 values for one particular selector. Represents either a
+// power of two-sized range, or a single value. A separate type ensures this is
+// only passed to the U32Enc ctor.
+struct U32Distr {
+  // No need to validate - all `d` are legitimate.
+  constexpr explicit U32Distr(uint32_t d) : d(d) {}
+
+  static constexpr uint32_t kDirect = 0x80000000u;
+
+  constexpr bool IsDirect() const { return (d & kDirect) != 0; }
+
+  // Only call if IsDirect().
+  constexpr uint32_t Direct() const { return d & (kDirect - 1); }
+
+  // Only call if !IsDirect().
+  constexpr size_t ExtraBits() const { return (d & 0x1F) + 1; }
+  uint32_t Offset() const { return (d >> 5) & 0x3FFFFFF; }
+
+  uint32_t d;
+};
+
+// A direct-coded 31-bit value occupying 2 bits in the bitstream.
+constexpr U32Distr Val(uint32_t value) {
+  return U32Distr(value | U32Distr::kDirect);
+}
+
+// Value - `offset` will be signaled in `bits` extra bits.
+constexpr U32Distr BitsOffset(uint32_t bits, uint32_t offset) {
+  return U32Distr(((bits - 1) & 0x1F) + ((offset & 0x3FFFFFF) << 5));
+}
+
+// Value will be signaled in `bits` extra bits.
+constexpr U32Distr Bits(uint32_t bits) { return BitsOffset(bits, 0); }
+
+// See U32Coder documentation in fields.h.
+class U32Enc {
+ public:
+  constexpr U32Enc(const U32Distr d0, const U32Distr d1, const U32Distr d2,
+                   const U32Distr d3)
+      : d_{d0, d1, d2, d3} {}
+
+  // Returns the U32Distr at `selector` = 0..3, least-significant first.
+  U32Distr GetDistr(const uint32_t selector) const {
+    JXL_ASSERT(selector < 4);
+    return d_[selector];
+  }
+
+ private:
+  U32Distr d_[4];
+};
+
+// Returns bit with the given `index` (0 = least significant).
+template <typename T>
+static inline constexpr uint64_t MakeBit(T index) {
+  return 1ULL << static_cast<uint32_t>(index);
+}
+
+// Returns vector of all possible values of an Enum type. Relies on each Enum
+// providing an overload of EnumBits() that returns a bit array of its values,
+// which implies values must be in [0, 64).
+template <typename Enum>
+std::vector<Enum> Values() {
+  uint64_t bits = EnumBits(Enum());
+
+  std::vector<Enum> values;
+  values.reserve(hwy::PopCount(bits));
+
+  // For each 1-bit in bits: add its index as value
+  while (bits != 0) {
+    const int index = Num0BitsBelowLS1Bit_Nonzero(bits);
+    values.push_back(static_cast<Enum>(index));
+    bits &= bits - 1;  // clear least-significant bit
+  }
+  return values;
+}
+
+// Returns true if value is one of Values<Enum>().
+template <class Enum>
+Status EnumValid(const Enum value) {
+  if (static_cast<uint32_t>(value) >= 64) {
+    return JXL_FAILURE("Value %u too large for %s\n",
+                       static_cast<uint32_t>(value), EnumName(Enum()));
+  }
+  const uint64_t bit = MakeBit(value);
+  if ((EnumBits(Enum()) & bit) == 0) {
+    return JXL_FAILURE("Invalid value %u for %s\n",
+                       static_cast<uint32_t>(value), EnumName(Enum()));
+  }
+  return true;
+}
+
+}  // namespace jxl
+
+#endif  // LIB_JXL_FIELD_ENCODINGS_H_