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+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// -----------------------------------------------------------------------------
+// File: internal/proto.h
+// -----------------------------------------------------------------------------
+//
+// Declares functions for serializing and deserializing data to and from memory
+// buffers in protocol buffer wire format.  This library takes no steps to
+// ensure that the encoded data matches with any message specification.
+
+#ifndef ABSL_LOG_INTERNAL_PROTO_H_
+#define ABSL_LOG_INTERNAL_PROTO_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/casts.h"
+#include "absl/base/config.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// absl::Span<char> represents a view into the available space in a mutable
+// buffer during encoding.  Encoding functions shrink the span as they go so
+// that the same view can be passed to a series of Encode functions.  If the
+// data do not fit, nothing is encoded, the view is set to size zero (so that
+// all subsequent encode calls fail), and false is returned.  Otherwise true is
+// returned.
+
+// In particular, attempting to encode a series of data into an insufficient
+// buffer has consistent and efficient behavior without any caller-side error
+// checking.  Individual values will be encoded in their entirety or not at all
+// (unless one of the `Truncate` functions is used).  Once a value is omitted
+// because it does not fit, no subsequent values will be encoded to preserve
+// ordering; the decoded sequence will be a prefix of the original sequence.
+
+// There are two ways to encode a message-typed field:
+//
+// * Construct its contents in a separate buffer and use `EncodeBytes` to copy
+//   it into the primary buffer with type, tag, and length.
+// * Use `EncodeMessageStart` to write type and tag fields and reserve space for
+//   the length field, then encode the contents directly into the buffer, then
+//   use `EncodeMessageLength` to write the actual length into the reserved
+//   bytes.  This works fine if the actual length takes fewer bytes to encode
+//   than were reserved, although you don't get your extra bytes back.
+//   This approach will always produce a valid encoding, but your protocol may
+//   require that the whole message field by omitted if the buffer is too small
+//   to contain all desired subfields.  In this case, operate on a copy of the
+//   buffer view and assign back only if everything fit, i.e. if the last
+//   `Encode` call returned true.
+
+// Encodes the specified integer as a varint field and returns true if it fits.
+// Used for int32_t, int64_t, uint32_t, uint64_t, bool, and enum field types.
+// Consumes up to kMaxVarintSize * 2 bytes (20).
+bool EncodeVarint(uint64_t tag, uint64_t value, absl::Span<char> *buf);
+inline bool EncodeVarint(uint64_t tag, int64_t value, absl::Span<char> *buf) {
+  return EncodeVarint(tag, static_cast<uint64_t>(value), buf);
+}
+inline bool EncodeVarint(uint64_t tag, uint32_t value, absl::Span<char> *buf) {
+  return EncodeVarint(tag, static_cast<uint64_t>(value), buf);
+}
+inline bool EncodeVarint(uint64_t tag, int32_t value, absl::Span<char> *buf) {
+  return EncodeVarint(tag, static_cast<uint64_t>(value), buf);
+}
+
+// Encodes the specified integer as a varint field using ZigZag encoding and
+// returns true if it fits.
+// Used for sint32 and sint64 field types.
+// Consumes up to kMaxVarintSize * 2 bytes (20).
+inline bool EncodeVarintZigZag(uint64_t tag, int64_t value,
+                               absl::Span<char> *buf) {
+  if (value < 0)
+    return EncodeVarint(tag, 2 * static_cast<uint64_t>(-(value + 1)) + 1, buf);
+  return EncodeVarint(tag, 2 * static_cast<uint64_t>(value), buf);
+}
+
+// Encodes the specified integer as a 64-bit field and returns true if it fits.
+// Used for fixed64 and sfixed64 field types.
+// Consumes up to kMaxVarintSize + 8 bytes (18).
+bool Encode64Bit(uint64_t tag, uint64_t value, absl::Span<char> *buf);
+inline bool Encode64Bit(uint64_t tag, int64_t value, absl::Span<char> *buf) {
+  return Encode64Bit(tag, static_cast<uint64_t>(value), buf);
+}
+inline bool Encode64Bit(uint64_t tag, uint32_t value, absl::Span<char> *buf) {
+  return Encode64Bit(tag, static_cast<uint64_t>(value), buf);
+}
+inline bool Encode64Bit(uint64_t tag, int32_t value, absl::Span<char> *buf) {
+  return Encode64Bit(tag, static_cast<uint64_t>(value), buf);
+}
+
+// Encodes the specified double as a 64-bit field and returns true if it fits.
+// Used for double field type.
+// Consumes up to kMaxVarintSize + 8 bytes (18).
+inline bool EncodeDouble(uint64_t tag, double value, absl::Span<char> *buf) {
+  return Encode64Bit(tag, absl::bit_cast<uint64_t>(value), buf);
+}
+
+// Encodes the specified integer as a 32-bit field and returns true if it fits.
+// Used for fixed32 and sfixed32 field types.
+// Consumes up to kMaxVarintSize + 4 bytes (14).
+bool Encode32Bit(uint64_t tag, uint32_t value, absl::Span<char> *buf);
+inline bool Encode32Bit(uint64_t tag, int32_t value, absl::Span<char> *buf) {
+  return Encode32Bit(tag, static_cast<uint32_t>(value), buf);
+}
+
+// Encodes the specified float as a 32-bit field and returns true if it fits.
+// Used for float field type.
+// Consumes up to kMaxVarintSize + 4 bytes (14).
+inline bool EncodeFloat(uint64_t tag, float value, absl::Span<char> *buf) {
+  return Encode32Bit(tag, absl::bit_cast<uint32_t>(value), buf);
+}
+
+// Encodes the specified bytes as a length-delimited field and returns true if
+// they fit.
+// Used for string, bytes, message, and packed-repeated field type.
+// Consumes up to kMaxVarintSize * 2 + value.size() bytes (20 + value.size()).
+bool EncodeBytes(uint64_t tag, absl::Span<const char> value,
+                 absl::Span<char> *buf);
+
+// Encodes as many of the specified bytes as will fit as a length-delimited
+// field and returns true as long as the field header (`tag_type` and `length`)
+// fits.
+// Used for string, bytes, message, and packed-repeated field type.
+// Consumes up to kMaxVarintSize * 2 + value.size() bytes (20 + value.size()).
+bool EncodeBytesTruncate(uint64_t tag, absl::Span<const char> value,
+                         absl::Span<char> *buf);
+
+// Encodes the specified string as a length-delimited field and returns true if
+// it fits.
+// Used for string, bytes, message, and packed-repeated field type.
+// Consumes up to kMaxVarintSize * 2 + value.size() bytes (20 + value.size()).
+inline bool EncodeString(uint64_t tag, absl::string_view value,
+                         absl::Span<char> *buf) {
+  return EncodeBytes(tag, value, buf);
+}
+
+// Encodes as much of the specified string as will fit as a length-delimited
+// field and returns true as long as the field header (`tag_type` and `length`)
+// fits.
+// Used for string, bytes, message, and packed-repeated field type.
+// Consumes up to kMaxVarintSize * 2 + value.size() bytes (20 + value.size()).
+inline bool EncodeStringTruncate(uint64_t tag, absl::string_view value,
+                                 absl::Span<char> *buf) {
+  return EncodeBytesTruncate(tag, value, buf);
+}
+
+// Encodes the header for a length-delimited field containing up to `max_size`
+// bytes or the number remaining in the buffer, whichever is less.  If the
+// header fits, a non-nullptr `Span` is returned; this must be passed to
+// `EncodeMessageLength` after all contents are encoded to finalize the length
+// field.  If the header does not fit, a nullptr `Span` is returned which is
+// safe to pass to `EncodeMessageLength` but need not be.
+// Used for string, bytes, message, and packed-repeated field type.
+// Consumes up to kMaxVarintSize * 2 bytes (20).
+ABSL_MUST_USE_RESULT absl::Span<char> EncodeMessageStart(uint64_t tag,
+                                                         uint64_t max_size,
+                                                         absl::Span<char> *buf);
+
+// Finalizes the length field in `msg` so that it encompasses all data encoded
+// since the call to `EncodeMessageStart` which returned `msg`.  Does nothing if
+// `msg` is a `nullptr` `Span`.
+void EncodeMessageLength(absl::Span<char> msg, const absl::Span<char> *buf);
+
+enum class WireType : uint64_t {
+  kVarint = 0,
+  k64Bit = 1,
+  kLengthDelimited = 2,
+  k32Bit = 5,
+};
+
+constexpr size_t VarintSize(uint64_t value) {
+  return value < 128 ? 1 : 1 + VarintSize(value >> 7);
+}
+constexpr size_t MinVarintSize() {
+  return VarintSize((std::numeric_limits<uint64_t>::min)());
+}
+constexpr size_t MaxVarintSize() {
+  return VarintSize((std::numeric_limits<uint64_t>::max)());
+}
+
+constexpr uint64_t MaxVarintForSize(size_t size) {
+  return size >= 10 ? (std::numeric_limits<uint64_t>::max)()
+                    : (static_cast<uint64_t>(1) << size * 7) - 1;
+}
+
+// `BufferSizeFor` returns a number of bytes guaranteed to be sufficient to
+// store encoded fields of the specified WireTypes regardless of tag numbers and
+// data values.  This only makes sense for `WireType::kLengthDelimited` if you
+// add in the length of the contents yourself, e.g. for string and bytes fields
+// by adding the lengths of any encoded strings to the return value or for
+// submessage fields by enumerating the fields you may encode into their
+// contents.
+constexpr size_t BufferSizeFor() { return 0; }
+template <typename... T>
+constexpr size_t BufferSizeFor(WireType type, T... tail) {
+  // tag_type + data + ...
+  return MaxVarintSize() +
+         (type == WireType::kVarint ? MaxVarintSize() :              //
+              type == WireType::k64Bit ? 8 :                         //
+                  type == WireType::k32Bit ? 4 : MaxVarintSize()) +  //
+         BufferSizeFor(tail...);
+}
+
+// absl::Span<const char> represents a view into the un-processed space in a
+// buffer during decoding.  Decoding functions shrink the span as they go so
+// that the same view can be decoded iteratively until all data are processed.
+// In general, if the buffer is exhausted but additional bytes are expected by
+// the decoder, it will return values as if the additional bytes were zeros.
+// Length-delimited fields are an exception - if the encoded length field
+// indicates more data bytes than are available in the buffer, the `bytes_value`
+// and `string_value` accessors will return truncated views.
+
+class ProtoField final {
+ public:
+  // Consumes bytes from `data` and returns true if there were any bytes to
+  // decode.
+  bool DecodeFrom(absl::Span<const char> *data);
+  uint64_t tag() const { return tag_; }
+  WireType type() const { return type_; }
+
+  // These value accessors will return nonsense if the data were not encoded in
+  // the corresponding wiretype from the corresponding C++ (or other language)
+  // type.
+
+  double double_value() const { return absl::bit_cast<double>(value_); }
+  float float_value() const {
+    return absl::bit_cast<float>(static_cast<uint32_t>(value_));
+  }
+  int32_t int32_value() const { return static_cast<int32_t>(value_); }
+  int64_t int64_value() const { return static_cast<int64_t>(value_); }
+  int32_t sint32_value() const {
+    if (value_ % 2) return static_cast<int32_t>(0 - ((value_ - 1) / 2) - 1);
+    return static_cast<int32_t>(value_ / 2);
+  }
+  int64_t sint64_value() const {
+    if (value_ % 2) return 0 - ((value_ - 1) / 2) - 1;
+    return value_ / 2;
+  }
+  uint32_t uint32_value() const { return static_cast<uint32_t>(value_); }
+  uint64_t uint64_value() const { return value_; }
+  bool bool_value() const { return value_ != 0; }
+  // To decode an enum, call int32_value() and cast to the appropriate type.
+  // Note that the official C++ proto compiler treats enum fields with values
+  // that do not correspond to a defined enumerator as unknown fields.
+
+  // To decode fields within a submessage field, call
+  // `DecodeNextField(field.BytesValue())`.
+  absl::Span<const char> bytes_value() const { return data_; }
+  absl::string_view string_value() const {
+    const auto data = bytes_value();
+    return absl::string_view(data.data(), data.size());
+  }
+  // Returns the encoded length of a length-delimited field.  This equals
+  // `bytes_value().size()` except when the latter has been truncated due to
+  // buffer underrun.
+  uint64_t encoded_length() const { return value_; }
+
+ private:
+  uint64_t tag_;
+  WireType type_;
+  // For `kTypeVarint`, `kType64Bit`, and `kType32Bit`, holds the decoded value.
+  // For `kTypeLengthDelimited`, holds the decoded length.
+  uint64_t value_;
+  absl::Span<const char> data_;
+};
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_PROTO_H_