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diff --git a/absl/log/internal/log_format.h b/absl/log/internal/log_format.h
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+// Copyright 2022 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: log/internal/log_format.h
+// -----------------------------------------------------------------------------
+//
+// This file declares routines implementing formatting of log message and log
+// prefix.
+
+#ifndef ABSL_LOG_INTERNAL_LOG_FORMAT_H_
+#define ABSL_LOG_INTERNAL_LOG_FORMAT_H_
+
+#include <stddef.h>
+
+#include <string>
+
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
+#include "absl/log/internal/config.h"
+#include "absl/strings/string_view.h"
+#include "absl/time/civil_time.h"
+#include "absl/time/time.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace log_internal {
+
+// Formats log message based on provided data.
+std::string FormatLogMessage(absl::LogSeverity severity,
+                             absl::CivilSecond civil_second,
+                             absl::Duration subsecond, log_internal::Tid tid,
+                             absl::string_view basename, int line,
+                             absl::string_view message);
+
+// Formats various entry metadata into a text string meant for use as a
+// prefix on a log message string.  Writes into `buf`, advances `buf` to point
+// at the remainder of the buffer (i.e. past any written bytes), and returns the
+// number of bytes written.
+//
+// In addition to calling `buf->remove_prefix()` (or the equivalent), this
+// function may also do `buf->remove_suffix(buf->size())` in cases where no more
+// bytes (i.e. no message data) should be written into the buffer.  For example,
+// if the prefix ought to be:
+//   I0926 09:00:00.000000 1234567 foo.cc:123]
+// `buf` is too small, the function might fill the whole buffer:
+//   I0926 09:00:00.000000 1234
+// (note the apparrently incorrect thread ID), or it might write less:
+//   I0926 09:00:00.000000
+// In this case, it might also empty `buf` prior to returning to prevent
+// message data from being written into the space where a reader would expect to
+// see a thread ID.
+size_t FormatLogPrefix(absl::LogSeverity severity, absl::Time timestamp,
+                       log_internal::Tid tid, absl::string_view basename,
+                       int line, absl::Span<char>& buf);
+
+}  // namespace log_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_INTERNAL_LOG_FORMAT_H_