1 files changed, 200 insertions, 0 deletions
diff --git a/src/log/slog/level.go b/src/log/slog/level.go
new file mode 100644
index 0000000..cd1213a
--- /dev/null
+++ b/src/log/slog/level.go
@@ -0,0 +1,200 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package slog
+
+import (
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+	"sync/atomic"
+)
+
+// A Level is the importance or severity of a log event.
+// The higher the level, the more important or severe the event.
+type Level int
+
+// Level numbers are inherently arbitrary,
+// but we picked them to satisfy three constraints.
+// Any system can map them to another numbering scheme if it wishes.
+//
+// First, we wanted the default level to be Info, Since Levels are ints, Info is
+// the default value for int, zero.
+//
+// Second, we wanted to make it easy to use levels to specify logger verbosity.
+// Since a larger level means a more severe event, a logger that accepts events
+// with smaller (or more negative) level means a more verbose logger. Logger
+// verbosity is thus the negation of event severity, and the default verbosity
+// of 0 accepts all events at least as severe as INFO.
+//
+// Third, we wanted some room between levels to accommodate schemes with named
+// levels between ours. For example, Google Cloud Logging defines a Notice level
+// between Info and Warn. Since there are only a few of these intermediate
+// levels, the gap between the numbers need not be large. Our gap of 4 matches
+// OpenTelemetry's mapping. Subtracting 9 from an OpenTelemetry level in the
+// DEBUG, INFO, WARN and ERROR ranges converts it to the corresponding slog
+// Level range. OpenTelemetry also has the names TRACE and FATAL, which slog
+// does not. But those OpenTelemetry levels can still be represented as slog
+// Levels by using the appropriate integers.
+//
+// Names for common levels.
+const (
+	LevelDebug Level = -4
+	LevelInfo  Level = 0
+	LevelWarn  Level = 4
+	LevelError Level = 8
+)
+
+// String returns a name for the level.
+// If the level has a name, then that name
+// in uppercase is returned.
+// If the level is between named values, then
+// an integer is appended to the uppercased name.
+// Examples:
+//
+//	LevelWarn.String() => "WARN"
+//	(LevelInfo+2).String() => "INFO+2"
+func (l Level) String() string {
+	str := func(base string, val Level) string {
+		if val == 0 {
+			return base
+		}
+		return fmt.Sprintf("%s%+d", base, val)
+	}
+
+	switch {
+	case l < LevelInfo:
+		return str("DEBUG", l-LevelDebug)
+	case l < LevelWarn:
+		return str("INFO", l-LevelInfo)
+	case l < LevelError:
+		return str("WARN", l-LevelWarn)
+	default:
+		return str("ERROR", l-LevelError)
+	}
+}
+
+// MarshalJSON implements [encoding/json.Marshaler]
+// by quoting the output of [Level.String].
+func (l Level) MarshalJSON() ([]byte, error) {
+	// AppendQuote is sufficient for JSON-encoding all Level strings.
+	// They don't contain any runes that would produce invalid JSON
+	// when escaped.
+	return strconv.AppendQuote(nil, l.String()), nil
+}
+
+// UnmarshalJSON implements [encoding/json.Unmarshaler]
+// It accepts any string produced by [Level.MarshalJSON],
+// ignoring case.
+// It also accepts numeric offsets that would result in a different string on
+// output. For example, "Error-8" would marshal as "INFO".
+func (l *Level) UnmarshalJSON(data []byte) error {
+	s, err := strconv.Unquote(string(data))
+	if err != nil {
+		return err
+	}
+	return l.parse(s)
+}
+
+// MarshalText implements [encoding.TextMarshaler]
+// by calling [Level.String].
+func (l Level) MarshalText() ([]byte, error) {
+	return []byte(l.String()), nil
+}
+
+// UnmarshalText implements [encoding.TextUnmarshaler].
+// It accepts any string produced by [Level.MarshalText],
+// ignoring case.
+// It also accepts numeric offsets that would result in a different string on
+// output. For example, "Error-8" would marshal as "INFO".
+func (l *Level) UnmarshalText(data []byte) error {
+	return l.parse(string(data))
+}
+
+func (l *Level) parse(s string) (err error) {
+	defer func() {
+		if err != nil {
+			err = fmt.Errorf("slog: level string %q: %w", s, err)
+		}
+	}()
+
+	name := s
+	offset := 0
+	if i := strings.IndexAny(s, "+-"); i >= 0 {
+		name = s[:i]
+		offset, err = strconv.Atoi(s[i:])
+		if err != nil {
+			return err
+		}
+	}
+	switch strings.ToUpper(name) {
+	case "DEBUG":
+		*l = LevelDebug
+	case "INFO":
+		*l = LevelInfo
+	case "WARN":
+		*l = LevelWarn
+	case "ERROR":
+		*l = LevelError
+	default:
+		return errors.New("unknown name")
+	}
+	*l += Level(offset)
+	return nil
+}
+
+// Level returns the receiver.
+// It implements Leveler.
+func (l Level) Level() Level { return l }
+
+// A LevelVar is a Level variable, to allow a Handler level to change
+// dynamically.
+// It implements Leveler as well as a Set method,
+// and it is safe for use by multiple goroutines.
+// The zero LevelVar corresponds to LevelInfo.
+type LevelVar struct {
+	val atomic.Int64
+}
+
+// Level returns v's level.
+func (v *LevelVar) Level() Level {
+	return Level(int(v.val.Load()))
+}
+
+// Set sets v's level to l.
+func (v *LevelVar) Set(l Level) {
+	v.val.Store(int64(l))
+}
+
+func (v *LevelVar) String() string {
+	return fmt.Sprintf("LevelVar(%s)", v.Level())
+}
+
+// MarshalText implements [encoding.TextMarshaler]
+// by calling [Level.MarshalText].
+func (v *LevelVar) MarshalText() ([]byte, error) {
+	return v.Level().MarshalText()
+}
+
+// UnmarshalText implements [encoding.TextUnmarshaler]
+// by calling [Level.UnmarshalText].
+func (v *LevelVar) UnmarshalText(data []byte) error {
+	var l Level
+	if err := l.UnmarshalText(data); err != nil {
+		return err
+	}
+	v.Set(l)
+	return nil
+}
+
+// A Leveler provides a Level value.
+//
+// As Level itself implements Leveler, clients typically supply
+// a Level value wherever a Leveler is needed, such as in HandlerOptions.
+// Clients who need to vary the level dynamically can provide a more complex
+// Leveler implementation such as *LevelVar.
+type Leveler interface {
+	Level() Level
+}