Source File
	attributes.go

Belonging Package
	google.golang.org/grpc/attributes

/*
 *
 * Copyright 2019 gRPC 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
 *
 *     http://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.
 *
 */

// Package attributes defines a generic key/value store used in various gRPC
// components.
//
// # Experimental
//
// Notice: This package is EXPERIMENTAL and may be changed or removed in a
// later release.
package attributes

import (
	"fmt"
	"iter"
	"maps"
	"strings"
)

// Attributes is an immutable struct for storing and retrieving generic
// key/value pairs.  Keys must be hashable, and users should define their own
// types for keys.  Values should not be modified after they are added to an
// Attributes or if they were received from one.  If values implement 'Equal(o
// any) bool', it will be called by (*Attributes).Equal to determine whether
// two values with the same key should be considered equal.
type Attributes struct {
	parent     *Attributes
	key, value any
}

// New returns a new Attributes containing the key/value pair.
func New(key, value any) *Attributes {
	return &Attributes{
		key:   key,
		value: value,
	}
}

// WithValue returns a new Attributes containing the previous keys and values
// and the new key/value pair.  If the same key appears multiple times, the
// last value overwrites all previous values for that key.  value should not be
// modified later.
//
// Note that Attributes do not support deletion. Avoid using untyped nil values.
// Since the Value method returns an untyped nil when a key is absent, it is
// impossible to distinguish between a missing key and a key explicitly set to
// an untyped nil. If you need to represent a value being unset, consider
// storing a specific sentinel type or a wrapper struct with a boolean field
// indicating presence.
func (a *Attributes) WithValue(key, value any) *Attributes {
	return &Attributes{
		parent: a,
		key:    key,
		value:  value,
	}
}

// Value returns the value associated with these attributes for key, or nil if
// no value is associated with key.  The returned value should not be modified.
func (a *Attributes) Value(key any) any {
	for cur := a; cur != nil; cur = cur.parent {
		if cur.key == key {
			return cur.value
		}
	}
	return nil
}

// Equal returns whether a and o are equivalent.  If 'Equal(o any) bool' is
// implemented for a value in the attributes, it is called to determine if the
// value matches the one stored in the other attributes.  If Equal is not
// implemented, standard equality is used to determine if the two values are
// equal. Note that some types (e.g. maps) aren't comparable by default, so
// they must be wrapped in a struct, or in an alias type, with Equal defined.
func (a *Attributes) Equal(o *Attributes) bool {
	if a == nil && o == nil {
		return true
	}
	if a == nil || o == nil {
		return false
	}
	if a == o {
		return true
	}
	m := maps.Collect(o.all())
	lenA := 0

	for k, v := range a.all() {
		lenA++
		ov, ok := m[k]
		if !ok {
			// o missing element of a
			return false
		}
		if eq, ok := v.(interface{ Equal(o any) bool }); ok {
			if !eq.Equal(ov) {
				return false
			}
		} else if v != ov {
			// Fallback to a standard equality check if Value is unimplemented.
			return false
		}
	}
	return lenA == len(m)
}

// String prints the attribute map. If any key or values throughout the map
// implement fmt.Stringer, it calls that method and appends.
func (a *Attributes) String() string {
	var sb strings.Builder
	sb.WriteString("{")
	first := true
	for k, v := range a.all() {
		if !first {
			sb.WriteString(", ")
		}
		fmt.Fprintf(&sb, "%q: %q ", str(k), str(v))
		first = false
	}
	sb.WriteString("}")
	return sb.String()
}

func str(x any) (s string) {
	if v, ok := x.(fmt.Stringer); ok {
		return fmt.Sprint(v)
	} else if v, ok := x.(string); ok {
		return v
	}
	return fmt.Sprintf("<%p>", x)
}

// MarshalJSON helps implement the json.Marshaler interface, thereby rendering
// the Attributes correctly when printing (via pretty.JSON) structs containing
// Attributes as fields.
//
// Is it impossible to unmarshal attributes from a JSON representation and this
// method is meant only for debugging purposes.
func (a *Attributes) MarshalJSON() ([]byte, error) {
	return []byte(a.String()), nil
}

// all returns an iterator that yields all key-value pairs in the Attributes
// chain. If a key appears multiple times, only the most recently added value
// is yielded.
func (a *Attributes) all() iter.Seq2[any, any] {
	return func(yield func(any, any) bool) {
		seen := map[any]bool{}
		for cur := a; cur != nil; cur = cur.parent {
			if seen[cur.key] {
				continue
			}
			if !yield(cur.key, cur.value) {
				return
			}
			seen[cur.key] = true
		}
	}
}

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