attribute/internal/attribute.go

// Copyright The OpenTelemetry Authors
// SPDX-License-Identifier: Apache-2.0

/*
Package attribute provide several helper functions for some commonly used
logic of processing attributes.
*/
package attribute

import (
	"reflect"
)

// sliceElem is the exact set of element types stored in attribute slice values.
// Using a closed set prevents accidental instantiations for unsupported types.
type sliceElem interface {
	bool | int64 | float64 | string
}

// SliceValue converts a slice into an array with the same elements.
func SliceValue[T sliceElem](v []T) any {
	// Keep only the common tiny-slice cases out of reflection. Extending this
	// much further increases code size for diminishing benefit while larger
	// slices still need the generic reflective path to preserve comparability.
	// This matches the short lengths that show up most often in local
	// benchmarks and semantic convention examples while leaving larger, less
	// predictable slices on the generic reflective path.
	switch len(v) {
	case 0:
		return [0]T{}
	case 1:
		return [1]T{v[0]}
	case 2:
		return [2]T{v[0], v[1]}
	case 3:
		return [3]T{v[0], v[1], v[2]}
	}

	return sliceValueReflect(v)
}

// AsSlice converts an array into a slice with the same elements.
func AsSlice[T sliceElem](v any) []T {
	// Mirror the small fixed-array fast path used by SliceValue.
	switch a := v.(type) {
	case [0]T:
		return []T{}
	case [1]T:
		return []T{a[0]}
	case [2]T:
		return []T{a[0], a[1]}
	case [3]T:
		return []T{a[0], a[1], a[2]}
	}

	return asSliceReflect[T](v)
}

func sliceValueReflect[T sliceElem](v []T) any {
	cp := reflect.New(reflect.ArrayOf(len(v), reflect.TypeFor[T]())).Elem()
	reflect.Copy(cp, reflect.ValueOf(v))
	return cp.Interface()
}

func asSliceReflect[T sliceElem](v any) []T {
	rv := reflect.ValueOf(v)
	if !rv.IsValid() || rv.Kind() != reflect.Array || rv.Type().Elem() != reflect.TypeFor[T]() {
		return nil
	}
	cpy := make([]T, rv.Len())
	if len(cpy) > 0 {
		_ = reflect.Copy(reflect.ValueOf(cpy), rv)
	}
	return cpy
}