text: // Copyright 2017, OpenCensus 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 internal
  
  import (
  	"strings"
  	"unicode"
  )
  
  const labelKeySizeLimit = 100
  
  // Sanitize returns a string that is trunacated to 100 characters if it's too
  // long, and replaces non-alphanumeric characters to underscores.
  func Sanitize(s string) string {
  	if len(s) == 0 {
  		return s
  	}
  	if len(s) > labelKeySizeLimit {
  		s = s[:labelKeySizeLimit]
  	}
  	s = strings.Map(sanitizeRune, s)
  	if unicode.IsDigit(rune(s[0])) {
  		s = "key_" + s
  	}
  	if s[0] == '_' {
  		s = "key" + s
  	}
  	return s
  }
  
  // converts anything that is not a letter or digit to an underscore
  func sanitizeRune(r rune) rune {
  	if unicode.IsLetter(r) || unicode.IsDigit(r) {
  		return r
  	}
  	// Everything else turns into an underscore
  	return '_'
  }
  