Improve PascalFromSnake behavior

utilities/name.go

@@ -1,14 +1,82 @@
 package utilities
 
-import (
-	"strings"
-)
-
 // PascalFromSnake converts an identifier in snake_case into PascalCase.
-func PascalFromSnake(str string) string {
-	var components []string
-	for _, c := range strings.Split(str, "_") {
-		components = append(components, strings.Title(strings.ToLower(c)))
+func PascalFromSnake(s string) string {
+	// adopted from github.com/golang/protobuf/protoc-gen-go/generator/generator.go
+	//
+	// Copyright 2010 The Go Authors.  All rights reserved.
+	// https://github.com/golang/protobuf
+	//
+	// Redistribution and use in source and binary forms, with or without
+	// modification, are permitted provided that the following conditions are
+	// met:
+	//
+	//     * Redistributions of source code must retain the above copyright
+	// notice, this list of conditions and the following disclaimer.
+	//     * Redistributions in binary form must reproduce the above
+	// copyright notice, this list of conditions and the following disclaimer
+	// in the documentation and/or other materials provided with the
+	// distribution.
+	//     * Neither the name of Google Inc. nor the names of its
+	// contributors may be used to endorse or promote products derived from
+	// this software without specific prior written permission.
+	//
+	// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+	// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+	// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+	// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+	// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+	// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+	// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+	// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+	// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+	// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+	// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+	if s == "" {
+		return ""
+	}
+	t := make([]byte, 0, 32)
+	i := 0
+	if s[0] == '_' {
+		// Need a capital letter; drop the '_'.
+		t = append(t, 'X')
+		i++
+	}
+	// Invariant: if the next letter is lower case, it must be converted
+	// to upper case.
+	// That is, we process a word at a time, where words are marked by _ or
+	// upper case letter. Digits are treated as words.
+	for ; i < len(s); i++ {
+		c := s[i]
+		if c == '_' && i+1 < len(s) && isASCIILower(s[i+1]) {
+			continue // Skip the underscore in s.
+		}
+		if isASCIIDigit(c) {
+			t = append(t, c)
+			continue
+		}
+		// Assume we have a letter now - if not, it's a bogus identifier.
+		// The next word is a sequence of characters that must start upper case.
+		if isASCIILower(c) {
+			c ^= ' ' // Make it a capital letter.
+		}
+		t = append(t, c) // Guaranteed not lower case.
+		// Accept lower case sequence that follows.
+		for i+1 < len(s) && isASCIILower(s[i+1]) {
+			i++
+			t = append(t, s[i])
+		}
 	}
-	return strings.Join(components, "")
+	return string(t)
+}
+
+// Is c an ASCII lower-case letter?
+func isASCIILower(c byte) bool {
+	return 'a' <= c && c <= 'z'
+}
+
+// Is c an ASCII digit?
+func isASCIIDigit(c byte) bool {
+	return '0' <= c && c <= '9'
 }