1 files changed, 113 insertions, 0 deletions
diff --git a/compare/compare_strings.go b/compare/compare_strings.go
new file mode 100644
index 000000000..1fd954081
--- /dev/null
+++ b/compare/compare_strings.go
@@ -0,0 +1,113 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// 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 compare
+
+import (
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// Strings returns an integer comparing two strings lexicographically.
+func Strings(s, t string) int {
+	c := compareFold(s, t)
+
+	if c == 0 {
+		// "B" and "b" would be the same so we need a tiebreaker.
+		return strings.Compare(s, t)
+	}
+
+	return c
+}
+
+// This function is derived from strings.EqualFold in Go's stdlib.
+// https://github.com/golang/go/blob/ad4a58e31501bce5de2aad90a620eaecdc1eecb8/src/strings/strings.go#L893
+func compareFold(s, t string) int {
+	for s != "" && t != "" {
+		var sr, tr rune
+		if s[0] < utf8.RuneSelf {
+			sr, s = rune(s[0]), s[1:]
+		} else {
+			r, size := utf8.DecodeRuneInString(s)
+			sr, s = r, s[size:]
+		}
+		if t[0] < utf8.RuneSelf {
+			tr, t = rune(t[0]), t[1:]
+		} else {
+			r, size := utf8.DecodeRuneInString(t)
+			tr, t = r, t[size:]
+		}
+
+		if tr == sr {
+			continue
+		}
+
+		c := 1
+		if tr < sr {
+			tr, sr = sr, tr
+			c = -c
+		}
+
+		//  ASCII only.
+		if tr < utf8.RuneSelf {
+			if sr >= 'A' && sr <= 'Z' {
+				if tr <= 'Z' {
+					// Same case.
+					return -c
+				}
+
+				diff := tr - (sr + 'a' - 'A')
+
+				if diff == 0 {
+					continue
+				}
+
+				if diff < 0 {
+					return c
+				}
+
+				if diff > 0 {
+					return -c
+				}
+			}
+		}
+
+		// Unicode.
+		r := unicode.SimpleFold(sr)
+		for r != sr && r < tr {
+			r = unicode.SimpleFold(r)
+		}
+
+		if r == tr {
+			continue
+		}
+
+		return -c
+	}
+
+	if s == "" && t == "" {
+		return 0
+	}
+
+	if s == "" {
+		return -1
+	}
+
+	return 1
+}
+
+// LessStrings returns whether s is less than t lexicographically.
+func LessStrings(s, t string) bool {
+	return Strings(s, t) < 0
+}