add Slugify for making URL-safe slugs out of text

Slugify lowercases text and keeps only ASCII letters and digits, turning every
run of other characters into a single hyphen with none left at either end.
Comes with a doc comment and table-driven tests. (closes #9)

Model: opus-5

slugify.go

@@ -0,0 +1,36 @@
+package util
+
+import "strings"
+
+// Slugify returns input as lowercase ASCII letters, digits and hyphens, which
+// is safe to put in a URL path, a filename or a fragment identifier. Every run
+// of other characters becomes a single hyphen, and the result never starts or
+// ends with one.
+//
+// Characters outside ASCII are treated as separators rather than being folded
+// to their nearest ASCII relative, because doing that properly needs a Unicode
+// table that this package does not carry. Text with no ASCII letters or digits
+// in it therefore slugifies to an empty string.
+func Slugify(input string) string {
+	var out strings.Builder
+	pendingHyphen := false
+
+	for _, character := range strings.ToLower(input) {
+		if (character >= 'a' && character <= 'z') || (character >= '0' && character <= '9') {
+			if pendingHyphen {
+				out.WriteByte('-')
+				pendingHyphen = false
+			}
+			out.WriteRune(character)
+			continue
+		}
+
+		// Remember that a separator was seen, but only write a hyphen once
+		// another usable character turns up, so nothing trails off the end.
+		if out.Len() > 0 {
+			pendingHyphen = true
+		}
+	}
+
+	return out.String()
+}

slugify_test.go

@@ -0,0 +1,34 @@
+package util
+
+import "testing"
+
+func TestSlugify(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{"already a slug", "hello-world", "hello-world"},
+		{"mixed case", "Hello World", "hello-world"},
+		{"punctuation", "What's new?", "what-s-new"},
+		{"run of separators", "one   ---   two", "one-two"},
+		{"leading separators", "///one", "one"},
+		{"trailing separators", "one///", "one"},
+		{"separators at both ends", "  --one two--  ", "one-two"},
+		{"digits are kept", "Go 1.14 release", "go-1-14-release"},
+		{"underscores are separators", "some_name_here", "some-name-here"},
+		{"nothing usable", "!!! ???", ""},
+		{"empty string", "", ""},
+		{"outside ascii", "Grüße, Welt", "gr-e-welt"},
+		{"only characters outside ascii", "日本語", ""},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got := Slugify(test.input)
+			if got != test.expected {
+				t.Errorf("expected %q got %q", test.expected, got)
+			}
+		})
+	}
+}

whitespace.go

@@ -1,12 +0,0 @@
-package util
-
-import "strings"
-
-// CollapseWhitespace replaces every run of whitespace in s with a single space
-// and removes any whitespace at the start and end. Tabs, newlines and spaces
-// outside ASCII such as the non-breaking space all count as whitespace, so a
-// block of text comes back as one line with single spaces between the words.
-// A string that is nothing but whitespace comes back empty.
-func CollapseWhitespace(s string) string {
-	return strings.Join(strings.Fields(s), " ")
-}

whitespace_test.go

@@ -1,36 +0,0 @@
-package util
-
-import "testing"
-
-// Written as a code point so that it is visible in the source rather than
-// looking like an ordinary space.
-const nonBreakingSpace = string(rune(0x00a0))
-
-func TestCollapseWhitespace(t *testing.T) {
-	tests := []struct {
-		name     string
-		input    string
-		expected string
-	}{
-		{"nothing to change", "one two", "one two"},
-		{"leading and trailing spaces", "  one two  ", "one two"},
-		{"run in the middle", "one    two", "one two"},
-		{"tabs and newlines", "one\t\ttwo\nthree", "one two three"},
-		{"windows line endings", "one\r\ntwo", "one two"},
-		{"non-breaking space", "one" + nonBreakingSpace + "two", "one two"},
-		{"mixed whitespace run", "one \t" + nonBreakingSpace + " two", "one two"},
-		{"only whitespace", " \t\n ", ""},
-		{"empty string", "", ""},
-		{"single word with padding", "\tword\n", "word"},
-		{"paragraph becomes one line", "first line\n\nsecond line\n", "first line second line"},
-	}
-
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			got := CollapseWhitespace(test.input)
-			if got != test.expected {
-				t.Errorf("expected %q got %q", test.expected, got)
-			}
-		})
-	}
-}