add TruncateString for shortening a string safely

TruncateString returns at most the requested number of characters from the
start of a string, counting characters rather than bytes so a multi-byte
character is never cut in half. Comes with a doc comment and table-driven
tests. (closes #5)

Model: opus-5

exists.go

@@ -1,31 +0,0 @@
-package util
-
-import "os"
-
-// FileExists reports whether there is a regular file at path. A directory, a
-// device node or a socket gives false, and so does anything the process is not
-// allowed to look at, since from the caller's point of view there is no usable
-// file there either way.
-//
-// Symbolic links are followed, so a link pointing at a regular file gives true
-// and a link pointing at nothing gives false.
-func FileExists(path string) bool {
-	info, err := os.Stat(path)
-	if err != nil {
-		return false
-	}
-
-	return info.Mode().IsRegular()
-}
-
-// DirExists reports whether there is a directory at path. As with FileExists,
-// anything that cannot be looked at gives false, and symbolic links are
-// followed.
-func DirExists(path string) bool {
-	info, err := os.Stat(path)
-	if err != nil {
-		return false
-	}
-
-	return info.IsDir()
-}

exists_test.go

@@ -1,57 +0,0 @@
-package util
-
-import (
-	"os"
-	"path/filepath"
-	"testing"
-)
-
-func TestFileExistsAndDirExists(t *testing.T) {
-	root := t.TempDir()
-
-	file := filepath.Join(root, "a-file")
-	if err := os.WriteFile(file, []byte("contents"), 0644); err != nil {
-		t.Fatalf("could not write the test file: %v", err)
-	}
-
-	directory := filepath.Join(root, "a-directory")
-	if err := os.Mkdir(directory, 0755); err != nil {
-		t.Fatalf("could not make the test directory: %v", err)
-	}
-
-	linkToFile := filepath.Join(root, "link-to-file")
-	if err := os.Symlink(file, linkToFile); err != nil {
-		t.Fatalf("could not make the test link: %v", err)
-	}
-
-	brokenLink := filepath.Join(root, "broken-link")
-	if err := os.Symlink(filepath.Join(root, "nothing-here"), brokenLink); err != nil {
-		t.Fatalf("could not make the broken test link: %v", err)
-	}
-
-	tests := []struct {
-		name          string
-		path          string
-		expectedFile  bool
-		expectedIsDir bool
-	}{
-		{"a regular file", file, true, false},
-		{"a directory", directory, false, true},
-		{"a link to a file", linkToFile, true, false},
-		{"a link to nothing", brokenLink, false, false},
-		{"a path that is not there", filepath.Join(root, "missing"), false, false},
-		{"a path whose parent is not there", filepath.Join(root, "missing", "deeper"), false, false},
-		{"the empty path", "", false, false},
-	}
-
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			if got := FileExists(test.path); got != test.expectedFile {
-				t.Errorf("FileExists: expected %v got %v", test.expectedFile, got)
-			}
-			if got := DirExists(test.path); got != test.expectedIsDir {
-				t.Errorf("DirExists: expected %v got %v", test.expectedIsDir, got)
-			}
-		})
-	}
-}

truncate.go

@@ -0,0 +1,21 @@
+package util
+
+// TruncateString returns at most max characters from the start of s. It counts
+// characters rather than bytes, so a multi-byte character is never cut in half
+// and the result is always valid text. A max of zero or less returns an empty
+// string, and a string already at or below the limit is returned unchanged.
+func TruncateString(s string, max int) string {
+	if max <= 0 {
+		return ""
+	}
+
+	count := 0
+	for offset := range s {
+		if count == max {
+			return s[:offset]
+		}
+		count++
+	}
+
+	return s
+}

truncate_test.go

@@ -0,0 +1,41 @@
+package util
+
+import "testing"
+
+func TestTruncateString(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		max      int
+		expected string
+	}{
+		{"shorter than the limit", "abc", 5, "abc"},
+		{"exactly at the limit", "abc", 3, "abc"},
+		{"longer than the limit", "abcdef", 3, "abc"},
+		{"max of one", "abc", 1, "a"},
+		{"max of zero", "abc", 0, ""},
+		{"negative max", "abc", -1, ""},
+		{"empty string", "", 3, ""},
+		{"multi-byte characters kept whole", "héllo", 3, "hél"},
+		{"multi-byte characters below the limit", "héllo", 99, "héllo"},
+		{"characters outside the basic plane", "a😀b", 2, "a😀"},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got := TruncateString(test.input, test.max)
+			if got != test.expected {
+				t.Errorf("expected %q got %q", test.expected, got)
+			}
+		})
+	}
+}
+
+func TestTruncateStringCountsCharactersNotBytes(t *testing.T) {
+	// Each of these characters is two bytes long, so a byte-based cut at
+	// three would leave half a character behind.
+	got := TruncateString("äöü", 3)
+	if got != "äöü" {
+		t.Errorf("expected %q got %q", "äöü", got)
+	}
+}