add FileExists and DirExists for checking what is at a path

FileExists reports whether a path holds a regular file and DirExists whether it
holds a directory, both answering false for anything they cannot look at.
Comes with doc comments and table-driven tests. (closes #15)

Model: opus-5

exists.go

@@ -0,0 +1,31 @@
+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

@@ -0,0 +1,57 @@
+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

@@ -1,21 +0,0 @@
-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

@@ -1,41 +0,0 @@
-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)
-	}
-}