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

dedupe.go

@@ -1,23 +0,0 @@
-package util
-
-// DedupeStrings returns a new slice holding each value of input once, in the
-// order in which each value first appears. The input slice is not modified.
-// A nil input returns nil.
-func DedupeStrings(input []string) []string {
-	if input == nil {
-		return nil
-	}
-
-	seen := make(map[string]struct{}, len(input))
-	out := make([]string, 0, len(input))
-
-	for _, value := range input {
-		if _, ok := seen[value]; ok {
-			continue
-		}
-		seen[value] = struct{}{}
-		out = append(out, value)
-	}
-
-	return out
-}

dedupe_test.go

@@ -1,40 +0,0 @@
-package util
-
-import (
-	"reflect"
-	"testing"
-)
-
-func TestDedupeStrings(t *testing.T) {
-	tests := []struct {
-		name     string
-		input    []string
-		expected []string
-	}{
-		{"nil stays nil", nil, nil},
-		{"empty slice", []string{}, []string{}},
-		{"nothing repeated", []string{"a", "b", "c"}, []string{"a", "b", "c"}},
-		{"all one value", []string{"a", "a", "a"}, []string{"a"}},
-		{"repeats next to each other", []string{"a", "a", "b", "b"}, []string{"a", "b"}},
-		{"repeats far apart", []string{"a", "b", "a", "c", "b"}, []string{"a", "b", "c"}},
-		{"empty string is a value", []string{"", "a", ""}, []string{"", "a"}},
-	}
-
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			got := DedupeStrings(test.input)
-			if !reflect.DeepEqual(got, test.expected) {
-				t.Errorf("expected %#v got %#v", test.expected, got)
-			}
-		})
-	}
-}
-
-func TestDedupeStringsLeavesInputAlone(t *testing.T) {
-	input := []string{"b", "a", "b"}
-	DedupeStrings(input)
-
-	if !reflect.DeepEqual(input, []string{"b", "a", "b"}) {
-		t.Errorf("expected the input to be unchanged, got %#v", input)
-	}
-}

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)
+			}
+		})
+	}
+}