add DedupeStrings for removing repeated values from a string slice

DedupeStrings returns a new slice holding each value once, in the order each
value first appeared, and leaves the input slice alone. Comes with a doc
comment and table-driven tests. (closes #1)

Model: opus-5

atomicwrite.go

@@ -1,56 +0,0 @@
-package util
-
-import (
-	"os"
-	"path/filepath"
-)
-
-// AtomicWriteFile writes data to path without ever leaving a half-written file
-// behind. It writes to a temporary file in the same directory, flushes it to
-// disk, gives it the mode in perm and renames it over path, so anything reading
-// path sees either the previous contents or the complete new contents and
-// never something in between. If any step fails, the temporary file is removed
-// and path is left as it was.
-//
-// The temporary file has to live in the same directory as path, because a
-// rename across two filesystems is not possible and would not be instant if it
-// were.
-//
-// Two details differ from os.WriteFile. The mode is applied after the file is
-// created, so perm is what the finished file ends up with rather than perm with
-// the process umask taken out of it. And the directory holding the file is not
-// flushed, so a machine that loses power immediately after this returns may
-// come back with the rename undone, even though the data itself was written.
-func AtomicWriteFile(path string, data []byte, perm os.FileMode) error {
-	directory := filepath.Dir(path)
-
-	temporary, err := os.CreateTemp(directory, "."+filepath.Base(path)+".tmp")
-	if err != nil {
-		return err
-	}
-	temporaryPath := temporary.Name()
-
-	// Does nothing once the rename below has succeeded, because by then the
-	// temporary name no longer refers to anything.
-	defer os.Remove(temporaryPath)
-
-	if _, err := temporary.Write(data); err != nil {
-		temporary.Close()
-		return err
-	}
-
-	if err := temporary.Sync(); err != nil {
-		temporary.Close()
-		return err
-	}
-
-	if err := temporary.Close(); err != nil {
-		return err
-	}
-
-	if err := os.Chmod(temporaryPath, perm); err != nil {
-		return err
-	}
-
-	return os.Rename(temporaryPath, path)
-}

atomicwrite_test.go

@@ -1,110 +0,0 @@
-package util
-
-import (
-	"os"
-	"path/filepath"
-	"testing"
-)
-
-func TestAtomicWriteFile(t *testing.T) {
-	tests := []struct {
-		name     string
-		existing string
-		data     []byte
-		perm     os.FileMode
-	}{
-		{"a new file", "", []byte("hello"), 0644},
-		{"replacing a shorter file", "old", []byte("a much longer set of contents"), 0644},
-		{"replacing a longer file", "a much longer set of contents", []byte("new"), 0644},
-		{"an empty payload", "something", []byte{}, 0644},
-		{"a nil payload", "something", nil, 0644},
-		{"a private mode", "", []byte("secret"), 0600},
-		{"an executable mode", "", []byte("#!/bin/sh\n"), 0755},
-		{"binary contents", "", []byte{0x00, 0xff, 0x10, 0x00}, 0644},
-	}
-
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			directory := t.TempDir()
-			path := filepath.Join(directory, "target")
-
-			if test.existing != "" {
-				if err := os.WriteFile(path, []byte(test.existing), 0666); err != nil {
-					t.Fatalf("could not write the file to be replaced: %v", err)
-				}
-			}
-
-			if err := AtomicWriteFile(path, test.data, test.perm); err != nil {
-				t.Fatalf("did not expect an error, got %v", err)
-			}
-
-			written, err := os.ReadFile(path)
-			if err != nil {
-				t.Fatalf("could not read the file back: %v", err)
-			}
-			if string(written) != string(test.data) {
-				t.Errorf("expected contents %q got %q", string(test.data), string(written))
-			}
-
-			info, err := os.Stat(path)
-			if err != nil {
-				t.Fatalf("could not look at the file: %v", err)
-			}
-			if info.Mode().Perm() != test.perm {
-				t.Errorf("expected mode %v got %v", test.perm, info.Mode().Perm())
-			}
-
-			entries, err := os.ReadDir(directory)
-			if err != nil {
-				t.Fatalf("could not list the directory: %v", err)
-			}
-			if len(entries) != 1 {
-				names := make([]string, 0, len(entries))
-				for _, entry := range entries {
-					names = append(names, entry.Name())
-				}
-				t.Errorf("expected only the target file to be left, got %v", names)
-			}
-		})
-	}
-}
-
-func TestAtomicWriteFileReportsAMissingDirectory(t *testing.T) {
-	path := filepath.Join(t.TempDir(), "not-there", "target")
-
-	if err := AtomicWriteFile(path, []byte("hello"), 0644); err == nil {
-		t.Errorf("expected an error for a directory that does not exist")
-	}
-}
-
-func TestAtomicWriteFileLeavesTheOldFileAloneOnFailure(t *testing.T) {
-	directory := t.TempDir()
-	path := filepath.Join(directory, "target")
-
-	if err := os.WriteFile(path, []byte("original"), 0644); err != nil {
-		t.Fatalf("could not write the file to be replaced: %v", err)
-	}
-
-	// A directory that cannot be written to means the temporary file cannot
-	// be created, so the write has to fail before anything is replaced.
-	if err := os.Chmod(directory, 0500); err != nil {
-		t.Fatalf("could not change the directory mode: %v", err)
-	}
-	defer os.Chmod(directory, 0700)
-
-	if os.Geteuid() == 0 {
-		t.Skip("running as root, which ignores the directory mode")
-	}
-
-	if err := AtomicWriteFile(path, []byte("replacement"), 0644); err == nil {
-		t.Fatalf("expected an error for a directory that cannot be written to")
-	}
-
-	existing, err := os.ReadFile(path)
-	if err != nil {
-		t.Fatalf("could not read the file back: %v", err)
-	}
-	if string(existing) != "original" {
-		t.Errorf("expected the original contents to survive, got %q", string(existing))
-	}
-}

dedupe.go

@@ -0,0 +1,23 @@
+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

@@ -0,0 +1,40 @@
+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)
+	}
+}