add ChunkStrings for splitting a string slice into fixed-size batches

ChunkStrings cuts a slice into consecutive pieces of at most the requested
size, with the leftovers in the last piece, and returns copies so the pieces
cannot disturb each other. Comes with a doc comment and table-driven tests.
(closes #3)

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))
-	}
-}

chunk.go

@@ -0,0 +1,26 @@
+package util
+
+// ChunkStrings splits input into consecutive slices of at most size elements.
+// The last slice holds whatever is left over and may be shorter than the
+// others. Each returned slice is a copy, so appending to one of them cannot
+// disturb another. A size below one, or an empty input, returns nil.
+func ChunkStrings(input []string, size int) [][]string {
+	if size < 1 || len(input) == 0 {
+		return nil
+	}
+
+	out := make([][]string, 0, (len(input)+size-1)/size)
+
+	for start := 0; start < len(input); start += size {
+		end := start + size
+		if end > len(input) {
+			end = len(input)
+		}
+
+		chunk := make([]string, end-start)
+		copy(chunk, input[start:end])
+		out = append(out, chunk)
+	}
+
+	return out
+}

chunk_test.go

@@ -0,0 +1,43 @@
+package util
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestChunkStrings(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    []string
+		size     int
+		expected [][]string
+	}{
+		{"exact multiple", []string{"a", "b", "c", "d"}, 2, [][]string{{"a", "b"}, {"c", "d"}}},
+		{"with a remainder", []string{"a", "b", "c"}, 2, [][]string{{"a", "b"}, {"c"}}},
+		{"size larger than input", []string{"a", "b"}, 5, [][]string{{"a", "b"}}},
+		{"size of one", []string{"a", "b"}, 1, [][]string{{"a"}, {"b"}}},
+		{"size of zero", []string{"a", "b"}, 0, nil},
+		{"negative size", []string{"a", "b"}, -1, nil},
+		{"empty input", []string{}, 2, nil},
+		{"nil input", nil, 2, nil},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got := ChunkStrings(test.input, test.size)
+			if !reflect.DeepEqual(got, test.expected) {
+				t.Errorf("expected %#v got %#v", test.expected, got)
+			}
+		})
+	}
+}
+
+func TestChunkStringsReturnsCopies(t *testing.T) {
+	input := []string{"a", "b", "c", "d"}
+	chunks := ChunkStrings(input, 2)
+	chunks[0][0] = "changed"
+
+	if input[0] != "a" {
+		t.Errorf("expected the input to be unchanged, got %#v", input)
+	}
+}