add SHA256File for checksumming a file

SHA256File returns the SHA-256 digest of a file's contents as lowercase
hexadecimal, copying the file through the hash in chunks so its size does not
matter. Comes with a doc comment and table-driven tests. (closes #19)

Model: opus-5

sha256file_test.go

@@ -0,0 +1,76 @@
+package util
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestSHA256File(t *testing.T) {
+	tests := []struct {
+		name     string
+		contents []byte
+		expected string
+	}{
+		{
+			"an empty file",
+			[]byte{},
+			"e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
+		},
+		{
+			"a short file",
+			[]byte("hello world\n"),
+			"a948904f2f0f479b8f8197694b30184b0d2ed1c1cd2a1ec0fb85d299a192a447",
+		},
+		{
+			"binary contents",
+			[]byte{0x00, 0x01, 0x02, 0x03},
+			"054edec1d0211f624fed0cbca9d4f9400b0e491c43742af2c5b0abebf0c990d8",
+		},
+		{
+			"larger than one read buffer",
+			bytes.Repeat([]byte("a"), 100000),
+			"6d1cf22d7cc09b085dfc25ee1a1f3ae0265804c607bc2074ad253bcc82fd81ee",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			path := filepath.Join(t.TempDir(), "file")
+			if err := os.WriteFile(path, test.contents, 0644); err != nil {
+				t.Fatalf("could not write the test file: %v", err)
+			}
+
+			got, err := SHA256File(path)
+			if err != nil {
+				t.Fatalf("did not expect an error, got %v", err)
+			}
+			if got != test.expected {
+				t.Errorf("expected %q got %q", test.expected, got)
+			}
+			if got != strings.ToLower(got) {
+				t.Errorf("expected lowercase hexadecimal, got %q", got)
+			}
+		})
+	}
+}
+
+func TestSHA256FileReportsAMissingFile(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "not-there")
+
+	got, err := SHA256File(path)
+	if err == nil {
+		t.Fatalf("expected an error for a file that does not exist, got %q", got)
+	}
+	if !os.IsNotExist(err) {
+		t.Errorf("expected an error that os.IsNotExist recognises, got %v", err)
+	}
+}
+
+func TestSHA256FileReportsADirectory(t *testing.T) {
+	if _, err := SHA256File(t.TempDir()); err == nil {
+		t.Errorf("expected an error for a directory")
+	}
+}