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

random.go

@@ -1,28 +0,0 @@
-package util
-
-import (
-	"crypto/rand"
-	"encoding/hex"
-	"errors"
-)
-
-// RandomHexString returns byteLength random bytes from the operating system's
-// random source, written as lowercase hexadecimal, so the returned string is
-// twice as long as byteLength. The bytes come from crypto/rand, which means the
-// result is fit for session tokens, temporary filenames and anything else a
-// stranger should not be able to guess.
-//
-// An error comes back only for a negative length or if the random source
-// itself fails, which does not happen on a working system.
-func RandomHexString(byteLength int) (string, error) {
-	if byteLength < 0 {
-		return "", errors.New("byte length cannot be negative")
-	}
-
-	buffer := make([]byte, byteLength)
-	if _, err := rand.Read(buffer); err != nil {
-		return "", err
-	}
-
-	return hex.EncodeToString(buffer), nil
-}

random_test.go

@@ -1,57 +0,0 @@
-package util
-
-import (
-	"encoding/hex"
-	"testing"
-)
-
-func TestRandomHexStringLength(t *testing.T) {
-	tests := []struct {
-		name           string
-		byteLength     int
-		expectedLength int
-	}{
-		{"no bytes at all", 0, 0},
-		{"one byte", 1, 2},
-		{"eight bytes", 8, 16},
-		{"sixteen bytes", 16, 32},
-		{"thirty-two bytes", 32, 64},
-	}
-
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			got, err := RandomHexString(test.byteLength)
-			if err != nil {
-				t.Fatalf("did not expect an error, got %v", err)
-			}
-			if len(got) != test.expectedLength {
-				t.Errorf("expected a string of %d characters, got %d (%q)", test.expectedLength, len(got), got)
-			}
-			if _, err := hex.DecodeString(got); err != nil {
-				t.Errorf("expected valid hexadecimal, got %q: %v", got, err)
-			}
-		})
-	}
-}
-
-func TestRandomHexStringRejectsNegativeLength(t *testing.T) {
-	got, err := RandomHexString(-1)
-	if err == nil {
-		t.Errorf("expected an error for a negative length, got %q", got)
-	}
-}
-
-func TestRandomHexStringDiffersBetweenCalls(t *testing.T) {
-	seen := make(map[string]struct{})
-
-	for i := 0; i < 100; i++ {
-		value, err := RandomHexString(16)
-		if err != nil {
-			t.Fatalf("did not expect an error, got %v", err)
-		}
-		if _, repeated := seen[value]; repeated {
-			t.Fatalf("got the same string twice: %q", value)
-		}
-		seen[value] = struct{}{}
-	}
-}

sha256file.go

@@ -0,0 +1,30 @@
+package util
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"io"
+	"os"
+)
+
+// SHA256File returns the SHA-256 digest of the contents of the file at path,
+// written as lowercase hexadecimal, in the same form as the sha256sum command
+// prints. The file is copied through the hash in chunks rather than read into
+// memory, so its size does not matter.
+//
+// The error is whatever went wrong opening or reading the file, passed along
+// unchanged so a caller can test it with os.IsNotExist and the like.
+func SHA256File(path string) (string, error) {
+	file, err := os.Open(path)
+	if err != nil {
+		return "", err
+	}
+	defer file.Close()
+
+	digest := sha256.New()
+	if _, err := io.Copy(digest, file); err != nil {
+		return "", err
+	}
+
+	return hex.EncodeToString(digest.Sum(nil)), nil
+}

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")
+	}
+}