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.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
+}