From 8c76e5cbfff262f33e75c3a3fd473e810758bd09 Mon Sep 17 00:00:00 2001
From: Joonas Loppi <joonas+code@joonas.fi>
Date: Mon, 2 Sep 2024 12:54:08 +0300
Subject: [PATCH] `typeddigest` package

---
 pkg/typeddigest/typeddigest.go      | 78 +++++++++++++++++++++++++++++
 pkg/typeddigest/typeddigest_test.go | 60 ++++++++++++++++++++++
 2 files changed, 138 insertions(+)
 create mode 100644 pkg/typeddigest/typeddigest.go
 create mode 100644 pkg/typeddigest/typeddigest_test.go

diff --git a/pkg/typeddigest/typeddigest.go b/pkg/typeddigest/typeddigest.go
new file mode 100644
index 0000000..653e519
--- /dev/null
+++ b/pkg/typeddigest/typeddigest.go
@@ -0,0 +1,78 @@
+// A digest that contains the algoritm as a prefix. Example `sha256:d7a8fbb307d7809469ca9abcb0082e4f8d5651e46d3cdb762d02d0bf37c9e592`
+package typeddigest
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+)
+
+const (
+	algSha256 = "sha256"
+)
+
+type Hash struct {
+	alg    string // "sha256"
+	digest []byte
+}
+
+func (h *Hash) String() string {
+	return fmt.Sprintf("%s:%x", h.alg, h.digest)
+}
+
+func (h *Hash) Equal(other *Hash) bool {
+	return h.alg == other.alg && bytes.Equal(h.digest, other.digest)
+}
+
+func Parse(val string) (*Hash, error) {
+	withErr := func(err error) (*Hash, error) { return nil, fmt.Errorf("typeddigest.Parse: %w", err) }
+
+	pos := strings.Index(val, ":")
+	if pos == -1 {
+		return withErr(errors.New("bad format, '<alg>:' prefix not found"))
+	}
+
+	alg := val[:pos]
+	digestHex := val[pos+1:]
+
+	if alg != algSha256 {
+		return withErr(fmt.Errorf("unsupported algorithm: %s", alg))
+	}
+
+	digest, err := hex.DecodeString(digestHex)
+	if err != nil {
+		return withErr(err)
+	}
+
+	if expectedSize := sha256.Size; len(digest) != expectedSize {
+		return withErr(fmt.Errorf("wrong digest size: expected %d; got %d", expectedSize, len(digest)))
+	}
+
+	return &Hash{alg, digest}, nil
+}
+
+func DigesterForAlgOf(other *Hash) func(io.Reader) (*Hash, error) {
+	switch other.alg {
+	case algSha256:
+		return Sha256
+	default:
+		return func(io.Reader) (*Hash, error) {
+			return nil, fmt.Errorf("typeddigest.DigesterForAlgOf: unsupported algorithm: %s", other.alg)
+		}
+	}
+}
+
+func Sha256(input io.Reader) (*Hash, error) {
+	withErr := func(err error) (*Hash, error) { return nil, fmt.Errorf("typeddigest.Sha256: %w", err) }
+
+	hash := sha256.New()
+	if _, err := io.Copy(hash, input); err != nil {
+		return withErr(err)
+	}
+
+	return &Hash{algSha256, hash.Sum(nil)}, nil
+}
diff --git a/pkg/typeddigest/typeddigest_test.go b/pkg/typeddigest/typeddigest_test.go
new file mode 100644
index 0000000..d8ece59
--- /dev/null
+++ b/pkg/typeddigest/typeddigest_test.go
@@ -0,0 +1,60 @@
+package typeddigest
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/function61/gokit/testing/assert"
+)
+
+func TestParse(t *testing.T) {
+	for _, tc := range []struct {
+		input  string
+		output string
+	}{
+		{
+			"sha256:d7a8fbb307d7809469ca9abcb0082e4f8d5651e46d3cdb762d02d0bf37c9e592",
+			"ok",
+		},
+		{
+			"sha256:88",
+			"ERROR: typeddigest.Parse: wrong digest size: expected 32; got 1",
+		},
+		{
+			"md5:d7a8fbb307d7809469ca9abcb0082e4f8d5651e46d3cdb762d02d0bf37c9e592",
+			"ERROR: typeddigest.Parse: unsupported algorithm: md5",
+		},
+		{
+			"",
+			"ERROR: typeddigest.Parse: bad format, '<alg>:' prefix not found",
+		},
+		{
+			"sha256:nothex",
+			"ERROR: typeddigest.Parse: encoding/hex: invalid byte: U+006E 'n'",
+		},
+	} {
+		tc := tc // pin
+		t.Run(tc.input, func(t *testing.T) {
+			th, err := Parse(tc.input)
+			asOutput := func() string {
+				if err != nil {
+					return fmt.Sprintf("ERROR: %v", err)
+				} else {
+					// test stability
+					assert.EqualString(t, th.String(), tc.input)
+					return "ok"
+				}
+			}()
+
+			assert.EqualString(t, asOutput, tc.output)
+		})
+	}
+}
+
+func TestSha256(t *testing.T) {
+	th, err := Sha256(strings.NewReader("The quick brown fox jumps over the lazy dog"))
+	assert.Ok(t, err)
+
+	assert.EqualString(t, th.String(), "sha256:d7a8fbb307d7809469ca9abcb0082e4f8d5651e46d3cdb762d02d0bf37c9e592")
+}