Add package to calculate and compare artifact checksums

Refers to #28

pkg/compiler/checksum.go

@@ -0,0 +1,78 @@
+// Copyright © 2019 Ettore Di Giacinto <mudler@gentoo.org>
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program; if not, see <http://www.gnu.org/licenses/>.
+
+package compiler
+
+import (
+
+	//"strconv"
+
+	"crypto/sha256"
+	"fmt"
+	"hash"
+	"io"
+	"os"
+
+	//	. "github.com/mudler/luet/pkg/logger"
+	"github.com/pkg/errors"
+)
+
+type HashImplementation string
+
+const (
+	SHA256 HashImplementation = "sha256"
+)
+
+type Checksums map[string]string
+
+type HashOptions struct {
+	Hasher hash.Hash
+	Type   HashImplementation
+}
+
+// Generate generates all Checksums supported for the artifact
+func (c *Checksums) Generate(a Artifact) error {
+	return c.generateSHA256(a)
+}
+
+func (c Checksums) Compare(d Checksums) error {
+	for t, sum := range d {
+		if v, ok := c[t]; ok && v != sum {
+			return errors.New("Checksum mismsatch")
+		}
+	}
+	return nil
+}
+
+func (c *Checksums) generateSHA256(a Artifact) error {
+	return c.generateSum(a, HashOptions{Hasher: sha256.New(), Type: SHA256})
+}
+
+func (c *Checksums) generateSum(a Artifact, opts HashOptions) error {
+
+	f, err := os.Open(a.GetPath())
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	if _, err := io.Copy(opts.Hasher, f); err != nil {
+		return err
+	}
+
+	sum := fmt.Sprintf("%x", opts.Hasher.Sum(nil))
+
+	(*c)[string(opts.Type)] = sum
+	return nil
+}