feat: add part getter

pkg/message/parser/part_test.go

@@ -0,0 +1,56 @@
+package parser
+
+import (
+	"strconv"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestPart(t *testing.T) {
+	p := newTestParser(t, "complex_structure.eml")
+
+	wantParts := map[string]string{
+		"":        "multipart/mixed",
+		"1":       "text/plain",
+		"2":       "application/octet-stream",
+		"3":       "multipart/mixed",
+		"3.1":     "text/plain",
+		"3.2":     "application/octet-stream",
+		"4":       "multipart/mixed",
+		"4.1":     "image/gif",
+		"4.2":     "multipart/mixed",
+		"4.2.1":   "text/plain",
+		"4.2.2":   "multipart/alternative",
+		"4.2.2.1": "text/plain",
+		"4.2.2.2": "text/html",
+	}
+
+	for partNumber, wantContType := range wantParts {
+		part, err := p.Part(getPartNumber(partNumber))
+		require.NoError(t, err)
+
+		contType, _, err := part.Header.ContentType()
+		require.NoError(t, err)
+		assert.Equal(t, wantContType, contType)
+	}
+}
+
+func getPartNumber(s string) (part []int) {
+	if s == "" {
+		return
+	}
+
+	for _, number := range strings.Split(s, ".") {
+		i64, err := strconv.ParseInt(number, 10, 64)
+		if err != nil {
+			panic(err)
+		}
+
+		part = append(part, int(i64))
+	}
+
+	return
+}