ip/resolver/http/service.go: refactor default decoder function into its own module

ip/resolver/decoder/text.go

@@ -0,0 +1,19 @@
+package decoder
+
+import (
+	"bufio"
+	"io"
+	"strings"
+)
+
+// Text decoder that expects the IP address
+// to be on the first line of the response.
+// The decoder will only trim trailing and leading whitespace.
+func Text(r io.Reader) (string, error) {
+	line, err := bufio.NewReader(r).ReadString('\n')
+	if err == nil || err == io.EOF {
+		line = strings.TrimRight(line, "\r\n")
+		line = strings.TrimSpace(line)
+	}
+	return line, err
+}

ip/resolver/decoder/text_test.go

@@ -0,0 +1,46 @@
+package decoder_test
+
+import (
+	"io"
+	"strings"
+	"testing"
+
+	"dnsupdater/ip/resolver/decoder"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestService_Name(t *testing.T) {
+	tests := []struct {
+		name        string
+		input       string
+		expected    string
+		expectError error
+	}{
+		{
+			name:        "simple",
+			input:       "192.168.0.1",
+			expected:    "192.168.0.1",
+			expectError: io.EOF,
+		},
+		{
+			name:        "whitespace",
+			input:       "  192.168.0.1  ",
+			expected:    "192.168.0.1",
+			expectError: io.EOF,
+		},
+		{
+			name:     "newline",
+			input:    "192.168.0.1\r\nmore stuff",
+			expected: "192.168.0.1",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			ip, err := decoder.Text(strings.NewReader(test.input))
+			assert.ErrorIs(t, err, test.expectError)
+			assert.Equal(t, test.expected, ip)
+		})
+	}
+}