From fe8afc8b062e39bc972beb8ab812d4f28806dd59 Mon Sep 17 00:00:00 2001
From: oliverpool <git@olivier.pfad.fr>
Date: Thu, 24 Oct 2024 22:36:42 +0200
Subject: [PATCH] encode: wrap long lines

---
 encoder.go      | 28 ++++++++++++++++
 encoder_test.go | 87 ++++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 106 insertions(+), 9 deletions(-)

diff --git a/encoder.go b/encoder.go
index 974cefc..b20c5e1 100644
--- a/encoder.go
+++ b/encoder.go
@@ -5,6 +5,7 @@ import (
 	"io"
 	"sort"
 	"strings"
+	"unicode/utf8"
 )
 
 // An Encoder formats cards.
@@ -69,6 +70,33 @@ func formatLine(key string, field *Field) string {
 	}
 
 	s += ":" + formatValue(field.Value)
+
+	// Content lines SHOULD be folded to a maximum width of 75 octets, excluding the line break.
+	const maxLen = 74 // -1 for the leading space on the new line
+	if newlines := (len(s) - 2) / maxLen; newlines > 0 {
+		var sb strings.Builder
+		sb.Grow(len(s) + newlines*len("\r\n "))
+		end := 1 + maxLen
+		for !utf8.RuneStart(s[end]) { // Multi-octet characters MUST remain contiguous.
+			end--
+		}
+		sb.WriteString(s[:end])
+		start := end
+		for start < len(s) {
+			sb.WriteString("\r\n ")
+			end := start + maxLen
+			if end > len(s) {
+				end = len(s)
+			} else {
+				for !utf8.RuneStart(s[end]) { // Multi-octet characters MUST remain contiguous.
+					end--
+				}
+			}
+			sb.WriteString(s[start:end])
+			start = end
+		}
+		return sb.String()
+	}
 	return s
 }
 
diff --git a/encoder_test.go b/encoder_test.go
index 6cfff47..64ae026 100644
--- a/encoder_test.go
+++ b/encoder_test.go
@@ -3,6 +3,7 @@ package vcard
 import (
 	"bytes"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -14,7 +15,7 @@ func TestEncoder(t *testing.T) {
 
 	expected := "BEGIN:VCARD\r\nVERSION:4.0\r\nCLIENTPIDMAP:1;urn:uuid:53e374d9-337e-4727-8803-a1e9c14e0556\r\nEMAIL;PID=1.1:jdoe@example.com\r\nFN;PID=1.1:J. Doe\r\nN:Doe;J.;;;\r\nUID:urn:uuid:4fbe8971-0bc3-424c-9c26-36c3e1eff6b1\r\nEND:VCARD\r\n"
 	if b.String() != expected {
-		t.Errorf("Excpected vcard to be %q, but got %q", expected, b.String())
+		t.Errorf("Expected vcard to be %q, but got %q", expected, b.String())
 	}
 
 	card, err := NewDecoder(&b).Decode()
@@ -27,16 +28,84 @@ func TestEncoder(t *testing.T) {
 	}
 }
 
-func TestFormatLine_withGroup(t *testing.T) {
-	l := formatLine("FN", &Field{
-		Value: "Akiyama Mio",
-		Group: "item1",
+func TestFormatLine(t *testing.T) {
+	t.Run("withGroup", func(t *testing.T) {
+		l := formatLine("FN", &Field{
+			Value: "Akiyama Mio",
+			Group: "item1",
+		})
+
+		expected := "item1.FN:Akiyama Mio"
+		if l != expected {
+			t.Errorf("Expected formatted line with group to be %q, but got %q", expected, l)
+		}
 	})
+	t.Run("wrapping", func(t *testing.T) {
+		l := formatLine("FN", &Field{
+			Value: strings.Repeat("1234567890", 16),
+			Group: "wrapme",
+		})
 
-	expected := "item1.FN:Akiyama Mio"
-	if l != expected {
-		t.Errorf("Excpected formatted line with group to be %q, but got %q", expected, l)
-	}
+		expected := "wrapme.FN:12345678901234567890123456789012345678901234567890123456789012345\r\n 67890123456789012345678901234567890123456789012345678901234567890123456789\r\n 012345678901234567890"
+		if l != expected {
+			t.Errorf("Expected wrapped line to be %q, but got %q", expected, l)
+		}
+	})
+	t.Run("wrapping_at_limit", func(t *testing.T) {
+		l := formatLine("FN", &Field{
+			Value: strings.Repeat("1234567890", 6) + "12345",
+			Group: "wrapme",
+		})
+
+		expected := "wrapme.FN:12345678901234567890123456789012345678901234567890123456789012345"
+		if l != expected {
+			t.Errorf("Expected wrapped line to be %q, but got %q", expected, l)
+		}
+	})
+	t.Run("wrapping_after_limit", func(t *testing.T) {
+		l := formatLine("FN", &Field{
+			Value: strings.Repeat("1234567890", 6) + "123451",
+			Group: "wrapme",
+		})
+
+		expected := "wrapme.FN:12345678901234567890123456789012345678901234567890123456789012345\r\n 1"
+		if l != expected {
+			t.Errorf("Expected wrapped line to be %q, but got %q", expected, l)
+		}
+	})
+	t.Run("wrapping_limit_unicode_start", func(t *testing.T) {
+		l := formatLine("FN", &Field{
+			Value: strings.Repeat("1234567890", 6) + "1234€",
+			Group: "wrapme",
+		})
+
+		expected := "wrapme.FN:1234567890123456789012345678901234567890123456789012345678901234\r\n €"
+		if l != expected {
+			t.Errorf("Expected wrapped line to be %q, but got %q", expected, l)
+		}
+	})
+	t.Run("wrapping_limit_unicode_later", func(t *testing.T) {
+		l := formatLine("FN", &Field{
+			Value: strings.Repeat("1234567890", 13) + "12345678€",
+			Group: "wrapme",
+		})
+
+		expected := "wrapme.FN:12345678901234567890123456789012345678901234567890123456789012345\r\n 6789012345678901234567890123456789012345678901234567890123456789012345678\r\n €"
+		if l != expected {
+			t.Errorf("Expected wrapped line to be %q, but got %q", expected, l)
+		}
+	})
+	t.Run("wrapping_very_long", func(t *testing.T) {
+		l := formatLine("FN", &Field{
+			Value: strings.Repeat("1234567890", 16),
+			Group: "wrapme",
+		})
+
+		expected := "wrapme.FN:12345678901234567890123456789012345678901234567890123456789012345\r\n 67890123456789012345678901234567890123456789012345678901234567890123456789\r\n 012345678901234567890"
+		if l != expected {
+			t.Errorf("Expected wrapped line to be %q, but got %q", expected, l)
+		}
+	})
 }
 
 var testValue = []struct {