feat: add truncateText and truncateURL helpers for Discord field limits

Discord silently rejects presence updates when text fields exceed 128
characters or URL fields exceed 256 characters. Fixes #16

rpc.go

@@ -55,6 +55,30 @@ const (
 
 const heartbeatInterval = 41 // Heartbeat interval in seconds
 
+// Discord API field length limits
+const (
+	maxTextLength = 128 // Max characters for text fields (details, state, name, large_text)
+	maxURLLength  = 256 // Max characters for URL fields (details_url, state_url, etc.)
+)
+
+// truncateText truncates s to maxTextLength runes, appending "…" if truncated.
+func truncateText(s string) string {
+	runes := []rune(s)
+	if len(runes) <= maxTextLength {
+		return s
+	}
+	return string(runes[:maxTextLength-1]) + "…"
+}
+
+// truncateURL returns s unchanged if within maxURLLength, otherwise returns ""
+// (a truncated URL would be broken, so we omit it entirely).
+func truncateURL(s string) string {
+	if len(s) <= maxURLLength {
+		return s
+	}
+	return ""
+}
+
 // activity represents a Discord activity sent via Gateway opcode 3.
 type activity struct {
 	Name              string             `json:"name"`

rpc_test.go

@@ -448,4 +448,55 @@ var _ = Describe("discordRPC", func() {
 			Expect(err).ToNot(HaveOccurred())
 		})
 	})
+
+	Describe("truncateText", func() {
+		It("returns short strings unchanged", func() {
+			Expect(truncateText("hello")).To(Equal("hello"))
+		})
+
+		It("returns exactly 128-char strings unchanged", func() {
+			s := strings.Repeat("a", 128)
+			Expect(truncateText(s)).To(Equal(s))
+		})
+
+		It("truncates strings over 128 chars to 127 + ellipsis", func() {
+			s := strings.Repeat("a", 200)
+			result := truncateText(s)
+			Expect([]rune(result)).To(HaveLen(128))
+			Expect(result).To(HaveSuffix("…"))
+		})
+
+		It("handles multi-byte characters correctly", func() {
+			// 130 Japanese characters — each is one rune but 3 bytes
+			s := strings.Repeat("あ", 130)
+			result := truncateText(s)
+			runes := []rune(result)
+			Expect(runes).To(HaveLen(128))
+			Expect(string(runes[127])).To(Equal("…"))
+		})
+
+		It("returns empty string unchanged", func() {
+			Expect(truncateText("")).To(Equal(""))
+		})
+	})
+
+	Describe("truncateURL", func() {
+		It("returns short URLs unchanged", func() {
+			Expect(truncateURL("https://example.com")).To(Equal("https://example.com"))
+		})
+
+		It("returns exactly 256-char URLs unchanged", func() {
+			u := "https://example.com/" + strings.Repeat("a", 236)
+			Expect(truncateURL(u)).To(Equal(u))
+		})
+
+		It("returns empty string for URLs over 256 chars", func() {
+			u := "https://example.com/" + strings.Repeat("a", 237)
+			Expect(truncateURL(u)).To(Equal(""))
+		})
+
+		It("returns empty string unchanged", func() {
+			Expect(truncateURL("")).To(Equal(""))
+		})
+	})
 })