From 680df5f205ff6456e56f27ecb1080198977550d4 Mon Sep 17 00:00:00 2001
From: kevin9327 <5299031+kevin9327@users.noreply.github.com>
Date: Thu, 12 Nov 2026 11:50:44 +0900
Subject: [PATCH] fix: reject Roman numerals above 3999 in RomanToInt

The function documents a maximum of 3999, and IntToRoman already
rejects that range, but MMMM was accepted as 4000. Return an error
instead and cover it with a test.
---
 conversion/romantoint.go      | 4 ++++
 conversion/romantoint_test.go | 5 +++++
 2 files changed, 9 insertions(+)

diff --git a/conversion/romantoint.go b/conversion/romantoint.go
index 629952320..c6df21a53 100644
--- a/conversion/romantoint.go
+++ b/conversion/romantoint.go
@@ -55,5 +55,9 @@ func RomanToInt(input string) (int, error) {
 	if len(input) > 0 {
 		return 0, errors.New("invalid roman numeral")
 	}
+	// Only standard form in the range 1 to 3999 is supported.
+	if output > 3999 {
+		return 0, errors.New("roman numeral must be in range 1 to 3999")
+	}
 	return output, nil
 }
diff --git a/conversion/romantoint_test.go b/conversion/romantoint_test.go
index 0fd34b2c3..41a1b6a39 100644
--- a/conversion/romantoint_test.go
+++ b/conversion/romantoint_test.go
@@ -43,6 +43,11 @@ func TestRomanToInt(t *testing.T) {
 	if err != nil {
 		t.Errorf("RomanToInt(\"\") returned an error %s", err.Error())
 	}
+
+	// MMMM is 4000, which is outside the supported 1-3999 range.
+	if _, err := RomanToInt("MMMM"); err == nil {
+		t.Error("RomanToInt(MMMM) expected an error")
+	}
 }
 
 func BenchmarkRomanToInt(b *testing.B) {