From cf4c985e4646c0fc2ee3802fcdc89e5f2e810cc8 Mon Sep 17 00:00:00 2001
From: Hugo Melder <service@hugomelder.com>
Date: Mon, 28 Oct 2024 06:42:41 -0700
Subject: [PATCH] NSString: Fix -commonPrefixWithString:options: behaviour
 (#455)

* Update changelog

* NSString: fix -commonPrefixWithString:options: behaviour

* NSString: More test cases
---
 ChangeLog                           |  6 +++++
 Source/NSString.m                   |  5 ++++
 Tests/base/NSString/basic.m         |  3 +++
 Tests/base/NSString/common_prefix.m | 39 +++++++++++++++++++++++++++++
 4 files changed, 53 insertions(+)
 create mode 100644 Tests/base/NSString/common_prefix.m

diff --git a/ChangeLog b/ChangeLog
index 4be99f770f..2e58f28858 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2024-10-28 Hugo Melder <hugo@algoriddim.com>
+	* Source/NSString.m:
+	-commonPrefixWithString:options: returns nil when string supplied as
+	first argument is nil. On macOS, the empty string is returned instead.
+	Align implementation with macOS.
+
 2024-10-13 Richard Frith-Macdonald <rfm@gnu.org>
 
 	* Source/NSFileManager.m: Create an NSError object when we fail to
diff --git a/Source/NSString.m b/Source/NSString.m
index b0c85b4af9..8c7a296932 100644
--- a/Source/NSString.m
+++ b/Source/NSString.m
@@ -3227,6 +3227,11 @@ - (NSUInteger) hash
 - (NSString*) commonPrefixWithString: (NSString*)aString
 			     options: (NSUInteger)mask
 {
+  // Return empty string to match behaviour on macOS
+  if (nil == aString)
+    {
+      return @"";
+    }
   if (mask & NSLiteralSearch)
     {
       int prefix_len = 0;
diff --git a/Tests/base/NSString/basic.m b/Tests/base/NSString/basic.m
index d94b539c0b..2b87f05d2f 100644
--- a/Tests/base/NSString/basic.m
+++ b/Tests/base/NSString/basic.m
@@ -144,6 +144,9 @@ int main()
   PASS([@"" isEqual: nil] == NO, "an empty string is not null");
   PASS([@"" isEqualToString: nil] == NO, "an empty string is not null");
 
+  s = [@"test" commonPrefixWithString: nil options: 0];
+  PASS_EQUAL(s, @"", "Common prefix of some string with nil is empty string");
+
   [arp release]; arp = nil;
   return 0;
 }
diff --git a/Tests/base/NSString/common_prefix.m b/Tests/base/NSString/common_prefix.m
new file mode 100644
index 0000000000..447dc45074
--- /dev/null
+++ b/Tests/base/NSString/common_prefix.m
@@ -0,0 +1,39 @@
+#import <Foundation/NSAutoreleasePool.h>
+#import <Foundation/NSString.h>
+#import "Testing.h"
+
+int main()
+{
+  NSAutoreleasePool *arp = [NSAutoreleasePool new];
+  NSString *result;
+
+  result = [@"abc" commonPrefixWithString:nil options:0];
+  PASS_EQUAL(result, @"", "common prefix of some string with nil is empty string");
+
+  result = [@"abc" commonPrefixWithString:@"abc" options:0];
+  PASS_EQUAL(result, @"abc", "common prefix of identical strings is the entire string");
+
+  result = [@"abc" commonPrefixWithString:@"abx" options:0];
+  PASS_EQUAL(result, @"ab", "common prefix of 'abc' and 'abx' is 'ab'");
+
+  result = [@"abc" commonPrefixWithString:@"def" options:0];
+  PASS_EQUAL(result, @"", "common prefix of completely different strings is empty");
+
+  result = [@"abc" commonPrefixWithString:@"" options:0];
+  PASS_EQUAL(result, @"", "common prefix with an empty string is empty");
+
+  result = [@"abc" commonPrefixWithString:@"a" options:0];
+  PASS_EQUAL(result, @"a", "common prefix of 'abc' and 'a' is 'a'");
+
+  result = [@"abc" commonPrefixWithString:@"aöç" options:0];
+  PASS_EQUAL(result, @"a", "common prefix of 'abc' and 'aöç' is 'a'");
+
+  result = [@"" commonPrefixWithString:@"abc" options:0];
+  PASS_EQUAL(result, @"", "common prefix with an empty base string is empty");
+
+  result = [@"abc" commonPrefixWithString:@"abcx" options:0];
+  PASS_EQUAL(result, @"abc", "common prefix of 'abc' and 'abcx' is 'abc'");
+
+  [arp drain];
+}
+