AK+Tests: Correct off-by-one error when right-trimming text

If the entire string you want to right-trim consists of characters you
want to remove, we previously would incorrectly leave the first
character there.

For example: `trim("aaaaa", "a")` would return "a" instead of "".

We can't use `i >= 0` in the loop since that would fail to detect
underflow, so instead we keep `i` in the range `size .. 1` and then
subtract 1 from it when reading the character.

Added some trim() tests while I was at it. (And to confirm that this was
the issue.)

2 files changed, 13 insertions, 2 deletions

diff --git a/AK/StringUtils.cpp b/AK/StringUtils.cpp
index bc2ae22e7a..e283df0ce3 100644
--- a/AK/StringUtils.cpp
+++ b/AK/StringUtils.cpp
@@ -338,10 +338,10 @@ StringView trim(StringView str, StringView characters, TrimMode mode)
     }
 
     if (mode == TrimMode::Right || mode == TrimMode::Both) {
-        for (size_t i = str.length() - 1; i > 0; --i) {
+        for (size_t i = str.length(); i > 0; --i) {
             if (substring_length == 0)
                 return ""sv;
-            if (!characters.contains(str[i]))
+            if (!characters.contains(str[i - 1]))
                 break;
             --substring_length;
         }
diff --git a/Tests/AK/TestStringUtils.cpp b/Tests/AK/TestStringUtils.cpp
index 356024ce07..a06227c137 100644
--- a/Tests/AK/TestStringUtils.cpp
+++ b/Tests/AK/TestStringUtils.cpp
@@ -338,6 +338,17 @@ TEST_CASE(is_whitespace)
     EXPECT(!AK::StringUtils::is_whitespace("a\t"sv));
 }
 
+TEST_CASE(trim)
+{
+    EXPECT_EQ(AK::StringUtils::trim("aaa.a."sv, "."sv, TrimMode::Right), "aaa.a"sv);
+    EXPECT_EQ(AK::StringUtils::trim("...aaa"sv, "."sv, TrimMode::Left), "aaa"sv);
+    EXPECT_EQ(AK::StringUtils::trim("...aaa.a..."sv, "."sv, TrimMode::Both), "aaa.a"sv);
+    EXPECT_EQ(AK::StringUtils::trim("."sv, "."sv, TrimMode::Right), ""sv);
+    EXPECT_EQ(AK::StringUtils::trim("."sv, "."sv, TrimMode::Left), ""sv);
+    EXPECT_EQ(AK::StringUtils::trim("."sv, "."sv, TrimMode::Both), ""sv);
+    EXPECT_EQ(AK::StringUtils::trim("..."sv, "."sv, TrimMode::Both), ""sv);
+}
+
 TEST_CASE(find)
 {
     String test_string = "1234567";