base: tweak the asserts in the String class, add String::Sub() method

for substrings, and the corresponding unit tests.

src/lol/base/array.h

@@ -175,6 +175,7 @@ public:
     void Remove(int pos, int todelete = 1)
     {
         ASSERT(pos >= 0);
+        ASSERT(todelete >= 0);
         ASSERT(pos + todelete <= m_count);
         for (int i = pos; i + todelete < m_count; i++)
             m_data[i] = m_data[i + todelete];

src/lol/base/string.h

@@ -64,15 +64,17 @@ public:
 
     inline char &operator [](int n)
     {
+        /* Allow n == Count() because we might have reasonable reasons
+         * to access that hidden null character. */
         ASSERT(n >= 0);
-        ASSERT(n < Count() || (!n && !Count()));
+        ASSERT(n <= Count());
         return ((Super &)*this)[n];
     }
 
     inline char const &operator [](int n) const
     {
         ASSERT(n >= 0);
-        ASSERT(n < Count() || (!n && !Count()));
+        ASSERT(n <= Count());
         return ((Super const &)*this)[n];
     }
 
@@ -95,11 +97,19 @@ public:
 
     void Resize(int count)
     {
-        ASSERT(count >= 0, "count = %d", count);
+        ASSERT(count >= 0);
         ((Super &)*this).Resize(count + 1);
         ((Super &)*this).Last() = '\0';
     }
 
+    String Sub(int start, int count) const
+    {
+        ASSERT(start >= 0);
+        ASSERT(count >= 0);
+        ASSERT(start + count <= Count());
+        return String(&(*this)[start], count);
+    }
+
     inline String operator +(String const &s) const
     {
         String ret(*this);

test/unit/string.cpp

@@ -133,6 +133,19 @@ LOLUNIT_FIXTURE(StringTest)
 
         LOLUNIT_ASSERT(s3 == s4);
     }
+
+    LOLUNIT_TEST(SubString)
+    {
+        String s1 = "Hello World";
+
+        String s2 = "Hello";
+        String s3 = s1.Sub(0, 5);
+        LOLUNIT_ASSERT(s3 == s2);
+
+        String s4 = "World";
+        String s5 = s4.Sub(6, 5);
+        LOLUNIT_ASSERT(s5 == s4);
+    }
 };
 
 } /* namespace lol */