Copy constructor makes small strings when it can

We create a small string whenever it will fit, instead of only when r is
small. This is safe because we always memcpy the whole blob when we call
reserve, and optimizes the case when r has been truncated.

This also reworks the test to follow the idiom adopted elsewhere re stl,
and adds a helper function to tell if a string is small based on data().

ctl/string.cc

@@ -42,6 +42,10 @@ void
 string::init_big(const string& s) noexcept
 {
     char* p2;
+#ifndef NDEBUG
+    if (!s.isbig())
+        __builtin_trap();
+#endif
     if (s.size() >= s.capacity() >> 1) {
         if (!(p2 = (char*)malloc(s.capacity())))
             __builtin_trap();

ctl/string.h

@@ -84,10 +84,12 @@ class string
 
     string(const string& r) noexcept
     {
-        if (r.isbig())
+        if (r.size() <= __::sso_max) {
+            __builtin_memcpy(blob, r.data(), __::string_size);
+            set_small_size(r.size());
+        } else {
             init_big(r);
-        else
-            __builtin_memcpy(blob, r.blob, __::string_size);
+        }
     }
 
     string(const char* const p, const size_t n) noexcept

test/ctl/string_test.cc

@@ -23,9 +23,19 @@
 #include "libc/runtime/runtime.h"
 #include "libc/str/str.h"
 
-using String = ctl::string;
 // #include <string>
-// using String = std::string;
+// #define ctl std
+
+using String = ctl::string;
+
+#undef ctl
+
+inline bool
+issmall(const String& s)
+{
+    return s.capacity() == sizeof(s) &&
+           s.data() == reinterpret_cast<const char*>(&s);
+}
 
 int
 main()
@@ -358,15 +368,14 @@ main()
         String s;
         if constexpr (std::is_same_v<ctl::string, decltype(s)>) {
             // tests the small-string optimization on ctl::string
-            char* d = s.data();
             for (int i = 0; i < 23; ++i) {
                 s.append("a");
-                if (s.data() != d) {
+                if (!issmall(s)) {
                     return 79 + i;
                 }
             }
             s.append("a");
-            if (s.data() == d) {
+            if (issmall(s)) {
                 return 103;
             }
         } else {
@@ -380,6 +389,21 @@ main()
         }
     }
 
+    {
+        String s("arst", 4);
+        for (int i = 0; i < 30; ++i) {
+            s.append("a");
+        }
+        s.resize(4);
+        if (s != "arst")
+            return 105;
+        if constexpr (std::is_same_v<ctl::string, decltype(s)>) {
+            String r(s);
+            if (issmall(s) || !issmall(r))
+                return 106;
+        }
+    }
+
     CheckForMemoryLeaks();
     return 0;
 }