diff --git a/include/tlibc/collections/Array.h b/include/tlibc/collections/Array.h
index 70b529e..33857ca 100755
--- a/include/tlibc/collections/Array.h
+++ b/include/tlibc/collections/Array.h
@@ -27,9 +27,10 @@ static inline void Array_realloc_size(Array_* ar, u32 new_size){
     ar->size = new_size;
 }
 
-static inline Array_ Array_copy(Array_ src){
+static inline Array_ Array_copy(const Array_ src){
     Array_ copy = Array_alloc_size(src.size);
-    memcpy(copy.data, src.data, src.size);
+    if(copy.data != NULL)
+        memcpy(copy.data, src.data, src.size);
     return copy;
 }
 
@@ -39,11 +40,11 @@ static inline Array_ Array_copy(Array_ src){
 #define struct_castTo_Array(STRUCT_PTR) Array_construct_size((STRUCT_PTR), sizeof(*STRUCT_PTR))
 
 ///@return a[0..i-1]
-static inline Array(u8) Array_sliceTo(Array(u8) a, u32 i){
+static inline Array(u8) Array_sliceTo(const Array(u8) a, u32 i){
     return Array_construct_size(a.data, i);
 }
 
 ///@return a[i...]
-static inline Array(u8) Array_sliceFrom(Array(u8) a, u32 i){
+static inline Array(u8) Array_sliceFrom(const Array(u8) a, u32 i){
     return Array_construct_size((u8*)a.data + i, a.size - i);
 }
diff --git a/include/tlibc/collections/List.h b/include/tlibc/collections/List.h
index 2095920..fc4370d 100755
--- a/include/tlibc/collections/List.h
+++ b/include/tlibc/collections/List.h
@@ -19,7 +19,9 @@ static inline List_ List_construct_size(void* data_ptr, u32 occupied_size, u32 a
 #define List_alloc(T, INITIAL_COUNT) List_alloc_size((INITIAL_COUNT) * sizeof(T))
 List_ List_alloc_size(u32 initial_size);
 
-void* List_expand_size(List_* ptr, u32 expansion_size);
+List_ List_copy(List_ src);
+
+void* List_expand_size(List_* ptr, u32 size_to_add);
 #define List_push(L, T, VALUE) *(T*)(List_expand_size(L, sizeof(T))) = VALUE
 #define List_pushMany(L, T, VALUES_PTR, COUNT) List_push_size(L, VALUES_PTR, (COUNT) * sizeof(T))
 void List_push_size(List_* ptr, void* values, u32 size);
diff --git a/include/tlibc/string/StringBuilder.h b/include/tlibc/string/StringBuilder.h
index 799dc78..8e46689 100755
--- a/include/tlibc/string/StringBuilder.h
+++ b/include/tlibc/string/StringBuilder.h
@@ -13,6 +13,14 @@ static inline StringBuilder StringBuilder_alloc(u32 initial_size) {
 }
 void StringBuilder_destroy(StringBuilder* b);
 
+static inline StringBuilder StringBuilder_copy(const StringBuilder* b){
+    return (StringBuilder) { .buffer = List_copy(b->buffer) };
+}
+static inline void StringBuilder_expand(StringBuilder* b, u32 size_to_add){
+    List_expand_size(&b->buffer, size_to_add);
+}
+
+
 /// @param count set to -1 to clear StringBuilder
 void StringBuilder_removeFromEnd(StringBuilder* b, u32 count);
 void StringBuilder_append_char(StringBuilder* b, char c);
@@ -25,3 +33,5 @@ void StringBuilder_append_memory(StringBuilder* b, Array(u8) mem, bool uppercase
 
 // adds '\0' to the buffer and returns pointer to buffer content
 str StringBuilder_getStr(StringBuilder* b);
+
+bool StringBuilder_equals(const StringBuilder* a, const StringBuilder* b);
diff --git a/include/tlibc/string/str.h b/include/tlibc/string/str.h
index 43903ee..88d9741 100755
--- a/include/tlibc/string/str.h
+++ b/include/tlibc/string/str.h
@@ -20,6 +20,10 @@ static inline str str_from_cstr(cstr s_ptr){
     return str_construct((void*)s_ptr, strlen(s_ptr), true);
 }
 
+static inline void str_destroy(str s){
+    free(s.data);
+}
+
 static inline Array_ str_castTo_Array(str s) {
     return Array_construct_size(s.data, s.size);
 }
@@ -31,29 +35,29 @@ static inline str Array_castTo_str(Array_ a, bool isZeroTerminated) {
 static const str str_null = str_construct(NULL, 0, 0);
 
 /// copies src content to new string and adds \0 at the end
-str str_copy(str src);
+str str_copy(const str self);
 
 /// compares two strings, NullPtr-friendly
-bool str_equals(str str0, str str1);
+bool str_equals(const str self, const str other);
 
 /// allocates new string which is reversed variant of <s>
 str str_reverse(str s);
 
-i32 str_seek(str src, str fragment, u32 startIndex);
-i32 str_seekReverse(str src, str fragment, u32 startIndex);
+i32 str_seek(const str src, const str fragment, u32 startIndex);
+i32 str_seekReverse(const str src, const str fragment, u32 startIndex);
 
-i32 str_seekChar(str src, char c, u32 startIndex);
-i32 str_seekCharReverse(str src, char c, u32 startIndex);
+i32 str_seekChar(const str src, char c, u32 startIndex);
+i32 str_seekCharReverse(const str src, char c, u32 startIndex);
 
-bool str_startsWith(str src, str fragment);
-bool str_endsWith(str src, str fragment);
+bool str_startsWith(const str src, const str fragment);
+bool str_endsWith(const str src, const str fragment);
 
 /// @brief calculates string hash using sdbm32 algorythm (something like lightweight crc32)
 /// @return non-cryptografic hash of the string
-u32 str_hash32(str s);
+u32 str_hash32(const str s);
 
-str str_toUpper(str src);
-str str_toLower(str src);
+str str_toUpper(const str src);
+str str_toLower(const str src);
 
 str hex_to_str(Array(u8) buf, bool uppercase);
 
diff --git a/src/collections/List.c b/src/collections/List.c
index 18dffaa..c04f242 100755
--- a/src/collections/List.c
+++ b/src/collections/List.c
@@ -7,12 +7,19 @@ List_ List_alloc_size(u32 initial_size){
     return List_construct_size(malloc(allocated_size), 0, allocated_size);
 }
 
-void* List_expand_size(List_* ptr, u32 expansion_size){
+List_ List_copy(const List_ src){
+    List_ copy = List_alloc_size(src.allocated_size);
+    if(copy.data != NULL)
+        memcpy(copy.data, src.data, src.size);
+    return copy;
+}
+
+void* List_expand_size(List_* ptr, u32 size_to_add){
     u32 occupied_size = ptr->size;
     u32 expanded_alloc_size = ptr->allocated_size;
     if(expanded_alloc_size == 0)
         expanded_alloc_size = 64;
-    ptr->size += expansion_size;
+    ptr->size += size_to_add;
     while(ptr->size > expanded_alloc_size){
         expanded_alloc_size *= 2;
     }
diff --git a/src/string/StringBuilder.c b/src/string/StringBuilder.c
index 79ffed9..5de448e 100755
--- a/src/string/StringBuilder.c
+++ b/src/string/StringBuilder.c
@@ -5,16 +5,24 @@ void StringBuilder_destroy(StringBuilder* b){
         return;
 
     free(b->buffer.data);
-    b->buffer = List_construct_size(NULL, 0, 0);
 }
 
 str StringBuilder_getStr(StringBuilder* b){
-    if(b->buffer.size == 0 || ((char*)b->buffer.data)[b->buffer.size - 1] != '\0')
+    if(b->buffer.size == 0 || ((char*)b->buffer.data)[b->buffer.size - 1] != '\0'){
         List_push(&b->buffer, u8, '\0');
-    str result = str_construct((char*)b->buffer.data, b->buffer.size - 1, true);
+        // '\0' at the end doesn't increase buffer.size
+        b->buffer.size -= 1;
+    }
+    str result = str_construct((char*)b->buffer.data, b->buffer.size, true);
     return result;
 }
 
+bool StringBuilder_equals(const StringBuilder* a, const StringBuilder* b){
+    str a_str = str_construct((char*)a->buffer.data, a->buffer.size, false);
+    str b_str = str_construct((char*)b->buffer.data, b->buffer.size, false);
+    return str_equals(a_str, b_str);
+}
+
 void StringBuilder_removeFromEnd(StringBuilder* b, u32 count){
     if(count < b->buffer.size){
         b->buffer.size -= count;
diff --git a/src/string/str.c b/src/string/str.c
index 26b58c9..b0bee62 100755
--- a/src/string/str.c
+++ b/src/string/str.c
@@ -1,19 +1,21 @@
 #include "tlibc/string/str.h"
 #include "tlibc/string/StringBuilder.h"
 
-str str_copy(str src){
-    if(src.data == NULL || src.size == 0)
-        return src;
+str str_copy(const str self){
+    if(self.data == NULL || self.size == 0)
+        return self;
     
-    str nstr = str_construct((char*)malloc(src.size + 1), src.size, true);
-    memcpy(nstr.data, src.data, src.size);
-    nstr.data[nstr.size] = '\0';
-    return nstr;
+    str copy = str_construct((char*)malloc(self.size + 1), self.size, true);
+    memcpy(copy.data, self.data, self.size);
+    copy.data[copy.size] = '\0';
+    return copy;
 }
 
-bool str_equals(str s0, str s1){
-    if(s0.size != s1.size)
+bool str_equals(const str self, const str other){
+    if(self.size != other.size)
         return false;
+    if(self.data == other.data)
+        return true;
     /* 
     BENCHMARK:
         str_equals64:   2.967s
@@ -21,7 +23,7 @@ bool str_equals(str s0, str s1){
         strncmp:        1.611s
         memcmp:         0.710s
     */
-    return memcmp(s0.data, s1.data, s0.size) == 0;
+    return memcmp(self.data, other.data, self.size) == 0;
 }
 
 str str_reverse(str s){
@@ -34,7 +36,7 @@ str str_reverse(str s){
     return r;
 }
 
-i32 str_seek(str src, str fragment, u32 startIndex){
+i32 str_seek(const str src, const str fragment, u32 startIndex){
     if(src.size == 0 || fragment.size == 0)
         return -1;
     
@@ -49,7 +51,7 @@ i32 str_seek(str src, str fragment, u32 startIndex){
     return -1;
 }
 
-i32 str_seekReverse(str src, str fragment, u32 startIndex){
+i32 str_seekReverse(const str src, const str fragment, u32 startIndex){
     if(src.size == 0 || fragment.size == 0)
         return -1;
     
@@ -66,7 +68,7 @@ i32 str_seekReverse(str src, str fragment, u32 startIndex){
     return -1;
 }
 
-i32 str_seekChar(str src, char c, u32 startIndex){
+i32 str_seekChar(const str src, char c, u32 startIndex){
     for(u32 i = startIndex; i < src.size; i++){
         if(src.data[i] == c)
             return i;
@@ -74,7 +76,7 @@ i32 str_seekChar(str src, char c, u32 startIndex){
     return -1;
 }
 
-i32 str_seekCharReverse(str src, char c, u32 startIndex){
+i32 str_seekCharReverse(const str src, char c, u32 startIndex){
     if(startIndex > src.size - 1)
         startIndex = src.size - 1;
     for(u32 i = startIndex; i != (u32)-1; i--){
@@ -84,24 +86,27 @@ i32 str_seekCharReverse(str src, char c, u32 startIndex){
     return -1;
 }
 
-bool str_startsWith(str src, str fragment){
+bool str_startsWith(const str src, const str fragment){
     if(src.size < fragment.size)
         return false;
     
-    src.size = fragment.size;
-    return str_equals(src, fragment);
+    str src_fragment = str_null;
+    src_fragment.data = src.data;
+    src_fragment.size = fragment.size;
+    return str_equals(src_fragment, fragment);
 }
 
-bool str_endsWith(str src, str fragment){
+bool str_endsWith(const str src, const str fragment){
     if(src.size < fragment.size)
         return false;
     
-    src.data = (char*)(src.data + src.size - fragment.size);
-    src.size = fragment.size;
-    return str_equals(src, fragment);
+    str src_fragment = str_null;
+    src_fragment.data = (char*)(src.data + src.size - fragment.size);
+    src_fragment.size = fragment.size;
+    return str_equals(src_fragment, fragment);
 }
 
-u32 str_hash32(str s){
+u32 str_hash32(const str s){
     u8* ubuf = (u8*)s.data;
     u32 hash=0;
     for (u32 i = 0; i < s.size; i++)
@@ -109,7 +114,7 @@ u32 str_hash32(str s){
     return hash;
 }
 
-str str_toUpper(str src){
+str str_toUpper(const str src){
     str r = str_copy(src);
     for (u32 i = 0; i < r.size; i++){
         if(isAlphabeticalLower(r.data[i]))
@@ -118,7 +123,7 @@ str str_toUpper(str src){
     return r;
 }
 
-str str_toLower(str src){
+str str_toLower(const str src){
     str r = str_copy(src);
     for (u32 i = 0; i < r.size; i++){
         if(isAlphabeticalUpper(r.data[i]))