[Core] Optimize some `String` methods

Avoids unnecessary COW checks by using pointers directly.

core/string/ustring.cpp

@@ -1658,30 +1658,40 @@ char32_t String::char_lowercase(char32_t p_char) {
 }
 
 String String::to_upper() const {
-	String upper = *this;
-
-	for (int i = 0; i < upper.size(); i++) {
-		const char32_t s = upper[i];
-		const char32_t t = _find_upper(s);
-		if (s != t) { // avoid copy on write
-			upper[i] = t;
-		}
+	if (is_empty()) {
+		return *this;
 	}
 
+	String upper;
+	upper.resize(size());
+	const char32_t *old_ptr = ptr();
+	char32_t *upper_ptrw = upper.ptrw();
+
+	while (*old_ptr) {
+		*upper_ptrw++ = _find_upper(*old_ptr++);
+	}
+
+	*upper_ptrw = 0;
+
 	return upper;
 }
 
 String String::to_lower() const {
-	String lower = *this;
-
-	for (int i = 0; i < lower.size(); i++) {
-		const char32_t s = lower[i];
-		const char32_t t = _find_lower(s);
-		if (s != t) { // avoid copy on write
-			lower[i] = t;
-		}
+	if (is_empty()) {
+		return *this;
 	}
 
+	String lower;
+	lower.resize(size());
+	const char32_t *old_ptr = ptr();
+	char32_t *lower_ptrw = lower.ptrw();
+
+	while (*old_ptr) {
+		*lower_ptrw++ = _find_lower(*old_ptr++);
+	}
+
+	*lower_ptrw = 0;
+
 	return lower;
 }
 
@@ -1919,15 +1929,16 @@ String String::hex_encode_buffer(const uint8_t *p_buffer, int p_len) {
 	static const char hex[16] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };
 
 	String ret;
-	char v[2] = { 0, 0 };
+	ret.resize(p_len * 2 + 1);
+	char32_t *ret_ptrw = ret.ptrw();
 
 	for (int i = 0; i < p_len; i++) {
-		v[0] = hex[p_buffer[i] >> 4];
-		ret += v;
-		v[0] = hex[p_buffer[i] & 0xF];
-		ret += v;
+		*ret_ptrw++ = hex[p_buffer[i] >> 4];
+		*ret_ptrw++ = hex[p_buffer[i] & 0xF];
 	}
 
+	*ret_ptrw = 0;
+
 	return ret;
 }
 
@@ -1950,11 +1961,12 @@ Vector<uint8_t> String::hex_decode() const {
 	Vector<uint8_t> out;
 	int len = length() / 2;
 	out.resize(len);
+	uint8_t *out_ptrw = out.ptrw();
 	for (int i = 0; i < len; i++) {
 		char32_t c;
 		HEX_TO_BYTE(first, i * 2);
 		HEX_TO_BYTE(second, i * 2 + 1);
-		out.write[i] = first * 16 + second;
+		out_ptrw[i] = first * 16 + second;
 	}
 	return out;
 #undef HEX_TO_BYTE
@@ -1975,14 +1987,16 @@ CharString String::ascii(bool p_allow_extended) const {
 
 	CharString cs;
 	cs.resize(size());
+	char *cs_ptrw = cs.ptrw();
+	const char32_t *this_ptr = ptr();
 
 	for (int i = 0; i < size(); i++) {
-		char32_t c = operator[](i);
+		char32_t c = this_ptr[i];
 		if ((c <= 0x7f) || (c <= 0xff && p_allow_extended)) {
-			cs[i] = c;
+			cs_ptrw[i] = c;
 		} else {
 			print_unicode_error(vformat("Invalid unicode codepoint (%x), cannot represent as ASCII/Latin-1", (uint32_t)c));
-			cs[i] = 0x20; // ascii doesn't have a replacement character like unicode, 0x1a is sometimes used but is kinda arcane
+			cs_ptrw[i] = 0x20; // ASCII doesn't have a replacement character like unicode, 0x1a is sometimes used but is kinda arcane.
 		}
 	}
 
@@ -3115,8 +3129,9 @@ Vector<uint8_t> String::md5_buffer() const {
 
 	Vector<uint8_t> ret;
 	ret.resize(16);
+	uint8_t *ret_ptrw = ret.ptrw();
 	for (int i = 0; i < 16; i++) {
-		ret.write[i] = hash[i];
+		ret_ptrw[i] = hash[i];
 	}
 	return ret;
 }
@@ -3128,8 +3143,9 @@ Vector<uint8_t> String::sha1_buffer() const {
 
 	Vector<uint8_t> ret;
 	ret.resize(20);
+	uint8_t *ret_ptrw = ret.ptrw();
 	for (int i = 0; i < 20; i++) {
-		ret.write[i] = hash[i];
+		ret_ptrw[i] = hash[i];
 	}
 
 	return ret;
@@ -3142,8 +3158,9 @@ Vector<uint8_t> String::sha256_buffer() const {
 
 	Vector<uint8_t> ret;
 	ret.resize(32);
+	uint8_t *ret_ptrw = ret.ptrw();
 	for (int i = 0; i < 32; i++) {
-		ret.write[i] = hash[i];
+		ret_ptrw[i] = hash[i];
 	}
 	return ret;
 }
@@ -3871,8 +3888,9 @@ Vector<String> String::bigrams() const {
 		return b;
 	}
 	b.resize(n_pairs);
+	String *b_ptrw = b.ptrw();
 	for (int i = 0; i < n_pairs; i++) {
-		b.write[i] = substr(i, 2);
+		b_ptrw[i] = substr(i, 2);
 	}
 	return b;
 }
@@ -4757,8 +4775,9 @@ String String::xml_unescape() const {
 		return String();
 	}
 	str.resize(len + 1);
-	_xml_unescape(get_data(), l, str.ptrw());
-	str[len] = 0;
+	char32_t *str_ptrw = str.ptrw();
+	_xml_unescape(get_data(), l, str_ptrw);
+	str_ptrw[len] = 0;
 	return str;
 }