use StringBuilder instead of Buffer in builtin and json package

builtin/assert.mbt

@@ -13,7 +13,7 @@
 // limitations under the License.
 
 fn debug_string[T : Show](t : T) -> String {
-  let buf = Buffer::new(size_hint=50)
+  let buf = StringBuilder::new(size_hint=50)
   t.output(buf)
   buf.to_string()
 }

builtin/autoloc.mbt

@@ -27,7 +27,7 @@ pub fn ArgsLoc::to_string(self : ArgsLoc) -> String {
 }
 
 pub fn ArgsLoc::to_json(self : ArgsLoc) -> String {
-  let buf = Buffer::new(size_hint=10)
+  let buf = StringBuilder::new(size_hint=10)
   buf.write_char('[')
   for i = 0; i < self._.length(); i = i + 1 {
     if i != 0 {

builtin/buffer.mbt

@@ -92,7 +92,7 @@ pub fn write_string(self : Buffer, value : String) -> Unit {
 }
 
 pub fn write_object(self : Buffer, value : Show) -> Unit {
-  write_string(self, value.to_string())
+  self.write_string(value.to_string())
 }
 
 pub fn write_bytes(self : Buffer, value : Bytes) -> Unit {

builtin/builtin.mbti

@@ -315,6 +315,15 @@ impl SourceLoc {
   to_string(Self) -> String
 }
 
+type StringBuilder
+impl StringBuilder {
+  new(~size_hint : Int = ..) -> Self
+  to_string(Self) -> String
+  write_char(Self, Char) -> Unit
+  write_string(Self, String) -> Unit
+  write_sub_string(Self, String, Int, Int) -> Unit
+}
+
 pub type UnsafeMaybeUninit
 impl Unit {
   op_equal(Unit, Unit) -> Bool
@@ -536,6 +545,7 @@ impl String {
   op_add(String, String) -> String
   op_equal(String, String) -> Bool
   op_get(String, Int) -> Char
+  substring(String, ~start : Int = .., ~end : Int = ..) -> String
   to_js_string(String) -> Js_string
   to_json(String) -> Json
   to_string(String) -> String

builtin/console.mbt

@@ -43,7 +43,7 @@ pub fn to_string(self : Int64) -> String {
   // The min and max value of i64 are -9223372036854775808 and 9223372036854775807,
   // so max=20 is enough.
 
-  let buf = Buffer::new(size_hint=20)
+  let buf = StringBuilder::new(size_hint=20)
   if self < 0L {
     buf.write_char('-')
   }
@@ -71,7 +71,7 @@ pub fn to_string(self : Int) -> String {
   // The min and max value of i32 are -2147483648 and 2147483647,
   // so max=11 is enough.
 
-  let buf = Buffer::new()
+  let buf = StringBuilder::new()
   if self < 0 {
     buf.write_char('-')
   }
@@ -88,7 +88,7 @@ pub fn to_string(self : Int) -> String {
 }
 
 pub fn UInt::to_string(self : UInt) -> String {
-  let buf = Buffer::new()
+  let buf = StringBuilder::new()
   fn write_digits(num) {
     let num2 = num / 10U
     if num2 != 0U {
@@ -108,7 +108,7 @@ test "UInt::to_string" {
 }
 
 pub fn UInt64::to_string(self : UInt64) -> String {
-  let buf = Buffer::new()
+  let buf = StringBuilder::new()
   fn write_digits(num : UInt64) {
     let num2 = num / 10UL
     if num2 != 0UL {

builtin/json.mbt

@@ -61,7 +61,7 @@ fn escape_json_string(str : String) -> String {
   }
 
   let len = str.length()
-  let buf = Buffer::new(size_hint=len)
+  let buf = StringBuilder::new(size_hint=len)
   for i in 0..<len {
     let c = str[i]
     match c {

builtin/linked_hash_map.mbt

@@ -325,7 +325,7 @@ fn calc_grow_threshold(capacity : Int) -> Int {
 // Utils
 
 fn debug_entries[K : Show, V : Show](self : Map[K, V]) -> String {
-  let buf = Buffer::new()
+  let buf = StringBuilder::new()
   for i = 0; i < self.entries.length(); i = i + 1 {
     if i > 0 {
       buf.write_char(',')

builtin/moon.pkg.json

@@ -18,6 +18,8 @@
     "int64_nonjs.mbt": ["not", "js"],
     "arraycore_js.mbt": ["js"],
     "arraycore_nonjs.mbt": ["not", "js"],
-    "array_nonjs_test.mbt": ["not", "js"]
+    "array_nonjs_test.mbt": ["not", "js"],
+    "stringbuilder_buffer.mbt": ["not", ["js"]],
+    "stringbuilder_concat.mbt": ["js"]
   }
 }

builtin/show.mbt

@@ -60,7 +60,7 @@ pub impl Show for Bytes with output(self, logger) {
 }
 
 pub impl Show for Bytes with to_string(self) {
-  let buf = Buffer::new()
+  let buf = StringBuilder::new()
   Show::output(self, buf)
   buf.to_string()
 }
@@ -128,7 +128,7 @@ pub impl Show for String with to_string(self) { self }
 /// Returns a valid MoonBit string literal representation of a string,
 /// add quotes and escape special characters.
 pub fn escape(self : String) -> String {
-  let buf = Buffer::new()
+  let buf = StringBuilder::new()
   Show::output(self, buf)
   buf.to_string()
 }

builtin/string.mbt

@@ -0,0 +1,44 @@
+// Copyright 2024 International Digital Economy Academy
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/// @intrinsic %string.substring
+fn unsafe_substring(str : String, start : Int, end : Int) -> String {
+  let len = end - start
+  let buf = Buffer::new(size_hint=len)
+  buf.write_sub_string(str, start, len)
+  buf.to_string()
+}
+
+/// Get substring of the string from [start] to [end] exclusive.
+pub fn substring(
+  self : String,
+  ~start : Int = 0,
+  ~end : Int = self.length()
+) -> String {
+  if start < 0 {
+    abort("String::substring: start index is negative")
+  }
+  if end < 0 {
+    abort("String::substring: end index is negative")
+  }
+  if start > end {
+    abort("String::substring: start index is greater than end index")
+  }
+  if end > self.length() {
+    abort(
+      "String::substring: end index is greater than the length of the string",
+    )
+  }
+  unsafe_substring(self, start, end)
+}

builtin/string_test.mbt

@@ -38,3 +38,26 @@ test "String::escape" {
     ,
   )
 }
+
+test "substring" {
+  assert_eq!("abc".substring(), "abc")
+  assert_eq!("abc".substring(start=1), "bc")
+  assert_eq!("abc".substring(end=2), "ab")
+  assert_eq!("abc".substring(start=1, end=2), "b")
+}
+
+test "panic substring_start_index_error" {
+  "test".substring(start=-1, end=0) |> ignore
+}
+
+test "panic substring_end_index_error" {
+  "test".substring(start=0, end=-1) |> ignore
+}
+
+test "panic substring_start_end_index_error" {
+  "test".substring(start=1, end=0) |> ignore
+}
+
+test "panic substring_length_index_error" {
+  "test".substring(start=0, end=5) |> ignore
+}

builtin/stringbuilder_buffer.mbt

@@ -0,0 +1,40 @@
+// Copyright 2024 International Digital Economy Academy
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+type StringBuilder Buffer
+
+pub fn StringBuilder::new(~size_hint : Int = 0) -> StringBuilder {
+  Buffer::new(~size_hint)
+}
+
+pub fn StringBuilder::write_string(self : StringBuilder, str : String) -> Unit {
+  self._.write_string(str)
+}
+
+pub fn StringBuilder::write_char(self : StringBuilder, ch : Char) -> Unit {
+  self._.write_char(ch)
+}
+
+pub fn StringBuilder::write_sub_string(
+  self : StringBuilder,
+  str : String,
+  start : Int,
+  len : Int
+) -> Unit {
+  self._.write_sub_string(str, start, len)
+}
+
+pub fn StringBuilder::to_string(self : StringBuilder) -> String {
+  self._.to_string()
+}

builtin/stringbuilder_concat.mbt

@@ -0,0 +1,41 @@
+// Copyright 2024 International Digital Economy Academy
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+type StringBuilder Ref[String]
+
+pub fn StringBuilder::new(~size_hint : Int = 0) -> StringBuilder {
+  ignore(size_hint)
+  { val: "" }
+}
+
+pub fn StringBuilder::write_string(self : StringBuilder, str : String) -> Unit {
+  self.val += str
+}
+
+pub fn StringBuilder::write_char(self : StringBuilder, ch : Char) -> Unit {
+  self.val += Char::to_string(ch)
+}
+
+pub fn StringBuilder::write_sub_string(
+  self : StringBuilder,
+  str : String,
+  start : Int,
+  len : Int
+) -> Unit {
+  self.val += str.substring(~start, end=start + len)
+}
+
+pub fn StringBuilder::to_string(self : StringBuilder) -> String {
+  self.val
+}

builtin/stringbuilder_test.mbt

@@ -0,0 +1,21 @@
+// Copyright 2024 International Digital Economy Academy
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+test "stringbuilder" {
+  let buf = StringBuilder::new()
+  buf.write_string("hello")
+  buf.write_char(' ')
+  buf.write_sub_string("world", 0, 3)
+  inspect!(buf.to_string(), content="hello wor")
+}

builtin/traits.mbt

@@ -74,7 +74,7 @@ pub trait Show {
 
 // Default implementation for `Show::to_string`, uses a `Buffer`
 impl Show with to_string(self) {
-  let logger = Buffer::new()
+  let logger = StringBuilder::new()
   self.output(logger)
   logger.to_string()
 }

json/internal_types.mbt

@@ -61,40 +61,3 @@ priv enum Token {
   Comma
   Colon
 } derive(Eq, Show)
-
-priv struct StringBuilder {
-  mut buffer : String
-}
-
-fn StringBuilder::make() -> StringBuilder {
-  { buffer: "" }
-}
-
-fn add_string(self : StringBuilder, s : String) -> Unit {
-  self.buffer = self.buffer + s
-}
-
-fn add_substring(
-  self : StringBuilder,
-  s : String,
-  start : Int,
-  end : Int
-) -> Unit {
-  self.buffer = self.buffer + s.substring(~start, ~end)
-}
-
-fn add_char(self : StringBuilder, c : Char) -> Unit {
-  self.buffer = self.buffer + c.to_string()
-}
-
-fn to_string(self : StringBuilder) -> String {
-  self.buffer
-}
-
-test "add_string method coverage" {
-  let sb = StringBuilder::make()
-  add_string(sb, "Hello")
-  assert_eq!(sb.buffer, "Hello")
-  add_string(sb, " World")
-  assert_eq!(sb.buffer, "Hello World")
-}

json/json.mbt

@@ -96,7 +96,7 @@ pub fn stringify(
         if members.is_empty() {
           return "{}"
         }
-        let buf = Buffer::new(size_hint=0)
+        let buf = StringBuilder::new(size_hint=0)
         buf.write_char('{')
         buf.write_string(indent_str(level + 1, indent))
         let mut first = true
@@ -125,7 +125,7 @@ pub fn stringify(
         if arr.is_empty() {
           return "[]"
         }
-        let buf = Buffer::new(size_hint=0)
+        let buf = StringBuilder::new(size_hint=0)
         buf.write_char('[')
         buf.write_string(indent_str(level + 1, indent))
         for i, v in arr {
@@ -139,7 +139,7 @@ pub fn stringify(
         buf..write_char(']').to_string()
       }
       String(s) => {
-        let buf = Buffer::new(size_hint=0)
+        let buf = StringBuilder::new(size_hint=0)
         buf
         ..write_char('\"')
         ..write_string(escape(s, ~escape_slash))
@@ -165,7 +165,7 @@ fn escape(str : String, ~escape_slash : Bool) -> String {
     }
   }
 
-  let buf = Buffer::new(size_hint=str.length())
+  let buf = StringBuilder::new(size_hint=str.length())
   for c in str {
     match c {
       '"' | '\\' => buf.write_string("\\\{c}")