chore: apply review comment

Author: quake

json/internal_types.mbt

@@ -17,13 +17,17 @@ priv struct ParseContext {
   mut offset : Int
   input : String
   end_offset : Int
-  max_nesting_depth : Int
-  mut depth : Int
+  mut remaining_available_depth : Int
 }
 
 ///|
 fn ParseContext::make(input : String, max_nesting_depth : Int) -> ParseContext {
-  { offset: 0, input, end_offset: input.length(), max_nesting_depth, depth: 0 }
+  {
+    offset: 0,
+    input,
+    end_offset: input.length(),
+    remaining_available_depth: max_nesting_depth,
+  }
 }
 
 ///|

json/json_test.mbt

@@ -538,5 +538,16 @@ test "call stack overflow" {
     json_string.write_char(']')
   }
   let _ = @json.parse(json_string.to_string()) catch { _ => null }
+  let depth = 1500
+  let json_string = StringBuilder::new()
+  for _ in 0..=depth {
+    json_string.write_char('[')
+  }
+  for _ in 0..=depth {
+    json_string.write_char(']')
+  }
+  let _ = @json.parse(json_string.to_string(), max_nesting_depth=2000) catch {
+    _ => null
+  }
 
 }

json/parse.mbt

@@ -23,6 +23,7 @@ pub fn valid(input : String) -> Bool {
 }
 
 ///|
+/// Parse a JSON input string into a Json value, with an optional maximum nesting depth (default is 1024)
 pub fn parse(
   input : String,
   max_nesting_depth? : Int = 1024,
@@ -62,14 +63,14 @@ fn ParseContext::parse_value2(
 
 ///|
 fn ParseContext::parse_object(ctx : ParseContext) -> Json raise ParseError {
-  if ctx.depth >= ctx.max_nesting_depth {
+  if ctx.remaining_available_depth <= 0 {
     raise DepthLimitExceeded
   }
-  ctx.depth += 1
+  ctx.remaining_available_depth -= 1
   let map = Map::new()
   loop ctx.lex_property_name() {
     RBrace => {
-      ctx.depth -= 1
+      ctx.remaining_available_depth += 1
       Json::object(map)
     }
     String(name) => {
@@ -78,7 +79,7 @@ fn ParseContext::parse_object(ctx : ParseContext) -> Json raise ParseError {
       match ctx.lex_after_object_value() {
         Comma => continue ctx.lex_property_name2()
         RBrace => {
-          ctx.depth -= 1
+          ctx.remaining_available_depth += 1
           Json::object(map)
         }
         _ => abort("unreachable")
@@ -90,14 +91,14 @@ fn ParseContext::parse_object(ctx : ParseContext) -> Json raise ParseError {
 
 ///|
 fn ParseContext::parse_array(ctx : ParseContext) -> Json raise ParseError {
-  if ctx.depth >= ctx.max_nesting_depth {
+  if ctx.remaining_available_depth <= 0 {
     raise DepthLimitExceeded
   }
-  ctx.depth += 1
+  ctx.remaining_available_depth -= 1
   let vec = []
   loop ctx.lex_value(allow_rbracket=true) {
     RBracket => {
-      ctx.depth -= 1
+      ctx.remaining_available_depth += 1
       Json::array(vec)
     }
     tok => {
@@ -106,7 +107,7 @@ fn ParseContext::parse_array(ctx : ParseContext) -> Json raise ParseError {
       match tok2 {
         Comma => continue ctx.lex_value(allow_rbracket=false)
         RBracket => {
-          ctx.depth -= 1
+          ctx.remaining_available_depth += 1
           Json::array(vec)
         }
         _ => abort("unreachable")

json/types.mbt

@@ -53,7 +53,10 @@ pub impl Show for ParseError with output(self, logger) {
       ..write_object(line)
       ..write_string(", column ")
       ..write_object(column)
-    DepthLimitExceeded => logger.write_string("Depth limit exceeded, please increase the max_nesting_depth parameter")
+    DepthLimitExceeded =>
+      logger.write_string(
+        "Depth limit exceeded, please increase the max_nesting_depth parameter",
+      )
   }
 }