Delete global constant pool from the parser

docs/config.md

@@ -0,0 +1,171 @@
+# config.yml
+
+`config.yml` is the definition of data structure for parser results -- AST.
+
+It defines the data structure for the parser implementation in C and Rust `node` modules.
+
+## C parser code
+
+```sh
+$ rake templates
+```
+
+Our C parser consists of two components:
+
+1. Pure C Parser
+2. Translator from the pure C AST to Ruby object
+
+`config.yml` defines the AST for pure C parser in `ast.h`/`ast.c` and translator from the C AST to Ruby objects in `ast_translation.h`/`ast_translation.c`.
+
+## `ruby-rbs` crate
+
+```sh
+$ cd rust; cargo build
+```
+
+The `build.rs` in `ruby-rbs` crate defines the data structure derived from `config.yml` definitions under `node` module.
+
+## nodes
+
+`nodes` defines *node* data types in C or Rust.
+
+```yaml
+nodes:
+  - name: RBS::AST::Declarations::Class
+    rust_name: ClassNode
+    fields:
+      - name: name
+        c_type: rbs_type_name
+      - name: type_params
+        c_type: rbs_node_list
+      - name: super_class
+        c_type: rbs_ast_declarations_class_super
+        optional: true  # NULL when no superclass (e.g., `class Foo end` vs `class Foo < Bar end`)
+      - name: members
+        c_type: rbs_node_list
+      - name: annotations
+        c_type: rbs_node_list
+      - name: comment
+        c_type: rbs_ast_comment
+        optional: true  # NULL when no comment precedes the declaration
+```
+
+This defines `rbs_ast_declarations_class` struct so that the parser constructs the AST using the structs.
+
+```c
+typedef struct rbs_ast_declarations_class {
+    rbs_node_t base;
+
+    struct rbs_type_name *name;
+    struct rbs_node_list *type_params;
+    struct rbs_ast_declarations_class_super *super_class; /* Optional */
+    struct rbs_node_list *members;
+    struct rbs_node_list *annotations;
+    struct rbs_ast_comment *comment; /* Optional */
+
+    rbs_location_range keyword_range;     /* Required */
+    rbs_location_range name_range;        /* Required */
+    rbs_location_range end_range;         /* Required */
+    rbs_location_range type_params_range; /* Optional */
+    rbs_location_range lt_range;          /* Optional */
+} rbs_ast_declarations_class_t;
+```
+
+The `rbs_ast_declarations_class` struct is a pure C AST, and `ast_translation.c` defines translation into a Ruby object of `RBS::AST::Declarations::Class` class.
+
+```c
+    case RBS_AST_DECLARATIONS_CLASS: {
+        rbs_ast_declarations_class_t *node = (rbs_ast_declarations_class_t *) instance;
+
+        VALUE h = rb_hash_new();
+        VALUE location = rbs_location_range_to_ruby_location(ctx, node->base.location);
+        rbs_loc *loc = rbs_check_location(location);
+        rbs_loc_legacy_alloc_children(loc, 5);
+        rbs_loc_legacy_add_required_child(loc, rb_intern("keyword"), (rbs_loc_range) { .start = node->keyword_range.start_char, .end = node->keyword_range.end_char });
+        rbs_loc_legacy_add_required_child(loc, rb_intern("name"), (rbs_loc_range) { .start = node->name_range.start_char, .end = node->name_range.end_char });
+        rbs_loc_legacy_add_required_child(loc, rb_intern("end"), (rbs_loc_range) { .start = node->end_range.start_char, .end = node->end_range.end_char });
+        rbs_loc_legacy_add_optional_child(loc, rb_intern("type_params"), (rbs_loc_range) { .start = node->type_params_range.start_char, .end = node->type_params_range.end_char });
+        rbs_loc_legacy_add_optional_child(loc, rb_intern("lt"), (rbs_loc_range) { .start = node->lt_range.start_char, .end = node->lt_range.end_char });
+        rb_hash_aset(h, ID2SYM(rb_intern("location")), location);
+        rb_hash_aset(h, ID2SYM(rb_intern("name")), rbs_struct_to_ruby_value(ctx, (rbs_node_t *) node->name)); // rbs_type_name
+        rb_hash_aset(h, ID2SYM(rb_intern("type_params")), rbs_node_list_to_ruby_array(ctx, node->type_params));
+        rb_hash_aset(h, ID2SYM(rb_intern("super_class")), rbs_struct_to_ruby_value(ctx, (rbs_node_t *) node->super_class)); // rbs_ast_declarations_class_super
+        rb_hash_aset(h, ID2SYM(rb_intern("members")), rbs_node_list_to_ruby_array(ctx, node->members));
+        rb_hash_aset(h, ID2SYM(rb_intern("annotations")), rbs_node_list_to_ruby_array(ctx, node->annotations));
+        rb_hash_aset(h, ID2SYM(rb_intern("comment")), rbs_struct_to_ruby_value(ctx, (rbs_node_t *) node->comment)); // rbs_ast_comment
+
+        rb_funcall(
+            RBS_AST_TypeParam,
+            rb_intern("resolve_variables"),
+            1,
+            rb_hash_lookup(h, ID2SYM(rb_intern("type_params")))
+        );
+        return CLASS_NEW_INSTANCE(
+            RBS_AST_Declarations_Class,
+            1,
+            &h
+        );
+    }
+```
+
+## enums
+
+`enums` defines *enum* data types in C or Rust.
+
+```yaml
+enums:
+  attribute_visibility:
+    optional: true
+    symbols:
+      - unspecified
+      - public
+      - private
+```
+
+For example, the `attribute_visibility` enum is a data type for `visibility` attribute of `attr_reader`, `attr_writer`, and `attr_accessor` definitions.
+The `visibility` attribute can be one of `unspecified`, `public`, and `private`.
+
+### Symbol enums
+
+Enum definition with `symbols:` attribute defines *enum* data that is mapped to Ruby symbols.
+
+```yaml
+enums:
+  attribute_visibility:
+    optional: true
+    symbols:
+      - unspecified
+      - public
+      - private
+```
+
+It defines an `enum` in C AST definition.
+
+```c
+enum RBS_ATTRIBUTE_VISIBILITY_TAG {
+    RBS_ATTRIBUTE_VISIBILITY_TAG_UNSPECIFIED,
+    RBS_ATTRIBUTE_VISIBILITY_TAG_PUBLIC,
+    RBS_ATTRIBUTE_VISIBILITY_TAG_PRIVATE,
+};
+```
+
+The C extension also defines a translation:
+
+```c
+VALUE rbs_attribute_visibility_to_ruby(enum rbs_attribute_visibility value) {
+    switch (value) {
+    case RBS_ATTRIBUTE_VISIBILITY_UNSPECIFIED:
+        return Qnil;
+    case RBS_ATTRIBUTE_VISIBILITY_PUBLIC:
+        return rb_id2sym(rb_intern("public"));
+    case RBS_ATTRIBUTE_VISIBILITY_PRIVATE:
+        return rb_id2sym(rb_intern("private"));
+    default:
+        rb_fatal("unknown enum rbs_attribute_visibility value: %d", value);
+    }
+}
+```
+
+`RBS_ATTRIBUTE_VISIBILITY_PUBLIC` and `RBS_ATTRIBUTE_VISIBILITY_PRIVATE` are translated to Ruby symbols `:public` and `:private` respectively. 
+
+Note that the first `RBS_ATTRIBUTE_VISIBILITY_UNSPECIFIED` is translated to `nil` in Ruby. This is specified by the `optional: true` attribute in YAML. When `optional: true` is set, the first enum value is translated to `nil`.