De-magic number parser::Output encoding

Author: Veykril

crates/parser/src/output.rs

@@ -31,47 +31,70 @@ pub enum Step<'a> {
 }
 
 impl Output {
+    const EVENT_MASK: u32 = 0b1;
+    const TAG_MASK: u32 = 0x0000_00F0;
+    const N_INPUT_TOKEN_MASK: u32 = 0x0000_FF00;
+    const KIND_MASK: u32 = 0xFFFF_0000;
+
+    const ERROR_SHIFT: u32 = Self::EVENT_MASK.trailing_ones();
+    const TAG_SHIFT: u32 = Self::TAG_MASK.trailing_zeros();
+    const N_INPUT_TOKEN_SHIFT: u32 = Self::N_INPUT_TOKEN_MASK.trailing_zeros();
+    const KIND_SHIFT: u32 = Self::KIND_MASK.trailing_zeros();
+
+    const TOKEN_EVENT: u8 = 0;
+    const ENTER_EVENT: u8 = 1;
+    const EXIT_EVENT: u8 = 2;
+
     pub fn iter(&self) -> impl Iterator<Item = Step<'_>> {
         self.event.iter().map(|&event| {
-            if event & 0b1 == 0 {
-                return Step::Error { msg: self.error[(event as usize) >> 1].as_str() };
+            if event & Self::EVENT_MASK == 0 {
+                return Step::Error {
+                    msg: self.error[(event as usize) >> Self::ERROR_SHIFT].as_str(),
+                };
             }
-            let tag = ((event & 0x0000_00F0) >> 4) as u8;
+            let tag = ((event & Self::TAG_MASK) >> Self::TAG_SHIFT) as u8;
             match tag {
-                0 => {
-                    let kind: SyntaxKind = (((event & 0xFFFF_0000) >> 16) as u16).into();
-                    let n_input_tokens = ((event & 0x0000_FF00) >> 8) as u8;
+                Self::TOKEN_EVENT => {
+                    let kind: SyntaxKind =
+                        (((event & Self::KIND_MASK) >> Self::KIND_SHIFT) as u16).into();
+                    let n_input_tokens =
+                        ((event & Self::N_INPUT_TOKEN_MASK) >> Self::N_INPUT_TOKEN_SHIFT) as u8;
                     Step::Token { kind, n_input_tokens }
                 }
-                1 => {
-                    let kind: SyntaxKind = (((event & 0xFFFF_0000) >> 16) as u16).into();
+                Self::ENTER_EVENT => {
+                    let kind: SyntaxKind =
+                        (((event & Self::KIND_MASK) >> Self::KIND_SHIFT) as u16).into();
                     Step::Enter { kind }
                 }
-                2 => Step::Exit,
+                Self::EXIT_EVENT => Step::Exit,
                 _ => unreachable!(),
             }
         })
     }
 
     pub(crate) fn token(&mut self, kind: SyntaxKind, n_tokens: u8) {
-        let e = ((kind as u16 as u32) << 16) | ((n_tokens as u32) << 8) | 1;
+        let e = ((kind as u16 as u32) << Self::KIND_SHIFT)
+            | ((n_tokens as u32) << Self::N_INPUT_TOKEN_SHIFT)
+            | Self::EVENT_MASK;
         self.event.push(e)
     }
 
     pub(crate) fn enter_node(&mut self, kind: SyntaxKind) {
-        let e = ((kind as u16 as u32) << 16) | (1 << 4) | 1;
+        let e = ((kind as u16 as u32) << Self::KIND_SHIFT)
+            | ((Self::ENTER_EVENT as u32) << Self::TAG_SHIFT)
+            | Self::EVENT_MASK;
         self.event.push(e)
     }
 
     pub(crate) fn leave_node(&mut self) {
-        let e = 2 << 4 | 1;
+        let e = (Self::EXIT_EVENT as u32) << Self::TAG_SHIFT | Self::EVENT_MASK;
         self.event.push(e)
     }
 
     pub(crate) fn error(&mut self, error: String) {
         let idx = self.error.len();
         self.error.push(error);
-        let e = (idx as u32) << 1;
+        let e = (idx as u32) << Self::ERROR_SHIFT;
         self.event.push(e);
     }
 }