feat(tazjin/rlox): Implement variable depth resolver r/2113

Implements the first part of the resolver from
https://craftinginterpreters.com/resolving-and-binding.html

This is wired up to the execution paths in main, but not yet in the
tests. The resolved depth is also not actually used for variable
lookups (yet).

Change-Id: I3a8615252b7b9b12d5a290c5ddf85988f61b9184
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2403
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

1 files changed, 193 insertions, 0 deletions

diff --git a/users/tazjin/rlox/src/resolver.rs b/users/tazjin/rlox/src/resolver.rs
new file mode 100644
index 000000000000..f5ef91b99a19
--- /dev/null
+++ b/users/tazjin/rlox/src/resolver.rs
@@ -0,0 +1,193 @@
+// Resolves variable access to their specific instances in the
+// environment chain.
+//
+// https://craftinginterpreters.com/resolving-and-binding.html
+
+use std::collections::HashMap;
+use std::rc::Rc;
+
+use crate::errors::{Error, ErrorKind};
+use crate::parser::{self, Expr, Statement};
+use crate::scanner::Token;
+
+#[derive(Default)]
+struct Resolver<'a> {
+    scopes: Vec<HashMap<&'a str, bool>>,
+}
+
+impl<'a> Resolver<'a> {
+    // AST traversal
+    fn resolve(&mut self, program: &'a mut parser::Block) -> Result<(), Error> {
+        self.begin_scope();
+        for stmt in program {
+            self.resolve_stmt(stmt)?;
+        }
+        self.end_scope();
+
+        Ok(())
+    }
+
+    fn resolve_stmt(&mut self, stmt: &'a mut Statement) -> Result<(), Error> {
+        match stmt {
+            Statement::Expr(expr) => self.resolve_expr(expr),
+            Statement::Print(expr) => self.resolve_expr(expr),
+            Statement::Var(var) => self.resolve_var(var),
+            Statement::Return(ret) => self.resolve_expr(&mut ret.value),
+            Statement::Block(block) => self.resolve(block),
+
+            Statement::If(if_stmt) => {
+                self.resolve_expr(&mut if_stmt.condition)?;
+                self.resolve_stmt(&mut if_stmt.then_branch)?;
+
+                if let Some(branch) = if_stmt.else_branch.as_mut() {
+                    self.resolve_stmt(branch)?;
+                }
+
+                Ok(())
+            }
+
+            Statement::While(while_stmt) => {
+                self.resolve_expr(&mut while_stmt.condition)?;
+                self.resolve_stmt(&mut while_stmt.body)
+            }
+
+            Statement::Function(func) => match Rc::get_mut(func) {
+                Some(func) => self.resolve_function(func),
+                // The resolver does not clone references, so unless
+                // the interpreter is called before the resolver this
+                // case should never happen.
+                None => {
+                    return Err(Error {
+                        line: 0,
+                        kind: ErrorKind::InternalError(
+                            "multiple function references before interpretation".into(),
+                        ),
+                    })
+                }
+            },
+        }
+    }
+
+    fn resolve_var(&mut self, var: &'a mut parser::Var) -> Result<(), Error> {
+        self.declare(&var.name);
+
+        if let Some(init) = &mut var.initialiser {
+            self.resolve_expr(init)?;
+        }
+
+        self.define(&var.name);
+
+        Ok(())
+    }
+
+    fn resolve_function(&mut self, func: &'a mut parser::Function) -> Result<(), Error> {
+        self.declare(&func.name);
+        self.define(&func.name);
+
+        self.begin_scope();
+
+        for param in &func.params {
+            self.declare(param);
+            self.define(param);
+        }
+
+        for stmt in &mut func.body {
+            self.resolve_stmt(stmt)?;
+        }
+
+        self.end_scope();
+
+        Ok(())
+    }
+
+    fn resolve_expr(&mut self, expr: &'a mut Expr) -> Result<(), Error> {
+        match expr {
+            Expr::Variable(var) => self.resolve_variable(var),
+            Expr::Assign(assign) => self.resolve_assign(assign),
+            Expr::Grouping(grouping) => self.resolve_expr(&mut grouping.0),
+            Expr::Call(call) => self.resolve_call(call),
+            Expr::Literal(_) => Ok(()),
+            Expr::Unary(unary) => self.resolve_expr(&mut unary.right),
+
+            Expr::Logical(log) => {
+                self.resolve_expr(&mut log.left)?;
+                self.resolve_expr(&mut log.right)
+            }
+
+            Expr::Binary(binary) => {
+                self.resolve_expr(&mut binary.left)?;
+                self.resolve_expr(&mut binary.right)
+            }
+        }
+    }
+
+    fn resolve_variable(&mut self, var: &'a mut parser::Variable) -> Result<(), Error> {
+        if let Some(scope) = self.scopes.last_mut() {
+            if let Some(false) = scope.get(var.name.lexeme.as_str()) {
+                return Err(Error {
+                    line: var.name.line,
+                    kind: ErrorKind::StaticError(
+                        "can't read local variable in its own initialiser".into(),
+                    ),
+                });
+            }
+        }
+
+        var.depth = self.resolve_local(&var.name);
+        Ok(())
+    }
+
+    fn resolve_assign(&mut self, assign: &'a mut parser::Assign) -> Result<(), Error> {
+        self.resolve_expr(&mut assign.value)?;
+        assign.depth = self.resolve_local(&assign.name);
+        Ok(())
+    }
+
+    fn resolve_local(&mut self, name: &'a Token) -> Option<usize> {
+        for (c, scope) in self.scopes.iter().rev().enumerate() {
+            if scope.contains_key(name.lexeme.as_str()) {
+                return Some(c);
+            }
+        }
+
+        None
+    }
+
+    fn resolve_call(&mut self, call: &'a mut parser::Call) -> Result<(), Error> {
+        self.resolve_expr(&mut call.callee)?;
+
+        for arg in call.args.iter_mut() {
+            self.resolve_expr(arg)?;
+        }
+
+        Ok(())
+    }
+
+    // Internal helpers
+
+    fn declare(&mut self, name: &'a Token) {
+        if let Some(scope) = self.scopes.last_mut() {
+            scope.insert(&name.lexeme, false);
+        }
+    }
+
+    fn define(&mut self, name: &'a Token) {
+        if let Some(scope) = self.scopes.last_mut() {
+            scope.insert(&name.lexeme, true);
+        }
+    }
+
+    fn begin_scope(&mut self) {
+        self.scopes.push(Default::default());
+    }
+
+    fn end_scope(&mut self) {
+        self.scopes.pop();
+    }
+}
+
+pub fn resolve(mut block: parser::Block) -> Result<parser::Block, Error> {
+    let mut resolver: Resolver = Default::default();
+    resolver.resolve(&mut block)?;
+    Ok(block)
+}