From 2f7f809fd6f943b31d0db9c82186ed23edcb5e20 Mon Sep 17 00:00:00 2001
From: Andrew Dirksen <andrew@dirksen.com>
Date: Mon, 19 Nov 2018 15:59:23 -0800
Subject: [PATCH] make interpreter reusable

---
 src/func.rs   | 12 ++++--------
 src/lib.rs    |  1 +
 src/runner.rs | 27 ++++++++++++++++++++++++---
 3 files changed, 29 insertions(+), 11 deletions(-)

diff --git a/src/func.rs b/src/func.rs
index 8a274c1..bf36b49 100644
--- a/src/func.rs
+++ b/src/func.rs
@@ -7,10 +7,7 @@ use host::Externals;
 use isa;
 use module::ModuleInstance;
 use parity_wasm::elements::Local;
-use runner::{
-	check_function_args, FunctionContext, Interpreter, InterpreterState, RuntimeValueInternal,
-	DEFAULT_CALL_STACK_LIMIT, DEFAULT_VALUE_STACK_LIMIT,
-};
+use runner::{check_function_args, Interpreter, InterpreterState, DEFAULT_CALL_STACK_LIMIT, DEFAULT_VALUE_STACK_LIMIT};
 use types::ValueType;
 use value::RuntimeValue;
 use {Signature, Trap};
@@ -136,13 +133,11 @@ impl FuncInstance {
 		func: &FuncRef,
 		args: &[RuntimeValue],
 		externals: &mut E,
-		value_stack: StackWithLimit<RuntimeValueInternal>,
-		call_stack: StackWithLimit<FunctionContext>,
+		interpreter: &mut Interpreter,
 	) -> Result<Option<RuntimeValue>, Trap> {
 		check_function_args(func.signature(), &args)?;
 		match *func.as_internal() {
 			FuncInstanceInternal::Internal { .. } => {
-				let mut interpreter = Interpreter::new(value_stack, call_stack);
 				interpreter.start_execution(externals, func, args, func.signature().return_type())
 			}
 			FuncInstanceInternal::Host {
@@ -167,7 +162,8 @@ impl FuncInstance {
 	) -> Result<Option<RuntimeValue>, Trap> {
 		let value_stack = StackWithLimit::with_size(StackSize::from_element_count(DEFAULT_VALUE_STACK_LIMIT));
 		let call_stack = StackWithLimit::with_size(StackSize::from_element_count(DEFAULT_CALL_STACK_LIMIT));
-		Self::invoke_configurable(func, args, externals, value_stack, call_stack)
+		let mut interpreter = Interpreter::new(value_stack, call_stack);
+		Self::invoke_configurable(func, args, externals, &mut interpreter)
 	}
 
 	/// Invoke the function, get a resumable handle. This handle can then be used to [`start_execution`]. If a
diff --git a/src/lib.rs b/src/lib.rs
index cad8752..c6b75dc 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -405,6 +405,7 @@ pub use self::global::{GlobalInstance, GlobalRef};
 pub use self::func::{FuncInstance, FuncRef, FuncInvocation, ResumableError};
 pub use self::types::{Signature, ValueType, GlobalDescriptor, TableDescriptor, MemoryDescriptor};
 pub use self::common::stack::{StackWithLimit, StackSize, StackSizeLimit, StackSizeInitial};
+pub use self::runner::Interpreter;
 
 /// WebAssembly-specific sizes and units.
 pub mod memory_units {
diff --git a/src/runner.rs b/src/runner.rs
index 57f0a89..c578821 100644
--- a/src/runner.rs
+++ b/src/runner.rs
@@ -172,6 +172,9 @@ pub struct Interpreter {
 }
 
 impl Interpreter {
+	/// Initialize an interpreter that will use `value_stack` and `call_stack`.
+	///
+	/// `value_stack` `call_stack` determine the allowed stack size during later executions.
 	pub fn new(
 		value_stack: StackWithLimit<RuntimeValueInternal>,
 		call_stack: StackWithLimit<FunctionContext>,
@@ -183,11 +186,20 @@ impl Interpreter {
 		}
 	}
 
-	pub fn state(&self) -> &InterpreterState {
+	// Todo, use types to prevent user from calling start_execution on an inretpreter not in Initialized
+	// state.
+	/// Wipe all data in this interpreter, so it can be safely reused.
+	pub fn reset(&mut self) {
+		self.value_stack.truncate(0);
+		self.call_stack.truncate(0);
+		self.state = InterpreterState::Initialized;
+	}
+
+	pub(crate) fn state(&self) -> &InterpreterState {
 		&self.state
 	}
 
-	pub fn start_execution<E: Externals>(
+	pub(crate) fn start_execution<E: Externals>(
 		&mut self,
 		externals: &mut E,
 		func: &FuncRef,
@@ -220,7 +232,7 @@ impl Interpreter {
 		Ok(opt_return_value)
 	}
 
-	pub fn resume_execution<'a, E: Externals + 'a>(
+	pub(crate) fn resume_execution<'a, E: Externals + 'a>(
 		&mut self,
 		return_val: Option<RuntimeValue>,
 		externals: &'a mut E,
@@ -1321,6 +1333,15 @@ impl ValueStack {
 	fn len(&self) -> usize {
 		self.0.len()
 	}
+
+	/// Shortens the stack, keeping the first `len` elements and dropping
+	/// the rest.
+	///
+	/// If `len` is greater than the stack's current length, this has no
+	/// effect.
+	fn truncate(&mut self, new_len: usize) {
+		self.0.truncate(new_len)
+	}
 }
 
 impl From<StackOverflow> for TrapKind {