From 1b262941cafb906da0aa814cea6b3dbdc15b1663 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Juhani=20Krekel=C3=A4?= <juhani.haverinen@gmail.com>
Date: Sun, 27 May 2018 12:46:47 +0300
Subject: [PATCH] Implement a readMemory() in runVM() that returns 0 on reads
 to nonexistent memory

---
 README.md |  2 --
 gir.js    | 49 ++++++++++++++++++++++++++-----------------------
 2 files changed, 26 insertions(+), 25 deletions(-)

diff --git a/README.md b/README.md
index 54ee23b..d715710 100644
--- a/README.md
+++ b/README.md
@@ -21,8 +21,6 @@ Gir supports following optimizations:
 TODO
 ----
 ### gir.js
-* Make VM and transformMultiplyLoops use a Proxied object that gives out 0
-  for nonexistent elements for tape and allows using [] interface
 * Support for other types of EOF?
 
 ### gir.html
diff --git a/gir.js b/gir.js
index 1532938..addf9e8 100644
--- a/gir.js
+++ b/gir.js
@@ -588,7 +588,7 @@ function newVM(program, input) {
 	};
 }
 
-// (girVMState, int) → {state: girVMState, complete: bool, cycles: int,
+// (girVMState, int/null) → {state: girVMState, complete: bool, cycles: int,
 //                      intParseFailed: bool, breakPointReached: bool,
 //                      lastIndex: int/null}
 // complete is set to true is the program completed its execution
@@ -598,6 +598,16 @@ function newVM(program, input) {
 // lastIndex tells the last memory index accessed by the VM
 // If maxCycles is null, the program runs until completion
 function runVM(state, maxCycles = null) {
+	// (int) → int
+	function readMemory(index) {
+		// Return 0 if index doesn't exist
+		if(memory.has(index)) {
+			return memory.get(index);
+		} else {
+			return 0;
+		}
+	}
+
 	let program = state.program;
 	let ip = state.ip;
 
@@ -630,8 +640,7 @@ function runVM(state, maxCycles = null) {
 
 		let command = program[ip];
 
-		// See if we need to make sure the cell we're on exists and
-		// calculate the index into the array of the cell we're
+		// Calculate the index into the array of the cell we're
 		// accessing
 		let index = tapeHead;
 		switch(command.type) {
@@ -643,21 +652,13 @@ function runVM(state, maxCycles = null) {
 			case readInt:
 				// These have an offset property, add it
 				index += command.offset;
-				// Fall through
-			case multiply:
-			case jumpIfZero:
-			case jumpIfNonZero:
-				// Ensure the cell exists
-				if(!memory.has(index)) {
-					memory.set(index, 0);
-				}
-				lastIndex = index;
 		}
+		lastIndex = index;
 
 		// Run the command
 		switch(command.type) {
 			case add:
-				let old = memory.get(index);
+				let old = readMemory(index);
 				memory.set(index, (command.value + old) & 0xFF);
 
 				ip++;
@@ -665,12 +666,18 @@ function runVM(state, maxCycles = null) {
 
 			case moveHead:
 				tapeHead += command.value;
+				// Set lastIndex to the new tape head
+				// position. Technically we do not access
+				// the cell, but otherwise it will point
+				// to the cell we were in previously, so
+				// this allows better debugging
+				lastIndex = tapeHead;
 
 				ip++;
 				break;
 
 			case writeByte:
-				output.push(memory.get(index));
+				output.push(readMemory(index));
 				ip++;
 				break;
 
@@ -697,14 +704,11 @@ function runVM(state, maxCycles = null) {
 						`multiply where change for 0 is ${command.changes.get(0)}`);
 				}
 
-				let multiplier = memory.get(index);
+				let multiplier = readMemory(index);
 
 				for(let [offset, change] of command.changes.entries()) {
 					let index = tapeHead + offset;
-					if(!memory.has(index)) {
-						memory.set(index, 0);
-					}
-					let old = memory.get(index);
+					let old = readMemory(index);
 					let value = old + multiplier * change;
 					memory.set(index, value & 0xff);
 				}
@@ -713,7 +717,7 @@ function runVM(state, maxCycles = null) {
 				break;
 
 			case jumpIfZero:
-				if(memory.get(index) == 0) {
+				if(readMemory(index) == 0) {
 					ip = command.target;
 				} else {
 					ip++;
@@ -721,7 +725,7 @@ function runVM(state, maxCycles = null) {
 				break;
 
 			case jumpIfNonZero:
-				if(memory.get(index) != 0) {
+				if(readMemory(index) != 0) {
 					ip = command.target;
 				} else {
 					ip++;
@@ -729,7 +733,7 @@ function runVM(state, maxCycles = null) {
 				break;
 
 			case writeInt:
-				let outputStr = memory.get(index).toString();
+				let outputStr = readMemory(index).toString();
 				output = output.concat(encodeUTF8(outputStr));
 				ip++;
 				break;
@@ -984,7 +988,6 @@ function cachedCompile(program, enableExtensions = true) {
 
 // (string, string, int) → string
 function ircbotRun(program, input, maxCycles = 400000) {
-	// TODO; Cache programs
 	let compiled = cachedCompile(program);
 	let vm = newVM(compiled, encodeUTF8(input));