From 377c65e56b6be0081f3af35f746f6644eb6c3598 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Juhani=20Krekel=C3=A4?= <juhani.haverinen@gmail.com>
Date: Thu, 24 May 2018 00:11:25 +0300
Subject: [PATCH] Implement the VM

---
 gir.js | 174 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 168 insertions(+), 6 deletions(-)

diff --git a/gir.js b/gir.js
index bd055b0..dddb751 100644
--- a/gir.js
+++ b/gir.js
@@ -33,12 +33,14 @@ const jumpIfNonZero = Symbol('jumpIfNonZero');
 
 // TODO: Add extensions from Eldis
 
+class ParsingError extends Error {}
+
+class UnknownIRError extends Error {}
+
 // ------------------------------------------------------------------
 // Parsing
 // ------------------------------------------------------------------
 
-class ParsingError extends Error {}
-
 // (string) → [commandObjects]
 // May throw ParsingError
 function parse(program) {
@@ -224,8 +226,6 @@ function prettifyIR(parsed) {
 // Optimization passes
 // ------------------------------------------------------------------
 
-class UnknownIRError extends Error {}
-
 // ([commandObjects]) → [commandObjects]
 function joinAdjacentOps(parsed) {
 	// ([commandObjects], commandType) → [commandObjects]
@@ -420,7 +420,7 @@ function flattenLoops(offsetted) {
 	return worker(offsetted);
 }
 
-// ([commandObjects]) → [offsetCommandObjects]
+// ([commandObjects]) → [flatCommandObjects]
 function optimize(parsed) {
 	const optimizations = [
 		joinAdjacentOps,
@@ -436,4 +436,166 @@ function optimize(parsed) {
 // Virtual machine
 // ------------------------------------------------------------------
 
-// TODO: Implement a brainfuck VM for running the optimized programs
+// ([flatCommandObject]) → girVMState
+function newVM(program, input) {
+	return {
+		// Initial state for the machine
+		program: program,
+		ip: 0,
+
+		memory: Object.create(null),
+		tapeHead: 0,
+
+		input: input,
+		output: ''
+	};
+}
+
+// (girVMState, int) → {state: girVMState, reachedLimit: bool}
+// reachedLimit is set to true if the program ran for maxCycles and did not
+// terminate
+// If maxCycles is null, the program runs until completion
+function runVM(state, maxCycles = null) {
+	let program = state.program;
+	let ip = state.ip;
+
+	// Create a copy of the memory, since we're going to modify it
+	// TODO: Make memory into a Proxied thing that returns 0 if it
+	// doesn't have the requested cell
+	let memory = Object.create(null);
+	for(let key in state.memory) {
+		memory[key] = state.memory[key];
+	}
+	let tapeHead = state.tapeHead;
+
+	let input = state.input;
+	let output = state.output;
+
+	let reachedLimit = true;
+	for(let cycle = 0; maxCycles === null || cycle < maxCycles; cycle++) {
+		// Exit the loop if we run to the end of the program
+		if(ip >= program.length) {
+			// Did not reach limit, program finished
+			reachedLimit = false;
+			break;
+		}
+
+		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
+		// accessing
+		let index = tapeHead;
+		switch(command.type) {
+			case add:
+			case writeByte:
+			case readByte:
+			case clear:
+				// These have an offset property, add it
+				index += command.offset;
+				// Fall through
+			case jumpIfZero:
+			case jumpIfNonZero:
+				// Ensure the cell exists
+				if(!(index in memory)) {
+					memory[index] = 0;
+				}
+		}
+
+		// Run the command
+		switch(command.type) {
+			case add:
+				if(!(index in memory)) memory[index] = 0;
+				memory[index] += command.value;
+				// Implement wraparound
+				memory[index] = memory[index] & 0xFF;
+				ip++;
+				break;
+
+			case moveHead:
+				tapeHead += command.value;
+				ip++;
+				break;
+
+			case writeByte:
+				if(!(index in memory)) memory[index] = 0;
+				// TODO: utf-8
+				output += String.fromCodePoint(memory[index]);
+				ip++;
+				break;
+
+			case readByte:
+				// TODO: utf-8
+				// Have we reached EOF?
+				if(input.length == 0) {
+					// Yes, return 0
+					memory[index] = 0;
+				} else {
+					// No, return character
+					memory[index] = input.codePointAt(0);
+					// FIXME: This only works for BMP
+					input = input.slice(1);
+				}
+				ip++;
+				break;
+
+			case clear:
+				memory[index] = 0;
+				ip++;
+				break;
+
+			case jumpIfZero:
+				if(!(index in memory)) memory[index] = 0;
+				if(memory[index] == 0) {
+					ip = command.target;
+				} else {
+					ip++;
+				}
+				break;
+
+			case jumpIfNonZero:
+				if(!(index in memory)) memory[index] = 0;
+				if(memory[index] != 0) {
+					ip = command.target;
+				} else {
+					ip++;
+				}
+				break;
+
+			default:
+				// Unknown command type
+				throw new UnknownIRError();
+		}
+	}
+
+	let newState = {
+		program,
+		ip,
+
+		memory,
+		tapeHead,
+
+		input,
+		output
+	};
+
+	return {state: newState, reachedLimit: reachedLimit};
+}
+
+// ------------------------------------------------------------------
+// User-facing functions
+// ------------------------------------------------------------------
+
+// (string) → [flatCommandObjects]
+function compile(program) {
+	return optimize(parse(program));
+}
+
+// (string, string) → string
+function run(program, input) {
+	// TODO: Allow setting cycle maximum
+	// TODO; Cache programs
+	let compiled = compile(program);
+	let vm = newVM(compiled, input);
+	return runVM(vm).state.output;
+}