Ouroboros, 11 7 bytes
Inspired by Sp3000's ><> answer.
r.*.n!(
Each line of code in an Ouroboros program represents a snake eating its tail.
r reads a number from input, or -1 for EOF. .* squares it, keeping 0 and 1 the same but mapping -1 to 1. .n outputs the number, leaving a copy on the stack. Finally, !( logically negates and eats that many characters of the end of the snake. If the number was 0, this eats one character, the (, which is also the current instruction. Since the instruction pointer was swallowed, the snake dies and the program ends. If the number was 1, this eats zero characters, and execution loops back to the beginning of the snake, repeating indefinitely.
Try it:
// Define Stack class
function Stack() {
this.stack = [];
this.length = 0;
}
Stack.prototype.push = function(item) {
this.stack.push(item);
this.length++;
}
Stack.prototype.pop = function() {
var result = 0;
if (this.length > 0) {
result = this.stack.pop();
this.length--;
}
return result;
}
Stack.prototype.top = function() {
var result = 0;
if (this.length > 0) {
result = this.stack[this.length - 1];
}
return result;
}
Stack.prototype.toString = function() {
return "" + this.stack;
}
// Define Snake class
function Snake(code) {
this.code = code;
this.length = this.code.length;
this.ip = 0;
this.ownStack = new Stack();
this.currStack = this.ownStack;
this.alive = true;
this.wait = 0;
this.partialString = this.partialNumber = null;
}
Snake.prototype.step = function() {
if (!this.alive) {
return null;
}
if (this.wait > 0) {
this.wait--;
return null;
}
var instruction = this.code.charAt(this.ip);
var output = null;
console.log("Executing instruction " + instruction);
if (this.partialString !== null) {
// We're in the middle of a double-quoted string
if (instruction == '"') {
// Close the string and push its character codes in reverse order
for (var i = this.partialString.length - 1; i >= 0; i--) {
this.currStack.push(this.partialString.charCodeAt(i));
}
this.partialString = null;
} else {
this.partialString += instruction;
}
} else if (instruction == '"') {
this.partialString = "";
} else if ("0" <= instruction && instruction <= "9") {
if (this.partialNumber !== null) {
this.partialNumber = this.partialNumber + instruction; // NB: concatenation!
} else {
this.partialNumber = instruction;
}
next = this.code.charAt((this.ip + 1) % this.length);
if (next < "0" || "9" < next) {
// Next instruction is non-numeric, so end number and push it
this.currStack.push(+this.partialNumber);
this.partialNumber = null;
}
} else if ("a" <= instruction && instruction <= "f") {
// a-f push numbers 10 through 15
var value = instruction.charCodeAt(0) - 87;
this.currStack.push(value);
} else if (instruction == "$") {
// Toggle the current stack
if (this.currStack === this.ownStack) {
this.currStack = this.program.sharedStack;
} else {
this.currStack = this.ownStack;
}
} else if (instruction == "s") {
this.currStack = this.ownStack;
} else if (instruction == "S") {
this.currStack = this.program.sharedStack;
} else if (instruction == "l") {
this.currStack.push(this.ownStack.length);
} else if (instruction == "L") {
this.currStack.push(this.program.sharedStack.length);
} else if (instruction == ".") {
var item = this.currStack.pop();
this.currStack.push(item);
this.currStack.push(item);
} else if (instruction == "m") {
var item = this.ownStack.pop();
this.program.sharedStack.push(item);
} else if (instruction == "M") {
var item = this.program.sharedStack.pop();
this.ownStack.push(item);
} else if (instruction == "y") {
var item = this.ownStack.top();
this.program.sharedStack.push(item);
} else if (instruction == "Y") {
var item = this.program.sharedStack.top();
this.ownStack.push(item);
} else if (instruction == "\\") {
var top = this.currStack.pop();
var next = this.currStack.pop()
this.currStack.push(top);
this.currStack.push(next);
} else if (instruction == "@") {
var c = this.currStack.pop();
var b = this.currStack.pop();
var a = this.currStack.pop();
this.currStack.push(c);
this.currStack.push(a);
this.currStack.push(b);
} else if (instruction == ";") {
this.currStack.pop();
} else if (instruction == "+") {
var b = this.currStack.pop();
var a = this.currStack.pop();
this.currStack.push(a + b);
} else if (instruction == "-") {
var b = this.currStack.pop();
var a = this.currStack.pop();
this.currStack.push(a - b);
} else if (instruction == "*") {
var b = this.currStack.pop();
var a = this.currStack.pop();
this.currStack.push(a * b);
} else if (instruction == "/") {
var b = this.currStack.pop();
var a = this.currStack.pop();
this.currStack.push(a / b);
} else if (instruction == "%") {
var b = this.currStack.pop();
var a = this.currStack.pop();
this.currStack.push(a % b);
} else if (instruction == "_") {
this.currStack.push(-this.currStack.pop());
} else if (instruction == "I") {
var value = this.currStack.pop();
if (value < 0) {
this.currStack.push(Math.ceil(value));
} else {
this.currStack.push(Math.floor(value));
}
} else if (instruction == ">") {
var b = this.currStack.pop();
var a = this.currStack.pop();
this.currStack.push(+(a > b));
} else if (instruction == "<") {
var b = this.currStack.pop();
var a = this.currStack.pop();
this.currStack.push(+(a < b));
} else if (instruction == "=") {
var b = this.currStack.pop();
var a = this.currStack.pop();
this.currStack.push(+(a == b));
} else if (instruction == "!") {
this.currStack.push(+ !this.currStack.pop());
} else if (instruction == "?") {
this.currStack.push(Math.random());
} else if (instruction == "n") {
output = "" + this.currStack.pop();
} else if (instruction == "o") {
output = String.fromCharCode(this.currStack.pop());
} else if (instruction == "r") {
var input = this.program.io.getNumber();
this.currStack.push(input);
} else if (instruction == "i") {
var input = this.program.io.getChar();
this.currStack.push(input);
} else if (instruction == "(") {
this.length -= Math.floor(this.currStack.pop());
this.length = Math.max(this.length, 0);
} else if (instruction == ")") {
this.length += Math.floor(this.currStack.pop());
this.length = Math.min(this.length, this.code.length);
} else if (instruction == "w") {
this.wait = this.currStack.pop();
}
// Any unrecognized character is a no-op
if (this.ip >= this.length) {
// We've swallowed the IP, so this snake dies
this.alive = false;
this.program.snakesLiving--;
} else {
// Increment IP and loop if appropriate
this.ip = (this.ip + 1) % this.length;
}
return output;
}
Snake.prototype.getHighlightedCode = function() {
var result = "";
for (var i = 0; i < this.code.length; i++) {
if (i == this.length) {
result += '<span class="swallowedCode">';
}
if (i == this.ip) {
if (this.wait > 0) {
result += '<span class="nextActiveToken">';
} else {
result += '<span class="activeToken">';
}
result += escapeEntities(this.code.charAt(i)) + '</span>';
} else {
result += escapeEntities(this.code.charAt(i));
}
}
if (this.length < this.code.length) {
result += '</span>';
}
return result;
}
// Define Program class
function Program(source, speed, io) {
this.sharedStack = new Stack();
this.snakes = source.split(/\r?\n/).map(function(snakeCode) {
var snake = new Snake(snakeCode);
snake.program = this;
snake.sharedStack = this.sharedStack;
return snake;
}.bind(this));
this.snakesLiving = this.snakes.length;
this.io = io;
this.speed = speed || 10;
this.halting = false;
}
Program.prototype.run = function() {
this.step();
if (this.snakesLiving) {
this.timeout = window.setTimeout(this.run.bind(this), 1000 / this.speed);
}
}
Program.prototype.step = function() {
for (var s = 0; s < this.snakes.length; s++) {
var output = this.snakes[s].step();
if (output) {
this.io.print(output);
}
}
this.io.displaySource(this.snakes.map(function (snake) {
return snake.getHighlightedCode();
}).join("<br>"));
}
Program.prototype.halt = function() {
window.clearTimeout(this.timeout);
}
var ioFunctions = {
print: function (item) {
var stdout = document.getElementById('stdout');
stdout.value += "" + item;
},
getChar: function () {
if (inputData) {
var inputChar = inputData[0];
inputData = inputData.slice(1);
result = inputChar.charCodeAt(0);
} else {
result = -1;
}
var stdinDisplay = document.getElementById('stdin-display');
stdinDisplay.innerHTML = escapeEntities(inputData);
return result;
},
getNumber: function () {
while (inputData && (inputData[0] < "0" || "9" < inputData[0])) {
inputData = inputData.slice(1);
}
if (inputData) {
var inputNumber = inputData.match(/\d+/)[0];
inputData = inputData.slice(inputNumber.length);
result = +inputNumber;
} else {
result = -1;
}
var stdinDisplay = document.getElementById('stdin-display');
stdinDisplay.innerHTML = escapeEntities(inputData);
return result;
},
displaySource: function (formattedCode) {
var sourceDisplay = document.getElementById('source-display');
sourceDisplay.innerHTML = formattedCode;
}
};
var program = null;
var inputData = null;
function showEditor() {
var source = document.getElementById('source'),
sourceDisplayWrapper = document.getElementById('source-display-wrapper'),
stdin = document.getElementById('stdin'),
stdinDisplayWrapper = document.getElementById('stdin-display-wrapper');
source.style.display = "block";
stdin.style.display = "block";
sourceDisplayWrapper.style.display = "none";
stdinDisplayWrapper.style.display = "none";
source.focus();
}
function hideEditor() {
var source = document.getElementById('source'),
sourceDisplay = document.getElementById('source-display'),
sourceDisplayWrapper = document.getElementById('source-display-wrapper'),
stdin = document.getElementById('stdin'),
stdinDisplay = document.getElementById('stdin-display'),
stdinDisplayWrapper = document.getElementById('stdin-display-wrapper');
source.style.display = "none";
stdin.style.display = "none";
sourceDisplayWrapper.style.display = "block";
stdinDisplayWrapper.style.display = "block";
var sourceHeight = getComputedStyle(source).height,
stdinHeight = getComputedStyle(stdin).height;
sourceDisplayWrapper.style.minHeight = sourceHeight;
sourceDisplayWrapper.style.maxHeight = sourceHeight;
stdinDisplayWrapper.style.minHeight = stdinHeight;
stdinDisplayWrapper.style.maxHeight = stdinHeight;
sourceDisplay.textContent = source.value;
stdinDisplay.textContent = stdin.value;
}
function escapeEntities(input) {
return input.replace(/&/g, '&').replace(/</g, '<').replace(/>/g, '>');
}
function resetProgram() {
var stdout = document.getElementById('stdout');
stdout.value = null;
if (program !== null) {
program.halt();
}
program = null;
inputData = null;
showEditor();
}
function initProgram() {
var source = document.getElementById('source'),
stepsPerSecond = document.getElementById('steps-per-second'),
stdin = document.getElementById('stdin');
program = new Program(source.value, +stepsPerSecond.innerHTML, ioFunctions);
hideEditor();
inputData = stdin.value;
}
function runBtnClick() {
if (program === null || program.snakesLiving == 0) {
resetProgram();
initProgram();
} else {
program.halt();
var stepsPerSecond = document.getElementById('steps-per-second');
program.speed = +stepsPerSecond.innerHTML;
}
program.run();
}
function stepBtnClick() {
if (program === null) {
initProgram();
} else {
program.halt();
}
program.step();
}
function sourceDisplayClick() {
resetProgram();
}.container {
width: 100%;
}
.so-box {
font-family:'Helvetica Neue', Arial, sans-serif;
font-weight: bold;
color: #fff;
text-align: center;
padding: .3em .7em;
font-size: 1em;
line-height: 1.1;
border: 1px solid #c47b07;
-webkit-box-shadow: 0 2px 2px rgba(0, 0, 0, 0.3), 0 2px 0 rgba(255, 255, 255, 0.15) inset;
text-shadow: 0 0 2px rgba(0, 0, 0, 0.5);
background: #f88912;
box-shadow: 0 2px 2px rgba(0, 0, 0, 0.3), 0 2px 0 rgba(255, 255, 255, 0.15) inset;
}
.control {
display: inline-block;
border-radius: 6px;
float: left;
margin-right: 25px;
cursor: pointer;
}
.option {
padding: 10px 20px;
margin-right: 25px;
float: left;
}
h1 {
text-align: center;
font-family: Georgia, 'Times New Roman', serif;
}
a {
text-decoration: none;
}
input, textarea {
box-sizing: border-box;
}
textarea {
display: block;
white-space: pre;
overflow: auto;
height: 50px;
width: 100%;
max-width: 100%;
min-height: 25px;
}
span[contenteditable] {
padding: 2px 6px;
background: #cc7801;
color: #fff;
}
#stdout-container, #stdin-container {
height: auto;
padding: 6px 0;
}
#reset {
float: right;
}
#source-display-wrapper , #stdin-display-wrapper{
display: none;
width: 100%;
height: 100%;
overflow: auto;
border: 1px solid black;
box-sizing: border-box;
}
#source-display , #stdin-display{
font-family: monospace;
white-space: pre;
padding: 2px;
}
.activeToken {
background: #f93;
}
.nextActiveToken {
background: #bbb;
}
.swallowedCode{
color: #999;
}
.clearfix:after {
content:".";
display: block;
height: 0;
clear: both;
visibility: hidden;
}
.clearfix {
display: inline-block;
}
* html .clearfix {
height: 1%;
}
.clearfix {
display: block;
}<!--
Designed and written 2015 by D. Loscutoff
Much of the HTML and CSS was taken from this Befunge interpreter by Ingo Bürk: https://codegolf.stackexchange.com/a/40331/16766
-->
<div class="container">
<textarea id="source" placeholder="Enter your program here" wrap="off">r.*.n!(</textarea>
<div id="source-display-wrapper" onclick="sourceDisplayClick()"><div id="source-display"></div></div></div><div id="stdin-container" class="container">
<textarea id="stdin" placeholder="Input" wrap="off">1</textarea>
<div id="stdin-display-wrapper" onclick="stdinDisplayClick()"><div id="stdin-display"></div></div></div><div id="controls-container" class="container clearfix"><input type="button" id="run" class="control so-box" value="Run" onclick="runBtnClick()" /><input type="button" id="pause" class="control so-box" value="Pause" onclick="program.halt()" /><input type="button" id="step" class="control so-box" value="Step" onclick="stepBtnClick()" /><input type="button" id="reset" class="control so-box" value="Reset" onclick="resetProgram()" /></div><div id="stdout-container" class="container"><textarea id="stdout" placeholder="Output" wrap="off" readonly></textarea></div><div id="options-container" class="container"><div class="option so-box">Steps per Second:
<span id="steps-per-second" contenteditable>20</span></div></div>Previous two-snake solution:
rm1(
S.n.!(
Snake 1
r reads a number from input; m moves it to the shared stack. Then 1( causes the snake to eat its instruction pointer and die.
Snake 2
S switches to the shared stack. .n duplicates the value and outputs it as a number. .!( duplicates again, logically negates, and eats that many characters. If the number was 0, this eats one character, killing the second snake and ending the program. If the number was 1, this eats zero characters and execution loops back to the beginning of the snake, repeating indefinitely.
