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class StringBuilder {
constructor() {
this.stringPieces = [];
}
add(s) {
this.stringPieces.push(s);
}
build() {
return this.stringPieces.join("");
}
}
const compileGoto = (gotoNode, stringBuilder) => {
stringBuilder.add(`this.followGoto("${gotoNode.label.symbol}");\nreturn;\n`);
};
const compileConditional = (conditionalNode, stringBuilder) => {
const {
variable: { symbol: variable },
goto: gotoNode,
} = conditionalNode;
stringBuilder.add(`if (this.get("${variable}") != 0) {\n`);
compileGoto(gotoNode, stringBuilder);
stringBuilder.add(`}\n`);
};
const compileAssignment = (assignmentNode, stringBuilder) => {
const {
variable: { symbol: variable },
expr,
} = assignmentNode;
if (expr.opr) {
if (expr.opr == "+") stringBuilder.add(`this.addOne("${variable}");\n`);
else if (expr.opr == "-")
stringBuilder.add(`this.subtractOne("${variable}");\n`);
} else {
stringBuilder.add("// noop \n");
}
};
const compileInstruction = (instruction, stringBuilder) => {
if (instruction.goto) {
compileGoto(instruction.goto, stringBuilder);
return; // ignore unreachable addition to instructionPointer
}
if (instruction.conditional) {
compileConditional(instruction.conditional, stringBuilder);
} else if (instruction.assignment) {
compileAssignment(instruction.assignment, stringBuilder);
}
stringBuilder.add("this.instructionPointer++;\n");
};
const compile = (ast) => {
const godelSequence = [];
const stringBuilder = new StringBuilder();
stringBuilder.add(`
class Program {
constructor() {
this.variables = new Map(); // variable -> natural number val
this.labelInstructions = new Map(); // labels to instruction indices
this.instructions = new Map(); // instruction indices to procedures
this.instructions.set(0, () => this.main());
this.instructionPointer = 0;
this.variables.set("Y", 0);
// -- program-specific state init --
this.finalInstruction = ${
ast.instructions.length + 1
}; // instruction of the implied "exit" label
// "E1" is the exit label
this.labelInstructions.set("E1", this.finalInstruction);
}
get(variable) {
if (!this.variables.has(variable)) {
this.variables.set(variable, 0);
}
return this.variables.get(variable);
}
addOne(variable) {
const val = this.get(variable);
this.variables.set(variable, val + 1);
}
subtractOne(variable) {
const val = this.get(variable);
this.variables.set(variable, val - 1);
}
followGoto(label) {
this.instructionPointer = this.labelInstructions.get(label);
}
step() {
if (!this.isCompleted()) {
const procedure = this.instructions.get(this.instructionPointer);
procedure();
}
return this.instructionPointer;
}
isCompleted() {
return this.instructionPointer == this.finalInstruction;
}
getResult() {
return this.variables.get("Y");
}
run(maxIter=500_000) {
let iter = 0;
while (!this.isCompleted() && (++iter) < maxIter) this.step();
if (iter < maxIter) {
return this.getResult();
}
throw new Error("Too many iterations. To resolve, please ask"
+ " Turing how we can find if a program will halt during compilation.");
}
main() {
`);
stringBuilder.add("// -- build label -> instruction map --\n");
for (let i = 0; i < ast.instructions.length; i++) {
const instruction = ast.instructions[i];
godelSequence.push(instruction.godel);
const line = instruction.instruction;
const instructionIdx = i + 1;
if (line.label) {
const symbol = line.label.symbol;
stringBuilder.add(
`this.instructions.set(${instructionIdx}, () => this.${symbol}());\n`
);
stringBuilder.add(
`this.labelInstructions.set("${symbol}", ${instructionIdx});\n`
);
}
}
stringBuilder.add("// -- compiled instructions --\n");
for (let i = 0; i < ast.instructions.length; i++) {
let instruction = ast.instructions[i].instruction;
const instructionIdx = i + 1;
if (instruction.label) {
const symbol = instruction.label.symbol;
stringBuilder.add(
` this.followGoto("${symbol}");\n}\n\n${symbol}() {\n`
);
stringBuilder.add(`this.instructionPointer = ${instructionIdx};\n`);
instruction = instruction.instruction;
}
compileInstruction(instruction, stringBuilder);
}
stringBuilder.add(` }\n}\n`);
stringBuilder.add("// -- \n");
stringBuilder.add("const program = new Program();\n\n");
stringBuilder.add("// !! set the initial Snapshot here !!\n");
stringBuilder.add('// program.variables.set("X1", 2);\n\n');
stringBuilder.add("program.run();\n");
stringBuilder.add("console.log(program.variables);\n");
stringBuilder.add("program.getResult();\n");
return {
js: js_beautify(stringBuilder.build(), {
indent_size: 2,
wrap_line_length: 100,
}),
godelSequence,
};
};
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