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, }; };