1 files changed, 163 insertions, 0 deletions
diff --git a/godel/js/compiler.js b/godel/js/compiler.js
new file mode 100644
index 0000000..65ebbcd
--- /dev/null
+++ b/godel/js/compiler.js
@@ -0,0 +1,163 @@
+class StringBuilder {
+  constructor() {
+    this.stringPieces = [];
+  }
+  add(s) {
+    this.stringPieces.push(s);
+  }
+  build() {
+    return this.stringPieces.join("");
+  }
+}
+
+const compileGoto = (gotoNode, stringBuilder) => {
+  stringBuilder.add(`this.followGoto("${gotoNode.label}");\nreturn;\n`);
+};
+
+const compileConditional = (conditionalNode, stringBuilder) => {
+  const { variable, goto: gotoNode } = conditionalNode;
+
+  stringBuilder.add(`if (this.get("${variable}") != 0) {\n`);
+  compileGoto(gotoNode, stringBuilder);
+  stringBuilder.add(`}\n`);
+};
+
+const compileAssignment = (assignmentNode, stringBuilder) => {
+  const { 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 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 = ${
+          ast.instructions.length + 1
+        }; // instruction of the implied "exit" label
+        this.labelInstructions.set("E", this.finalInstruction); // "E" is the exit label
+      }
+
+      get(variable) {
+        if (!this.variables.has(variable)) {
+          this.variables.set(variable, 0);
+        }
+        return this.variables.get(variable);
+      }
+
+      addOne(variable) {
+        const val = this.get(variable);
+        this.variables.set(variable, val + 1);
+      }
+
+      subtractOne(variable) {
+        const val = this.get(variable);
+        this.variables.set(variable, val - 1);
+      }
+
+      followGoto(label) {
+        this.instructionPointer = this.labelInstructions.get(label);
+      }
+
+      step() {
+        if (!this.isCompleted()) {
+          const procedure = this.instructions.get(this.instructionPointer);
+          procedure();
+ 
+        }
+        return this.instructionPointer;
+      }
+
+      isCompleted() {
+        return this.instructionPointer == this.finalInstruction;
+      }
+
+      getResult() {
+        return this.variables.get("Y");
+      }
+
+      run(maxIter=50000) {
+        let iter = 0;
+        while (!this.isCompleted() && (++iter) < maxIter) this.step();
+        if (iter < maxIter) {
+          return this.getResult();
+        }
+        throw new Error("Iterations went over maxIter=(" + maxIter + "). To resolve, please ask Turing how we can tell if a program will halt during compilation.");
+      }
+
+      main() {
+`);
+
+  stringBuilder.add("// -- build label -> instruction map --\n");
+  for (let i = 0; i < ast.instructions.length; i++) {
+    const line = ast.instructions[i];
+    const instructionIdx = i + 1;
+    if (line.label) {
+      stringBuilder.add(
+        `this.instructions.set(${instructionIdx}, () => this.${line.label}());\n`,
+      );
+      stringBuilder.add(
+        `this.labelInstructions.set("${line.label}", ${instructionIdx});\n`,
+      );
+    }
+  }
+
+  stringBuilder.add("// -- compiled instructions --\n");
+  for (let i = 0; i < ast.instructions.length; i++) {
+    let instruction = ast.instructions[i];
+    const instructionIdx = i + 1;
+    if (instruction.label) {
+      stringBuilder.add(
+        `  this.followGoto("${instruction.label}");\n}\n\n${instruction.label}() {\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(50_000); // 50_000 is the max iterations before throwing an exception\n",
+  );
+
+  return js_beautify(stringBuilder.build(), {
+    indent_size: 2,
+    wrap_line_length: 120,
+  });
+};