Observe gas limits in MOS declaration

ref #327

src/parser/__tests__/source.spec.ts

@@ -1823,4 +1823,24 @@ describe('SonicWeave parser', () => {
       '27\\27',
     ]);
   });
+
+  it('supports guard rails against absurd MOS declarations', () => {
+    const ast = parseAST('MOS 420L 69s');
+
+    const visitor = getSourceVisitor(false);
+    visitor.rootContext!.gas = 100;
+    expect(() => visitor.executeProgram(ast)).toThrow();
+  });
+
+  it('supports guard rails against unreasonable automos', () => {
+    const ast = parseAST(`
+      MOS 20L 20s
+      automos()
+    `);
+
+    const visitor = getSourceVisitor(false);
+    visitor.rootContext!.gas = 100;
+    visitor.visit(ast.body[0]);
+    expect(() => visitor.visit(ast.body[1])).toThrow();
+  });
 });

src/parser/mos.ts

@@ -43,10 +43,15 @@ export class Tardigrade {
     this.subVisitor = subVisitor;
   }
 
+  spendGas(amount?: number) {
+    this.subVisitor.spendGas(amount);
+  }
+
   createMosConfig(): MosConfig {
     if (!this.pattern) {
       throw new Error('Mode must be given in MOS declaration.');
     }
+    this.spendGas(this.pattern.length);
     const notation = generateNotation(this.pattern);
     const N = new Fraction(this.pattern.length);
     const countL = new Fraction((this.pattern.match(/L/g) ?? []).length);
@@ -179,6 +184,7 @@ export class Tardigrade {
   }
 
   visit(node: MosExpression) {
+    this.spendGas();
     switch (node.type) {
       case 'AbstractStepPattern':
         return this.visitAbstractStepPattern(node);
@@ -202,13 +208,15 @@ export class Tardigrade {
   }
 
   visitAbstractStepPattern(node: AbstractStepPattern) {
+    this.spendGas(node.pattern.length);
     this.pattern = node.pattern.join('');
     if (node.equave) {
       this.equave = this.visitRationalEquave(node.equave);
     }
   }
 
   visitIntegerPattern(node: IntegerPattern) {
+    this.spendGas(node.pattern.length);
     const small = Math.min(...node.pattern);
     const large = Math.max(...node.pattern);
     this.pattern = node.pattern.map(i => (i === small ? 's' : 'L')).join('');
@@ -223,6 +231,7 @@ export class Tardigrade {
   }
 
   visitPatternUpDownPeriod(node: PatternUpDownPeriod) {
+    this.spendGas(Math.abs(node.countLarge) + Math.abs(node.countSmall));
     const options: MosOptions = {};
     if (node.udp) {
       options.up = node.udp.up;

src/stdlib/builtin.ts

@@ -1933,6 +1933,7 @@ function automos(this: ExpressionVisitor) {
   if (!this.rootContext?.mosConfig || scale.length) {
     return;
   }
+  this.spendGas(this.rootContext.mosConfig.scale.size);
   const J4 = this.rootContext.C4;
   const monzos = scaleMonzos(this.rootContext.mosConfig).map(m => J4.mul(m));
   return monzos.map(