Feat: lots of convenience stuff

crates/forth/src/vm.rs

@@ -140,6 +140,7 @@ impl Forth {
         var_writes: &mut HashMap<String, Value>,
     ) -> Result<(), String> {
         let mut pc = 0;
+        let mut marks: Vec<usize> = Vec::new();
         let trace_cell = std::cell::RefCell::new(trace);
         let var_writes_cell = std::cell::RefCell::new(Some(var_writes));
 
@@ -1541,6 +1542,29 @@ impl Forth {
                         .unwrap_or(0);
                     stack.push(Value::Int(val as i64, None));
                 }
+                Op::Mark => {
+                    marks.push(stack.len());
+                }
+                Op::Count(span) => {
+                    let mark = marks.pop().ok_or("count without mark")?;
+                    stack.push(Value::Int((stack.len() - mark) as i64, *span));
+                }
+                Op::Index(word_span) => {
+                    let idx = pop_int(stack)?;
+                    let count = pop_int(stack)? as usize;
+                    if count == 0 {
+                        return Err("index count must be > 0".into());
+                    }
+                    let resolved_idx = ((idx % count as i64 + count as i64) % count as i64) as usize;
+                    if let Some(span) = word_span {
+                        if stack.len() >= count {
+                            let start = stack.len() - count;
+                            let selected = &stack[start + resolved_idx];
+                            record_resolved_from_value(&trace_cell, Some(*span), selected);
+                        }
+                    }
+                    drain_select_run(count, resolved_idx, stack, outputs, cmd)?;
+                }
                 Op::Forget => {
                     let name = pop(stack)?;
                     self.dict.lock().remove(name.as_str()?);