Add base branch logic

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/vm/inst.rs

@@ -1,6 +1,13 @@
 use crate::obj::ObjPtr;
 use crate::vm::name::Name;
 
+#[derive(Debug, Clone)]
+pub enum JumpCondition {
+    False,
+    True,
+    Always,
+}
+
 #[derive(Debug, Clone)]
 pub enum Inst {
     /// Pushes an object to the stack.
@@ -29,4 +36,37 @@ pub enum Inst {
     /// resize the stack to its last size (pre-call), and push the return value
     /// back to the top of the stack.
     Return,
+
+    /// Adds the given value to the current program counter (positive or
+    /// negative) when the given condition holds..
+    ///
+    /// `Inst::JumpRelative(1)` functions as a no-op.
+    /// `Inst::JumpRelative(0)` functions as a "hang"
+    /// `Inst::JumpRelative(-1)` jumps to the previous instruction.
+    JumpRelative(isize, JumpCondition),
+
+    /// Pops the top value off of the stack and sets the condition flag to
+    /// "true" or "false".
+    Compare,
+
+    Comment(String),
+}
+
+impl Inst {
+    pub fn disassemble_string(&self) -> String {
+        match self {
+            Inst::Push(ptr) => format!("push <{:#x}>", (ptr as *const _ as usize)),
+            Inst::Load(name) => format!("load {name:?}"),
+            Inst::Store(name) => format!("store {name:?}"),
+            Inst::GetAttr(attr) => format!("getattr {attr}"),
+            Inst::Pop => format!("pop"),
+            Inst::Call(argc) => format!("call {argc}"),
+            Inst::Return => format!("return"),
+            Inst::JumpRelative(distance, condition) => {
+                format!("jump {:+} (when condition is {:?})", distance, condition)
+            }
+            Inst::Compare => format!("compare"),
+            Inst::Comment(comment) => format!("# {comment}"),
+        }
+    }
 }