4 files changed, 10 insertions, 0 deletions
diff --git a/pit/include/lcq/pit/runtime.h b/pit/include/lcq/pit/runtime.h
index feeb267..96d0351 100644
--- a/pit/include/lcq/pit/runtime.h
+++ b/pit/include/lcq/pit/runtime.h
@@ -98,6 +98,7 @@ typedef struct pit_runtime {
     pit_vec(pit_traversal_entry) *traversal; /* intermediate stack used during tree traversal */
     pit_vec(pit_callstack_entry) *callstack; /* stack of enclosing function calls */
     /* bookkeeping */
+    i64 fuel; /* fuel total - ignored if negative. decrements on each instruction executed. */
     /* "frozen" values offsets: values before these offsets are immutable, and we can reset here later */
     i64 frozen_values, frozen_symtab;
     pit_value error; /* error value - if this is non-nil, an error has occured! only tracks the first error */
@@ -108,6 +109,7 @@ typedef struct pit_runtime {
 } pit_runtime;
 pit_runtime *pit_runtime_new(u8 *buf, i64 len);
 
+void pit_runtime_set_fuel(pit_runtime *rt, i64 fuel);
 void pit_runtime_freeze(pit_runtime *rt); /* freeze the runtime at the current point - everything currently defined becomes immutable */
 void pit_runtime_reset(pit_runtime *rt); /* restore the runtime to the frozen point, resetting everything that has happened since */
 bool pit_runtime_print_error(pit_runtime *rt); /* return true if an error has occured, and print to stderr */
diff --git a/pit/src/main.c b/pit/src/main.c
index 157ad87..2b741c1 100644
--- a/pit/src/main.c
+++ b/pit/src/main.c
@@ -11,6 +11,7 @@ int main(int argc, char **argv) {
     i64 sz = 256 * 1024 * 1024;
     u8 *buf = malloc((size_t) sz);
     pit_runtime *rt = pit_runtime_new(buf, sz);
+    rt->fuel = 10000000;
     pit_install_library_essential(rt);
     pit_install_library_io(rt);
     pit_install_library_plist(rt);
diff --git a/pit/src/runtime.c b/pit/src/runtime.c
index d4a0a62..ab65d71 100644
--- a/pit/src/runtime.c
+++ b/pit/src/runtime.c
@@ -45,6 +45,7 @@ pit_runtime *pit_runtime_new(u8 *buf, i64 len) {
     ret->traversal = pit_vec_new(pit_traversal_entry)(pit_arena_alloc_back(a, stack_size), stack_size);
     ret->callstack = pit_vec_new(pit_callstack_entry)(pit_arena_alloc_back(a, stack_size), stack_size);
     ret->saved_bindings = pit_vec_new(pit_value)(pit_arena_alloc_back(a, stack_size), stack_size);
+    ret->fuel = -1;
     ret->frozen_values = 0;
     ret->frozen_symtab = 0;
     ret->error = PIT_NIL;
@@ -60,6 +61,10 @@ pit_runtime *pit_runtime_new(u8 *buf, i64 len) {
     return ret;
 }
 
+void pit_runtime_set_fuel(pit_runtime *rt, i64 fuel) {
+    rt->fuel = fuel;
+}
+
 void pit_runtime_freeze(pit_runtime *rt) {
     rt->frozen_values = rt->heap->next;
     rt->frozen_symtab = rt->symtab->next;
diff --git a/pit/src/runtime/eval.c b/pit/src/runtime/eval.c
index eba9715..a2ed166 100644
--- a/pit/src/runtime/eval.c
+++ b/pit/src/runtime/eval.c
@@ -155,6 +155,8 @@ bool pit_vm_run_one(pit_runtime *rt) {
     if (rt->callstack->next < 1 || rt->error != PIT_NIL) return false;
     pit_callstack_entry *ent = pit_vec_get(pit_callstack_entry)(rt->callstack, rt->callstack->next - 1);
     if (ent == NULL) { pit_error(rt, "malformed call stack"); return false; }
+    if (rt->fuel == 0) { pit_error(rt, "out of fuel"); return false; }
+    rt->fuel -= 1;
     pit_value ins = pit_value_cons_car(rt, ent->code);
     if (ins == PIT_NIL) { pit_error(rt, "malformed vm instruction: %v %v", ins); return false; }
     ent->code = pit_value_cons_cdr(rt, ent->code);