diff --git a/nac3core/src/toplevel/builtins.rs b/nac3core/src/toplevel/builtins.rs
index fc0c0006b..c2e5be4ae 100644
--- a/nac3core/src/toplevel/builtins.rs
+++ b/nac3core/src/toplevel/builtins.rs
@@ -1,5 +1,5 @@
 use std::cell::RefCell;
-use inkwell::{IntPredicate, FloatPredicate, values::BasicValueEnum};
+use inkwell::{IntPredicate, FloatPredicate, values::{BasicValueEnum, IntValue}};
 use crate::{symbol_resolver::SymbolValue, codegen::expr::destructure_range};
 use super::*;
 
@@ -573,7 +573,7 @@ pub fn get_builtins(primitives: &mut (PrimitiveStore, Unifier)) -> BuiltinInfo {
                     ret: int32,
                     vars: vec![(list_var.1, list_var.0), (arg_ty.1, arg_ty.0)].into_iter().collect(),
                 }))),
-                var_id: Default::default(),
+                var_id: vec![arg_ty.1],
                 instance_to_symbol: Default::default(),
                 instance_to_stmt: Default::default(),
                 resolver: None,
@@ -585,41 +585,9 @@ pub fn get_builtins(primitives: &mut (PrimitiveStore, Unifier)) -> BuiltinInfo {
                         let int32 = ctx.ctx.i32_type();
                         let zero = int32.const_zero();
                         if ctx.unifier.unioned(arg_ty, range_ty) {
-                            let int1 = ctx.ctx.bool_type();
-                            let one = int32.const_int(1, false);
-                            let falze = int1.const_int(0, false);
-                            let abs_intrinsic =
-                                ctx.module.get_function("llvm.abs.i32").unwrap_or_else(|| {
-                                    let fn_type = int32.fn_type(&[int32.into(), int1.into()], false);
-                                    ctx.module.add_function("llvm.abs.i32", fn_type, None)
-                                });
                             let arg = arg.into_pointer_value();
                             let (start, end, step) = destructure_range(ctx, arg);
-                            let diff = ctx.builder.build_int_sub(end, start, "diff");
-                            let diff = if let BasicValueEnum::IntValue(val) = ctx
-                                .builder
-                                .build_call(abs_intrinsic, &[diff.into(), falze.into()], "absdiff")
-                                .try_as_basic_value()
-                                .left()
-                                .unwrap() {
-                                    val
-                                } else {
-                                    unreachable!();
-                                };
-                            let diff = ctx.builder.build_int_sub(diff, one, "diff");
-                            let step = if let BasicValueEnum::IntValue(val) = ctx
-                                .builder
-                                .build_call(abs_intrinsic, &[step.into(), falze.into()], "absstep")
-                                .try_as_basic_value()
-                                .left()
-                                .unwrap() {
-                                    val
-                                } else {
-                                    unreachable!();
-                                };
-                            let length = ctx.builder.build_int_signed_div(diff, step, "div");
-                            let length = ctx.builder.build_int_add(length, int32.const_int(1, false), "add1");
-                            Some(length.into())
+                            Some(calculate_len_for_slice_range(ctx, start, end, step).into())
                         } else {
                             Some(ctx.build_gep_and_load(arg.into_pointer_value(), &[zero, zero]))
                         }
@@ -648,4 +616,72 @@ pub fn get_builtins(primitives: &mut (PrimitiveStore, Unifier)) -> BuiltinInfo {
             "len",
         ]
     )
+}
+
+// equivalent code:
+// def length(start, end, step != 0):
+//     diff = end - start
+//     # if diff == 0 OR `diff` and `step` are of different signs, always zero
+//     if diff * step <= 0:
+//         return 0
+//     else:
+//         return ((abs(diff) - 1) // abs(step)) + 1
+pub fn calculate_len_for_slice_range<'ctx, 'a>(
+    ctx: &mut CodeGenContext<'ctx, 'a>,
+    start: IntValue<'ctx>,
+    end: IntValue<'ctx>,
+    step: IntValue<'ctx>,
+) -> IntValue<'ctx> {
+    let int32 = ctx.ctx.i32_type();
+    let int1 = ctx.ctx.bool_type();
+    let falze = int1.const_int(0, false);
+    let abs_intrinsic =
+        ctx.module.get_function("llvm.abs.i32").unwrap_or_else(|| {
+            let fn_type = int32.fn_type(&[int32.into(), int1.into()], false);
+            ctx.module.add_function("llvm.abs.i32", fn_type, None)
+        });
+    let diff = ctx.builder.build_int_sub(end, start, "diff");
+    let test_mult = ctx.builder.build_int_mul(diff, step, "test_mult");
+    let test =
+        ctx.builder.build_int_compare(inkwell::IntPredicate::SLE, test_mult, int32.const_zero(), "cmp");
+    let current = ctx.builder.get_insert_block().unwrap().get_parent().unwrap();
+    let then_bb = ctx.ctx.append_basic_block(current, "then");
+    let else_bb = ctx.ctx.append_basic_block(current, "else");
+    let cont_bb = ctx.ctx.append_basic_block(current, "cont");
+    ctx.builder.build_conditional_branch(test, then_bb, else_bb);
+    
+    ctx.builder.position_at_end(then_bb);
+    let length_zero = int32.const_zero();
+    ctx.builder.build_unconditional_branch(cont_bb);
+    
+    ctx.builder.position_at_end(else_bb);
+    let diff = if let BasicValueEnum::IntValue(val) = ctx
+        .builder
+        .build_call(abs_intrinsic, &[diff.into(), falze.into()], "absdiff")
+        .try_as_basic_value()
+        .left()
+        .unwrap() {
+            val
+        } else {
+            unreachable!();
+        };
+    let diff = ctx.builder.build_int_sub(diff, int32.const_int(1, false), "diff");
+    let step = if let BasicValueEnum::IntValue(val) = ctx
+        .builder
+        .build_call(abs_intrinsic, &[step.into(), falze.into()], "absstep")
+        .try_as_basic_value()
+        .left()
+        .unwrap() {
+            val
+        } else {
+            unreachable!();
+        };
+    let length = ctx.builder.build_int_signed_div(diff, step, "div");
+    let length = ctx.builder.build_int_add(length, int32.const_int(1, false), "add1");
+    ctx.builder.build_unconditional_branch(cont_bb);
+    
+    ctx.builder.position_at_end(cont_bb);
+    let phi = ctx.builder.build_phi(length_zero.get_type(), "lenphi");
+    phi.add_incoming(&[(&length_zero, then_bb), (&length, else_bb)]);
+    phi.as_basic_value().into_int_value()
 }
\ No newline at end of file