diff --git a/lang/src/vec.asm b/lang/src/vec.asm
index e30adf0..9ab3e93 100644
--- a/lang/src/vec.asm
+++ b/lang/src/vec.asm
@@ -21,6 +21,7 @@ global vec_get
 global vec_drop
 global vec_find
 global vec_insert
+global vec_insert_sorted
 global vec_binary_search_by
 
 global vec_tests
@@ -507,7 +508,7 @@ vec_insert:
 ;; rsi: pointer to key
 ;; rdx: compare function fn(ctx: *const (), a: *const T, b: *const T) -> i32
 ;; rcx: compare_fn context
-;;fn vec_binary_search_by(vec: *mut Vec, key: *const T, compare: fn(ctx: *const (), a: *const T, b: *const T) -> i32, ctx: *const ()) -> isize
+;;fn vec_binary_search_by(vec: *mut Vec, key: *const T, compare: fn(ctx: *const (), a: *const T, b: *const T) -> i32, ctx: *const ()) -> (usize, bool)
 vec_binary_search_by:
     push rbp
     mov rbp, rsp
@@ -518,6 +519,7 @@ vec_binary_search_by:
     mov [rsp + 0x18], rcx       ; save compare fn context
 
     mov rax, [rdi + 8]          ; len
+    dec rax                     ; high
     mov qword [rsp + 0x20], 0         ; low
     mov [rsp + 0x28], rax       ; mid
     mov [rsp + 0x30], rax       ; high
@@ -551,31 +553,55 @@ vec_binary_search_by:
     ; greater
     mov rax, [rsp + 0x28]       ; mid
     inc rax
+    mov [rsp + 0x28], rax       ; mid
     mov [rsp + 0x20], rax       ; low = mid + 1
     jmp .loop
 .less:
     mov rax, [rsp + 0x28]       ; mid
+    cmp rax, 0
+    je .not_found               ; if mid == 0, not found
     dec rax
     mov [rsp + 0x30], rax       ; high = mid - 1
     jmp .loop
 .found:
     mov rax, [rsp + 0x28]
+    xor rdx, rdx
     jmp .epilogue
 .not_found:
-    mov rax, [rsp + 0x20]
-    not rax
+    mov rax, [rsp + 0x28]
+    mov rdx, 1
 
 .epilogue:
     add rsp, 0x40
     pop rbp
     ret
-    
 
+;; rdi: pointer to Vec struct
+;; rsi: pointer to key
+;; rdx: compare function fn(ctx: *const (), a: *const T, b: *const T) -> i32
+;; rcx: compare_fn context
+;;fn vec_insert_sorted(vec: *mut Vec, key: *const T, compare: fn(ctx: *const (), a: *const T, b: *const T) -> i32, ctx: *const ()) -> (usize, bool)
+vec_insert_sorted:
+    push rbp
+    mov rbp, rsp
+    sub rsp, 0x18
+    mov [rsp], rdi              ; vec
+    mov [rsp + 8], rsi          ; element ptr
+    call vec_binary_search_by
+    test rdx, rdx
+    jnz .insert
+    ; element already exists
+    jmp .epilogue
+.insert:
+    mov [rsp + 0x10], rax       ; index
+    mov rdi, [rsp]              ; vec
+    mov rsi, rax                ; index
+    mov rdx, [rsp + 8]          ; element ptr
+    call vec_insert
+    mov rax, [rsp + 0x10]       ; index
+    mov rdx, 1                  ; indicate inserted
 
-    
-    
-
-
-
-    
-
+.epilogue:
+    add rsp, 0x18
+    pop rbp
+    ret
diff --git a/lang/tests/vec.rs b/lang/tests/vec.rs
index 44aa9bf..282b53f 100644
--- a/lang/tests/vec.rs
+++ b/lang/tests/vec.rs
@@ -60,11 +60,7 @@ impl<T> VecT<T> {
     fn push(&mut self, value: T) {
         let value = core::mem::ManuallyDrop::new(value);
         unsafe {
-            vec_push(
-                &mut self.vec,
-                &raw const value as *const T as *const u8,
-                core::mem::size_of::<T>(),
-            );
+            vec_push(&mut self.vec, &raw const value as *const T as *const u8);
         }
     }
 
@@ -172,19 +168,47 @@ impl<T> VecT<T> {
         }
 
         unsafe {
-            let index = vec_binary_search_by(
+            let (index, vacant) = vec_binary_search_by(
                 &raw const self.vec as *mut _,
                 elem as *const T as *const u8,
                 cmp_trampoline::<T, F>,
                 &raw mut cmp as *mut F as *mut (),
             );
-            if (index as isize) < 0 {
-                Err(!index)
+            if vacant {
+                Err(index)
             } else {
                 Ok(index)
             }
         }
     }
+
+    fn insert_sorted<F>(&self, elem: T, mut cmp: F) -> Result<usize, usize>
+    where
+        F: FnMut(&T, &T) -> i32,
+    {
+        extern "C" fn cmp_trampoline<T, F: FnMut(&T, &T) -> i32>(
+            f: *const (),
+            a: *const u8,
+            b: *const u8,
+        ) -> i32 {
+            let f = unsafe { &mut *(f as *mut F) };
+            let a = unsafe { &*(a as *const T) };
+            let b = unsafe { &*(b as *const T) };
+            f(a, b)
+        }
+
+        let mut elem = core::mem::ManuallyDrop::new(elem);
+
+        unsafe {
+            let (index, inserted) = vec_insert_sorted(
+                &raw const self.vec as *mut _,
+                &raw const elem as *const u8,
+                cmp_trampoline::<T, F>,
+                &raw mut cmp as *mut F as *mut (),
+            );
+            Ok(index)
+        }
+    }
 }
 
 #[unsafe(no_mangle)]
@@ -203,7 +227,7 @@ unsafe extern "C" {
         drop: Option<extern "C" fn(*mut u8)>,
         cap: usize,
     );
-    unsafe fn vec_push(vec: *mut BlobVec, elem: *const u8, size: usize);
+    unsafe fn vec_push(vec: *mut BlobVec, elem: *const u8);
     unsafe fn vec_insert(vec: *mut BlobVec, index: usize, elem: *const u8);
     unsafe fn vec_pop(vec: *mut BlobVec);
     unsafe fn vec_drop_last(vec: *mut BlobVec);
@@ -226,7 +250,13 @@ unsafe extern "C" {
         elem: *const u8,
         cmp: extern "C" fn(*const (), *const u8, *const u8) -> i32,
         cmp_data: *mut (),
-    ) -> usize;
+    ) -> (usize, bool);
+    unsafe fn vec_insert_sorted(
+        vec: *mut BlobVec,
+        elem: *const u8,
+        cmp: extern "C" fn(*const (), *const u8, *const u8) -> i32,
+        cmp_data: *mut (),
+    ) -> (usize, bool);
 }
 
 fn main() {
@@ -266,7 +296,7 @@ fn main() {
         vec_init(&mut vec, 4, Some(drop));
         let mut value: u32 = 2;
         assert_eq!(vec.len, 0);
-        vec_push(&mut vec, &value as *const u32 as *const u8, 4);
+        vec_push(&mut vec, &value as *const u32 as *const u8);
         assert_eq!(vec.len, 1);
         assert_eq!(as_slice::<u32>(&vec), &[2]);
         let retrieved = *(vec_get(&mut vec, 0) as *mut u32);
@@ -276,10 +306,10 @@ fn main() {
         assert_eq!(vec.len, 0);
         assert_eq!(get_drops(), 2);
         value = 3;
-        vec_push(&mut vec, &value as *const u32 as *const u8, 4);
+        vec_push(&mut vec, &value as *const u32 as *const u8);
         assert_eq!(as_slice::<u32>(&vec), &[3]);
         value = 5;
-        vec_push(&mut vec, &value as *const u32 as *const u8, 4);
+        vec_push(&mut vec, &value as *const u32 as *const u8);
         assert_eq!(as_slice::<u32>(&vec), &[3, 5]);
         assert_eq!(vec.len, 2);
         vec_drop_last(&mut vec);
@@ -322,8 +352,12 @@ fn main() {
         core::cmp::Ordering::Equal => 0,
         core::cmp::Ordering::Greater => 1,
     };
+    assert_eq!(vec.binary_search_by(&35, cmp), Err(2));
     assert_eq!(vec.binary_search_by(&25, cmp), Err(1));
     assert_eq!(vec.binary_search_by(&30, cmp), Ok(1));
     assert_eq!(vec.binary_search_by(&5, cmp), Err(0));
-    assert_eq!(vec.binary_search_by(&55, cmp), Err(5));
+    assert_eq!(vec.binary_search_by(&55, cmp), Err(4));
+
+    vec.insert_sorted(35, cmp);
+    assert_eq!(vec.as_slice(), &[20, 30, 35, 40, 50]);
 }