diff --git a/examples/euler13.zr b/examples/euler13.zr
index 27cf2d0..12b5d88 100644
--- a/examples/euler13.zr
+++ b/examples/euler13.zr
@@ -1,5 +1,5 @@
 func main[] : I64
     37107287533 + 46376937677 + 74324986199 + 91942213363 + 23067588207 + 89261670696 + 28112879812 + 44274228917 + 47451445736 + 70386486105 + 62176457141 + 64906352462 + 92575867718 + 58203565325 + 80181199384 + 35398664372 + 86515506006 + 71693888707 + 54370070576 + 53282654108 + 36123272525 + 45876576172 + 17423706905 + 81142660418 + 51934325451 + 62467221648 + 15732444386 + 55037687525 + 18336384825 + 80386287592 + 78182833757 + 16726320100 + 48403098129 + 87086987551 + 59959406895 + 69793950679 + 41052684708 + 65378607361 + 35829035317 + 94953759765 + 88902802571 + 25267680276 + 36270218540 + 24074486908 + 91430288197 + 34413065578 + 23053081172 + 11487696932 + 63783299490 + 67720186971 + 95548255300 + 76085327132 + 37774242535 + 23701913275 + 29798860272 + 18495701454 + 38298203783 + 34829543829 + 40957953066 + 29746152185 + 41698116222 + 62467957194 + 23189706772 + 86188088225 + 11306739708 + 82959174767 + 97623331044 + 42846280183 + 55121603546 + 32238195734 + 75506164965 + 62177842752 + 32924185707 + 99518671430 + 73267460800 + 76841822524 + 97142617910 + 87783646182 + 10848802521 + 71329612474 + 62184073572 + 66627891981 + 60661826293 + 85786944089 + 66024396409 + 64913982680 + 16730939319 + 94809377245 + 78639167021 + 15368713711 + 40789923115 + 44889911501 + 41503128880 + 81234880673 + 82616570773 + 22918802058 + 77158542502 + 72107838435 + 20849603980 + 53503534226
     |> I64.to_string()
-    |> substr(0, 10)
+    |> String.substr(0, 10)
     |> print()
\ No newline at end of file
diff --git a/examples/quicksort.zr b/examples/quicksort.zr
new file mode 100644
index 0000000..ae3e0a7
--- /dev/null
+++ b/examples/quicksort.zr
@@ -0,0 +1,46 @@
+func quicksort[arr: Array] : I64
+    do_quicksort(arr, 0, Array.size(arr)-1)
+
+func do_quicksort[arr: Array, low: I64, high: I64] : I64
+    if low < high
+        let i: I64 = partition(arr, low, high)
+        do_quicksort(arr, low, i - 1)
+        do_quicksort(arr, i + 1, high)
+
+func partition[arr: Array, low: I64, high: I64] : I64
+    let pivot: I64 = Array.nth(arr, high)
+    let i: I64 = low - 1
+    for j in low:high
+        if Array.nth(arr, j) <= pivot
+            i = i + 1
+            let temp: I64 = Array.nth(arr, i)
+            Array.set(arr, i, Array.nth(arr, j))
+            Array.set(arr, j, temp)
+    let temp: I64 = Array.nth(arr, i + 1)
+    Array.set(arr, i + 1, Array.nth(arr, high))
+    Array.set(arr, high, temp)
+    return i + 1
+
+func main[] : I64
+    let arr: Array = Array.new()
+    Array.push(arr, 340)
+    Array.push(arr, 252)
+    Array.push(arr, 352)
+    Array.push(arr, 117)
+    Array.push(arr, 650)
+    Array.push(arr, 652)
+    Array.push(arr, 322)
+    Array.push(arr, 175)
+    Array.push(arr, 714)
+    Array.push(arr, 268)
+    Array.push(arr, 725)
+    Array.push(arr, 664)
+
+    for i in 0:Array.size(arr)
+        print_i64(Array.nth(arr, i))
+    print("------------")
+
+    quicksort(arr)
+
+    for i in 0:Array.size(arr)
+        print_i64(Array.nth(arr, i))
\ No newline at end of file
diff --git a/examples/rule110.zr b/examples/rule110.zr
index 02f349a..3293302 100644
--- a/examples/rule110.zr
+++ b/examples/rule110.zr
@@ -19,9 +19,9 @@ func to_str[state: Array]: String
     let o: String = ""
     for i in 0:Array.size(state)
         if Array.nth(state, i)
-            o = concat(o, "#")
+            o = String.concat(o, "#")
         else
-            o = concat(o, " ")
+            o = String.concat(o, " ")
     return o
 
 func main[] : I64
diff --git a/examples/strings.zr b/examples/strings.zr
index ad28f4f..70d7b61 100644
--- a/examples/strings.zr
+++ b/examples/strings.zr
@@ -1,7 +1,7 @@
 func main[] : I64
     let a: String = I64.to_string(58394)
     print_i64(strlen(a))
-    let b: String = concat(a, "test")
+    let b: String = String.concat(a, "test")
     print_i64(strlen(b))
     free(a)
     free(b)
\ No newline at end of file
diff --git a/src/builtin.s b/src/builtin.s
new file mode 100644
index 0000000..3cd13c4
--- /dev/null
+++ b/src/builtin.s
@@ -0,0 +1,152 @@
+String.nth:
+    movzx rax, byte [rdi + rsi]
+    ret
+
+String.set:
+    mov [rdi + rsi], dl
+    ret
+
+OS.time:
+    push rbx
+    sub rsp, 16
+    mov rbx, rsp
+    mov rdi, rbx
+    xor esi, esi
+    call gettimeofday
+    imul rcx, qword [rbx], 1000
+    mov rax, qword [rbx+8]
+    mov esi, 1000
+    cqo
+    idiv rsi
+    add rax, rcx
+    add rsp, 16
+    pop rbx
+    ret
+
+OS.listdir:
+    push r14
+    push rbx
+    push rax
+    mov r14, rdi
+    call Array.new
+    mov rbx, rax
+    mov rdi, r14
+    call opendir
+    mov r14, rax
+.LBB5_1:
+    mov rdi, r14
+    call readdir
+    test rax, rax
+    je .LBB5_7
+    cmp byte [rax+19], 46
+    jne .LBB5_6
+    movzx ecx, byte [rax+20]
+    test ecx, ecx
+    je .LBB5_1
+    cmp ecx, 46
+    jne .LBB5_6
+    cmp byte [rax+21], 0
+    je .LBB5_1
+.LBB5_6:
+    add rax, 19
+    mov rdi, rax
+    call strdup
+    mov rsi, rax
+    mov rdi, rbx
+    call Array.push
+    jmp .LBB5_1
+.LBB5_7:
+    mov rdi, r14
+    call closedir
+    mov rax, rbx
+    add rsp, 8
+    pop rbx
+    pop r14
+    ret
+
+Array.new:
+    mov rdi, 1
+    mov rsi, 24
+    jmp calloc
+
+Array.nth:
+    mov rax, [rdi]
+    mov rax, [rax + rsi*8]
+    ret
+
+Array.set:
+    mov rax, [rdi]
+    mov [rax + rsi*8], rdx
+    ret
+
+Array.push:
+    push r14
+    push rbx
+    push rax
+    mov r14, rsi
+    mov rbx, rdi
+    mov rax, [rdi]
+    mov rcx, [rdi + 16]
+    cmp rcx, [rdi + 8]
+    jne .no_realloc
+    lea rdx, [rcx + rcx]
+    mov rsi, 4
+    test rcx, rcx
+    cmovnz rsi, rdx
+    mov [rbx + 8], rsi
+    shl rsi, 3
+    mov rdi, rax
+    call realloc
+    mov [rbx], rax
+    mov rcx, [rbx + 16]
+.no_realloc:
+    mov [rax + rcx*8], r14
+    inc qword [rbx + 16]
+    add rsp, 8
+    pop rbx
+    pop r14
+    ret
+
+Array.size:
+    mov rax, [rdi + 16]
+    ret
+
+Array.free:
+    push rbx
+    mov rbx, rdi
+    mov rdi, [rdi]
+    call free
+    mov rdi, rbx
+    pop rbx
+    jmp free
+
+Math.isqrt:
+    xor rax, rax
+    mov rcx, 1
+    mov rbx, rdi
+    shl rcx, 62
+.isqrt.1:
+    cmp rcx, 0
+    je .isqrt.5
+    cmp rcx, rbx
+    jbe .isqrt.2
+    shr rcx, 2
+    jmp .isqrt.1
+.isqrt.2:
+    cmp rcx, 0
+    je .isqrt.5
+    mov rdx, rax
+    add rdx, rcx
+    cmp rbx, rdx
+    jb .isqrt.3
+    sub rbx, rdx
+    shr rax, 1
+    add rax, rcx
+    jmp .isqrt.4
+.isqrt.3:
+    shr rax, 1
+.isqrt.4:
+    shr rcx, 2
+    jmp .isqrt.2
+.isqrt.5:
+    ret
\ No newline at end of file
diff --git a/src/codegen_x86_64.rs b/src/codegen_x86_64.rs
index d4fd18b..2b8e1d6 100644
--- a/src/codegen_x86_64.rs
+++ b/src/codegen_x86_64.rs
@@ -102,6 +102,7 @@ extern realloc
 extern free
 extern printf
 extern sprintf
+extern strtol
 extern strlen
 extern strcmp
 extern strcat
@@ -113,6 +114,8 @@ extern fopen
 extern fseek
 extern ftell
 extern fread
+extern fwrite
+extern fclose
 extern rewind
 extern system
 extern opendir
@@ -120,156 +123,9 @@ extern readdir
 extern closedir
 extern exit
 extern gettimeofday
-
-String.nth:
-    movzx rax, byte [rdi + rsi]
-    ret
-
-String.set:
-    mov [rdi + rsi], dl
-    ret
-
-OS.time:
-    push rbx
-    sub rsp, 16
-    mov rbx, rsp
-    mov rdi, rbx
-    xor esi, esi
-    call gettimeofday
-    imul rcx, qword [rbx], 1000
-    mov rax, qword [rbx+8]
-    mov esi, 1000
-    cqo
-    idiv rsi
-    add rax, rcx
-    add rsp, 16
-    pop rbx
-    ret
-
-OS.listdir:
-    push r14
-    push rbx
-    push rax
-    mov r14, rdi
-    call Array.new
-    mov rbx, rax
-    mov rdi, r14
-    call opendir
-    mov r14, rax
-.LBB5_1:
-    mov rdi, r14
-    call readdir
-    test rax, rax
-    je .LBB5_7
-    cmp byte [rax+19], 46
-    jne .LBB5_6
-    movzx ecx, byte [rax+20]
-    test ecx, ecx
-    je .LBB5_1
-    cmp ecx, 46
-    jne .LBB5_6
-    cmp byte [rax+21], 0
-    je .LBB5_1
-.LBB5_6:
-    add rax, 19
-    mov rdi, rax
-    call strdup
-    mov rsi, rax
-    mov rdi, rbx
-    call Array.push
-    jmp .LBB5_1
-.LBB5_7:
-    mov rdi, r14
-    call closedir
-    mov rax, rbx
-    add rsp, 8
-    pop rbx
-    pop r14
-    ret
-
-Array.new:
-    mov rdi, 1
-    mov rsi, 24
-    jmp calloc
-
-Array.nth:
-    mov rax, [rdi]
-    mov rax, [rax + rsi*8]
-    ret
-
-Array.push:
-    push r14
-    push rbx
-    push rax
-    mov r14, rsi
-    mov rbx, rdi
-    mov rax, [rdi]
-    mov rcx, [rdi + 16]
-    cmp rcx, [rdi + 8]
-    jne .no_realloc
-    lea rdx, [rcx + rcx]
-    mov rsi, 4
-    test rcx, rcx
-    cmovnz rsi, rdx
-    mov [rbx + 8], rsi
-    shl rsi, 3
-    mov rdi, rax
-    call realloc
-    mov [rbx], rax
-    mov rcx, [rbx + 16]
-.no_realloc:
-    mov [rax + rcx*8], r14
-    inc qword [rbx + 16]
-    add rsp, 8
-    pop rbx
-    pop r14
-    ret
-
-Array.size:
-    mov rax, [rdi + 16]
-    ret
-
-Array.free:
-    push rbx
-    mov rbx, rdi
-    mov rdi, [rdi]
-    call free
-    mov rdi, rbx
-    pop rbx
-    jmp free
-
-Math.isqrt:
-    xor rax, rax
-    mov rcx, 1
-    mov rbx, rdi
-    shl rcx, 62
-.isqrt.1:
-    cmp rcx, 0
-    je .isqrt.5
-    cmp rcx, rbx
-    jbe .isqrt.2
-    shr rcx, 2
-    jmp .isqrt.1
-.isqrt.2:
-    cmp rcx, 0
-    je .isqrt.5
-    mov rdx, rax
-    add rdx, rcx
-    cmp rbx, rdx
-    jb .isqrt.3
-    sub rbx, rdx
-    shr rax, 1
-    add rax, rcx
-    jmp .isqrt.4
-.isqrt.3:
-    shr rax, 1
-.isqrt.4:
-    shr rcx, 2
-    jmp .isqrt.2
-.isqrt.5:
-    ret
 ",
         );
+        emit!(&mut self.output, "{}", include_str!("builtin.s"));
         Ok(())
     }
 
diff --git a/src/main.rs b/src/main.rs
index db946c7..f5b91c5 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -44,6 +44,7 @@ fn compile_file(path: String) -> Result<(), ZernError> {
     compile_file_to(&mut codegen, filename, source)?;
     codegen.emit_epilogue()?;
 
+    // TODO
     if fs::write("out.s", codegen.get_output()).is_err() {
         eprintln!("\x1b[91mERROR\x1b[0m: failed to write to out.s");
         process::exit(1);
diff --git a/src/std.zr b/src/std.zr
index 6b8556c..0f08fb2 100644
--- a/src/std.zr
+++ b/src/std.zr
@@ -8,13 +8,13 @@ func print[x: String] : I64
 func print_i64[x: I64] : I64
     printf("%ld\n", x)
 
-func concat[a: String, b: String] : String
+func String.concat[a: String, b: String] : String
     let c: String = malloc(strlen(a) + strlen(b) + 1)
     strcpy(c, a)
     strcat(c, b)
     return c
 
-func substr[s: String, start: I64, length: I64] : String
+func String.substr[s: String, start: I64, length: I64] : String
     let out: String = malloc(length + 1)
     strlcpy(out, s + start, length + 1)
     return out
@@ -42,8 +42,17 @@ func IO.read_file[path: String]: String
 
     let n: I64 = fread(buffer, 1, size, file)
     String.set(buffer, n, 0)
+    fclose(file)
     return buffer
 
+func IO.write_file[path: String, content: String] : I64
+    let file: Ptr = fopen(path, "wb")
+    if !file
+        panic("failed to open file")
+
+    fwrite(content, 1, strlen(content), file)
+    fclose(file)
+
 func Char.to_i64[c: Char]: I64
     return c - 48
 
@@ -52,6 +61,9 @@ func I64.to_string[n: I64] : String
     sprintf(x, "%ld", n)
     return x
 
+func I64.parse[s: String] : I64
+    return strtol(s, 0, 0)
+
 func Math.gcd[a: I64, b: I64] : I64
     while b != 0
         let tmp: I64 = b
diff --git a/test.zr b/test.zr
index d1f2532..4359b0e 100644
--- a/test.zr
+++ b/test.zr
@@ -1,6 +1,6 @@
 func run_test[x: String] : I64
     printf("\033[93mBuilding %s...\033[0m", x)
-    let cmd: String = concat("./target/release/zern examples/", x)
+    let cmd: String = String.concat("./target/release/zern examples/", x)
 
     let build_start_time: I64 = OS.time()
     if system(cmd) != 0