Coding Cairo

Rules and patterns for writing efficient Cairo code. Sourced from audit findings and production profiling.

Attribution: Originally authored by feltroidprime (cairo-skills). Integrated with permission into starknet-agentic.

When to Use

• Implementing arithmetic (modular, parity checks, quotient/remainder)
• Optimizing loops (slow iteration, repeated .len() calls, index-based access)
• Splitting or assembling integer limbs (u256 → u128, u32s → u128, felt252 → u96)
• Packing struct fields into storage slots
• Using BoundedInt for zero-overhead arithmetic with compile-time bounds
• Choosing integer types (u128 vs u256, BoundedInt vs native types)

Not for: Profiling/benchmarking (use benchmarking-cairo if available)

Quick Reference — All Rules

Always / Never Rules

1. Always use DivRem::div_rem — never separate % and /

Cairo computes quotient and remainder in a single operation. Using both % and / on the same value doubles the cost.

// BAD
let q = x / m;
let r = x % m;

// GOOD
let (q, r) = DivRem::div_rem(x, m);

2. Never use < or > in while loop conditions — use !=

Equality checks are cheaper than comparisons in Cairo.

// BAD
while i < n { ... i += 1; }

// GOOD
while i != n { ... i += 1; }

3. Never compute 2^k with pow() — use a lookup table

u32::pow() is expensive. Use a match lookup for known ranges.

// BAD
let p = 2_u32.pow(depth.into());

// GOOD — match-based lookup
fn pow2(n: u32) -> u32 {
    match n {
        0 => 1, 1 => 2, 2 => 4, 3 => 8, 4 => 16, 5 => 32,
        6 => 64, 7 => 128, 8 => 256, 9 => 512, 10 => 1024,
        // extend as needed
        _ => core::panic_with_felt252('pow2 out of range'),
    }
}

4. Always iterate arrays with pop_front / for / multi_pop_front — never index-loop

Index-based access (array.at(i)) is more expensive than pointer-based iteration.

// BAD
let mut i = 0;
while i != data.len() {
    let val = *data.at(i);
    i += 1;
}

// GOOD — pop_front
while let Option::Some(val) = data.pop_front() { ... }

// GOOD — for loop (equivalent)
for val in data { ... }

// GOOD — batch iteration
while let Option::Some(chunk) = data.multi_pop_front::<4>() { ... }

5. Never call .len() inside a loop condition — cache it

.len() recomputes every iteration. Store it once.

// BAD
while i != data.len() { ... i += 1; }

// GOOD
let n = data.len();
while i != n { ... i += 1; }

6. Always use span.slice() instead of manual loop extraction

slice() manipulates pointers directly — no element-by-element copying.

// BAD
let mut result: Array = array![];
let mut i = 0;
while i != length {
    result.append(*data.at(start + i));
    i += 1;
}

// GOOD
let result = data.slice(start, length);

7. Always use DivRem for parity checks — never use bitwise ops

Bitwise AND is more expensive than div_rem in Cairo. Use DivRem::div_rem(x, 2) to get both the halved value and parity in one operation.

// BAD
let is_odd = (index & 1) == 1;
index = index / 2;

// GOOD
let (q, r) = DivRem::div_rem(index, 2);
if r == 1 { /* odd branch */ }
index = q;

8. Always use the smallest integer type that fits the value range

u128 instead of u256 when the range is known. Adds clarity, prevents intermediate overflow.

// BAD — u256 for a value known to be < 2^128
fn deposit(value: u256) { assert(value < MAX_U128, '...'); ... }

// GOOD — type encodes the constraint
fn deposit(value: u128) { ... }

9. Always use StorePacking to pack small fields into one storage slot

Multiple small fields (basis points, flags, bounded amounts) can share a single felt252 slot.

use starknet::storage_access::StorePacking;

const POW_2_128: felt252 = 0x100000000000000000000000000000000;

impl MyStorePacking of StorePacking<MyStruct, felt252> {
    fn pack(value: MyStruct) -> felt252 {
        value.amount.into() + value.fee_bps.into() * POW_2_128
    }
    fn unpack(value: felt252) -> MyStruct {
        let u256 { low, high } = value.into();
        MyStruct { amount: low, fee_bps: high.try_into().unwrap() }
    }
}

10. Always use BoundedInt for byte cutting, limb assembly, and type conversions

Never use bitwise ops (&, |, shifts) or raw u128/u256 arithmetic for splitting or combining integer limbs. Use bounded_int::div_rem to extract parts and bounded_int::mul + bounded_int::add to assemble them. BoundedInt tracks bounds at compile time, eliminating overflow checks.

Assembling limbs (e.g., 4 x u32 → u128):

// BAD — direct u128 arithmetic (28,340 gas)
fn u32s_to_u128(d0: u32, d1: u32, d2: u32, d3: u32) -> u128 {
    d0.into() + d1.into() * POW_2_32 + d2.into() * POW_2_64 + d3.into() * POW_2_96
}

// GOOD — BoundedInt (13,840 gas, 2x faster)
fn u32s_to_u128(d0: u32, d1: u32, d2: u32, d3: u32) -> u128 {
    let d0_bi: u32_bi = upcast(d0);
    let d1_bi: u32_bi = upcast(d1);
    let d2_bi: u32_bi = upcast(d2);
    let d3_bi: u32_bi = upcast(d3);
    let r: u128_bi = add(add(add(d0_bi, mul(d1_bi, POW_32_UI)), mul(d2_bi, POW_64_UI)), mul(d3_bi, POW_96_UI));
    upcast(r)
}

Splitting values (e.g., felt252 → two u96 limbs):

// GOOD — div_rem to split, mul+add to reassemble
fn felt252_to_two_u96(value: felt252) -> (u96, u96) {
    match u128s_from_felt252(value) {
        U128sFromFelt252Result::Narrow(low) => {
            let (hi32, lo96) = bounded_int::div_rem(low, NZ_POW96_TYPED);
            (lo96, upcast(hi32))
        },
        U128sFromFelt252Result::Wide((high, low)) => {
            let (lo_hi32, lo96) = bounded_int::div_rem(low, NZ_POW96_TYPED);
            let hi64: BoundedInt<0, { POW64 - 1 }> = downcast(high).unwrap();
            (lo96, bounded_int::add(bounded_int::mul(hi64, POW32_TYPED), lo_hi32))
        },
    }
}

Extracting bits (e.g., building a 4-bit selector):

// GOOD — div_rem by 2 extracts LSB, quotient is right-shifted value
let (qu1, bit0) = bounded_int::div_rem(u1, TWO_NZ); // bit0 in {0,1}
let (qu2, bit1) = bounded_int::div_rem(u2, TWO_NZ);
let selector = add(bit0, mul(bit1, TWO_UI)); // selector in {0..3}

See garaga/selectors.cairo and cairo-perfs-snippets for production examples.

11. Always use hades_permutation for 2-input Poseidon hashes

poseidon_hash_span has overhead for span construction. For exactly two inputs, use the permutation directly.

// BAD
let hash = poseidon_hash_span(array![x, y].span());

// GOOD
let (hash, _, _) = hades_permutation(x, y, 2);

Code Quality

• DRY: Extract repeated validation into helper functions. If two functions validate-then-write the same struct, extract a shared _set_config().
• scarb fmt: Run before every commit.
• .tool-versions: Pin Scarb (2.15.1) and Starknet Foundry (0.56.0) versions with ASDF for reproducible builds.
• Keep dependencies updated: Newer Scarb/Foundry versions include gas optimizations and compiler improvements.

BoundedInt Optimization

BoundedInt<MIN, MAX> encodes value constraints in the type system, eliminating runtime overflow checks. Use the CLI tool (bounded_int_calc.py in this skill directory) to compute bounds — do NOT calculate manually.

Critical Architecture Decision: Avoid Downcast

The #1 optimization pitfall: Converting between u16/u32/u64 and BoundedInt at function boundaries.

The Problem

If your functions take u16 and return u16, you must:

1. downcast input to BoundedInt (expensive — requires range check)
2. Do bounded arithmetic (cheap)
3. upcast result back to u16 (cheap but wasteful)

The downcast operation adds a range check that dominates the savings from bounded arithmetic. In profiling:

• downcast: 161,280 steps (18.86%)
• bounded_int_div_rem: 204,288 steps (23.89%)
• Total bounded approach: worse than original!

The Solution: BoundedInt Throughout

Use BoundedInt types as function inputs AND outputs. This eliminates downcast entirely.

// BAD: Converts at every call (downcast overhead kills performance)
pub fn add_mod(a: u16, b: u16) -> u16 {
    let a: Zq = downcast(a).expect('overflow'); // EXPENSIVE
    let b: Zq = downcast(b).expect('overflow'); // EXPENSIVE
    let sum: ZqSum = add(a, b);
    let (_q, rem) = bounded_int_div_rem(sum, nz_q);
    upcast(rem)
}

// GOOD: BoundedInt in, BoundedInt out (no downcast)
pub fn add_mod(a: Zq, b: Zq) -> Zq {
    let sum: ZqSum = add(a, b);
    let (_q, rem) = bounded_int_div_rem(sum, nz_q);
    rem
}

Refactoring Strategy

When optimizing existing code:

1. Identify the hot path — profile to find which functions use modular arithmetic heavily
2. Change signatures — update function inputs/outputs to use BoundedInt types
3. Propagate types outward — callers must also use BoundedInt
4. Downcast only at boundaries — convert from u16/u32 only at system entry points (e.g., deserialization)

Type Conversion Rules

Key insight: upcast only works when target range is a superset of source range. You cannot upcast u32 to BoundedInt<0, 150994944> because u32 max (4294967295) > 150994944.

Prerequisites

# Scarb.toml
[dependencies]
corelib_imports = "0.1.2"

// CORRECT imports — copy exactly
use corelib_imports::bounded_int::{
    BoundedInt, upcast, downcast, bounded_int_div_rem,
    AddHelper, MulHelper, DivRemHelper, UnitInt,
};
use corelib_imports::bounded_int::bounded_int::{SubHelper, add, sub, mul};

Copy-Paste Template

Working example for modular arithmetic mod 100:

use corelib_imports::bounded_int::{
    BoundedInt, upcast, downcast, bounded_int_div_rem,
    AddHelper, MulHelper, DivRemHelper, UnitInt,
};
use corelib_imports::bounded_int::bounded_int::{SubHelper, add, sub, mul};

type Val = BoundedInt<0, 99>;          // [0, 99]
type ValSum = BoundedInt<0, 198>;      // [0, 198]
type ValConst = UnitInt<100>;          // singleton {100}

impl AddValImpl of AddHelper<Val, Val> {
    type Result = ValSum;
}

impl DivRemValImpl of DivRemHelper<ValSum, ValConst> {
    type DivT = BoundedInt<0, 1>;
    type RemT = Val;
}

fn add_mod_100(a: Val, b: Val) -> Val {
    let sum: ValSum = add(a, b);
    let nz_100: NonZero<ValConst> = 100;
    let (_q, rem) = bounded_int_div_rem(sum, nz_100);
    rem
}

CLI Tool

Use bounded_int_calc.py in this skill directory. Always use CLI — never calculate manually.

# Addition: [a_lo, a_hi] + [b_lo, b_hi]
python3 bounded_int_calc.py add 0 12288 0 12288
# -> BoundedInt<0, 24576>

# Subtraction: [a_lo, a_hi] - [b_lo, b_hi]
python3 bounded_int_calc.py sub 0 12288 0 12288
# -> BoundedInt<-12288, 12288>

# Multiplication
python3 bounded_int_calc.py mul 0 12288 0 12288
# -> BoundedInt<0, 150994944>

# Division: quotient and remainder bounds
python3 bounded_int_calc.py div 0 24576 12289 12289
# -> DivT: BoundedInt<0, 1>, RemT: BoundedInt<0, 12288>

# Custom impl name
python3 bounded_int_calc.py mul 0 12288 0 12288 --name MulZqImpl

BoundedInt Bounds Quick Reference

Negative Dividends: SHIFT Pattern

bounded_int_div_rem doesn’t support negative lower bounds. When a subtraction produces a negative-bounded result that needs reduction, add a multiple of the modulus first:

// sub_mod: (a - b) mod Q via SHIFT
pub fn sub_mod(a: Zq, b: Zq) -> Zq {
    let a_plus_q: BoundedInt<12289, 24577> = add(a, Q_CONST); // shift by +Q
    let diff: BoundedInt<1, 24577> = sub(a_plus_q, b);        // now non-negative
    let (_q, rem) = bounded_int_div_rem(diff, nz_q());
    rem
}

// fused_sub_mul_mod: a - (b*c) mod Q via large SHIFT
// OFFSET = 12288 * Q = 151007232 (smallest multiple of Q >= max product)
pub fn fused_sub_mul_mod(a: Zq, b: Zq, c: Zq) -> Zq {
    let prod: ZqProd = mul(b, c);
    let a_offset: BoundedInt<151007232, 151019520> = add(a, OFFSET_CONST);
    let diff: BoundedInt<12288, 151019520> = sub(a_offset, prod);
    let (_q, rem) = bounded_int_div_rem(diff, nz_q());
    rem
}

Rule: SHIFT = ceil(|min_possible_value| / modulus) * modulus. Adding SHIFT preserves the result mod Q (since SHIFT ≡ 0 mod Q) while making all values non-negative.

Common BoundedInt Mistakes

• Downcast at every function call — the biggest performance killer. Use BoundedInt types throughout, not just inside arithmetic functions.
• Trying to upcast to a narrower type — upcast(val: u32) to BoundedInt<0, 150994944> fails because u32 max > 150994944.
• Wrong imports — use exact imports from Prerequisites section above.
• Wrong subtraction bounds — it’s [a_lo - b_hi, a_hi - b_lo], NOT [a_lo - b_lo, a_hi - b_hi].
• Negative dividend in bounded_int_div_rem — div_rem doesn’t support negative lower bounds. Add a SHIFT (multiple of modulus) before reducing. See SHIFT pattern above.
• Missing intermediate types — always annotate: let sum: ZqSum = add(a, b);
• Division quotient off-by-one — integer division floors: 24576 / 12289 = 1, not 2.
• Using UnitInt vs BoundedInt for constants — use UnitInt<N> for singleton constants like divisors.
• Using div_rem vs bounded_int_div_rem — the function is bounded_int_div_rem, not div_rem.
• Bounds exceed u128::max — BoundedInt bounds are hard-capped at 2^128. Larger values crash the Sierra specializer: ‘Provided generic argument is unsupported.’