Fix WRITE_DIVMOD_SEGMENT hint (#2078)

* Minor fixes - use CAIRO_PRIME

* Add error handling

* Update CHANGELOG.md

Author: JulianGCalderon

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 #### Upcoming Changes
 
+* fix: Use Cairo prime instead of SECP_P in WRITE_DIVMOD_SEGMENT hint [#2078](https://github.com/lambdaclass/cairo-vm/pull/2078)
 
 * feat: add support for alias identifiers destination in program serde [#2071](https://github.com/lambdaclass/cairo-vm/pull/2071)
 

vm/src/hint_processor/builtin_hint_processor/kzg_da/mod.rs

@@ -1,25 +1,23 @@
 use core::str::FromStr;
 
-use super::{
-    hint_utils::get_relocatable_from_var_name,
-    secp::{bigint_utils::BigInt3, secp_utils::SECP_P},
-};
+use super::{hint_utils::get_relocatable_from_var_name, secp::bigint_utils::BigInt3};
 use crate::{
     hint_processor::hint_processor_definition::HintReference,
     serde::deserialize_program::ApTracking,
     types::relocatable::MaybeRelocatable,
+    utils::CAIRO_PRIME,
     vm::{errors::hint_errors::HintError, vm_core::VirtualMachine},
     Felt252,
 };
 use crate::{
-    stdlib::{collections::HashMap, ops::Deref, prelude::*},
+    stdlib::{collections::HashMap, prelude::*},
     types::exec_scope::ExecutionScopes,
 };
 use lazy_static::lazy_static;
-use num_bigint::BigInt;
+use num_bigint::{BigInt, BigUint};
 use num_integer::Integer;
 use num_traits::FromPrimitive;
-use num_traits::Zero;
+use num_traits::Signed;
 
 lazy_static! {
     static ref BLS_BASE: BigInt = BigInt::from_u64(2).unwrap().pow(86);
@@ -50,21 +48,21 @@ pub fn write_div_mod_segment(
 ) -> Result<(), HintError> {
     let a = bls_pack(
         &BigInt3::from_var_name("a", vm, ids_data, ap_tracking)?,
-        &SECP_P,
+        &CAIRO_PRIME,
     );
     let b = bls_pack(
         &BigInt3::from_var_name("b", vm, ids_data, ap_tracking)?,
-        &SECP_P,
+        &CAIRO_PRIME,
     );
     let (q, r) = (a * b).div_mod_floor(&BLS_PRIME);
     let q_reloc = get_relocatable_from_var_name("q", vm, ids_data, ap_tracking)?;
     let res_reloc = get_relocatable_from_var_name("res", vm, ids_data, ap_tracking)?;
 
-    let q_arg: Vec<MaybeRelocatable> = bls_split(q)
+    let q_arg: Vec<MaybeRelocatable> = bls_split(q)?
         .into_iter()
         .map(|ref n| Felt252::from(n).into())
         .collect::<Vec<MaybeRelocatable>>();
-    let res_arg: Vec<MaybeRelocatable> = bls_split(r)
+    let res_arg: Vec<MaybeRelocatable> = bls_split(r)?
         .into_iter()
         .map(|ref n| Felt252::from(n).into())
         .collect::<Vec<MaybeRelocatable>>();
@@ -74,35 +72,36 @@ pub fn write_div_mod_segment(
     Ok(())
 }
 
-fn bls_split(mut num: BigInt) -> Vec<BigInt> {
-    use num_traits::Signed;
-    let mut a = Vec::new();
+fn bls_split(mut num: BigInt) -> Result<Vec<BigInt>, HintError> {
+    let mut canonical = Vec::new();
     for _ in 0..2 {
-        let residue = &num % BLS_BASE.deref();
-        num /= BLS_BASE.deref();
-        a.push(residue);
+        let (new_num, residue) = num.div_rem(&BLS_BASE);
+        num = new_num;
+        canonical.push(residue);
+    }
+
+    if num.abs() >= BigInt::from(1u128 << 127) {
+        return Err(HintError::BlsSplitError(Box::new(num)));
     }
-    assert!(num.abs() < BigInt::from_u128(1 << 127).unwrap());
-    a.push(num);
-    a
+
+    canonical.push(num);
+    Ok(canonical)
 }
 
-fn as_int(value: BigInt, prime: &BigInt) -> BigInt {
-    let half_prime = prime / 2u32;
-    if value > half_prime {
-        value - prime
-    } else {
+fn as_int(value: BigInt, prime: &BigUint) -> BigInt {
+    let half_prime: BigInt = (prime / 2u32).into();
+    let prime: BigInt = prime.clone().into();
+    if value < half_prime {
         value
+    } else {
+        value - prime
     }
 }
 
-fn bls_pack(z: &BigInt3, prime: &BigInt) -> BigInt {
-    let limbs = &z.limbs;
-    limbs
+fn bls_pack(z: &BigInt3, prime: &BigUint) -> BigInt {
+    z.limbs
         .iter()
         .enumerate()
-        .fold(BigInt::zero(), |acc, (i, limb)| {
-            let limb_as_int = as_int(limb.to_bigint(), prime);
-            acc + limb_as_int * &BLS_BASE.pow(i as u32)
-        })
+        .map(|(i, limb)| as_int(limb.to_bigint(), prime) * &BLS_BASE.pow(i as u32))
+        .sum()
 }

vm/src/vm/errors/hint_errors.rs

@@ -196,6 +196,8 @@ pub enum HintError {
     EmptyNibbles,
     #[error("circuit evalution: {0}")]
     CircuitEvaluationFailed(Box<str>),
+    #[error("high limb magnitude should be smaller than 2 ** 127: {0}")]
+    BlsSplitError(Box<BigInt>),
 }
 
 #[cfg(test)]