chore!: refactor ToRadix to ToRadixLe and ToRadixBe

acir/src/circuit/directives.rs

@@ -42,11 +42,12 @@ pub enum Directive {
         bit_size: u32,
     },
 
-    //decomposition of a: a=\sum b[i]*radix^i where b is an array of witnesses < radix
+    //decomposition of a: a=\sum b[i]*radix^i where b is an array of witnesses < radix in either little endian or big endian form
     ToRadix {
         a: Expression,
         b: Vec<Witness>,
         radix: u32,
+        is_little_endian: bool,
     },
 
     // Sort directive, using a sorting network
@@ -122,13 +123,19 @@ impl Directive {
                 write_u32(&mut writer, r.witness_index())?;
                 write_u32(&mut writer, *bit_size)?;
             }
-            Directive::ToRadix { a, b, radix } => {
+            Directive::ToRadix {
+                a,
+                b,
+                radix,
+                is_little_endian,
+            } => {
                 a.write(&mut writer)?;
                 write_u32(&mut writer, b.len() as u32)?;
                 for bit in b {
                     write_u32(&mut writer, bit.witness_index())?;
                 }
                 write_u32(&mut writer, *radix)?;
+                write_u32(&mut writer, *is_little_endian as u32)?;
             }
             Directive::PermutationSort {
                 inputs: a,
@@ -222,8 +229,14 @@ impl Directive {
                 }
 
                 let radix = read_u32(&mut reader)?;
+                let is_little_endian = read_u32(&mut reader)?;
 
-                Ok(Directive::ToRadix { a, b, radix })
+                Ok(Directive::ToRadix {
+                    a,
+                    b,
+                    radix,
+                    is_little_endian: is_little_endian == 1,
+                })
             }
             6 => {
                 let tuple = read_u32(&mut reader)?;
@@ -314,10 +327,18 @@ fn serialization_roundtrip() {
         bit_size: 32,
     };
 
-    let to_radix = Directive::ToRadix {
+    let to_radix_le = Directive::ToRadix {
+        a: Expression::default(),
+        b: vec![Witness(1u32), Witness(2u32), Witness(3u32), Witness(4u32)],
+        radix: 4,
+        is_little_endian: true,
+    };
+
+    let to_radix_be = Directive::ToRadix {
         a: Expression::default(),
         b: vec![Witness(1u32), Witness(2u32), Witness(3u32), Witness(4u32)],
         radix: 4,
+        is_little_endian: false,
     };
 
     let directives = vec![
@@ -326,7 +347,8 @@ fn serialization_roundtrip() {
         quotient_predicate,
         truncate,
         odd_range,
-        to_radix,
+        to_radix_le,
+        to_radix_be,
     ];
 
     for directive in directives {

acir/src/circuit/opcodes.rs

@@ -152,16 +152,22 @@ impl std::fmt::Display for Opcode {
                 )
             }
             Opcode::BlackBoxFuncCall(g) => write!(f, "{g}"),
-            Opcode::Directive(Directive::ToRadix { a, b, radix: _ }) => {
+            Opcode::Directive(Directive::ToRadix {
+                a,
+                b,
+                radix: _,
+                is_little_endian,
+            }) => {
                 write!(f, "DIR::TORADIX ")?;
                 write!(
                     f,
                     // TODO (Note): this assumes that the decomposed bits have contiguous witness indices
                     // This should be the case, however, we can also have a function which checks this
-                    "(_{}, [_{}..._{}])",
+                    "(_{}, [_{}..._{}], endianness: {})",
                     a,
                     b.first().unwrap().witness_index(),
                     b.last().unwrap().witness_index(),
+                    if *is_little_endian { "little" } else { "big" }
                 )
             }
             Opcode::Directive(Directive::PermutationSort {

acvm/src/lib.rs

@@ -31,6 +31,8 @@ pub enum OpcodeNotSolvable {
     MissingAssignment(u32),
     #[error("expression has too many unknowns {0}")]
     ExpressionHasTooManyUnknowns(Expression),
+    #[error("compiler error: unreachable code")]
+    UnreachableCode,
 }
 
 #[derive(PartialEq, Eq, Debug, Error)]

acvm/src/pwg.rs

@@ -64,3 +64,26 @@ pub fn get_value(
 
     Ok(result)
 }
+
+// Inserts `value` into the initial witness map
+// under the key of `witness`.
+// Returns an error, if there was already a value in the map
+// which does not match the value that one is about to insert
+fn insert_value(
+    witness: &Witness,
+    value_to_insert: FieldElement,
+    initial_witness: &mut BTreeMap<Witness, FieldElement>,
+) -> Result<(), OpcodeResolutionError> {
+    let optional_old_value = initial_witness.insert(*witness, value_to_insert);
+
+    let old_value = match optional_old_value {
+        Some(old_value) => old_value,
+        None => return Ok(()),
+    };
+
+    if old_value != value_to_insert {
+        return Err(OpcodeResolutionError::UnsatisfiedConstrain);
+    }
+
+    Ok(())
+}

acvm/src/pwg/directives.rs

@@ -8,9 +8,9 @@ use acir::{
 use num_bigint::BigUint;
 use num_traits::{One, Zero};
 
-use crate::OpcodeResolutionError;
+use crate::{OpcodeNotSolvable, OpcodeResolutionError};
 
-use super::{get_value, sorting::route, witness_to_value};
+use super::{get_value, insert_value, sorting::route, witness_to_value};
 
 pub fn solve_directives(
     initial_witness: &mut BTreeMap<Witness, FieldElement>,
@@ -84,21 +84,58 @@ pub fn solve_directives(
 
             Ok(())
         }
-        Directive::ToRadix { a, b, radix } => {
-            let val_a = get_value(a, initial_witness)?;
+        Directive::ToRadix {
+            a,
+            b,
+            radix,
+            is_little_endian,
+        } => {
+            let value_a = get_value(a, initial_witness)?;
 
-            let a_big = BigUint::from_bytes_be(&val_a.to_be_bytes());
-            let a_dec = a_big.to_radix_le(*radix);
-            if b.len() < a_dec.len() {
-                return Err(OpcodeResolutionError::UnsatisfiedConstrain);
-            }
-            for i in 0..b.len() {
-                let v = if i < a_dec.len() {
-                    FieldElement::from_be_bytes_reduce(&[a_dec[i]])
-                } else {
-                    FieldElement::zero()
-                };
-                insert_witness(b[i], v, initial_witness)?;
+            let big_integer = BigUint::from_bytes_be(&value_a.to_be_bytes());
+
+            if *is_little_endian {
+                // Decompose the integer into its radix digits in little endian form.
+                let decomposed_integer = big_integer.to_radix_le(*radix);
+
+                if b.len() < decomposed_integer.len() {
+                    return Err(OpcodeResolutionError::UnsatisfiedConstrain);
+                }
+
+                for (i, witness) in b.iter().enumerate() {
+                    // Fetch the `i'th` digit from the decomposed integer list
+                    // and convert it to a field element.
+                    // If it is not available, which can happen when the decomposed integer
+                    // list is shorter than the witness list, we return 0.
+                    let value = match decomposed_integer.get(i) {
+                        Some(digit) => FieldElement::from_be_bytes_reduce(&[*digit]),
+                        None => FieldElement::zero(),
+                    };
+
+                    insert_value(witness, value, initial_witness)?
+                }
+            } else {
+                // Decompose the integer into its radix digits in big endian form.
+                let decomposed_integer = big_integer.to_radix_be(*radix);
+
+                // if it is big endian and the decompoased integer list is shorter
+                // than the witness list, pad the extra part with 0 first then
+                // add the decompsed interger list to the witness list.
+                let padding_len = b.len() - decomposed_integer.len();
+                let mut value = FieldElement::zero();
+                for (i, witness) in b.iter().enumerate() {
+                    if i >= padding_len {
+                        value = match decomposed_integer.get(i - padding_len) {
+                            Some(digit) => FieldElement::from_be_bytes_reduce(&[*digit]),
+                            None => {
+                                return Err(OpcodeResolutionError::OpcodeNotSolvable(
+                                    OpcodeNotSolvable::UnreachableCode,
+                                ))
+                            }
+                        };
+                    }
+                    insert_value(witness, value, initial_witness)?
+                }
             }
 
             Ok(())

stdlib/src/fallback.rs

@@ -42,6 +42,7 @@ pub(crate) fn bit_decomposition(
         a: gate.clone(),
         b: bit_vector.clone(),
         radix: 2,
+        is_little_endian: true,
     }));
 
     // Now apply constraints to the bits such that they are the bit decomposition