Add SmallPowers struct

0xPolygonZero · Jul 31, 2023 · 3293870 · 3293870
1 parent c661576
commit 3293870
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Showing 2 changed files with 29 additions and 3 deletions.
diff --git a/evm/src/cross_table_lookup.rs b/evm/src/cross_table_lookup.rs
@@ -6,7 +6,7 @@ use itertools::Itertools;
 use plonky2::field::extension::{Extendable, FieldExtension};
 use plonky2::field::packed::PackedField;
 use plonky2::field::polynomial::PolynomialValues;
-use plonky2::field::types::Field;
+use plonky2::field::types::{Field, SmallPowers};
 use plonky2::hash::hash_types::RichField;
 use plonky2::iop::ext_target::ExtensionTarget;
 use plonky2::iop::target::Target;
@@ -79,14 +79,14 @@ impl<F: Field> Column<F> {
     }
 
     pub fn le_bits<I: IntoIterator<Item = impl Borrow<usize>>>(cs: I) -> Self {
-        Self::linear_combination(cs.into_iter().map(|c| *c.borrow()).zip(F::TWO.powers()))
+        Self::linear_combination(cs.into_iter().map(|c| *c.borrow()).zip(SmallPowers::new(2)))
     }
 
     pub fn le_bytes<I: IntoIterator<Item = impl Borrow<usize>>>(cs: I) -> Self {
         Self::linear_combination(
             cs.into_iter()
                 .map(|c| *c.borrow())
-                .zip(F::from_canonical_u16(256).powers()),
+                .zip(SmallPowers::new(256)),
         )
     }
 

diff --git a/field/src/types.rs b/field/src/types.rs
@@ -598,3 +598,29 @@ impl<F: Field> Powers<F> {
         }
     }
 }
+
+/// An iterator similar to `Powers`, but which base fits in a `u32`.
+#[derive(Clone)]
+pub struct SmallPowers<F: Field> {
+    base: u32,
+    current: F,
+}
+
+impl<F: Field> SmallPowers<F> {
+    pub fn new(base: u32) -> Self {
+        Self {
+            base,
+            current: F::ONE,
+        }
+    }
+}
+
+impl<F: Field> Iterator for SmallPowers<F> {
+    type Item = F;
+
+    fn next(&mut self) -> Option<F> {
+        let result = self.current;
+        self.current = self.current.mul_u32(self.base);
+        Some(result)
+    }
+}