polynomial: use FieldVec in Polynomial

We can now use `PrintImpl` even without an allocator. This is of limited
use of course, but it paves the ground for use to do error correction
without an allocator, which is interesting.

Author: apoelstra

src/primitives/checksum.rs

@@ -4,20 +4,14 @@
 //!
 //! [BCH]: <https://en.wikipedia.org/wiki/BCH_code>
 
-#[cfg(all(feature = "alloc", not(feature = "std"), not(test)))]
+#[cfg(all(feature = "alloc", not(feature = "std")))]
 use alloc::vec::Vec;
-#[cfg(feature = "alloc")]
-use core::fmt;
-#[cfg(feature = "alloc")]
 use core::marker::PhantomData;
-use core::{mem, ops};
+use core::{fmt, mem, ops};
 
-#[cfg(feature = "alloc")]
 use super::Polynomial;
 use crate::primitives::hrp::Hrp;
-#[cfg(feature = "alloc")]
-use crate::Fe1024;
-use crate::Fe32;
+use crate::{Fe1024, Fe32};
 
 /// Trait defining a particular checksum.
 ///
@@ -169,18 +163,16 @@ pub trait Checksum {
 /// to compute the values yourself. The reason is that the specific values
 /// used depend on the representation of extension fields, which may differ
 /// between implementations (and between specifications) of your BCH code.
-#[cfg(feature = "alloc")]
 pub struct PrintImpl<'a, ExtField = Fe1024> {
     name: &'a str,
-    generator: &'a [Fe32],
+    generator: Polynomial<Fe32>,
     target: &'a [Fe32],
     bit_len: usize,
     hex_width: usize,
     midstate_repr: &'static str,
     phantom: PhantomData<ExtField>,
 }
 
-#[cfg(feature = "alloc")]
 impl<'a, ExtField> PrintImpl<'a, ExtField> {
     /// Constructor for an object to print an impl-block for the [`Checksum`] trait.
     ///
@@ -225,7 +217,7 @@ impl<'a, ExtField> PrintImpl<'a, ExtField> {
         // End sanity checks.
         PrintImpl {
             name,
-            generator,
+            generator: Polynomial::with_monic_leading_term(generator),
             target,
             bit_len,
             hex_width,
@@ -235,23 +227,16 @@ impl<'a, ExtField> PrintImpl<'a, ExtField> {
     }
 }
 
-#[cfg(feature = "alloc")]
 impl<'a, ExtField> fmt::Display for PrintImpl<'a, ExtField>
 where
     ExtField: super::Bech32Field + super::ExtensionField<BaseField = Fe32>,
 {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         // Generator polynomial as a polynomial over GF1024
-        let gen_poly = {
-            let mut v = Vec::with_capacity(self.generator.len() + 1);
-            v.push(ExtField::ONE);
-            v.extend(self.generator.iter().cloned().map(ExtField::from));
-            Polynomial::new(v)
-        };
-        let (gen, length, exponents) = gen_poly.bch_generator_primitive_element();
+        let (gen, length, exponents) = self.generator.bch_generator_primitive_element::<ExtField>();
 
         write!(f, "// Code block generated by Checksum::print_impl polynomial ")?;
-        for fe in self.generator {
+        for fe in self.generator.as_inner() {
             write!(f, "{}", fe)?;
         }
         write!(f, " target ")?;
@@ -278,9 +263,9 @@ where
         )?;
         f.write_str("\n")?;
         writeln!(f, "    const CODE_LENGTH: usize = {};", length)?;
-        writeln!(f, "    const CHECKSUM_LENGTH: usize = {};", gen_poly.degree())?;
+        writeln!(f, "    const CHECKSUM_LENGTH: usize = {};", self.generator.degree())?;
         writeln!(f, "    const GENERATOR_SH: [{}; 5] = [", self.midstate_repr)?;
-        let mut gen5 = self.generator.to_vec();
+        let mut gen5 = self.generator.clone().into_inner();
         for _ in 0..5 {
             let gen_packed = u128::pack(gen5.iter().copied().map(From::from));
             writeln!(f, "        0x{:0width$x},", gen_packed, width = self.hex_width)?;

src/primitives/fieldvec.rs

@@ -186,6 +186,34 @@ impl<F: Field> FieldVec<F> {
     }
 }
 
+impl<F: Default> FieldVec<F> {
+    /// Pushes an item onto the end of the vector.
+    ///
+    /// # Panics
+    ///
+    /// Panics if [`Self::has_data`] is false, or if it would be false after the push.
+    pub fn push(&mut self, item: F) {
+        self.len += 1;
+        self.assert_has_data();
+
+        #[cfg(not(feature = "alloc"))]
+        {
+            self.inner_a[self.len - 1] = item;
+        }
+
+        #[cfg(feature = "alloc")]
+        if self.len < NO_ALLOC_MAX_LENGTH + 1 {
+            self.inner_a[self.len - 1] = item;
+        } else {
+            if self.len == NO_ALLOC_MAX_LENGTH + 1 {
+                let inner_a = core::mem::take(&mut self.inner_a);
+                self.inner_v = inner_a.into();
+            }
+            self.inner_v.push(item);
+        }
+    }
+}
+
 impl<F: Clone + Default> iter::FromIterator<F> for FieldVec<F> {
     /// Constructor from an iterator of elements.
     ///

src/primitives/mod.rs

@@ -12,14 +12,13 @@ pub mod gf32;
 pub mod gf32_ext;
 pub mod hrp;
 pub mod iter;
-#[cfg(feature = "alloc")]
 mod polynomial;
 pub mod segwit;
 
 use checksum::{Checksum, PackedNull};
 use field::impl_ops_for_fe;
 pub use field::{Bech32Field, ExtensionField, Field};
-#[cfg(feature = "alloc")]
+use fieldvec::FieldVec;
 use polynomial::Polynomial;
 
 use crate::{Fe1024, Fe32};