Add pippenger under multiexp

crypto/multiexp/src/straus.rs

@@ -0,0 +1,69 @@
+use group::{ff::PrimeField, Group};
+
+fn prep<G: Group>(pairs: &[(G::Scalar, G)], little: bool) -> (Vec<Vec<u8>>, Vec<[G; 16]>) {
+  let mut nibbles = vec![];
+  let mut tables = vec![];
+  for pair in pairs {
+    let p = nibbles.len();
+    nibbles.push(vec![]);
+    {
+      let mut repr = pair.0.to_repr();
+      let bytes = repr.as_mut();
+      if !little {
+        bytes.reverse();
+      }
+
+      nibbles[p].resize(bytes.len() * 2, 0);
+      for i in 0 .. bytes.len() {
+        nibbles[p][i * 2] = bytes[i] & 0b1111;
+        nibbles[p][(i * 2) + 1] = (bytes[i] >> 4) & 0b1111;
+      }
+    }
+
+    tables.push([G::identity(); 16]);
+    let mut accum = G::identity();
+    for i in 1 .. 16 {
+      accum += pair.1;
+      tables[p][i] = accum;
+    }
+  }
+
+  (nibbles, tables)
+}
+
+pub(crate) fn straus<G: Group>(pairs: &[(G::Scalar, G)], little: bool) -> G {
+  let (nibbles, tables) = prep(pairs, little);
+
+  let mut res = G::identity();
+  for b in (0 .. nibbles[0].len()).rev() {
+    for _ in 0 .. 4 {
+      res = res.double();
+    }
+
+    for s in 0 .. tables.len() {
+      res += tables[s][usize::from(nibbles[s][b])];
+    }
+  }
+  res
+}
+
+pub(crate) fn straus_vartime<G: Group>(pairs: &[(G::Scalar, G)], little: bool) -> G {
+  let (nibbles, tables) = prep(pairs, little);
+
+  let mut res = G::identity();
+  for b in (0 .. nibbles[0].len()).rev() {
+    if b != (nibbles[0].len() - 1) {
+      for _ in 0 .. 4 {
+        res = res.double();
+      }
+    }
+
+    for s in 0 .. tables.len() {
+      if nibbles[s][b] != 0 {
+        res += tables[s][usize::from(nibbles[s][b])];
+      }
+    }
+  }
+
+  res
+}