feat: add pack_bytes

Author: olehmisar

README.md

@@ -9,7 +9,7 @@ Put this into your Nargo.toml.
 If you are using Noir:
 
 ```toml
-nodash = { git = "https://github.com/olehmisar/nodash/", tag = "v0.35.3" }
+nodash = { git = "https://github.com/olehmisar/nodash/", tag = "v0.35.4" }
 ```
 
 The version of nodash matches the version of Noir. The patch version may be different if a bugfix or a new feature is added for the same version of Noir. E.g., [email protected] and [email protected] are compatible with [email protected].
@@ -137,3 +137,14 @@ use nodash::ArrayExtensions;
 
 assert([1, 2, 3].pad_end::<5>(0) == [1, 2, 3, 0, 0]);
 ```
+
+### `pack_bytes`
+
+Packs `[u8; N]` into `[Field; N / 31 + 1]`. Useful, if you need to get a hash over bytes. I.e., `pedersen_hash(pack_bytes(bytes))` will be MUCH cheaper than `pedersen_hash(bytes)`.
+
+```rs
+use nodash::pack_bytes;
+
+let bytes: [u8; 32] = [0; 32];
+let packed = pack_bytes(bytes);
+```

src/array.nr

@@ -39,6 +39,29 @@ impl<T, let N: u32> crate::ArrayExtensions<T, N> for [T; N] {
     }
 }
 
+// TODO: write tests
+pub fn pack_bytes<let N: u32>(bytes: [u8; N]) -> [Field; N / 31 + 1] {
+    let bytes_padded = bytes.pad_end::<(N / 31 + 1) * 31>(N / 31 + 1) * 31>( 0);
+    let mut res = [0 as Field; N / 31 + 1];
+    for i in 0..N / 31 + 1 {
+        let chunk = bytes_padded.slice::<31>(i * 31, i * 31 + 31);
+        res[i] = field_from_bytes(chunk);
+    }
+    res
+}
+
+// copied from https://github.com/AztecProtocol/aztec-packages/blob/a2ed567ad42b237088c110ce12ce8212d5099da2/noir-projects/noir-protocol-circuits/crates/types/src/utils/field.nr#L4
+fn field_from_bytes<let N: u32>(bytes: [u8; N]) -> Field {
+    assert(bytes.len() < 32, "field_from_bytes: N must be less than 32");
+    let mut as_field = 0;
+    let mut offset = 1;
+    for i in 0..N {
+        as_field += (bytes[i] as Field) * offset;
+        offset *= 256;
+    }
+    as_field
+}
+
 mod tests {
     #[test]
     fn test_slice() {