Refactor equihash code

components/equihash/src/lib.rs

@@ -20,6 +20,8 @@
 // Catch documentation errors caused by code changes.
 #![deny(rustdoc::broken_intra_doc_links)]
 
+mod minimal;
+mod params;
 mod verify;
 
 #[cfg(test)]

components/equihash/src/minimal.rs

@@ -0,0 +1,190 @@
+use std::io::Cursor;
+use std::mem::size_of;
+
+use byteorder::{BigEndian, ReadBytesExt};
+
+use crate::params::Params;
+
+pub(crate) fn expand_array(vin: &[u8], bit_len: usize, byte_pad: usize) -> Vec<u8> {
+    assert!(bit_len >= 8);
+    assert!(u32::BITS as usize >= 7 + bit_len);
+
+    let out_width = (bit_len + 7) / 8 + byte_pad;
+    let out_len = 8 * out_width * vin.len() / bit_len;
+
+    // Shortcut for parameters where expansion is a no-op
+    if out_len == vin.len() {
+        return vin.to_vec();
+    }
+
+    let mut vout: Vec<u8> = vec![0; out_len];
+    let bit_len_mask: u32 = (1 << bit_len) - 1;
+
+    // The acc_bits least-significant bits of acc_value represent a bit sequence
+    // in big-endian order.
+    let mut acc_bits = 0;
+    let mut acc_value: u32 = 0;
+
+    let mut j = 0;
+    for b in vin {
+        acc_value = (acc_value << 8) | u32::from(*b);
+        acc_bits += 8;
+
+        // When we have bit_len or more bits in the accumulator, write the next
+        // output element.
+        if acc_bits >= bit_len {
+            acc_bits -= bit_len;
+            for x in byte_pad..out_width {
+                vout[j + x] = ((
+                    // Big-endian
+                    acc_value >> (acc_bits + (8 * (out_width - x - 1)))
+                ) & (
+                    // Apply bit_len_mask across byte boundaries
+                    (bit_len_mask >> (8 * (out_width - x - 1))) & 0xFF
+                )) as u8;
+            }
+            j += out_width;
+        }
+    }
+
+    vout
+}
+
+/// Returns `None` if the parameters are invalid for this minimal encoding.
+pub(crate) fn indices_from_minimal(p: Params, minimal: &[u8]) -> Option<Vec<u32>> {
+    let c_bit_len = p.collision_bit_length();
+    // Division is exact because k >= 3.
+    if minimal.len() != ((1 << p.k) * (c_bit_len + 1)) / 8 {
+        return None;
+    }
+
+    assert!(((c_bit_len + 1) + 7) / 8 <= size_of::<u32>());
+    let len_indices = u32::BITS as usize * minimal.len() / (c_bit_len + 1);
+    let byte_pad = size_of::<u32>() - ((c_bit_len + 1) + 7) / 8;
+
+    let mut csr = Cursor::new(expand_array(minimal, c_bit_len + 1, byte_pad));
+    let mut ret = Vec::with_capacity(len_indices);
+
+    // Big-endian so that lexicographic array comparison is equivalent to integer
+    // comparison
+    while let Ok(i) = csr.read_u32::<BigEndian>() {
+        ret.push(i);
+    }
+
+    Some(ret)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{expand_array, indices_from_minimal, Params};
+
+    #[test]
+    fn array_expansion() {
+        let check_array = |(bit_len, byte_pad), compact, expanded| {
+            assert_eq!(expand_array(compact, bit_len, byte_pad), expanded);
+        };
+
+        // 8 11-bit chunks, all-ones
+        check_array(
+            (11, 0),
+            &[
+                0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+            ],
+            &[
+                0x07, 0xff, 0x07, 0xff, 0x07, 0xff, 0x07, 0xff, 0x07, 0xff, 0x07, 0xff, 0x07, 0xff,
+                0x07, 0xff,
+            ][..],
+        );
+        // 8 21-bit chunks, alternating 1s and 0s
+        check_array(
+            (21, 0),
+            &[
+                0xaa, 0xaa, 0xad, 0x55, 0x55, 0x6a, 0xaa, 0xab, 0x55, 0x55, 0x5a, 0xaa, 0xaa, 0xd5,
+                0x55, 0x56, 0xaa, 0xaa, 0xb5, 0x55, 0x55,
+            ],
+            &[
+                0x15, 0x55, 0x55, 0x15, 0x55, 0x55, 0x15, 0x55, 0x55, 0x15, 0x55, 0x55, 0x15, 0x55,
+                0x55, 0x15, 0x55, 0x55, 0x15, 0x55, 0x55, 0x15, 0x55, 0x55,
+            ][..],
+        );
+        // 8 21-bit chunks, based on example in the spec
+        check_array(
+            (21, 0),
+            &[
+                0x00, 0x02, 0x20, 0x00, 0x0a, 0x7f, 0xff, 0xfe, 0x00, 0x12, 0x30, 0x22, 0xb3, 0x82,
+                0x26, 0xac, 0x19, 0xbd, 0xf2, 0x34, 0x56,
+            ],
+            &[
+                0x00, 0x00, 0x44, 0x00, 0x00, 0x29, 0x1f, 0xff, 0xff, 0x00, 0x01, 0x23, 0x00, 0x45,
+                0x67, 0x00, 0x89, 0xab, 0x00, 0xcd, 0xef, 0x12, 0x34, 0x56,
+            ][..],
+        );
+        // 16 14-bit chunks, alternating 11s and 00s
+        check_array(
+            (14, 0),
+            &[
+                0xcc, 0xcf, 0x33, 0x3c, 0xcc, 0xf3, 0x33, 0xcc, 0xcf, 0x33, 0x3c, 0xcc, 0xf3, 0x33,
+                0xcc, 0xcf, 0x33, 0x3c, 0xcc, 0xf3, 0x33, 0xcc, 0xcf, 0x33, 0x3c, 0xcc, 0xf3, 0x33,
+            ],
+            &[
+                0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33,
+                0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33, 0x33,
+                0x33, 0x33, 0x33, 0x33,
+            ][..],
+        );
+        // 8 11-bit chunks, all-ones, 2-byte padding
+        check_array(
+            (11, 2),
+            &[
+                0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+            ],
+            &[
+                0x00, 0x00, 0x07, 0xff, 0x00, 0x00, 0x07, 0xff, 0x00, 0x00, 0x07, 0xff, 0x00, 0x00,
+                0x07, 0xff, 0x00, 0x00, 0x07, 0xff, 0x00, 0x00, 0x07, 0xff, 0x00, 0x00, 0x07, 0xff,
+                0x00, 0x00, 0x07, 0xff,
+            ][..],
+        );
+    }
+
+    #[test]
+    fn minimal_solution_repr() {
+        let check_repr = |minimal, indices| {
+            assert_eq!(
+                indices_from_minimal(Params { n: 80, k: 3 }, minimal).unwrap(),
+                indices,
+            );
+        };
+
+        // The solutions here are not intended to be valid.
+        check_repr(
+            &[
+                0x00, 0x00, 0x08, 0x00, 0x00, 0x40, 0x00, 0x02, 0x00, 0x00, 0x10, 0x00, 0x00, 0x80,
+                0x00, 0x04, 0x00, 0x00, 0x20, 0x00, 0x01,
+            ],
+            &[1, 1, 1, 1, 1, 1, 1, 1],
+        );
+        check_repr(
+            &[
+                0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+                0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+            ],
+            &[
+                2097151, 2097151, 2097151, 2097151, 2097151, 2097151, 2097151, 2097151,
+            ],
+        );
+        check_repr(
+            &[
+                0x0f, 0xff, 0xf8, 0x00, 0x20, 0x03, 0xff, 0xfe, 0x00, 0x08, 0x00, 0xff, 0xff, 0x80,
+                0x02, 0x00, 0x3f, 0xff, 0xe0, 0x00, 0x80,
+            ],
+            &[131071, 128, 131071, 128, 131071, 128, 131071, 128],
+        );
+        check_repr(
+            &[
+                0x00, 0x02, 0x20, 0x00, 0x0a, 0x7f, 0xff, 0xfe, 0x00, 0x4d, 0x10, 0x01, 0x4c, 0x80,
+                0x0f, 0xfc, 0x00, 0x00, 0x2f, 0xff, 0xff,
+            ],
+            &[68, 41, 2097151, 1233, 665, 1023, 1, 1048575],
+        );
+    }
+}

components/equihash/src/params.rs

@@ -0,0 +1,37 @@
+#[derive(Clone, Copy)]
+pub(crate) struct Params {
+    pub(crate) n: u32,
+    pub(crate) k: u32,
+}
+
+impl Params {
+    /// Returns `None` if the parameters are invalid.
+    pub(crate) fn new(n: u32, k: u32) -> Option<Self> {
+        // We place the following requirements on the parameters:
+        // - n is a multiple of 8, so the hash output has an exact byte length.
+        // - k >= 3 so the encoded solutions have an exact byte length.
+        // - k < n, so the collision bit length is at least 1.
+        // - n is a multiple of k + 1, so we have an integer collision bit length.
+        if (n % 8 == 0) && (k >= 3) && (k < n) && (n % (k + 1) == 0) {
+            Some(Params { n, k })
+        } else {
+            None
+        }
+    }
+    pub(crate) fn indices_per_hash_output(&self) -> u32 {
+        512 / self.n
+    }
+    pub(crate) fn hash_output(&self) -> u8 {
+        (self.indices_per_hash_output() * self.n / 8) as u8
+    }
+    pub(crate) fn collision_bit_length(&self) -> usize {
+        (self.n / (self.k + 1)) as usize
+    }
+    pub(crate) fn collision_byte_length(&self) -> usize {
+        (self.collision_bit_length() + 7) / 8
+    }
+    #[cfg(test)]
+    pub(crate) fn hash_length(&self) -> usize {
+        ((self.k as usize) + 1) * self.collision_byte_length()
+    }
+}

components/equihash/src/test_vectors/invalid.rs

@@ -1,4 +1,4 @@
-use crate::verify::{Kind, Params};
+use crate::{params::Params, verify::Kind};
 
 pub(crate) struct TestVector {
     pub(crate) params: Params,

components/equihash/src/test_vectors/valid.rs

@@ -1,4 +1,4 @@
-use crate::verify::Params;
+use crate::params::Params;
 
 pub(crate) struct TestVector {
     pub(crate) params: Params,