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