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use std::io::{Read, Result as IoResult};
const TABLE: [u32; 256] = generate_table();
const fn generate_table() -> [u32; 256] {
let mut table = [0; 256];
let mut i = 0;
while i <= 255 {
let mut t = i as u32;
let mut j = 0;
while j < 8 {
if (t & 1) > 0 {
t = (t >> 1) ^ 0xEDB88320
} else {
t >>= 1
}
j += 1;
}
table[i] = t;
i += 1
}
table
}
fn crc32(byte: u8, crc: u32) -> u32 {
(crc >> 8) ^ TABLE[((crc & 0xFF) as u8 ^ byte) as usize]
}
pub struct Keys {
key0: u32,
key1: u32,
key2: u32,
}
impl Keys {
pub fn new() -> Self {
Self {
key0: 305419896,
key1: 591751049,
key2: 878082192,
}
}
pub fn update(&mut self, byte: u8) {
self.key0 = crc32(byte, self.key0);
self.key1 = self
.key1
.wrapping_add(self.key0 & 0xFF)
.wrapping_mul(134775813)
.wrapping_add(1);
self.key2 = crc32((self.key1 >> 24) as u8, self.key2);
}
#[allow(dead_code)]
pub fn encode_bytes(&mut self, byte: u8) -> u8 {
let key = self.key2 | 2;
self.update(byte);
byte ^ ((key.wrapping_mul(key ^ 1)) >> 8) as u8
}
pub fn decode_byte(&mut self, byte: u8) -> u8 {
let key = self.key2 | 2;
let byte = byte ^ ((key.wrapping_mul(key ^ 1)) >> 8) as u8;
self.update(byte);
byte
}
}
pub struct WeakDecoder<Io: Read> {
io: Io,
keys: Keys,
}
impl<Io: Read> WeakDecoder<Io> {
pub fn new(io: Io, keys: Keys) -> Self {
WeakDecoder { io, keys }
}
}
impl<Io: Read> Read for WeakDecoder<Io> {
fn read(&mut self, buf: &mut [u8]) -> IoResult<usize> {
let bytes = self.io.read(buf)?;
for i in 0..bytes {
buf[i] = self.keys.decode_byte(buf[i]);
}
Ok(bytes)
}
}
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