Binary search a vector in chunks - rust

Question

I have a file of ipv4 addresses, which as we know are 4 bytes each. I wish to do a binary search over the file contents to find a given IP address. Rust has a built-in binary search but it doesn't let you pass a len and it instead reads it from the vector.
I have tried to adapt the built-in rust binary search but am a bit lost. This is where i am so far. Maybe there is a way to use the built in method?
fn binary_search(s: &Vec<&u8>, x: &u32) -> Result<usize, usize> {
let f = |p: &[u8]| p.cmp(x); // need to compare byte slices somehow
let mut size = s.len() / 4;
if size == 0 {
return Err(0);
}
let mut base = 0usize;
while size > 1 {
let half = size / 2;
let mid = base + half;
let cmp = f(s[mid..mid+4]);
base = if cmp == Greater { base } else { mid };
size -= half;
}
let cmp = f(s[base..base+4]);
if cmp == Equal {
Ok(base)
} else {
Err(base + (cmp == Less) as usize)
}
}

Answer 1

It’d be better to have a slice with one element per address, either of 4-byte arrays ([u8; 4]), some equivalent struct (hey, Ipv4Addr), or just u32. Unfortunately, I don’t think it’s possible to reinterpret a &[u8] with a length divisible by 4 as &[[u8; 4]] yet (and the other options would need alignment). You could do this conversion while reading the file in chunks, though.
So first, in an equivalent example program:
use std::net::Ipv4Addr;
fn main() {
let vec: Vec<Ipv4Addr> = vec![
[10, 0, 0, 0].into(),
[20, 0, 0, 0].into(),
[30, 0, 0, 0].into(),
];
println!("vec {:?}", vec);
let found = vec.binary_search(&Ipv4Addr::from_str("20.0.0.0").unwrap());
println!("found {:?}", found);
}
(playground)
Then reading from a file would look something like:
let mut vec: Vec<Ipv4Addr> = vec![];
loop {
let mut address = [0; 4];
match f.read_exact(&mut address) {
Ok(()) => {},
Err(err) if err.kind() == ErrorKind::UnexpectedEof => break,
err => err?,
}
vec.push(address.into());
}
(although this one is slightly lax in that it ignores any trailing bytes that don’t form a multiple of 4)
where f is a BufReader around a file.

Answer 2

I think i have a working solution now, but i'm not a master at rust so please critique it harshly.
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6e3102ea622f1ae0d66465f4007ccb03
use std::cmp::Ordering::{self, Equal, Greater, Less};
use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};
use std::str::FromStr;
fn binary_search(s: Vec<u8>, x: Vec<u8>) -> Result<usize, usize> {
let f = |p: &[u8]| p.cmp(&x);
let mut size = s.len() / 4;
if size == 0 {
return Err(0);
}
let mut base = 0usize;
while size > 1 {
let half = size / 2;
let mid = base + half;
// mid is always in [0, size), that means mid is >= 0 and < size.
// mid >= 0: by definition
// mid < size: mid = size / 2 + size / 4 + size / 8 ...
let cmp = f(s[mid*4..(mid+1)*4].to_vec());
base = if cmp == Greater { base } else { mid };
size -= half;
}
// base is always in [0, size) because base <= mid.
let cmp = f(s[base*4..(base+1)*4].to_vec());
if cmp == Equal {
Ok(base*4)
} else {
Err(base*4 + ((cmp == Less) as usize) * 4)
}
}
fn main() {
let vec: Vec<u8> = vec![10, 0, 0, 0, 20, 0, 0, 0, 30, 0, 0, 0];
println!("vec {:?}", vec);
let found = binary_search(vec, Ipv4Addr::from_str("20.0.0.0").unwrap().octets().to_vec());
println!("found {:?}", found);
}