use bytes::buf::Buf;
use std::{
    hash::{BuildHasher, Hasher},
    io::Cursor,
};

pub fn txids_from_buffer(buffer: &[u8]) -> Vec<u32> {
    let mut txids: Vec<u32> = Vec::new();
    let mut cursor: Cursor<&[u8]> = Cursor::new(buffer);
    let size: u32 = cursor.get_u32();
    for _ in 0..size {
        txids.push(cursor.get_u32());
    }

    txids
}

pub struct U32HasherState;

impl BuildHasher for U32HasherState {
    type Hasher = U32Hasher;

    fn build_hasher(&self) -> Self::Hasher {
        U32Hasher(0)
    }
}

pub struct U32Hasher(u32);

impl Hasher for U32Hasher {
    fn finish(&self) -> u64 {
        // Safety: Two u32s next to each other will make a u64
        unsafe { core::mem::transmute::<(u32, u32), u64>((self.0, 0_u32)) }
    }

    fn write(&mut self, bytes: &[u8]) {
        // Assert in debug builds (testing too) that only 4 byte keys (u32, i32, f32, etc.) run
        debug_assert!(bytes.len() == 4);
        // Safety: We know that the size of the key is at least 4 bytes
        self.0 = unsafe { *bytes.as_ptr().cast::<u32>() };
    }
}

#[cfg(test)]
mod tests {
    use std::collections::HashMap;

    use super::U32HasherState;

    #[test]
    fn test_me() {
        let mut hm: HashMap<u32, String, U32HasherState> = HashMap::with_hasher(U32HasherState);

        hm.insert(0, String::from("0"));
        hm.insert(42, String::from("42"));
        hm.insert(256, String::from("256"));
        hm.insert(u32::MAX, String::from("MAX"));
        hm.insert(u32::MAX >> 2, String::from("MAX >> 2"));

        assert_eq!(hm.get(&0), Some(&String::from("0")));
        assert_eq!(hm.get(&42), Some(&String::from("42")));
        assert_eq!(hm.get(&256), Some(&String::from("256")));
        assert_eq!(hm.get(&u32::MAX), Some(&String::from("MAX")));
        assert_eq!(hm.get(&(u32::MAX >> 2)), Some(&String::from("MAX >> 2")));
        assert_eq!(hm.get(&(u32::MAX >> 4)), None);
        assert_eq!(hm.get(&3), None);
        assert_eq!(hm.get(&43), None);
    }
}