use anyhow::{anyhow, Result};
use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
use crc32c::crc32c;

use std::io::Cursor;

const BLOCK_SIZE: u64 = 4096;
const THIN_SUPERBLOCK_CSUM_XOR: u32 = 160774;
const CACHE_SUPERBLOCK_CSUM_XOR: u32 = 9031977;
const ERA_SUPERBLOCK_CSUM_XOR: u32 = 146538381;
const BITMAP_CSUM_XOR: u32 = 240779;
const INDEX_CSUM_XOR: u32 = 160478;
const BTREE_CSUM_XOR: u32 = 121107;
const ARRAY_CSUM_XOR: u32 = 595846735;

fn checksum(buf: &[u8]) -> u32 {
    crc32c(&buf[4..]) ^ 0xffffffff
}

#[derive(Debug, PartialEq)]
#[allow(clippy::upper_case_acronyms)]
#[allow(non_camel_case_types)]
pub enum BT {
    THIN_SUPERBLOCK,
    CACHE_SUPERBLOCK,
    ERA_SUPERBLOCK,
    NODE,
    INDEX,
    BITMAP,
    ARRAY,
    UNKNOWN,
}

pub fn metadata_block_type(buf: &[u8]) -> BT {
    if buf.len() != BLOCK_SIZE as usize {
        return BT::UNKNOWN;
    }

    // The checksum is always stored in the first u32 of the buffer.
    let mut rdr = Cursor::new(buf);
    let sum_on_disk = rdr.read_u32::<LittleEndian>().unwrap();
    let csum = checksum(buf);
    let btype = csum ^ sum_on_disk;

    match btype {
        THIN_SUPERBLOCK_CSUM_XOR => BT::THIN_SUPERBLOCK,
        CACHE_SUPERBLOCK_CSUM_XOR => BT::CACHE_SUPERBLOCK,
        ERA_SUPERBLOCK_CSUM_XOR => BT::ERA_SUPERBLOCK,
        BTREE_CSUM_XOR => BT::NODE,
        BITMAP_CSUM_XOR => BT::BITMAP,
        INDEX_CSUM_XOR => BT::INDEX,
        ARRAY_CSUM_XOR => BT::ARRAY,
        _ => BT::UNKNOWN,
    }
}

pub fn write_checksum(buf: &mut [u8], kind: BT) -> Result<()> {
    if buf.len() != BLOCK_SIZE as usize {
        return Err(anyhow!("block is wrong size"));
    }

    use BT::*;
    let salt = match kind {
        THIN_SUPERBLOCK => THIN_SUPERBLOCK_CSUM_XOR,
        CACHE_SUPERBLOCK => CACHE_SUPERBLOCK_CSUM_XOR,
        ERA_SUPERBLOCK => ERA_SUPERBLOCK_CSUM_XOR,
        NODE => BTREE_CSUM_XOR,
        BITMAP => BITMAP_CSUM_XOR,
        INDEX => INDEX_CSUM_XOR,
        ARRAY => ARRAY_CSUM_XOR,
        UNKNOWN => {
            return Err(anyhow!("Invalid block type"));
        }
    };

    let csum = checksum(buf) ^ salt;
    let mut out = std::io::Cursor::new(buf);
    out.write_u32::<LittleEndian>(csum)?;
    Ok(())
}