use std::convert;
use std::env;
use std::error;
use std::fmt;
use std::io;
use std::io::prelude::*;
use std::iter::repeat;
use std::num;

use getopts::Options;
use rand::RngCore;
use rustc_serialize::base64::{self, FromBase64, ToBase64};

use gf256::Gf256;

mod gf256;

fn new_vec<T: Clone>(n: usize, x: T) -> Vec<T> {
    repeat(x).take(n).collect()
}

#[derive(Debug)]
pub struct Error {
    descr: &'static str,
    detail: Option<String>,
}

impl Error {
    fn new(descr: &'static str, detail: Option<String>) -> Error {
        Error { descr: descr, detail: detail }
    }
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self.detail {
            None => write!(f, "{}", self.descr),
            Some(ref detail) => write!(f, "{} ({})", self.descr, detail)
        }
    }
}

impl error::Error for Error {
    fn description(&self) -> &str { self.descr }
    fn cause(&self) -> Option<&dyn error::Error> { None }
}

impl convert::From<Error> for io::Error {
    fn from(me: Error) -> io::Error {
        io::Error::new(io::ErrorKind::Other, me)
    }
}

// a try!-like macro for Option<T> expressions that takes
// a &'static str as error message as 2nd parameter
// and creates an Error out of it if necessary.
macro_rules! otry {
	($o:expr, $e:expr) => (
		match $o {
			Some(thing_) => thing_,
			None => return Err(convert::From::from(Error::new($e, None)))
		}
	)
}

/// maps a ParseIntError to an io::Error
fn pie2io(p: num::ParseIntError) -> io::Error {
    convert::From::from(
        Error::new("Integer parsing error", Some(p.to_string()))
    )
}

fn other_io_err(descr: &'static str, detail: Option<String>) -> io::Error {
    convert::From::from(
        Error::new(descr, detail)
    )
}

/// evaluates a polynomial at x=1, 2, 3, ... n (inclusive)
fn encode<W: Write>(src: &[u8], n: u8, w: &mut W) -> io::Result<()> {
    for raw_x in 1..((n as u16) + 1) {
        let x = Gf256::from_byte(raw_x as u8);
        let mut fac = Gf256::one();
        let mut acc = Gf256::zero();
        for &coeff in src.iter() {
            acc = acc + fac * Gf256::from_byte(coeff);
            fac = fac * x;
        }
        w.write(&[acc.to_byte()])?;
    }
    Ok(())
}

/// evaluates an interpolated polynomial at `raw_x` where
/// the polynomial is determined using Lagrangian interpolation
/// based on the given x/y coordinates `src`.
fn lagrange_interpolate(src: &[(u8, u8)], raw_x: u8) -> u8 {
    let x = Gf256::from_byte(raw_x);
    let mut sum = Gf256::zero();
    for (i, &(raw_xi, raw_yi)) in src.iter().enumerate() {
        let xi = Gf256::from_byte(raw_xi);
        let yi = Gf256::from_byte(raw_yi);
        let mut lix = Gf256::one();
        for (j, &(raw_xj, _)) in src.iter().enumerate() {
            if i != j {
                let xj = Gf256::from_byte(raw_xj);
                let delta = xi - xj;
                assert!(delta.poly != 0, "Duplicate shares");
                lix = lix * (x - xj) / delta;
            }
        }
        sum = sum + lix * yi;
    }
    sum.to_byte()
}

fn secret_share(src: &[u8], k: u8, n: u8) -> io::Result<Vec<Vec<u8>>> {
    let mut result = Vec::with_capacity(n as usize);
    for _ in 0..(n as usize) {
        result.push(new_vec(src.len(), 0u8));
    }
    let mut col_in = new_vec(k as usize, 0u8);
    let mut col_out = Vec::with_capacity(n as usize);
    let mut osrng = rand::thread_rng();
    for (c, &s) in src.iter().enumerate() {
        col_in[0] = s;
        osrng.fill_bytes(&mut col_in[1..]);
        col_out.clear();
        encode(&*col_in, n, &mut col_out)?;
        for (&y, share) in col_out.iter().zip(result.iter_mut()) {
            share[c] = y;
        }
    }
    Ok(result)
}

enum Action {
    Encode(u8, u8),
    // k and n parameter
    Decode,
}

fn parse_k_n(s: &str) -> io::Result<(u8, u8)> {
    let mut iter = s.split(',');
    let msg = "K and N have to be separated with a comma";
    let s1 = otry!(iter.next(), msg).trim();
    let s2 = otry!(iter.next(), msg).trim();
    let k = s1.parse().map_err(pie2io)?;
    let n = s2.parse().map_err(pie2io)?;
    Ok((k, n))
}

/// computes a CRC-24 hash over the concatenated coding parameters k, n
/// and the raw share data
fn crc24_as_bytes(k: u8, n: u8, octets: &[u8]) -> [u8; 3] {
    use std::hash::Hasher;

    let mut h = crc24::Crc24Hasher::new();
    h.write(&[k, n]);
    h.write(octets);
    let v = h.finish();

    [((v >> 16) & 0xFF) as u8,
        ((v >> 8) & 0xFF) as u8,
        (v & 0xFF) as u8]
}

fn perform_encode(k: u8, n: u8, with_checksums: bool) -> io::Result<()> {
    let secret = {
        let limit: usize = 0x10000;
        let stdin = io::stdin();
        let mut locked = stdin.lock();
        let mut tmp: Vec<u8> = Vec::new();
        locked.by_ref().take(limit as u64).read_to_end(&mut tmp)?;
        if tmp.len() == limit {
            let mut dummy = [0u8];
            if locked.read(&mut dummy)? > 0 {
                return Err(other_io_err("Secret too large",
                                        Some(format!("My limit is at {} bytes.", limit))));
            }
        }
        tmp
    };
    let shares = secret_share(&*secret, k, n)?;
    let config = base64::Config {
        pad: false,
        ..base64::STANDARD
    };
    for (index, share) in shares.iter().enumerate() {
        let salad = share.to_base64(config);
        if with_checksums {
            let crc_bytes = crc24_as_bytes(k, (index + 1) as u8, &**share);
            println!("{}-{}-{}-{}", k, index + 1, salad, crc_bytes.to_base64(config));
        } else {
            println!("{}-{}-{}", k, index + 1, salad);
        }
    }
    Ok(())
}

/// reads shares from stdin and returns Ok(k, shares) on success
/// where shares is a Vec<(u8, Vec<u8>)> representing x-coordinates
/// and share data.
fn read_shares() -> io::Result<(u8, Vec<(u8, Vec<u8>)>)> {
    let stdin = io::stdin();
    let stdin = io::BufReader::new(stdin.lock());
    let mut opt_k_l: Option<(u8, usize)> = None;
    let mut counter = 0u8;
    let mut shares: Vec<(u8, Vec<u8>)> = Vec::new();
    for line in stdin.lines() {
        let line = line?;
        let parts: Vec<_> = line.trim().split('-').collect();
        if parts.len() < 3 || parts.len() > 4 {
            return Err(other_io_err("Share parse error: Expected 3 or 4 \
			                         parts searated by a minus sign", None));
        }
        let (k, n, p3, opt_p4) = {
            let mut iter = parts.into_iter();
            let k = iter.next().unwrap().parse::<u8>().map_err(pie2io)?;
            let n = iter.next().unwrap().parse::<u8>().map_err(pie2io)?;
            let p3 = iter.next().unwrap();
            let opt_p4 = iter.next();
            (k, n, p3, opt_p4)
        };
        if k < 1 || n < 1 {
            return Err(other_io_err("Share parse error: Illegal K,N parameters", None));
        }
        let data = p3.from_base64().map_err(|_| other_io_err(
            "Share parse error: Base64 decoding of data block failed", None))?;
        if let Some(check) = opt_p4 {
            if check.len() != 4 {
                return Err(other_io_err("Share parse error: Checksum part is \
				                         expected to be four characters", None));
            }
            let crc_bytes = check.from_base64().map_err(|_| other_io_err(
                "Share parse error: Base64 decoding of checksum failed", None))?;
            let mychksum = crc24_as_bytes(k, n, &*data);
            if crc_bytes != mychksum {
                return Err(other_io_err("Share parse error: Checksum mismatch", None));
            }
        }
        if let Some((ck, cl)) = opt_k_l {
            if ck != k || cl != data.len() {
                return Err(other_io_err("Incompatible shares", None));
            }
        } else {
            opt_k_l = Some((k, data.len()));
        }
        if shares.iter().all(|s| s.0 != n) {
            shares.push((n, data));
            counter += 1;
            if counter == k {
                return Ok((k, shares));
            }
        }
    }
    Err(other_io_err("Not enough shares provided!", None))
}

fn perform_decode() -> io::Result<()> {
    let (k, shares) = read_shares()?;
    assert!(!shares.is_empty());
    let slen = shares[0].1.len();
    let mut col_in = Vec::with_capacity(k as usize);
    let mut secret = Vec::with_capacity(slen);
    for byteindex in 0..slen {
        col_in.clear();
        for s in shares.iter().take(k as usize) {
            col_in.push((s.0, s.1[byteindex]));
        }
        secret.push(lagrange_interpolate(&*col_in, 0u8));
    }
    let mut out = io::stdout();
    out.write_all(&*secret)?;
    out.flush()
}

fn main() {
    let mut stderr = io::stderr();
    let args: Vec<String> = env::args().collect();

    let mut opts = Options::new();
    opts.optflag("h", "help", "print this help text");
    opts.optflag("d", "decode", "for decoding");
    opts.optopt("e", "encode", "for encoding, K is the required number of \
	                            shares for decoding, N is the number of shares \
	                            to generate. 1 <= K <= N <= 255", "K,N");
    let opt_matches = match opts.parse(&args[1..]) {
        Ok(m) => m,
        Err(f) => {
            drop(writeln!(&mut stderr, "Error: {}", f));
            // env::set_exit_status(1); // FIXME: unstable feature
            return;
        }
    };

    if args.len() < 2 || opt_matches.opt_present("h") {
        println!(
            "The program secretshare is an implementation of Shamir's secret sharing scheme.\n\
 It is applied byte-wise within a finite field for arbitrarily long secrets.\n");
        println!("{}", opts.usage("Usage: secretshare [options]"));
        println!("Input is read from STDIN and output is written to STDOUT.");
        return;
    }

    let action: Result<_, _> =
        match (opt_matches.opt_present("e"), opt_matches.opt_present("d")) {
            (false, false) => Err("Nothing to do! Use -e or -d"),
            (true, true) => Err("Use either -e or -d and not both"),
            (false, true) => Ok(Action::Decode),
            (true, false) => {
                if let Some(param) = opt_matches.opt_str("e") {
                    if let Ok((k, n)) = parse_k_n(&*param) {
                        if 0 < k && k <= n {
                            Ok(Action::Encode(k, n))
                        } else {
                            Err("Invalid encoding parameters K,N")
                        }
                    } else {
                        Err("Could not parse K,N parameters")
                    }
                } else {
                    Err("No parameter for -e or -d provided")
                }
            }
        };

    let result =
        match action {
            Ok(Action::Encode(k, n)) => perform_encode(k, n, true),
            Ok(Action::Decode) => perform_decode(),
            Err(e) => Err(other_io_err(e, None))
        };

    if let Err(e) = result {
        drop(writeln!(&mut stderr, "{}", e));
        // env::set_exit_status(1); // FIXME: unstable feature
    }
}