use ring::digest;
use ring::hmac;

use std::io::Write;

fn convert_digest_to_hmac_alg(hash: &'static digest::Algorithm) -> hmac::Algorithm {
    if hash == &digest::SHA256 {
        hmac::HMAC_SHA256
    } else if hash == &digest::SHA384 {
        hmac::HMAC_SHA384
    } else if hash == &digest::SHA512 {
        hmac::HMAC_SHA512
    } else {
        panic!("bad digest for prf");
    }
}

fn concat_sign(key: &hmac::Key, a: &[u8], b: &[u8]) -> hmac::Tag {
    let mut ctx = hmac::Context::with_key(key);
    ctx.update(a);
    ctx.update(b);
    ctx.sign()
}

fn p(out: &mut [u8], hashalg: &'static digest::Algorithm, secret: &[u8], seed: &[u8]) {
    let hmac_key = hmac::Key::new(convert_digest_to_hmac_alg(hashalg), secret);

    // A(1)
    let mut current_a = hmac::sign(&hmac_key, seed);

    let mut offs = 0;

    while offs < out.len() {
        // P_hash[i] = HMAC_hash(secret, A(i) + seed)
        let p_term = concat_sign(&hmac_key, current_a.as_ref(), seed);
        offs += out[offs..].as_mut().write(p_term.as_ref()).unwrap();

        // A(i+1) = HMAC_hash(secret, A(i))
        current_a = hmac::sign(&hmac_key, current_a.as_ref());
    }
}

fn concat(a: &[u8], b: &[u8]) -> Vec<u8> {
    let mut ret = Vec::new();
    ret.extend_from_slice(a);
    ret.extend_from_slice(b);
    ret
}

pub fn prf(out: &mut [u8],
           hashalg: &'static digest::Algorithm,
           secret: &[u8],
           label: &[u8],
           seed: &[u8]) {
    let joined_seed = concat(label, seed);
    p(out, hashalg, secret, &joined_seed);
}

#[cfg(test)]
mod tests {
    use ring::digest::{SHA256, SHA512};

    #[test]
    fn check_sha256() {
        let secret = b"\x9b\xbe\x43\x6b\xa9\x40\xf0\x17\xb1\x76\x52\x84\x9a\x71\xdb\x35";
        let seed = b"\xa0\xba\x9f\x93\x6c\xda\x31\x18\x27\xa6\xf7\x96\xff\xd5\x19\x8c";
        let label = b"test label";
        let expect = include_bytes!("testdata/prf-result.1.bin");
        let mut output = [0u8; 100];

        super::prf(&mut output, &SHA256, secret, label, seed);
        assert_eq!(expect.len(), output.len());
        assert_eq!(expect.to_vec(), output.to_vec());
    }

    #[test]
    fn check_sha512() {
        let secret = b"\xb0\x32\x35\x23\xc1\x85\x35\x99\x58\x4d\x88\x56\x8b\xbb\x05\xeb";
        let seed = b"\xd4\x64\x0e\x12\xe4\xbc\xdb\xfb\x43\x7f\x03\xe6\xae\x41\x8e\xe5";
        let label = b"test label";
        let expect = include_bytes!("testdata/prf-result.2.bin");
        let mut output = [0u8; 196];

        super::prf(&mut output, &SHA512, secret, label, seed);
        assert_eq!(expect.len(), output.len());
        assert_eq!(expect.to_vec(), output.to_vec());
    }
}