use ed25519_dalek::{Signature, Signer, SigningKey, Verifier, VerifyingKey};

pub type Result<T> = std::result::Result<T, Error>;

#[derive(Debug)]
pub struct Error(String);

impl std::fmt::Display for Error {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        self.0.fmt(f)
    }
}

impl std::error::Error for Error {}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Profile {
    pub display_name: String,
    pub bio: String,
    pub xmr_addr: String,
    pub ts: i64,
    pub sig: Vec<u8>,
}

const PREFIX: &[u8] = b"prf ";
const NAME_MAX: usize = 64;
const BIO_MAX: usize = 512;

pub fn validate(display_name: &str, bio: &str, xmr_addr: &str) -> Result<()> {
    check_fields(display_name, bio, xmr_addr)
}

pub fn sign(
    identity_sk: &[u8],
    display_name: &str,
    bio: &str,
    xmr_addr: &str,
    ts: i64,
) -> Result<Profile> {
    check_fields(display_name, bio, xmr_addr)?;
    let sk_bytes: [u8; 32] = identity_sk
        .try_into()
        .map_err(|_| Error("identity secret key must be 32 bytes".into()))?;
    let sk = SigningKey::from_bytes(&sk_bytes);
    let sig = sk
        .sign(&sign_msg(display_name, bio, xmr_addr, ts))
        .to_bytes()
        .to_vec();
    Ok(Profile {
        display_name: display_name.to_string(),
        bio: bio.to_string(),
        xmr_addr: xmr_addr.to_string(),
        ts,
        sig,
    })
}

pub fn verify(identity_pk: &[u8], prf: &Profile) -> bool {
    if check_fields(&prf.display_name, &prf.bio, &prf.xmr_addr).is_err() {
        return false;
    }
    if identity_pk.len() != 32 || prf.sig.len() != 64 {
        return false;
    }
    let pk: [u8; 32] = match identity_pk.try_into() {
        Ok(p) => p,
        Err(_) => return false,
    };
    let sig: [u8; 64] = match prf.sig.as_slice().try_into() {
        Ok(s) => s,
        Err(_) => return false,
    };
    let Ok(vk) = VerifyingKey::from_bytes(&pk) else {
        return false;
    };
    vk.verify(
        &sign_msg(&prf.display_name, &prf.bio, &prf.xmr_addr, prf.ts),
        &Signature::from_bytes(&sig),
    )
    .is_ok()
}

pub fn encode(prf: &Profile) -> Vec<u8> {
    let mut out = Vec::from(PREFIX);
    out.extend_from_slice(prf.display_name.as_bytes());
    out.push(b'\n');
    out.extend_from_slice(prf.bio.as_bytes());
    out.push(b'\n');
    out.extend_from_slice(prf.xmr_addr.as_bytes());
    out.push(b'\n');
    out.extend_from_slice(prf.ts.to_string().as_bytes());
    out.push(b'\n');
    out.extend_from_slice(to_hex(&prf.sig).as_bytes());
    out
}

pub fn decode(pt: &[u8]) -> Option<Profile> {
    let rest = pt.strip_prefix(PREFIX)?;
    let text = std::str::from_utf8(rest).ok()?;
    let mut parts = text.splitn(5, '\n');
    let display_name = parts.next()?.to_string();
    let bio = parts.next()?.to_string();
    let xmr_addr = parts.next()?.to_string();
    let ts = parts.next()?.parse::<i64>().ok()?;
    let sig = from_hex(parts.next()?)?;
    if check_fields(&display_name, &bio, &xmr_addr).is_err() {
        return None;
    }
    Some(Profile {
        display_name,
        bio,
        xmr_addr,
        ts,
        sig,
    })
}

fn check_fields(display_name: &str, bio: &str, xmr_addr: &str) -> Result<()> {
    if display_name.len() > NAME_MAX {
        return Err(Error("display_name longer than 64 bytes".into()));
    }
    if bio.len() > BIO_MAX {
        return Err(Error("bio longer than 512 bytes".into()));
    }
    if display_name.contains('\n') || bio.contains('\n') || xmr_addr.contains('\n') {
        return Err(Error("profile fields must not contain newlines".into()));
    }
    Ok(())
}

fn sign_msg(display_name: &str, bio: &str, xmr_addr: &str, ts: i64) -> Vec<u8> {
    let mut msg = Vec::from(display_name.as_bytes());
    msg.push(b'\n');
    msg.extend_from_slice(bio.as_bytes());
    msg.push(b'\n');
    msg.extend_from_slice(xmr_addr.as_bytes());
    msg.push(b'\n');
    msg.extend_from_slice(ts.to_string().as_bytes());
    msg
}

fn to_hex(bytes: &[u8]) -> String {
    bytes.iter().map(|b| format!("{b:02x}")).collect()
}

fn from_hex(s: &str) -> Option<Vec<u8>> {
    if s.is_empty() || !s.len().is_multiple_of(2) {
        return None;
    }
    if !s.bytes().all(|c| c.is_ascii_hexdigit()) {
        return None;
    }
    (0..s.len())
        .step_by(2)
        .map(|i| u8::from_str_radix(&s[i..i + 2], 16).ok())
        .collect()
}