//! The 128-bit Secret Key and its printable form `K1-XXXXXX-XXXXX-XXXXX-XXXXX-XXXXX-C`.
//!
//! Body: 26 Crockford base32 characters (128 bits, MSB first, zero-padded).
//! Check: one Crockford symbol = first byte of SHA-256(raw key) mod 32.

use sha2::{Digest, Sha256};
use zeroize::Zeroizing;

use crate::{random_bytes, CryptoError, Result};

const ALPHABET: &[u8; 32] = b"0123456789ABCDEFGHJKMNPQRSTVWXYZ";
const PREFIX: &str = "K1";
const GROUPS: [usize; 5] = [6, 5, 5, 5, 5];

pub struct SecretKey(Zeroizing<[u8; 16]>);

impl SecretKey {
    pub fn generate() -> Self {
        Self(Zeroizing::new(random_bytes()))
    }

    pub fn from_bytes(b: [u8; 16]) -> Self {
        Self(Zeroizing::new(b))
    }

    pub fn expose(&self) -> &[u8; 16] {
        &self.0
    }

    pub fn to_display(&self) -> Zeroizing<String> {
        let body = encode(self.expose());
        let mut out = String::from(PREFIX);
        let mut i = 0;
        for g in GROUPS {
            out.push('-');
            out.push_str(&body[i..i + g]);
            i += g;
        }
        out.push('-');
        out.push(ALPHABET[checksum(self.expose())] as char);
        Zeroizing::new(out)
    }

    /// Accepts any case, spaces or hyphens, and the usual Crockford confusables (O→0, I/L→1).
    pub fn parse(s: &str) -> Result<Self> {
        let cleaned: Zeroizing<String> = Zeroizing::new(
            s.chars().filter(|c| !c.is_whitespace() && *c != '-').map(|c| c.to_ascii_uppercase()).collect(),
        );
        let rest = cleaned.strip_prefix(PREFIX).ok_or(CryptoError::BadSecretKey)?;
        // ASCII only: `split_at` below works on bytes and must not land inside a character.
        if !rest.is_ascii() || rest.len() != 27 {
            return Err(CryptoError::BadSecretKey);
        }
        let (body, check) = rest.split_at(26);
        let bytes = decode(body)?;
        let check_val = symbol_value(check.as_bytes()[0])?;
        if check_val != checksum(&bytes) {
            return Err(CryptoError::BadSecretKey);
        }
        Ok(Self::from_bytes(bytes))
    }
}

fn checksum(b: &[u8; 16]) -> usize {
    (Sha256::digest(b)[0] % 32) as usize
}

fn encode(b: &[u8; 16]) -> String {
    let n = u128::from_be_bytes(*b);
    // 26 symbols * 5 bits = 130 bits; the top two bits are zero.
    (0..26).rev().map(|i| ALPHABET[((n >> (i * 5)) & 0x1f) as usize] as char).collect()
}

fn symbol_value(c: u8) -> Result<usize> {
    let c = match c {
        b'O' => b'0',
        b'I' | b'L' => b'1',
        other => other,
    };
    ALPHABET.iter().position(|&a| a == c).ok_or(CryptoError::BadSecretKey)
}

fn decode(s: &str) -> Result<[u8; 16]> {
    let mut n: u128 = 0;
    for (i, c) in s.bytes().enumerate() {
        let v = symbol_value(c)? as u128;
        // 26 symbols carry 130 bits; the 2 spare bits sit at the top, so the first symbol holds 3 data bits.
        if i == 0 && v > 0b111 {
            return Err(CryptoError::BadSecretKey); // would overflow 128 bits
        }
        n = (n << 5) | v;
    }
    Ok(n.to_be_bytes())
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn non_ascii_is_rejected_without_panicking() {
        // Found by fuzz/secret_key_parse: 27 bytes whose 26th..27th byte is a Cyrillic 'е'.
        assert!(SecretKey::parse("K1-10Y5-00Y0-000000p00-000000p0\u{435}-").is_err());
        assert!(SecretKey::parse("K1-000000-00000-00000-00000-0000\u{e9}-0").is_err());
    }

    #[test]
    fn roundtrip() {
        for _ in 0..200 {
            let k = SecretKey::generate();
            let s = k.to_display();
            assert_eq!(s.len(), "K1-XXXXXX-XXXXX-XXXXX-XXXXX-XXXXX-X".len());
            assert_eq!(SecretKey::parse(&s).unwrap().expose(), k.expose());
            assert_eq!(SecretKey::parse(&s.to_lowercase()).unwrap().expose(), k.expose());
        }
    }

    #[test]
    fn extremes_roundtrip() {
        for b in [[0u8; 16], [0xff; 16], [0x80; 16]] {
            let k = SecretKey::from_bytes(b);
            assert_eq!(SecretKey::parse(&k.to_display()).unwrap().expose(), &b);
        }
        // First symbol 8 would need bit 128: reject.
        let max = SecretKey::from_bytes([0xff; 16]).to_display().to_string();
        assert!(max.starts_with("K1-7"));
        assert!(SecretKey::parse(&max.replacen("K1-7", "K1-8", 1)).is_err());
    }

    #[test]
    fn detects_typo() {
        let k = SecretKey::from_bytes([0xAB; 16]);
        let mut s = k.to_display().to_string();
        let pos = 5;
        let orig = s.as_bytes()[pos];
        let replacement = if orig == b'2' { "3" } else { "2" };
        s.replace_range(pos..pos + 1, replacement);
        // A typo is either rejected (31/32 of the time) or yields a different key, never the same one.
        assert!(SecretKey::parse(&s).is_err() || SecretKey::parse(&s).unwrap().expose() != k.expose());
    }
}