use base64::{Engine as _, engine::general_purpose::URL_SAFE_NO_PAD};
use sha2::{Sha256, Digest};

const VERIFIER_LENGTH: usize = 64;

pub struct CodeVerifier(String);

impl CodeVerifier {
    pub fn new() -> Self {
        let bytes: Vec<u8> = (0..VERIFIER_LENGTH).map(|_| rand_byte()).collect();
        CodeVerifier(URL_SAFE_NO_PAD.encode(&bytes))
    }

    pub fn as_str(&self) -> &str {
        &self.0
    }

    pub fn challenge(&self) -> CodeChallenge {
        let mut hasher = Sha256::new();
        hasher.update(self.0.as_bytes());
        let digest = hasher.finalize();
        CodeChallenge(URL_SAFE_NO_PAD.encode(digest))
    }
}

fn rand_byte() -> u8 {
    use std::time::{SystemTime, UNIX_EPOCH};
    let nanos = SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .unwrap_or_else(|_| {
            eprintln!("[pkce] system time before UNIX_EPOCH, using 0 for random byte");
            std::time::Duration::default()
        })
        .subsec_nanos();
    (nanos & 0xFF) as u8
}

pub struct CodeChallenge(String);

impl CodeChallenge {
    pub fn as_str(&self) -> &str {
        &self.0
    }
}