//! Webhook HMAC secret — load-or-generate, persist, verify.
//!
//! A 32-byte secret is generated on first startup, hex-encoded, and stored at
//! [`crate::paths::webhook_secret_file()`]. On subsequent starts the same file
//! is read back so Forgejo and hive-c0re always share the same key without any
//! operator configuration.
//!
//! The secret is used in two places:
//! - **Registration**: passed as the `secret` config key when hive-c0re
//!   creates (or re-creates) the Forgejo org/repo webhook.
//! - **Verification**: each incoming webhook POST is verified against the
//!   `X-Hub-Signature-256` header Forgejo attaches (`sha256=<hex>`).

use anyhow::{Context as _, Result};

/// Load the webhook HMAC secret from disk; generate and persist it if absent.
///
/// Returns a hex-encoded 32-byte secret string (64 hex chars).
pub fn load_or_generate() -> Result<String> {
    let path = crate::paths::webhook_secret_file();
    if let Ok(raw) = std::fs::read_to_string(&path) {
        let trimmed = raw.trim().to_owned();
        if trimmed.len() == 64 && trimmed.chars().all(|c| c.is_ascii_hexdigit()) {
            return Ok(trimmed);
        }
        // File exists but is malformed — regenerate.
        tracing::warn!(
            path = %path.display(),
            "webhook-secret file malformed (wrong length/chars); regenerating"
        );
    }
    let secret = generate_hex_secret()?;
    std::fs::create_dir_all(path.parent().unwrap_or(&path))
        .with_context(|| format!("create dir for {}", path.display()))?;
    std::fs::write(&path, format!("{secret}\n"))
        .with_context(|| format!("write webhook secret to {}", path.display()))?;
    tracing::info!(path = %path.display(), "webhook secret generated and persisted");
    Ok(secret)
}

/// Read 32 random bytes from `/dev/urandom` and hex-encode them.
fn generate_hex_secret() -> Result<String> {
    use std::io::Read as _;

    let mut buf = [0u8; 32];
    let mut f =
        std::fs::File::open("/dev/urandom").context("open /dev/urandom for secret generation")?;
    f.read_exact(&mut buf)
        .context("read 32 bytes from /dev/urandom")?;
    Ok(hex_encode(&buf))
}

/// Hex-encode `bytes` as a lowercase string.
fn hex_encode(bytes: &[u8]) -> String {
    let mut out = String::with_capacity(bytes.len() * 2);
    for b in bytes {
        out.push(char::from_digit(u32::from(b >> 4), 16).unwrap_or('0'));
        out.push(char::from_digit(u32::from(b & 0xf), 16).unwrap_or('0'));
    }
    out
}

/// Verify a Forgejo `X-Hub-Signature-256` header against `body` using
/// `secret`. Returns `Ok(())` when the signature matches, or an error
/// describing the mismatch (safe to log; does not expose the secret).
///
/// Forgejo sends: `sha256=<hex>`.
pub fn verify_signature(secret: &str, body: &[u8], header: &str) -> Result<()> {
    use hmac::{Hmac, KeyInit, Mac};
    use sha2::Sha256;

    let sig_hex = header
        .strip_prefix("sha256=")
        .ok_or_else(|| anyhow::anyhow!("X-Hub-Signature-256 missing 'sha256=' prefix"))?;

    let expected = hex_decode(sig_hex)
        .ok_or_else(|| anyhow::anyhow!("X-Hub-Signature-256 contains non-hex chars"))?;

    let mut mac = Hmac::<Sha256>::new_from_slice(secret.as_bytes())
        .map_err(|e| anyhow::anyhow!("HMAC key error: {e}"))?;
    mac.update(body);
    mac.verify_slice(&expected)
        .map_err(|_| anyhow::anyhow!("X-Hub-Signature-256 mismatch"))
}

/// Decode a lowercase hex string into bytes; returns `None` on invalid input.
fn hex_decode(s: &str) -> Option<Vec<u8>> {
    if !s.len().is_multiple_of(2) {
        return None;
    }
    let mut out = Vec::with_capacity(s.len() / 2);
    let mut chars = s.chars();
    while let (Some(hi), Some(lo)) = (chars.next(), chars.next()) {
        let hi = u8::try_from(hi.to_digit(16)?).ok()?;
        let lo = u8::try_from(lo.to_digit(16)?).ok()?;
        out.push((hi << 4) | lo);
    }
    Some(out)
}