fn parse_key_hex(value: &str) -> Option<Key> {
    if value.len() != 64 || !value.bytes().all(|b| b.is_ascii_hexdigit()) {
        return None;
    }
    let mut key = [0_u8; 32];
    for (index, chunk) in value.as_bytes().chunks_exact(2).enumerate() {
        let text = std::str::from_utf8(chunk).ok()?;
        key[index] = u8::from_str_radix(text, 16).ok()?;
    }
    Some(key)
}

fn default_cache_scheme() -> String {
    "http".to_owned()
}

fn cache_tags(headers: &HeaderMap) -> Vec<String> {
    let mut tags = Vec::new();
    for value in headers.get_all("cache-tag") {
        let Ok(value) = value.to_str() else {
            continue;
        };
        for raw in value.split(|c: char| c == ',' || c.is_ascii_whitespace()) {
            let tag = raw.trim();
            if tag.is_empty()
                || tag.len() > 128
                || !tag.bytes().all(|b| {
                    b.is_ascii_alphanumeric() || matches!(b, b'_' | b'-' | b'.' | b':' | b'/')
                })
            {
                continue;
            }
            if !tags.iter().any(|existing| existing == tag) {
                tags.push(tag.to_owned());
                if tags.len() >= 64 {
                    return tags;
                }
            }
        }
    }
    tags
}

fn now_ms() -> u64 {
    SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .unwrap_or_default()
        .as_millis()
        .min(u64::MAX as u128) as u64
}

fn system_time_ms(value: u64) -> SystemTime {
    UNIX_EPOCH
        .checked_add(Duration::from_millis(value))
        .unwrap_or(UNIX_EPOCH)
}

fn cold_age(entry: &ColdEntry) -> u64 {
    entry
        .age_at_store_seconds
        .saturating_add(entry.stored_at.elapsed().unwrap_or_default().as_secs())
}

fn cold_resident_age(entry: &ColdEntry) -> u64 {
    entry
        .resident_age_at_store_seconds
        .saturating_add(entry.stored_at.elapsed().unwrap_or_default().as_secs())
}

fn retention_deadline(
    _ttl: u64,
    _stale_while_revalidate: u64,
    _stale_if_error: u64,
    _has_validator: bool,
) -> u64 {
    u64::MAX
}

fn has_validator(headers: &HeaderMap) -> bool {
    headers.contains_key("etag") || headers.contains_key("last-modified")
}

fn stale_forbidden(must_revalidate: bool, proxy_revalidate: bool) -> bool {
    must_revalidate || proxy_revalidate
}

fn client_max_stale(headers: &HeaderMap) -> Option<Option<u64>> {
    let control = directives(headers)?;
    let value = control.get("max-stale")?;
    match value {
        None => Some(None),
        Some(value) => value.parse::<u64>().ok().map(Some),
    }
}

fn request_freshness_allows(age: u64, ttl: u64, headers: &HeaderMap) -> bool {
    let Some(control) = directives(headers) else {
        return false;
    };
    if let Some(Some(value)) = control.get("max-age") {
        let Ok(limit) = value.parse::<u64>() else {
            return false;
        };
        if age > limit {
            return false;
        }
    }
    if let Some(Some(value)) = control.get("min-fresh") {
        let Ok(required) = value.parse::<u64>() else {
            return false;
        };
        if ttl.saturating_sub(age) < required {
            return false;
        }
    }
    true
}

fn stale_mode(
    age: u64,
    ttl: u64,
    stale_while_revalidate: u64,
    must_revalidate: bool,
    proxy_revalidate: bool,
    requires_revalidation: bool,
    request_headers: &HeaderMap,
) -> Option<bool> {
    if age < ttl || requires_revalidation || stale_forbidden(must_revalidate, proxy_revalidate) {
        return None;
    }
    if !request_freshness_allows(age, ttl, request_headers) {
        return None;
    }
    let stale_for = age.saturating_sub(ttl);
    let max_stale = client_max_stale(request_headers);

    if matches!(max_stale, Some(Some(limit)) if stale_for > limit) {
        return None;
    }
    if stale_while_revalidate > 0 && stale_for <= stale_while_revalidate {
        return Some(true);
    }
    match max_stale {
        Some(None) | Some(Some(_)) => Some(false),
        None => None,
    }
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum CacheState {
    Fresh,
    StaleButServable { background_revalidate: bool },
    Expired,
}

impl CacheState {
    fn servable(self) -> bool {
        !matches!(self, Self::Expired)
    }

    fn stale(self) -> bool {
        matches!(self, Self::StaleButServable { .. })
    }

    fn background_revalidate(self) -> bool {
        matches!(
            self,
            Self::StaleButServable {
                background_revalidate: true
            }
        )
    }
}

fn cache_state(
    age: u64,
    ttl: u64,
    stale_while_revalidate: u64,
    must_revalidate: bool,
    proxy_revalidate: bool,
    requires_revalidation: bool,
    request_headers: &HeaderMap,
) -> CacheState {
    if age < ttl && !requires_revalidation && request_freshness_allows(age, ttl, request_headers) {
        return CacheState::Fresh;
    }
    match stale_mode(
        age,
        ttl,
        stale_while_revalidate,
        must_revalidate,
        proxy_revalidate,
        requires_revalidation,
        request_headers,
    ) {
        Some(background_revalidate) => CacheState::StaleButServable {
            background_revalidate,
        },
        None => CacheState::Expired,
    }
}

fn cold_paths(root: &Path, key: &Key) -> (PathBuf, PathBuf, PathBuf, PathBuf) {
    let key = key_hex(key);
    let shard = root.join(&key[..2]);
    (
        shard.join(format!("{key}.body")),
        shard.join(format!("{key}.json")),
        shard.join(format!(".{key}.body.tmp")),
        shard.join(format!(".{key}.json.tmp")),
    )
}

fn cold_header_pairs(headers: &HeaderMap) -> Option<Vec<(String, String)>> {
    let mut result = Vec::new();
    for (name, value) in headers {
        result.push((name.as_str().to_owned(), value.to_str().ok()?.to_owned()));
    }
    Some(result)
}

fn pairs_to_headers(values: &[(String, String)]) -> Option<HeaderMap> {
    let mut headers = HeaderMap::new();
    for (name, value) in values {
        let name = name.parse::<http::HeaderName>().ok()?;
        let value = HeaderValue::from_str(value).ok()?;
        headers.append(name, value);
    }
    Some(headers)
}