//! Some utility functions for manipulating cosmetic filter rules.
//! Used by `CosmeticFilterCacheBuilder` and `CosmeticFilterCache`.

use crate::resources::PermissionMask;
use memchr::memchr as find_char;

/// Returns the first token of a CSS selector.
///
/// This should only be called once `selector` has been verified to start with either a "#" or "."
/// character.
pub(crate) fn key_from_selector(selector: &str) -> Option<String> {
    use regex::Regex;
    use std::sync::LazyLock;

    static RE_PLAIN_SELECTOR: LazyLock<Regex> =
        LazyLock::new(|| Regex::new(r"^[#.][\w\\-]+").unwrap());
    static RE_PLAIN_SELECTOR_ESCAPED: LazyLock<Regex> =
        LazyLock::new(|| Regex::new(r"^[#.](?:\\[0-9A-Fa-f]+ |\\.|\w|-)+").unwrap());
    static RE_ESCAPE_SEQUENCE: LazyLock<Regex> =
        LazyLock::new(|| Regex::new(r"\\([0-9A-Fa-f]+ |.)").unwrap());

    // If there are no escape characters in the selector, just take the first class or id token.
    let mat = RE_PLAIN_SELECTOR.find(selector);
    if let Some(location) = mat {
        let key = &location.as_str();
        if find_char(b'\\', key.as_bytes()).is_none() {
            return Some((*key).into());
        }
    } else {
        return None;
    }

    // Otherwise, the characters in the selector must be escaped.
    let mat = RE_PLAIN_SELECTOR_ESCAPED.find(selector);
    if let Some(location) = mat {
        let mut key = String::with_capacity(selector.len());
        let escaped = &location.as_str();
        let mut beginning = 0;
        let mat = RE_ESCAPE_SEQUENCE.captures_iter(escaped);
        for capture in mat {
            // Unwrap is safe because the 0th capture group is the match itself
            let location = capture.get(0).unwrap();
            key += &escaped[beginning..location.start()];
            beginning = location.end();
            // Unwrap is safe because there is a capture group specified in the regex
            let capture = capture.get(1).unwrap().as_str();
            if capture.chars().count() == 1 {
                // Check number of unicode characters rather than byte length
                key += capture;
            } else {
                // This u32 conversion can overflow
                let codepoint = u32::from_str_radix(&capture[..capture.len() - 1], 16).ok()?;

                // Not all u32s are valid Unicode codepoints
                key += &core::char::from_u32(codepoint)?.to_string();
            }
        }
        Some(key + &escaped[beginning..])
    } else {
        None
    }
}

/// Exists to use common logic for binning filters correctly
#[derive(Clone)]
pub(crate) enum SpecificFilterType {
    Hide(String),
    Unhide(String),
    InjectScript((String, PermissionMask)),
    UninjectScript((String, PermissionMask)),
    ProceduralOrAction(String),
    ProceduralOrActionException(String),
}

impl SpecificFilterType {
    pub(crate) fn negated(self) -> Self {
        match self {
            Self::Hide(s) => Self::Unhide(s),
            Self::Unhide(s) => Self::Hide(s),
            Self::InjectScript(s) => Self::UninjectScript(s),
            Self::UninjectScript(s) => Self::InjectScript(s),
            Self::ProceduralOrAction(s) => Self::ProceduralOrActionException(s),
            Self::ProceduralOrActionException(s) => Self::ProceduralOrAction(s),
        }
    }
}

/// Encodes permission bits in the last 2 ascii chars of a script string
/// Returns the script with permission appended
pub(crate) fn encode_script_with_permission(script: &str, permission: &PermissionMask) -> String {
    const HEX_CHARS: &[u8; 16] = b"0123456789abcdef";
    let high = (permission.to_bits() >> 4) as usize;
    let low = (permission.to_bits() & 0x0f) as usize;

    let mut encoded_script = String::with_capacity(script.len() + 2);
    encoded_script.push_str(script);
    encoded_script.push(HEX_CHARS[high] as char);
    encoded_script.push(HEX_CHARS[low] as char);
    encoded_script
}

/// Decodes permission bits from the last 2 ascii chars of a script string
/// Returns (permission, script) tuple
pub(crate) fn decode_script_with_permission(encoded_script: &str) -> (PermissionMask, &str) {
    if encoded_script.len() < 2 {
        return (PermissionMask::default(), "");
    }
    let mut last_chars = encoded_script.char_indices().rev();
    // unwrap: length >= 2 asserted above
    let (digit2, digit1) = (last_chars.next().unwrap(), last_chars.next().unwrap());
    fn parse_hex(c: char) -> Option<u8> {
        match c {
            '0'..='9' => Some(c as u8 - b'0'),
            'a'..='f' => Some(c as u8 - b'a' + 10),
            _ => None,
        }
    }
    if let (Some(d1), Some(d2)) = (parse_hex(digit1.1), parse_hex(digit2.1)) {
        let permission = PermissionMask::from_bits(d1 << 4 | d2);
        (permission, &encoded_script[..digit1.0])
    } else {
        (PermissionMask::default(), "")
    }
}

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

    #[test]
    fn test_encode_decode_script_with_permission() {
        for permission in 0u8..=255u8 {
            let script = "console.log('测试 🚀 emoji')".to_string();
            let permission = PermissionMask::from_bits(permission);

            let encoded = encode_script_with_permission(&script, &permission);
            let (decoded_permission, decoded_script) = decode_script_with_permission(&encoded);

            assert_eq!(decoded_permission.to_bits(), permission.to_bits());
            assert_eq!(decoded_script, script);
        }
    }

    #[test]
    fn test_encode_decode_script_with_permission_empty() {
        let (permission, script) = decode_script_with_permission("");
        assert_eq!(permission.to_bits(), 0);
        assert_eq!(script, "");
    }
}
