use std::marker::PhantomData;

use array_vec::ArrayStr;

use super::Buffer;

/// Holds computed constants for common escaping scheme identity checks.
pub struct CommonIdents<E: Escape>(PhantomData<E>);

/// Converts an array of ASCII bytes to an array of characters.
pub const fn ascii_chars<const N: usize>(s: &[u8; N]) -> [char; N] {
    assert!(s.is_ascii());
    let mut chars = ['\0'; N];
    let mut i = 0;
    while i < s.len() {
        chars[i] = s[i] as char;
        i += 1;
    }
    chars
}

/// Returns `true` if, for the given escaping scheme `E`, every character in `set` requires no escaping.
pub const fn are_all_chars_identity<E: Escape + ~const EscapeMeta>(set: &[char]) -> bool {
    let mut i = 0;
    while i < set.len() {
        if !E::is_identity(set[i]) {
            return false;
        }
        i += 1;
    }
    true
}

const BOOL_CHARS: &[char] = &ascii_chars(b"truefals");
const UINT_CHARS: &[char] = &ascii_chars(b"0123456789");

impl<E: Escape> CommonIdents<E> {
    /// True if `true` and `false` will never need escaping.
    pub const BOOLS: bool = are_all_chars_identity::<E>(BOOL_CHARS);
    /// True if unsigned integers will never need escaping.
    pub const UINTS: bool = are_all_chars_identity::<E>(UINT_CHARS);
    /// True if signed integers will never need escaping.
    pub const INTS: bool = Self::UINTS && are_all_chars_identity::<E>(&['-']);
    /// True if floats (using [`ryu`]'s formatting) will never need escaping.
    pub const FLOATS: bool = Self::INTS && are_all_chars_identity::<E>(&['.', 'e']);
}

/// Constant metadata about an impl of [`Escape`].
#[const_trait]
pub trait EscapeMeta {
    /// Returns `true` if the escaping scheme will never map the given character, regardless of its
    /// configuration.
    fn is_identity(c: char) -> bool;
}

/// A scheme for escaping strings.
pub trait Escape: const EscapeMeta {
    /// The type of an escaped character.
    type Escaped: AsRef<str>;

    /// If the character needs to be escaped, does so and returns it as a string. Otherwise,
    /// returns `None`.
    fn escape(&self, c: char) -> Option<Self::Escaped>;

    /// Writes the `string` to the `buffer`, applying any necessary escaping.
    #[inline]
    fn escape_to_buf(&self, buffer: &mut Buffer, string: &str) {
        buffer.reserve(string.len());
        let mut i = 0;
        for (j, c) in string.char_indices() {
            if let Some(rep) = self.escape(c) {
                buffer.push_str(&string[i..j]);
                buffer.push_str(rep.as_ref());
                i = j + c.len_utf8();
            }
        }
    }

    /// Writes the `string` to the `buffer`, applying any necessary escaping.
    ///
    /// # Examples
    ///
    /// ```
    /// use sailfish::{Escape, EscapeHtml};
    ///
    /// let mut buf = String::new();
    /// EscapeHtml.escape_to_string(&mut buf, "<h1>Hello, world!</h1>");
    /// assert_eq!(buf, "&lt;h1&gt;Hello, world!&lt;/h1&gt;");
    /// ```
    #[inline]
    fn escape_to_string(&self, buffer: &mut String, string: &str) {
        let mut buf = Buffer::from(std::mem::take(buffer));
        self.escape_to_buf(&mut buf, string);
        *buffer = buf.into_string();
    }
}

/// A scheme for escaping strings for safe insertion into JSON strings.
pub struct EscapeJsonString;

impl const EscapeMeta for EscapeJsonString {
    #[inline]
    fn is_identity(c: char) -> bool {
        !matches!(c, '"' | '\\' | '\u{0000}'..='\u{001F}')
    }
}

impl Escape for EscapeJsonString {
    type Escaped = ArrayStr<4>;

    #[inline]
    fn escape(&self, c: char) -> Option<Self::Escaped> {
        match c {
            '"' => Some(ArrayStr::try_from(r#"\""#).unwrap()),
            '\\' => Some(ArrayStr::try_from(r"\\").unwrap()),
            '\u{0000}'..='\u{001F}' => {
                let c = c as u8;
                let mut s = ArrayStr::try_from(r"\u").unwrap();
                unsafe {
                    const HEX_DIGITS: [u8; 16] = *b"0123456789ABCDEF";
                    // SAFETY: we only write valid UTF-8
                    let arr = s.data_mut();
                    arr.unused_mut()[0].write(HEX_DIGITS[usize::from(c >> 4)]);
                    arr.unused_mut()[1].write(HEX_DIGITS[usize::from(c & 0xF)]);
                    // SAFETY: we just initialized the last 2 bytes
                    arr.set_len(arr.len() + 2);
                }
                Some(s)
            }
            _ => None,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::{CommonIdents, EscapeJsonString};

    #[test]
    fn check_idents() {
        assert!(CommonIdents::<EscapeJsonString>::BOOLS);
        assert!(CommonIdents::<EscapeJsonString>::UINTS);
        assert!(CommonIdents::<EscapeJsonString>::INTS);
        assert!(CommonIdents::<EscapeJsonString>::FLOATS);
    }
}