use core::char;
use core::fmt::{self, Write};
use core::mem;
use core::str as core_str;

// https://tools.ietf.org/html/rfc3629
static UTF8_CHAR_WIDTH: [u8; 256] = [
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, // 0x1F
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, // 0x3F
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, // 0x5F
    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
    1, // 0x7F
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, // 0x9F
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, // 0xBF
    0, 0, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
    2, // 0xDF
    3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, // 0xEF
    4, 4, 4, 4, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0xFF
];

/// Given a first byte, determines how many bytes are in this UTF-8 character.
#[inline]
pub fn utf8_char_width(b: u8) -> usize {
    UTF8_CHAR_WIDTH[b as usize] as usize
}

/// Lossy UTF-8 string.
pub struct Utf8Lossy {
    bytes: [u8],
}

impl Utf8Lossy {
    pub fn from_bytes(bytes: &[u8]) -> &Utf8Lossy {
        // SAFETY: Both use the same memory layout, and UTF-8 correctness isn't required.
        unsafe { mem::transmute(bytes) }
    }

    pub fn chunks(&self) -> Utf8LossyChunksIter<'_> {
        Utf8LossyChunksIter {
            source: &self.bytes,
        }
    }
}

/// Iterator over lossy UTF-8 string
pub struct Utf8LossyChunksIter<'a> {
    source: &'a [u8],
}

#[derive(PartialEq, Eq, Debug)]
pub struct Utf8LossyChunk<'a> {
    /// Sequence of valid chars.
    /// Can be empty between broken UTF-8 chars.
    pub valid: &'a str,
    /// Single broken char, empty if none.
    /// Empty iff iterator item is last.
    pub broken: &'a [u8],
}

impl<'a> Iterator for Utf8LossyChunksIter<'a> {
    type Item = Utf8LossyChunk<'a>;

    fn next(&mut self) -> Option<Utf8LossyChunk<'a>> {
        if self.source.is_empty() {
            return None;
        }

        const TAG_CONT_U8: u8 = 128;
        fn safe_get(xs: &[u8], i: usize) -> u8 {
            *xs.get(i).unwrap_or(&0)
        }

        let mut i = 0;
        while i < self.source.len() {
            let i_ = i;

            // SAFETY: `i` starts at `0`, is less than `self.source.len()`, and
            // only increases, so `0 <= i < self.source.len()`.
            let byte = unsafe { *self.source.get_unchecked(i) };
            i += 1;

            if byte < 128 {
            } else {
                let w = utf8_char_width(byte);

                macro_rules! error {
                    () => {{
                        // SAFETY: We have checked up to `i` that source is valid UTF-8.
                        unsafe {
                            let r = Utf8LossyChunk {
                                valid: core_str::from_utf8_unchecked(&self.source[0..i_]),
                                broken: &self.source[i_..i],
                            };
                            self.source = &self.source[i..];
                            return Some(r);
                        }
                    }};
                }

                match w {
                    2 => {
                        if safe_get(self.source, i) & 192 != TAG_CONT_U8 {
                            error!();
                        }
                        i += 1;
                    }
                    3 => {
                        match (byte, safe_get(self.source, i)) {
                            (0xE0, 0xA0..=0xBF) => (),
                            (0xE1..=0xEC, 0x80..=0xBF) => (),
                            (0xED, 0x80..=0x9F) => (),
                            (0xEE..=0xEF, 0x80..=0xBF) => (),
                            _ => {
                                error!();
                            }
                        }
                        i += 1;
                        if safe_get(self.source, i) & 192 != TAG_CONT_U8 {
                            error!();
                        }
                        i += 1;
                    }
                    4 => {
                        match (byte, safe_get(self.source, i)) {
                            (0xF0, 0x90..=0xBF) => (),
                            (0xF1..=0xF3, 0x80..=0xBF) => (),
                            (0xF4, 0x80..=0x8F) => (),
                            _ => {
                                error!();
                            }
                        }
                        i += 1;
                        if safe_get(self.source, i) & 192 != TAG_CONT_U8 {
                            error!();
                        }
                        i += 1;
                        if safe_get(self.source, i) & 192 != TAG_CONT_U8 {
                            error!();
                        }
                        i += 1;
                    }
                    _ => {
                        error!();
                    }
                }
            }
        }

        let r = Utf8LossyChunk {
            // SAFETY: We have checked that the entire source is valid UTF-8.
            valid: unsafe { core_str::from_utf8_unchecked(self.source) },
            broken: &[],
        };
        self.source = &[];
        Some(r)
    }
}

impl fmt::Display for Utf8Lossy {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        // If we're the empty string then our iterator won't actually yield
        // anything, so perform the formatting manually
        if self.bytes.is_empty() {
            return "".fmt(f);
        }

        for Utf8LossyChunk { valid, broken } in self.chunks() {
            // If we successfully decoded the whole chunk as a valid string then
            // we can return a direct formatting of the string which will also
            // respect various formatting flags if possible.
            if valid.len() == self.bytes.len() {
                assert!(broken.is_empty());
                return valid.fmt(f);
            }

            f.write_str(valid)?;
            if !broken.is_empty() {
                f.write_char(char::REPLACEMENT_CHARACTER)?;
            }
        }
        Ok(())
    }
}