use std::borrow::Borrow;
use std::cmp::Ordering;
use std::fmt::{self, Debug, Display, Formatter, Write};
use std::hash::{Hash, Hasher};
use std::ops::Deref;
use std::rc::Rc;

/// An economical string with inline storage and clone-on-write semantics.
#[derive(Clone)]
pub struct EcoString(Repr);

/// The internal representation. Either:
/// - inline when below a certain number of bytes,
/// - or reference-counted on the heap with COW semantics.
#[derive(Clone)]
enum Repr {
    Small { buf: [u8; LIMIT], len: u8 },
    Large(Rc<String>),
}

/// The maximum number of bytes that can be stored inline.
///
/// The value is chosen such that an `EcoString` fits exactly into 16 bytes
/// (which are needed anyway due to the `Rc`s alignment, at least on 64-bit
/// platforms).
///
/// Must be at least 4 to hold any char.
const LIMIT: usize = 14;

impl EcoString {
    /// Create a new, empty string.
    pub fn new() -> Self {
        Self(Repr::Small { buf: [0; LIMIT], len: 0 })
    }

    /// Create a new, empty string with the given `capacity`.
    pub fn with_capacity(capacity: usize) -> Self {
        if capacity <= LIMIT {
            Self::new()
        } else {
            Self(Repr::Large(Rc::new(String::with_capacity(capacity))))
        }
    }

    /// Create an instance from an existing string-like type.
    pub fn from_str<S>(s: S) -> Self
    where
        S: AsRef<str> + Into<String>,
    {
        let slice = s.as_ref();
        let len = slice.len();
        Self(if len <= LIMIT {
            let mut buf = [0; LIMIT];
            buf[.. len].copy_from_slice(slice.as_bytes());
            Repr::Small { buf, len: len as u8 }
        } else {
            Repr::Large(Rc::new(s.into()))
        })
    }

    /// Whether the string is empty.
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// The length of the string in bytes.
    pub fn len(&self) -> usize {
        match &self.0 {
            Repr::Small { len, .. } => usize::from(*len),
            Repr::Large(string) => string.len(),
        }
    }

    /// A string slice containing the entire string.
    pub fn as_str(&self) -> &str {
        self
    }

    /// Append the given character at the end.
    pub fn push(&mut self, c: char) {
        match &mut self.0 {
            Repr::Small { buf, len } => {
                let prev = usize::from(*len);
                if c.len_utf8() == 1 && prev < LIMIT {
                    buf[prev] = c as u8;
                    *len += 1;
                } else {
                    self.push_str(c.encode_utf8(&mut [0; 4]));
                }
            }
            Repr::Large(rc) => Rc::make_mut(rc).push(c),
        }
    }

    /// Append the given string slice at the end.
    pub fn push_str(&mut self, string: &str) {
        match &mut self.0 {
            Repr::Small { buf, len } => {
                let prev = usize::from(*len);
                let new = prev + string.len();
                if new <= LIMIT {
                    buf[prev .. new].copy_from_slice(string.as_bytes());
                    *len = new as u8;
                } else {
                    let mut spilled = String::with_capacity(new);
                    spilled.push_str(self);
                    spilled.push_str(string);
                    self.0 = Repr::Large(Rc::new(spilled));
                }
            }
            Repr::Large(rc) => Rc::make_mut(rc).push_str(string),
        }
    }

    /// Remove the last character from the string.
    pub fn pop(&mut self) -> Option<char> {
        let c = self.as_str().chars().rev().next()?;
        match &mut self.0 {
            Repr::Small { len, .. } => {
                *len -= c.len_utf8() as u8;
            }
            Repr::Large(rc) => {
                Rc::make_mut(rc).pop();
            }
        }
        Some(c)
    }

    /// Clear the string.
    pub fn clear(&mut self) {
        match &mut self.0 {
            Repr::Small { len, .. } => *len = 0,
            Repr::Large(rc) => {
                if Rc::strong_count(rc) == 1 {
                    Rc::make_mut(rc).clear();
                } else {
                    *self = Self::new();
                }
            }
        }
    }

    /// Repeat this string `n` times.
    pub fn repeat(&self, n: usize) -> Self {
        if n == 0 {
            return Self::new();
        }

        if let Repr::Small { buf, len } = &self.0 {
            let prev = usize::from(*len);
            let new = prev.saturating_mul(n);
            if new <= LIMIT {
                let src = &buf[.. prev];
                let mut buf = [0; LIMIT];
                for i in 0 .. n {
                    buf[prev * i .. prev * (i + 1)].copy_from_slice(src);
                }
                return Self(Repr::Small { buf, len: new as u8 });
            }
        }

        self.as_str().repeat(n).into()
    }
}

impl Deref for EcoString {
    type Target = str;

    fn deref(&self) -> &str {
        match &self.0 {
            // Safety:
            // The buffer contents stem from correct UTF-8 sources:
            // - Valid ASCII characters
            // - Other string slices
            // - Chars that were encoded with char::encode_utf8
            // Furthermore, we still do the bounds-check on the len in case
            // it gets corrupted somehow.
            Repr::Small { buf, len } => unsafe {
                std::str::from_utf8_unchecked(&buf[.. usize::from(*len)])
            },
            Repr::Large(string) => string.as_str(),
        }
    }
}

impl Default for EcoString {
    fn default() -> Self {
        Self::new()
    }
}

impl Debug for EcoString {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        Debug::fmt(self.as_str(), f)
    }
}

impl Display for EcoString {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        Display::fmt(self.as_str(), f)
    }
}

impl Eq for EcoString {}

impl PartialEq for EcoString {
    fn eq(&self, other: &Self) -> bool {
        self.as_str().eq(other.as_str())
    }
}

impl PartialEq<str> for EcoString {
    fn eq(&self, other: &str) -> bool {
        self.as_str().eq(other)
    }
}

impl PartialEq<&str> for EcoString {
    fn eq(&self, other: &&str) -> bool {
        self.as_str().eq(*other)
    }
}

impl Ord for EcoString {
    fn cmp(&self, other: &Self) -> Ordering {
        self.as_str().cmp(other.as_str())
    }
}

impl PartialOrd for EcoString {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        self.as_str().partial_cmp(other.as_str())
    }
}

impl Hash for EcoString {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.as_str().hash(state);
    }
}

impl Write for EcoString {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        self.push_str(s);
        Ok(())
    }

    fn write_char(&mut self, c: char) -> fmt::Result {
        self.push(c);
        Ok(())
    }
}

impl AsRef<str> for EcoString {
    fn as_ref(&self) -> &str {
        self
    }
}

impl Borrow<str> for EcoString {
    fn borrow(&self) -> &str {
        self
    }
}

impl From<&Self> for EcoString {
    fn from(s: &Self) -> Self {
        s.clone()
    }
}

impl From<char> for EcoString {
    fn from(c: char) -> Self {
        let mut buf = [0; LIMIT];
        let len = c.encode_utf8(&mut buf).len();
        Self(Repr::Small { buf, len: len as u8 })
    }
}

impl From<&str> for EcoString {
    fn from(s: &str) -> Self {
        Self::from_str(s)
    }
}

impl From<String> for EcoString {
    fn from(s: String) -> Self {
        Self::from_str(s)
    }
}

impl From<EcoString> for String {
    fn from(s: EcoString) -> Self {
        match s.0 {
            Repr::Small { .. } => s.as_str().to_owned(),
            Repr::Large(rc) => match Rc::try_unwrap(rc) {
                Ok(string) => string,
                Err(rc) => (*rc).clone(),
            },
        }
    }
}

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

    const ALPH: &str = "abcdefghijklmnopqrstuvwxyz";

    #[test]
    fn test_str_new() {
        // Test inline strings.
        assert_eq!(EcoString::new(), "");
        assert_eq!(EcoString::from('a'), "a");
        assert_eq!(EcoString::from('😀'), "😀");
        assert_eq!(EcoString::from("abc"), "abc");

        // Test around the inline limit.
        assert_eq!(EcoString::from(&ALPH[.. LIMIT - 1]), ALPH[.. LIMIT - 1]);
        assert_eq!(EcoString::from(&ALPH[.. LIMIT]), ALPH[.. LIMIT]);
        assert_eq!(EcoString::from(&ALPH[.. LIMIT + 1]), ALPH[.. LIMIT + 1]);

        // Test heap string.
        assert_eq!(EcoString::from(ALPH), ALPH);
    }

    #[test]
    fn test_str_push() {
        let mut v = EcoString::new();
        v.push('a');
        v.push('b');
        v.push_str("cd😀");
        assert_eq!(v, "abcd😀");
        assert_eq!(v.len(), 8);

        // Test fully filling the inline storage.
        v.push_str("efghij");
        assert_eq!(v.len(), LIMIT);

        // Test spilling with `push`.
        let mut a = v.clone();
        a.push('k');
        assert_eq!(a, "abcd😀efghijk");
        assert_eq!(a.len(), 15);

        // Test spilling with `push_str`.
        let mut b = v.clone();
        b.push_str("klmn");
        assert_eq!(b, "abcd😀efghijklmn");
        assert_eq!(b.len(), 18);

        // v should be unchanged.
        assert_eq!(v.len(), LIMIT);
    }

    #[test]
    fn test_str_pop() {
        // Test with inline string.
        let mut v = EcoString::from("Hello World!");
        assert_eq!(v.pop(), Some('!'));
        assert_eq!(v, "Hello World");

        // Remove one-by-one.
        for _ in 0 .. 10 {
            v.pop();
        }

        assert_eq!(v, "H");
        assert_eq!(v.pop(), Some('H'));
        assert_eq!(v, "");
        assert!(v.is_empty());

        // Test with large string.
        let mut v = EcoString::from(ALPH);
        assert_eq!(v.pop(), Some('z'));
        assert_eq!(v.len(), 25);
    }

    #[test]
    fn test_str_index() {
        // Test that we can use the index syntax.
        let v = EcoString::from("abc");
        assert_eq!(&v[.. 2], "ab");
    }

    #[test]
    fn test_str_repeat() {
        // Test with empty string.
        assert_eq!(EcoString::new().repeat(0), "");
        assert_eq!(EcoString::new().repeat(100), "");

        // Test non-spilling and spilling case.
        let v = EcoString::from("abc");
        assert_eq!(v.repeat(0), "");
        assert_eq!(v.repeat(3), "abcabcabc");
        assert_eq!(v.repeat(5), "abcabcabcabcabc");
    }
}