//! A lightweight, self-contained s-expression parser and data format.
//! Use `parse` to get an s-expression from its string representation, and the
//! `Display` trait to serialize it, potentially by doing `sexp.to_string()`.

#![deny(unsafe_code)]

use std::borrow::Cow;
use std::fmt;
use std::str::{self, FromStr};

use error::{ERes, err};
pub use error::Error;
pub use position::SourcePosition;
use position::spos;

#[cfg(test)]
mod test;

mod error;
mod position;

/// A single data element in an s-expression. Floats are excluded to ensure
/// atoms may be used as keys in ordered and hashed data structures.
///
/// All strings must be valid utf-8.
#[derive(Debug, PartialEq, Clone, PartialOrd)]
pub enum Atom {
    /// Simple string atom
    S(String),
    /// Quoted string
    QS(String),
    /// Character literal (with single quotes)
    C(char),
    /// Signed integer (normally only used for negative values)
    I(i64),
    /// Unsigned integer
    U(u64),
    /// Float
    F(f64),
}

/// An s-expression is either an atom or a list of s-expressions. This is
/// similar to the data format used by lisp.
#[derive(Debug, Clone)]
pub enum Sexp {
    /// Atom
    Atom(Atom, SourcePosition),
    /// List of expressions
    List(Vec<Sexp>, SourcePosition),
}

impl Sexp {
    pub fn pos(&self) -> &SourcePosition {
        match self {
            Sexp::List(_, pos) | Sexp::Atom(_, pos) => pos
        }
    }

    /// Check fi thsi Sexp is an atom
    pub fn is_atom(&self) -> bool {
        match self {
            Sexp::Atom(_, _) => true,
            _ => false,
        }
    }

    /// Check fi thsi Sexp is a list
    pub fn is_list(&self) -> bool {
        match self {
            Sexp::List(_, _) => true,
            _ => false,
        }
    }
}

impl PartialEq for Sexp {
    fn eq(&self, other: &Self) -> bool {
        match (self, other) {
            (Sexp::Atom(a, _), Sexp::Atom(b, _)) => {
                a == b
            }
            (Sexp::List(a, _), Sexp::List(b, _)) => {
                a == b
            }
            _ => false
        }
    }
}

fn without_underscores(s: &str) -> Cow<str> {
    if s.contains('_') {
        s.chars()
            .filter(|c| *c != '_')
            .collect::<String>().into()
    } else {
        s.into()
    }
}

pub fn atom_of_string(s: String) -> Atom {
    if s.starts_with('#') {
        match u64::from_str_radix(&without_underscores(&s[1..]), 16) {
            Ok(u) => return Atom::U(u),
            Err(_) => {}
        };
    }

    if s.starts_with("0x") {
        match u64::from_str_radix(&without_underscores(&s[2..]), 16) {
            Ok(u) => return Atom::U(u),
            Err(_) => {}
        };
    }

    if s.starts_with("0b") {
        match u64::from_str_radix(&without_underscores(&s[2..]), 2) {
            Ok(u) => return Atom::U(u),
            Err(_) => {}
        };
    }

    if !s.starts_with('_') {
        let filtered = without_underscores(&s);

        match FromStr::from_str(&filtered) {
            Ok(u) => return Atom::U(u),
            Err(_) => {}
        };

        match FromStr::from_str(&filtered) {
            Ok(i) => return Atom::I(i),
            Err(_) => {}
        };

        match FromStr::from_str(&filtered) {
            Ok(f) => return Atom::F(f),
            Err(_) => {}
        };
    }

    Atom::S(s)
}

// returns the char it found, and the new pos if you wish to consume that char
fn peek(s: &str, pos: usize) -> ERes<(char, usize)> {
    //trace!("peek {}", pos);
    if pos == s.len() { return err("unexpected eof", s, pos); }
    if s.is_char_boundary(pos) {
        let ch = s[pos..].chars().next().unwrap();
        let next = pos + ch.len_utf8();
        Ok((ch, next))
    } else {
        // strings must be composed of valid utf-8 chars.
        unreachable!()
    }
}

// returns the char it found, and the new pos if you wish to consume that char
fn peekn(s: &str, nth: usize, pos: usize) -> Option<(char, usize)> {
    //trace!("peekn {}", pos);
    if nth == 0 {
        panic!("peekn with nth=0");
    }
    if s.is_char_boundary(pos) {
        let mut iter = s[pos..].chars();
        let mut bytelen = 0;
        for n in 0..nth {
            if let Some(ch) = iter.next() {
                bytelen += ch.len_utf8();
                if n == (nth - 1) {
                    return Some((ch, pos + bytelen));
                }
            } else {
                return None;
            }
        }
        unreachable!()
    } else {
        // strings must be composed of valid utf-8 chars.
        unreachable!()
    }
}

fn expect(s: &str, pos: &mut usize, c: char) -> ERes<()> {
    //trace!("expect {}", pos);
    let (ch, next) = peek(s, *pos)?;
    *pos = next;
    if ch == c {
        Ok(())
    } else {
        err("unexpected character", s, *pos)
    }
}

fn consume_until_newline(s: &str, pos: &mut usize) -> ERes<()> {
    loop {
        if *pos == s.len() {
            return Ok(());
        }
        let (ch, next) = peek(s, *pos)?;
        *pos = next;
        if ch == '\n' {
            return Ok(());
        }
    }
}

// zero or more spaces
fn zspace(s: &str, pos: &mut usize) -> ERes<()> {
    //trace!("zspace {}", pos);
    loop {
        if *pos == s.len() {
            return Ok(());
        }
        let (ch, next) = peek(s, *pos)?;

        if ch == ';' {
            consume_until_newline(s, pos)?
        } else if ch.is_whitespace() {
            *pos = next;
        } else {
            return Ok(());
        }
    }
}

fn parse_quoted_atom(s: &str, quote: char, pos: &mut usize) -> ERes<Atom> {
    //trace!("parse_quoted_atom {}", pos);
    let pos0 = *pos;
    let mut cs: String = String::new();

    expect(s, pos, quote)?;

    loop {
        let (ch, next) = peek(s, *pos)?;
        if ch == quote {
            *pos = next;
            break;
        } else if ch == '\\' {
            let (postslash, nextnext) = peek(s, next)?;
            match postslash {
                'r' => cs.push('\r'),
                'n' => cs.push('\n'),
                't' => cs.push('\t'),
                other => cs.push(other)
            }
            *pos = nextnext;
        } else {
            cs.push(ch);
            *pos = next;
        }
    }

    if quote == '\'' {
        // This is a character literal
        if cs.chars().count() == 1 {
            return Ok(Atom::C(cs.chars().next().unwrap()));
        } else {
            return err("Too long character literal!", s, pos0);
        }
    }

    // Do not try i64 conversion, since this atom was explicitly quoted.
    Ok(Atom::QS(cs))
}

fn parse_unquoted_atom(s: &str, pos: &mut usize) -> ERes<Atom> {
    //trace!("parse_unquoted_atom {}", pos);
    let mut cs: String = String::new();

    loop {
        if *pos == s.len() { break; }
        let (c, next) = peek(s, *pos)?;

        if c == ';' {
            consume_until_newline(s, pos)?;
            break;
        }
        if c.is_whitespace() || c == '(' || c == ')' {
            break;
        }
        cs.push(c);
        *pos = next;
    }

    Ok(atom_of_string(cs))
}

fn parse_atom(s: &str, pos: &mut usize) -> ERes<Atom> {
    //trace!("parse_atom {}", pos);
    let (ch, _) = peek(s, *pos)?;

    if ch == '"' {
        return parse_quoted_atom(s, ch, pos);
    } else if ch == '\'' {
        if let Some(('\\', _)) = peekn(s, 2, *pos) {
            if let Some(('\'', _)) = peekn(s, 4, *pos) {
                // Character literal with an escape sequence
                return parse_quoted_atom(s, '\'', pos);
            }
        } else if let Some(('\'', _)) = peekn(s, 3, *pos) {
            // Simple character literal
            return parse_quoted_atom(s, '\'', pos);
        }
    }

    parse_unquoted_atom(s, pos)
}

fn parse_list(s: &str, pos: &mut usize) -> ERes<Vec<Sexp>> {
    //trace!("parse_list {}", pos);
    zspace(s, pos)?;
    expect(s, pos, '(')?;

    let mut sexps: Vec<Sexp> = Vec::new();

    loop {
        zspace(s, pos)?;
        let (c, next) = peek(s, *pos)?;
        if c == ')' {
            *pos = next;
            break;
        }
        sexps.push(parse_sexp(s, pos)?);
    }

    zspace(s, pos)?;

    Ok(sexps)
}

fn parse_sexp(s: &str, pos: &mut usize) -> ERes<Sexp> {
    //trace!("parse_sexp {}", pos);
    zspace(s, pos)?;
    let (c, _) = peek(s, *pos)?;
    let pos0 = *pos;
    let r = if c == '(' {
        Ok(Sexp::List(parse_list(s, pos)?, spos(s, pos0)))
    } else {
        Ok(Sexp::Atom(parse_atom(s, pos)?, spos(s, pos0)))
    };
    zspace(s, pos)?;
    r
}

/// Constructs an atomic s-expression from a string.
pub fn atom_s(s: &str) -> Sexp {
    Sexp::Atom(Atom::S(s.to_owned()), Default::default())
}

/// Constructs an atomic s-expression from a string.
pub fn atom_qs(s: &str) -> Sexp {
    Sexp::Atom(Atom::QS(s.to_owned()), Default::default())
}

/// Constructs an atomic s-expression from an int.
pub fn atom_i(i: i64) -> Sexp {
    Sexp::Atom(Atom::I(i), Default::default())
}

/// Constructs an atomic s-expression from an unsigned int.
pub fn atom_u(u: u64) -> Sexp {
    Sexp::Atom(Atom::U(u), Default::default())
}

/// Constructs an atomic s-expression from a char
pub fn atom_c(c: char) -> Sexp {
    Sexp::Atom(Atom::C(c), Default::default())
}

/// Constructs an atomic s-expression from a float.
pub fn atom_f(f: f64) -> Sexp {
    Sexp::Atom(Atom::F(f), Default::default())
}

/// Constructs a list s-expression given a slice of s-expressions.
pub fn list(xs: &[Sexp]) -> Sexp {
    Sexp::List(xs.to_owned(), Default::default())
}

/// Reads an s-expression out of a `&str`.
#[inline(never)]
pub fn parse(s: &str) -> Result<Sexp, Box<Error>> {
    let mut pos = 0;
    let ret = parse_sexp(s, &mut pos)?;
    if pos == s.len() {
        Ok(ret)
    } else {
        err("unrecognized post-s-expression data", s, pos)
    }
}

fn quote(s: &str) -> String {
    s.chars().fold(String::new(), |mut s, ch| {
        match ch {
            '\'' | '\\' | '"' => {
                s.push('\\');
                s.push(ch);
            }
            '\n' => {
                s.push_str("\\n");
            }
            '\r' => {
                s.push_str("\\n");
            }
            '\t' => {
                s.push_str("\\t");
            }
            other => {
                s.push(other);
            }
        }
        s
    })
}

impl fmt::Display for Atom {
    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        match *self {
            Atom::QS(ref s) => write!(f, "\"{}\"", quote(s)),
            Atom::S(ref s) => write!(f, "{}", s),
            Atom::C(c) => write!(f, "'{}'", quote(&c.to_string())),
            Atom::I(i) => write!(f, "{}", i),
            Atom::U(u) => write!(f, "{}", u),
            Atom::F(d) => write!(f, "{}", d),
        }
    }
}

impl fmt::Display for Sexp {
    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        match *self {
            Sexp::Atom(ref a, _) => write!(f, "{}", a),
            Sexp::List(ref xs, _) => {
                write!(f, "(")?;
                for (i, x) in xs.iter().enumerate() {
                    let s = if i == 0 { "" } else { " " };
                    write!(f, "{}{}", s, x)?;
                }
                write!(f, ")")
            }
        }
    }
}