expression.rs

// Miniscript
// Written in 2019 by
//     Andrew Poelstra <apoelstra@wpsoftware.net>
//
// To the extent possible under law, the author(s) have dedicated all
// copyright and related and neighboring rights to this software to
// the public domain worldwide. This software is distributed without
// any warranty.
//
// You should have received a copy of the CC0 Public Domain Dedication
// along with this software.
// If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
//

//! # Function-like Expression Language
//!
use core::str::FromStr;

use crate::prelude::*;
use crate::{errstr, Error, MAX_RECURSION_DEPTH};

#[derive(Debug)]
/// A token of the form `x(...)` or `x`
pub struct Tree<'a> {
    /// The name `x`
    pub name: &'a str,
    /// The comma-separated contents of the `(...)`, if any
    pub args: Vec<Tree<'a>>,
}
// or_b(pk(A),pk(B))
//
// A = musig(musig(B,C),D,E)
// or_b()
// pk(A), pk(B)

/// A trait for extracting a structure from a Tree representation in token form
pub trait FromTree: Sized {
    /// Extract a structure from Tree representation
    fn from_tree(top: &Tree) -> Result<Self, Error>;
}

enum Found {
    Nothing,
    LBracket(usize), // Either a left ( or {
    Comma(usize),
    RBracket(usize), // Either a right ) or }
}

fn next_expr(sl: &str, delim: char) -> Found {
    let mut found = Found::Nothing;
    if delim == '(' {
        for (n, ch) in sl.char_indices() {
            match ch {
                '(' => {
                    found = Found::LBracket(n);
                    break;
                }
                ',' => {
                    found = Found::Comma(n);
                    break;
                }
                ')' => {
                    found = Found::RBracket(n);
                    break;
                }
                _ => {}
            }
        }
    } else if delim == '{' {
        let mut new_count = 0;
        for (n, ch) in sl.char_indices() {
            match ch {
                '{' => {
                    found = Found::LBracket(n);
                    break;
                }
                '(' => {
                    new_count += 1;
                }
                ',' => {
                    if new_count == 0 {
                        found = Found::Comma(n);
                        break;
                    }
                }
                ')' => {
                    new_count -= 1;
                }
                '}' => {
                    found = Found::RBracket(n);
                    break;
                }
                _ => {}
            }
        }
    } else {
        unreachable!("{}", "Internal: delimiters in parsing must be '(' or '{'");
    }
    found
}

// Get the corresponding delim
fn closing_delim(delim: char) -> char {
    match delim {
        '(' => ')',
        '{' => '}',
        _ => unreachable!("Unknown delimiter"),
    }
}

impl<'a> Tree<'a> {
    /// Parse an expression with round brackets
    pub fn from_slice(sl: &'a str) -> Result<(Tree<'a>, &'a str), Error> {
        // Parsing TapTree or just miniscript
        Self::from_slice_delim(sl, 0u32, '(')
    }

    pub(crate) fn from_slice_delim(
        mut sl: &'a str,
        depth: u32,
        delim: char,
    ) -> Result<(Tree<'a>, &'a str), Error> {
        if depth >= MAX_RECURSION_DEPTH {
            return Err(Error::MaxRecursiveDepthExceeded);
        }

        match next_expr(sl, delim) {
            // String-ending terminal
            Found::Nothing => Ok((
                Tree {
                    name: sl,
                    args: vec![],
                },
                "",
            )),
            // Terminal
            Found::Comma(n) | Found::RBracket(n) => Ok((
                Tree {
                    name: &sl[..n],
                    args: vec![],
                },
                &sl[n..],
            )),
            // Function call
            Found::LBracket(n) => {
                // Special case for musig
                // We want the complex musig to be just one node in our tree.
                if &sl[..n] == "musig" {
                    let mut open_brackets = 0;
                    let mut finish_index = 0;
                    for (i, ch) in sl[n..].char_indices() {
                        if ch == delim {
                            open_brackets = open_brackets + 1;
                        } else if ch == closing_delim(delim) {
                            open_brackets = open_brackets - 1;
                        }
                        if open_brackets == 0 {
                            finish_index = i;
                            break;
                        }
                    } 
                    if open_brackets != 0 {
                        return Err(Error::ExpectedChar(closing_delim(delim)));
                    }
                    Ok((
                        Tree {
                            name: &sl[..n + finish_index + 1],
                            args: vec![],
                        }, 
                        &sl[n + finish_index + 1..],
                    ))
                } else {
                    let mut ret = Tree {
                        name: &sl[..n],
                        args: vec![],
                    };
                    // update the string to parse
                    sl = &sl[n + 1..];
                    loop {
                        let (arg, new_sl) = Tree::from_slice_delim(sl, depth + 1, delim)?;
                        ret.args.push(arg);
                        // There has to be a closing bracket for the open bracket we encounter in this case
                        if new_sl.is_empty() {
                            return Err(Error::ExpectedChar(closing_delim(delim)));
                        }

                        sl = &new_sl[1..];
                        match new_sl.as_bytes()[0] {
                            b',' => {}
                            last_byte => {
                                if last_byte == closing_delim(delim) as u8 {
                                    break;
                                } else {
                                    return Err(Error::ExpectedChar(closing_delim(delim)));
                                }
                            }
                        }
                    }
                    Ok((ret, sl))
                }
            }
        }
    }

    /// Parses a tree from a string
    #[allow(clippy::should_implement_trait)] // Cannot use std::str::FromStr because of lifetimes.
    pub fn from_str(s: &'a str) -> Result<Tree<'a>, Error> {
        // Filter out non-ASCII because we byte-index strings all over the
        // place and Rust gets very upset when you splinch a string.
        for ch in s.bytes() {
            if !ch.is_ascii() {
                return Err(Error::Unprintable(ch));
            }
        }

        let (top, rem) = Tree::from_slice(s)?;
        if rem.is_empty() {
            Ok(top)
        } else {
            Err(errstr(rem))
        }
    }
}

/// Parse a string as a u32, for timelocks or thresholds
pub fn parse_num(s: &str) -> Result<u32, Error> {
    if s.len() > 1 {
        let ch = s.chars().next().unwrap();
        if !('1'..='9').contains(&ch) {
            return Err(Error::Unexpected(
                "Number must start with a digit 1-9".to_string(),
            ));
        }
    }
    u32::from_str(s).map_err(|_| errstr(s))
}

/// Attempts to parse a terminal expression
pub fn terminal<T, F, Err>(term: &Tree, convert: F) -> Result<T, Error>
where
    F: FnOnce(&str) -> Result<T, Err>,
    Err: ToString,
{
    if term.args.is_empty() {
        convert(term.name).map_err(|e| Error::Unexpected(e.to_string()))
    } else {
        Err(errstr(term.name))
    }
}

/// Attempts to parse an expression with exactly one child
pub fn unary<L, T, F>(term: &Tree, convert: F) -> Result<T, Error>
where
    L: FromTree,
    F: FnOnce(L) -> T,
{
    if term.args.len() == 1 {
        let left = FromTree::from_tree(&term.args[0])?;
        Ok(convert(left))
    } else {
        Err(errstr(term.name))
    }
}

/// Attempts to parse an expression with exactly two children
pub fn binary<L, R, T, F>(term: &Tree, convert: F) -> Result<T, Error>
where
    L: FromTree,
    R: FromTree,
    F: FnOnce(L, R) -> T,
{
    if term.args.len() == 2 {
        let left = FromTree::from_tree(&term.args[0])?;
        let right = FromTree::from_tree(&term.args[1])?;
        Ok(convert(left, right))
    } else {
        Err(errstr(term.name))
    }
}

#[cfg(test)]
mod tests {

    use super::parse_num;

    #[test]
    fn test_parse_num() {
        assert!(parse_num("0").is_ok());
        assert!(parse_num("00").is_err());
        assert!(parse_num("0000").is_err());
        assert!(parse_num("06").is_err());
        assert!(parse_num("+6").is_err());
        assert!(parse_num("-6").is_err());
    }
}