Crate bet

A library building and preparing expressions, for example boolean expressions such as (A | B) & !(C | D | E), which can be executed on dynamic contents.

An expression is built by sequentially pushing the parts: parenthesis, operators, atoms (the “variables”). You do that by calling the push_operator, open_par, close_par and push_atom functions, which build the tree for you.

It can then be evaluated with the eval function which takes as parameters

• a function which gives a value to an atom
• a function which, given an operator and one or two values, gives a new value
• a function deciding whether to short-circuit

Normal evaluation order is left to right but is modified with parenthesis.

bet is designed around separation of building, transformations, and evaluation, so that an expression can be efficiently applied on many inputs. bet is designed for very fast evaluation.

Examples: Known open-source usages

dysk

bet is used in dysk to filter filesystems.

Example: dysk -f '(type=xfs & remote=no) | size > 5T'.

Here, the atoms are type=xfs, remote=no, and size > 5T.

To parse such expression, the simplest solution is to first parse it with atoms being simple strings, then apply try_map_atoms on the tree to replace the strings with structs which can be efficiently evaluated on many entries.

Here’s how it’s done:

impl FromStr for Filter {
    type Err = ParseExprError;
    fn from_str(input: &str) -> Result<Self, ParseExprError> {

        // we start by reading the global structure
        let mut expr: BeTree<BoolOperator, String> = BeTree::new();
        for c in input.chars() {
            match c {
                '&' => expr.push_operator(BoolOperator::And),
                '|' => expr.push_operator(BoolOperator::Or),
                '!' => expr.push_operator(BoolOperator::Not),
                ' ' => {},
                '(' => expr.open_par(),
                ')' => expr.close_par(),
                _ => expr.mutate_or_create_atom(String::new).push(c),
            }
        }

        // then we parse each leaf
        let expr = expr.try_map_atoms(|raw| raw.parse())?;

        Ok(Self { expr })
    }
}

broot

In broot, bet enables composite queries on files.

For example, !lock&(carg|c/carg/) looks for files whose name or content contains “carg”, but excluding files whose name contains “lock”.

rhit

bet is used in rhit to filter log lines.

For example, with rhit -p 'y & !( \d{4} | sp | bl )', you get stats on hits on paths containing “y” but neither a 4 digits number, “sp”, nor “bl”.

Complet example : parsing and evaluating boolean expressions

Here we parse expressions like "(A | B) & !(C | D | E)" and evaluate them.

use bet::BeTree;

/// The operators in this example are AND, OR, and NOT operating on booleans.
/// `And` and `Or` are binary while `Not` is unary.
#[derive(Debug, Clone, Copy, PartialEq)]
enum BoolOperator {
    And,
    Or,
    Not,
}

/// simple but realistic example of an expression parsing.
///
/// You don't have to parse tokens in advance, you may accumulate
/// into atoms with `mutate_or_create_atom`.
///
/// For more reliable results on user inputs, you may want to check
/// the consistency (or use default operators) during parsing
/// with `accept_atom`, `accept_unary_operator`, etc.
fn parse(input: &str) -> BeTree<BoolOperator, char> {
    let mut expr = BeTree::new();
    for c in input.chars() {
        match c {
            '&' => expr.push_operator(BoolOperator::And),
            '|' => expr.push_operator(BoolOperator::Or),
            '!' => expr.push_operator(BoolOperator::Not),
            ' ' => {},
            '(' => expr.open_par(),
            ')' => expr.close_par(),
            _ => expr.push_atom(c),
        }
    }
    expr
}

/// evaluate the expression.
///
/// `trues` is the set of chars whose value is true.
///
/// If no operation is expected to fail, you may use
/// `eval` instead of `eval_faillible`, for a simpler
/// API.
fn eval(expr: &BeTree<BoolOperator, char>, trues: &[char]) -> bool {
    expr.eval_faillible(
        // the function evaluating leafs - here it's simple
        |c| Ok(trues.contains(c)),
        // the function applying an operator to one or two values
        |op, a, b| match (op, b) {
            (BoolOperator::And, Some(b)) => Ok(a & b),
            (BoolOperator::Or, Some(b)) => Ok(a | b),
            (BoolOperator::Not, None) => Ok(!a),
            _ => { Err("unexpected operation") }
        },
        // when to short-circuit. This is essential when leaf
        // evaluation is expensive or when the left part guards
        // for correctness of the right part evaluation
        |op, a| match (op, a) {
            (BoolOperator::And, false) => true,
            (BoolOperator::Or, true) => true,
            _ => false,
        },
    ).unwrap().unwrap()
}

// checking complex evaluations with T=true and F=false
assert_eq!(eval(&parse("!((T|F)&T)"), &['T']), false);
assert_eq!(eval(&parse("!(!((T|F)&(F|T)&T)) & !F & (T | (T|F))"), &['T']), true);

// we evaluate an expression with two different sets of values
let expr = parse("(A | B) & !(C | D | E)");
assert_eq!(eval(&expr, &['A', 'C', 'E']), false);
assert_eq!(eval(&expr, &['A', 'B']), true);

// Let's show the left to right evaluation order
// and importance of parenthesis
assert_eq!(eval(&parse("(A & B)|(C & D)"), &['A', 'B', 'C']), true);
assert_eq!(eval(&parse(" A & B | C & D "), &['A', 'B', 'C']), false);

Structs

• BeTree
  An expression which may contain unary and binary operations
• Node
  A node in the expression tree

Enums

• Child
  One of the children of a node
• Token
  Something that can be added to the tree

Type Aliases

• AtomId
• NodeId