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/*!
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](https://dystroy.org/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:
```ignore
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](https://dystroy.org/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](https://dystroy.org/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".
# Complete 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);
```
*/
pub use ;