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use nom::{
branch::alt,
bytes::complete::tag,
character::complete::char,
character::complete::{digit1 as digit, space0 as space},
combinator::map_res,
multi::fold_many0,
sequence::{delimited, pair},
IResult,
};
// Parser definition
use std::str::FromStr;
// We parse any expr surrounded by parens, ignoring all whitespaces around those
fn parens(i: &str) -> IResult<&str, i64> {
delimited(space, delimited(tag("("), expr, tag(")")), space)(i)
}
// We transform an integer string into a i64, ignoring surrounding whitespaces
// We look for a digit suite, and try to convert it.
// If either str::from_utf8 or FromStr::from_str fail,
// we fallback to the parens parser defined above
fn factor(i: &str) -> IResult<&str, i64> {
alt((
map_res(delimited(space, digit, space), FromStr::from_str),
parens,
))(i)
}
// We read an initial factor and for each time we find
// a * or / operator followed by another factor, we do
// the math by folding everything
fn term(i: &str) -> IResult<&str, i64> {
let (i, init) = factor(i)?;
fold_many0(
pair(alt((char('*'), char('/'))), factor),
move || init,
|acc, (op, val): (char, i64)| {
if op == '*' {
acc * val
} else {
acc / val
}
},
)(i)
}
fn expr(i: &str) -> IResult<&str, i64> {
let (i, init) = term(i)?;
fold_many0(
pair(alt((char('+'), char('-'))), term),
move || init,
|acc, (op, val): (char, i64)| {
if op == '+' {
acc + val
} else {
acc - val
}
},
)(i)
}
#[test]
fn factor_test() {
assert_eq!(factor("3"), Ok(("", 3)));
assert_eq!(factor(" 12"), Ok(("", 12)));
assert_eq!(factor("537 "), Ok(("", 537)));
assert_eq!(factor(" 24 "), Ok(("", 24)));
}
#[test]
fn term_test() {
assert_eq!(term(" 12 *2 / 3"), Ok(("", 8)));
assert_eq!(term(" 2* 3 *2 *2 / 3"), Ok(("", 8)));
assert_eq!(term(" 48 / 3/2"), Ok(("", 8)));
}
#[test]
fn expr_test() {
assert_eq!(expr(" 1 + 2 "), Ok(("", 3)));
assert_eq!(expr(" 12 + 6 - 4+ 3"), Ok(("", 17)));
assert_eq!(expr(" 1 + 2*3 + 4"), Ok(("", 11)));
}
#[test]
fn parens_test() {
assert_eq!(expr(" ( 2 )"), Ok(("", 2)));
assert_eq!(expr(" 2* ( 3 + 4 ) "), Ok(("", 14)));
assert_eq!(expr(" 2*2 / ( 5 - 1) + 3"), Ok(("", 4)));
}