1use nom::{
2  branch::alt,
3  bytes::complete::tag,
4  character::complete::char,
5  character::complete::{digit1 as digit, space0 as space},
6  combinator::map_res,
7  multi::fold_many0,
8  sequence::{delimited, pair},
9  IResult,
10};
11
12// Parser definition
13
14use std::str::FromStr;
15
16// We parse any expr surrounded by parens, ignoring all whitespaces around those
17fn parens(i: &str) -> IResult<&str, i64> {
18  delimited(space, delimited(tag("("), expr, tag(")")), space)(i)
19}
20
21// We transform an integer string into a i64, ignoring surrounding whitespaces
22// We look for a digit suite, and try to convert it.
23// If either str::from_utf8 or FromStr::from_str fail,
24// we fallback to the parens parser defined above
25fn factor(i: &str) -> IResult<&str, i64> {
26  alt((
27    map_res(delimited(space, digit, space), FromStr::from_str),
28    parens,
29  ))(i)
30}
31
32// We read an initial factor and for each time we find
33// a * or / operator followed by another factor, we do
34// the math by folding everything
35fn term(i: &str) -> IResult<&str, i64> {
36  let (i, init) = factor(i)?;
37
38  fold_many0(
39    pair(alt((char('*'), char('/'))), factor),
40    move || init,
41    |acc, (op, val): (char, i64)| {
42      if op == '*' {
43        acc * val
44      } else {
45        acc / val
46      }
47    },
48  )(i)
49}
50
51fn expr(i: &str) -> IResult<&str, i64> {
52  let (i, init) = term(i)?;
53
54  fold_many0(
55    pair(alt((char('+'), char('-'))), term),
56    move || init,
57    |acc, (op, val): (char, i64)| {
58      if op == '+' {
59        acc + val
60      } else {
61        acc - val
62      }
63    },
64  )(i)
65}
66
67#[test]
68fn factor_test() {
69  assert_eq!(factor("3"), Ok(("", 3)));
70  assert_eq!(factor(" 12"), Ok(("", 12)));
71  assert_eq!(factor("537  "), Ok(("", 537)));
72  assert_eq!(factor("  24   "), Ok(("", 24)));
73}
74
75#[test]
76fn term_test() {
77  assert_eq!(term(" 12 *2 /  3"), Ok(("", 8)));
78  assert_eq!(term(" 2* 3  *2 *2 /  3"), Ok(("", 8)));
79  assert_eq!(term(" 48 /  3/2"), Ok(("", 8)));
80}
81
82#[test]
83fn expr_test() {
84  assert_eq!(expr(" 1 +  2 "), Ok(("", 3)));
85  assert_eq!(expr(" 12 + 6 - 4+  3"), Ok(("", 17)));
86  assert_eq!(expr(" 1 + 2*3 + 4"), Ok(("", 11)));
87}
88
89#[test]
90fn parens_test() {
91  assert_eq!(expr(" (  2 )"), Ok(("", 2)));
92  assert_eq!(expr(" 2* (  3 + 4 ) "), Ok(("", 14)));
93  assert_eq!(expr("  2*2 / ( 5 - 1) + 3"), Ok(("", 4)));
94}
95