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day18 simplification. No subtraction of division necessary

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This commit is contained in:
Glenn Griffin 2020-12-18 16:31:35 -08:00
parent 40b618593f
commit 62601e0782
1 changed files with 7 additions and 21 deletions
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28
src/day18.rs
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@ -4,7 +4,7 @@ use nom::bytes::complete::tag;
use nom::character::complete::{char, digit1, space0};
use nom::combinator::map_res;
use nom::multi::fold_many0;
use nom::sequence::{delimited, pair};
use nom::sequence::{delimited, preceded};
use nom::IResult;
use std::str::FromStr;
@ -20,32 +20,18 @@ fn num_or_paren_expr(i: &str) -> IResult<&str, usize> {
alt((num, paren_expr))(i)
}
fn add_or_sub(i: &str) -> IResult<&str, usize> {
fn add_or_paren(i: &str) -> IResult<&str, usize> {
let (i, lhs) = num_or_paren_expr(i)?;
fold_many0(
pair(alt((char('+'), char('-'))), num_or_paren_expr),
lhs,
|lhs, (op, rhs)| match op {
'+' => lhs + rhs,
'-' => lhs - rhs,
_ => unreachable!("invalid op"),
},
)(i)
fold_many0(preceded(char('+'), num_or_paren_expr), lhs, |lhs, rhs| {
lhs + rhs
})(i)
}
fn expr(i: &str) -> IResult<&str, usize> {
let (i, lhs) = add_or_sub(i)?;
let (i, lhs) = add_or_paren(i)?;
fold_many0(
pair(alt((char('*'), char('/'))), add_or_sub),
lhs,
|lhs, (op, rhs)| match op {
'*' => lhs * rhs,
'/' => lhs / rhs,
_ => unreachable!("invalid op"),
},
)(i)
fold_many0(preceded(char('*'), add_or_paren), lhs, |lhs, rhs| lhs * rhs)(i)
}
#[aoc(day18, part1)]