day18 without nom

Cargo.toml

@@ -9,4 +9,3 @@ edition = "2018"
 [dependencies]
 aoc-runner = "0.3.0"
 aoc-runner-derive = "0.3.0"
-nom = "6.0.1"

src/day18.rs

@@ -1,45 +1,78 @@
 use aoc_runner_derive::aoc;
-use nom::branch::alt;
-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, preceded};
-use nom::IResult;
-use std::str::FromStr;
 
-fn num(i: &str) -> IResult<&str, usize> {
-    map_res(delimited(space0, digit1, space0), FromStr::from_str)(i)
+fn num(i: &str) -> Option<(&str, usize)> {
+    let i = i.trim_start();
+    let end_idx = i
+        .as_bytes()
+        .iter()
+        .copied()
+        .position(|b| !(b'0'..=b'9').contains(&b))
+        .unwrap_or(i.len());
+    let n = (&i[..end_idx]).parse().ok()?;
+    let rem = &i[end_idx..];
+    Some((rem, n))
 }
 
 #[aoc(day18, part1)]
 fn solve_d18_p1(input: &str) -> usize {
-    fn paren(i: &str) -> IResult<&str, usize> {
-        delimited(space0, delimited(tag("("), expr, tag(")")), space0)(i)
+    #[derive(Debug, Copy, Clone)]
+    enum Operator {
+        Add,
+        Mul,
     }
 
-    fn num_or_paren(i: &str) -> IResult<&str, usize> {
-        alt((num, paren))(i)
+    fn operator(i: &str) -> Option<(&str, Operator)> {
+        let i = i.trim_start();
+        let op = match i.as_bytes()[0] {
+            b'+' => Operator::Add,
+            b'*' => Operator::Mul,
+            _ => return None,
+        };
+        Some((&i[1..], op))
     }
 
-    fn add_or_paren(i: &str) -> IResult<&str, usize> {
-        let (i, lhs) = num_or_paren(i)?;
-
-        fold_many0(preceded(char('+'), num_or_paren), lhs, |lhs, rhs| lhs + rhs)(i)
+    fn paren(i: &str) -> Option<(&str, usize)> {
+        let i = i.trim_start();
+        if i.is_empty() || i.as_bytes()[0] != b'(' {
+            return None;
+        }
+        let (rem, n) = expr(&i[1..])?;
+        if rem.is_empty() || rem.as_bytes()[0] != b')' {
+            return None;
+        }
+        let rem = &rem[1..];
+        Some((rem, n))
     }
 
-    fn expr(i: &str) -> IResult<&str, usize> {
-        let (i, lhs) = add_or_paren(i)?;
+    fn num_or_paren(i: &str) -> Option<(&str, usize)> {
+        num(i).or_else(|| paren(i))
+    }
 
-        fold_many0(
-            pair(alt((char('+'), char('*'))), num_or_paren),
-            lhs,
-            |lhs, (op, rhs)| match op {
-                '+' => lhs + rhs,
-                '*' => lhs * rhs,
-                _ => unreachable!(),
-            },
-        )(i)
+    fn operator_and_rhs(i: &str) -> Option<(&str, (Operator, usize))> {
+        let (i, op) = operator(i)?;
+        let (i, rhs) = num_or_paren(i)?;
+        Some((i, (op, rhs)))
+    }
+
+    fn expr(i: &str) -> Option<(&str, usize)> {
+        let (mut rem, mut lhs) = num_or_paren(i)?;
+
+        loop {
+            if rem.is_empty() {
+                break;
+            }
+
+            if let Some((irem, (op, rhs))) = operator_and_rhs(rem) {
+                rem = irem;
+                lhs = match op {
+                    Operator::Add => lhs + rhs,
+                    Operator::Mul => lhs * rhs,
+                };
+            } else {
+                break;
+            }
+        }
+        Some((rem, lhs))
     }
 
     input.split('\n').map(|line| expr(line).unwrap().1).sum()
@@ -47,24 +80,59 @@ fn solve_d18_p1(input: &str) -> usize {
 
 #[aoc(day18, part2)]
 fn solve_d18_p2(input: &str) -> usize {
-    fn paren(i: &str) -> IResult<&str, usize> {
-        delimited(space0, delimited(tag("("), expr, tag(")")), space0)(i)
+    fn paren(i: &str) -> Option<(&str, usize)> {
+        let i = i.trim_start();
+        if i.is_empty() || i.as_bytes()[0] != b'(' {
+            return None;
+        }
+        let (rem, n) = expr(&i[1..])?;
+        if rem.is_empty() || rem.as_bytes()[0] != b')' {
+            return None;
+        }
+        let rem = &rem[1..];
+        Some((rem, n))
     }
 
-    fn num_or_paren(i: &str) -> IResult<&str, usize> {
-        alt((num, paren))(i)
+    fn num_or_paren(i: &str) -> Option<(&str, usize)> {
+        num(i).or_else(|| paren(i))
     }
 
-    fn add_or_paren(i: &str) -> IResult<&str, usize> {
-        let (i, lhs) = num_or_paren(i)?;
+    fn add_or_paren(i: &str) -> Option<(&str, usize)> {
+        let (mut i, mut lhs) = num_or_paren(i)?;
 
-        fold_many0(preceded(char('+'), num_or_paren), lhs, |lhs, rhs| lhs + rhs)(i)
+        loop {
+            i = i.trim_start();
+            if i.is_empty() || i.as_bytes()[0] != b'+' {
+                break;
+            }
+            i = &i[1..];
+            if let Some((rem, rhs)) = num_or_paren(i) {
+                lhs += rhs;
+                i = rem;
+            } else {
+                break;
+            }
+        }
+        Some((i, lhs))
     }
 
-    fn expr(i: &str) -> IResult<&str, usize> {
-        let (i, lhs) = add_or_paren(i)?;
+    fn expr(i: &str) -> Option<(&str, usize)> {
+        let (mut i, mut lhs) = add_or_paren(i)?;
 
-        fold_many0(preceded(char('*'), add_or_paren), lhs, |lhs, rhs| lhs * rhs)(i)
+        loop {
+            i = i.trim_start();
+            if i.is_empty() || i.as_bytes()[0] != b'*' {
+                break;
+            }
+            i = &i[1..];
+            if let Some((rem, rhs)) = add_or_paren(i) {
+                lhs *= rhs;
+                i = rem;
+            } else {
+                break;
+            }
+        }
+        Some((i, lhs))
     }
 
     input.split('\n').map(|line| expr(line).unwrap().1).sum()