From 48594a75e6d1c178d6be80c01810d9e9a494f1b0 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Thu, 1 Dec 2022 11:28:59 +0100
Subject: [PATCH] Make parsers more robust

---
 2022/src/common.rs | 81 ++++++++++++++++++++++++++++++++++++++++++++--
 2022/src/day01.rs  | 10 ++----
 2 files changed, 81 insertions(+), 10 deletions(-)

diff --git a/2022/src/common.rs b/2022/src/common.rs
index 636e230..6eff6c8 100644
--- a/2022/src/common.rs
+++ b/2022/src/common.rs
@@ -1,7 +1,11 @@
 //! Common helper utilities to all days
 
 use anyhow::Result;
+use nom::error::ErrorKind;
+use nom::error::ParseError;
 use nom::Finish;
+use nom::IResult;
+use nom::InputLength;
 use nom::Parser;
 
 /// Parse input from some nom parser and return as an anyhow result
@@ -12,8 +16,79 @@ pub fn parse_input<'a, O>(
     input: &'a [u8],
     mut parser: impl Parser<&'a [u8], O, nom::error::Error<&'a [u8]>>,
 ) -> Result<O> {
-    match parser.parse(input).finish() {
-        Ok((_, value)) => Ok(value),
-        Err(err) => anyhow::bail!("Failed to parse at: {err:?}"),
+    let (unparsed, output) = parser.parse(input).finish().map_err(|e| {
+        anyhow::anyhow!(
+            "Parser error {:?} to parse at {}",
+            e.code,
+            String::from_utf8_lossy(e.input)
+        )
+    })?;
+
+    if !unparsed.is_empty() {
+        Err(anyhow::anyhow!(
+            "Not all input consumed: {}",
+            String::from_utf8_lossy(unparsed)
+        ))
+    } else {
+        Ok(output)
+    }
+}
+
+/// Applies a parser iteratively and reduces the results using the given function. Fails if the
+/// embedded parser doesn't return at least one result.
+///
+/// # Arguments
+/// - `f`: the function to apply
+/// - `g`: the function that combines the result o `f` with previous results
+///
+/// This implementation is based on [`nom::multi::fold_many1`] with minor differences. If
+/// successful, this should probably be upstreamed.
+pub fn reduce_many1<I, O, E, F>(
+    mut f: F,
+    mut g: impl FnMut(O, O) -> O,
+) -> impl FnMut(I) -> IResult<I, O, E>
+where
+    I: Clone + InputLength,
+    E: ParseError<I>,
+    F: Parser<I, O, E>,
+{
+    // Cannot delegate to fold_many0 because that would make the function FnOnce rather than FnMut,
+    // since it would transfer ownership of the embedded parser to fold_many0.
+    move |i: I| {
+        let _i = i.clone();
+        match f.parse(_i) {
+            Err(nom::Err::Error(_)) => {
+                Err(nom::Err::Error(E::from_error_kind(i, ErrorKind::Many1)))
+            }
+            Err(e) => Err(e),
+            Ok((i1, mut acc)) => {
+                let mut input = i1;
+
+                loop {
+                    let _input = input.clone();
+                    let len = input.input_len();
+                    match f.parse(_input) {
+                        Err(nom::Err::Error(_)) => {
+                            break;
+                        }
+                        Err(e) => return Err(e),
+                        Ok((i, o)) => {
+                            // infinite loop check: the parser must always consume
+                            if i.input_len() == len {
+                                return Err(nom::Err::Failure(E::from_error_kind(
+                                    i,
+                                    ErrorKind::Many1,
+                                )));
+                            }
+
+                            acc = g(acc, o);
+                            input = i;
+                        }
+                    }
+                }
+
+                Ok((input, acc))
+            }
+        }
     }
 }
diff --git a/2022/src/day01.rs b/2022/src/day01.rs
index 98e14df..e5d59c8 100644
--- a/2022/src/day01.rs
+++ b/2022/src/day01.rs
@@ -3,23 +3,19 @@ use std::ops::Add;
 use anyhow::Result;
 use nom::character::complete::newline;
 use nom::combinator::opt;
-use nom::multi::fold_many1;
 use nom::multi::separated_list0;
 use nom::sequence::terminated;
 use nom::IResult;
 
 use crate::common::parse_input;
+use crate::common::reduce_many1;
 
 fn parse_elf(input: &[u8]) -> IResult<&[u8], i32> {
-    fold_many1(
-        terminated(nom::character::complete::i32, newline),
-        || 0,
-        Add::add,
-    )(input)
+    reduce_many1(terminated(nom::character::complete::i32, newline), Add::add)(input)
 }
 
 fn parse_max(input: &[u8]) -> IResult<&[u8], i32> {
-    fold_many1(terminated(parse_elf, opt(newline)), || 0, Ord::max)(input)
+    reduce_many1(terminated(parse_elf, opt(newline)), Ord::max)(input)
 }
 
 pub fn part1(input: &[u8]) -> Result<String> {