From 0220f88e27bbd6bcb4f3a87141e45648f1051444 Mon Sep 17 00:00:00 2001
From: Xiretza <xiretza@xiretza.xyz>
Date: Thu, 16 Dec 2021 20:36:23 +0100
Subject: [PATCH] 2021 day16/rust: clean up

---
 2021/day16/day16_rs/src/main.rs | 87 +++++++++++++++++----------------
 1 file changed, 46 insertions(+), 41 deletions(-)

diff --git a/2021/day16/day16_rs/src/main.rs b/2021/day16/day16_rs/src/main.rs
index de91976..0f4e01d 100644
--- a/2021/day16/day16_rs/src/main.rs
+++ b/2021/day16/day16_rs/src/main.rs
@@ -7,7 +7,6 @@ use std::{
 
 use nom::{
     bits::complete as bits,
-    combinator::flat_map,
     multi::{many0, many_m_n},
 };
 use nom::{combinator::map, sequence::pair};
@@ -53,58 +52,64 @@ enum PacketType {
 }
 
 impl PacketType {
-    pub fn parse(input: Input) -> IResult<PacketType> {
-        flat_map(bits::take(3_usize), |type_id: u8| {
-            move |i| match Operator::try_from(type_id) {
-                Ok(operator) => map(Self::operation_inner, |sub_packets| PacketType::Operation {
+    pub fn parse(i: Input) -> IResult<PacketType> {
+        let (i, operator) = map(bits::take(3_usize), |type_id: u8| {
+            Operator::try_from(type_id)
+        })(i)?;
+
+        match operator {
+            Ok(operator) => map(Self::parse_sub_packets, |sub_packets| {
+                PacketType::Operation {
                     operator,
                     sub_packets,
-                })(i),
-                Err(_) => map(Self::literal_inner, PacketType::Literal)(i),
-            }
-        })(input)
+                }
+            })(i),
+            Err(_) => map(Self::parse_literal_value, PacketType::Literal)(i),
+        }
     }
 
-    fn operation_inner(i: Input) -> IResult<Vec<Packet>> {
+    fn parse_sub_packets(i: Input) -> IResult<Vec<Packet>> {
         enum LengthType {
-            Bits(u16),
-            Packets(u16),
+            Bits(usize),
+            Packets(usize),
         }
 
-        flat_map(
-            flat_map(bits::take(1_usize), |length_type_id: u8| {
-                move |i| match length_type_id {
+        impl LengthType {
+            pub fn parse(i: Input) -> IResult<Self> {
+                let (i, length_type_id) = bits::take(1_usize)(i)?;
+                match length_type_id {
                     0 => map(bits::take(15_usize), LengthType::Bits)(i),
                     1 => map(bits::take(11_usize), LengthType::Packets)(i),
                     _ => unreachable!(),
                 }
-            }),
-            |length_type| {
-                move |i| match length_type {
-                    LengthType::Packets(n) => {
-                        many_m_n(usize::from(n), usize::from(n), Packet::parse)(i)
-                    }
-                    LengthType::Bits(n) => {
-                        // map_parser(recognize(bits::take(n)), many1(Packet::parse))(i)
-                        let extra_bytes_required = (usize::from(n) + i.1) / 8;
-                        let extra_bits_required = (usize::from(n) + i.1) % 8;
+            }
+        }
 
-                        let subpackets_slice = (&i.0[..=extra_bytes_required], i.1);
-                        let (subpackets_end, subpackets) = many0(Packet::parse)(subpackets_slice)?;
-                        if subpackets_end.0.len() > 1 {
-                            todo!()
-                        }
-                        Ok((
-                            (&i.0[extra_bytes_required..], extra_bits_required),
-                            subpackets,
-                        ))
-                    }
-                }
-            },
-        )(i)
+        let (i, length_type) = LengthType::parse(i)?;
+        match length_type {
+            LengthType::Packets(n) => many_m_n(n, n, Packet::parse)(i),
+            LengthType::Bits(n) => {
+                // manual implementation of something like the following:
+                // map_parser(recognize(bits::take(n)), many0(Packet::parse))(i)
+
+                let new_byte_offset = (n + i.1) / 8;
+                let new_bit_offset = (n + i.1) % 8;
+
+                let subpackets_input = (&i.0[..=new_byte_offset], i.1);
+                let (subpackets_end, subpackets) = many0(Packet::parse)(subpackets_input)?;
+
+                let new_input = (&i.0[new_byte_offset..], new_bit_offset);
+                assert_eq!(subpackets_end, new_input);
+
+                Ok((
+                    new_input,
+                    subpackets,
+                ))
+            }
+        }
     }
 
-    fn literal_inner(i: Input) -> IResult<usize> {
+    fn parse_literal_value(i: Input) -> IResult<usize> {
         fold_till(
             pair(bits::take(1_usize), bits::take(4_usize)),
             || 0,
@@ -122,8 +127,8 @@ impl PacketType {
         match self {
             Self::Literal(n) => *n,
             Self::Operation {
-                ref operator,
-                ref sub_packets,
+                operator,
+                sub_packets,
             } => operator.evaluate(sub_packets.iter().map(|p| p.typ.evaluate())),
         }
     }