Move parsing code to new file, convert more to nom

This commit also implements a custom error type to capture why a
World/Board failed to load -- which I might end up tearing out.

src/world.rs

@@ -0,0 +1,363 @@
+use std::{error::Error, fmt::Display};
+
+use nom::{
+    bytes::complete::take,
+    combinator::fail,
+    error::{ErrorKind, ParseError},
+    multi::count,
+    number::complete::{le_i16, le_u8},
+    sequence::tuple,
+    Err, IResult,
+};
+
+#[derive(Debug)]
+pub struct LoadError {
+    message: String,
+}
+
+impl<I> ParseError<I> for LoadError {
+    fn from_error_kind(_input: I, kind: ErrorKind) -> Self {
+        Self {
+            message: kind.description().into(),
+        }
+    }
+
+    fn append(_input: I, kind: ErrorKind, other: Self) -> Self {
+        Self {
+            message: format!("{}: {:?}", other.message, kind),
+        }
+    }
+}
+
+impl From<Err<LoadError>> for LoadError {
+    fn from(value: Err<LoadError>) -> Self {
+        let message = match value {
+            Err::Error(e) => e.message,
+            Err::Incomplete(x) => format!("{:?}", x),
+            Err::Failure(e) => e.message,
+        };
+        Self { message }
+    }
+}
+
+impl From<&str> for LoadError {
+    fn from(value: &str) -> Self {
+        Self {
+            message: value.into(),
+        }
+    }
+}
+
+impl Display for LoadError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.message.fmt(f)
+    }
+}
+
+impl Error for LoadError {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        None
+    }
+
+    fn description(&self) -> &str {
+        "description() is deprecated; use Display"
+    }
+
+    fn cause(&self) -> Option<&dyn Error> {
+        self.source()
+    }
+}
+
+pub struct World {
+    pub header: Vec<u8>,
+    pub boards: Vec<Board>,
+}
+
+pub struct Board {
+    pub name: Vec<u8>,
+    pub terrain: Vec<[u8; 2]>,
+    pub max_shots: u8,
+    pub is_dark: bool,
+    pub board_n: u8,
+    pub board_s: u8,
+    pub board_w: u8,
+    pub board_e: u8,
+    pub reenter_when_zapped: bool,
+    pub message: Vec<u8>,
+    pub enter_x: u8,
+    pub enter_y: u8,
+    pub time_limit: i16,
+    pub stats: Vec<Stat>,
+}
+
+pub struct Stat {
+    pub x: u8,
+    pub y: u8,
+    pub x_step: i16,
+    pub y_step: i16,
+    pub cycle: i16,
+    pub p1: u8,
+    pub p2: u8,
+    pub p3: u8,
+    pub follower: i16,
+    pub leader: i16,
+    pub under_element: u8,
+    pub under_color: u8,
+    pub instruction_pointer: i16,
+    pub bind_index: i16,
+    pub code: Vec<u8>,
+}
+
+impl World {
+    pub fn from_bytes(bytes: &[u8]) -> Result<World, LoadError> {
+        let mut world = World {
+            header: Vec::from(&bytes[0..512]),
+            boards: vec![],
+        };
+        let num_boards = 1 + u16::from_le_bytes((&world.header[2..4]).try_into().unwrap());
+        let mut offset = world.header.len();
+        for _ in 0..num_boards {
+            // Ideally, this wouldn't panic if we run out of bytes
+            let board_len =
+                u16::from_le_bytes((&bytes[offset..offset + 2]).try_into().unwrap()) as usize;
+            offset += 2;
+            world
+                .boards
+                .push(Board::from_bytes(&bytes[offset..offset + board_len])?);
+            offset += board_len;
+        }
+        Ok(world)
+    }
+
+    pub fn to_bytes(&self) -> Result<Vec<u8>, Box<dyn Error>> {
+        let mut result = self.header.clone();
+        for board in &self.boards {
+            result.extend_from_slice(&board.to_bytes()?);
+        }
+        Ok(result)
+    }
+}
+
+impl Board {
+    pub fn from_bytes(bytes: &[u8]) -> Result<Board, LoadError> {
+        // Read board name
+        let (input, name) = pstring(50)(bytes)?;
+
+        // Read terrain
+        const NUM_TILES: usize = 60 * 25;
+        let mut input = input;
+        let mut terrain = Vec::with_capacity(NUM_TILES);
+        while terrain.len() < NUM_TILES {
+            let (next_input, (count, element, color)) = tuple((le_u8, le_u8, le_u8))(input)?;
+            input = next_input;
+            let count: u32 = if count == 0 { 256 } else { count.into() };
+            for _ in 0..count {
+                terrain.push([element, color]);
+                if terrain.len() > NUM_TILES {
+                    return Err("too many tiles of board terrain".into());
+                }
+            }
+        }
+
+        // Read board info
+        let (input, (max_shots, is_dark)) = tuple((le_u8, bool_u8))(input)?;
+        let (input, (board_n, board_s, board_w, board_e)) =
+            tuple((le_u8, le_u8, le_u8, le_u8))(input)?;
+        let (input, (reenter_when_zapped, message)) = tuple((bool_u8, pstring(58)))(input)?;
+        let (input, (enter_x, enter_y, time_limit)) = tuple((le_u8, le_u8, le_i16))(input)?;
+        let (input, _) = take(16usize)(input)?;
+
+        // Read stats
+        let (input, num_stats) = le_i16(input)?;
+        let num_stats = num_stats + 1;
+        if num_stats < 0 {
+            return Err("cannot have a negative number of stats".into());
+        }
+        let (_input, stats) = count(Stat::from_bytes, num_stats as usize)(input)?;
+
+        Ok(Board {
+            name,
+            terrain,
+            max_shots,
+            is_dark,
+            board_n,
+            board_s,
+            board_e,
+            board_w,
+            reenter_when_zapped,
+            message,
+            enter_x,
+            enter_y,
+            time_limit,
+            stats,
+        })
+    }
+
+    pub fn to_bytes(&self) -> Result<Vec<u8>, &'static str> {
+        let mut result = vec![];
+        result.push_padding(2); // reserve space for board size
+        result.push_string(50, &self.name)?;
+
+        // Encode terrain
+        if self.terrain.len() != 1500 {
+            return Err("invalid number of tiles for board terrain");
+        }
+        let mut iter = self.terrain.iter().peekable();
+        while let Some(tile) = iter.next() {
+            let mut count = 1;
+            while count < 255 && iter.peek().map_or(false, |&next_tile| next_tile == tile) {
+                count += 1;
+                iter.next();
+            }
+            result.push(count);
+            result.extend_from_slice(tile);
+        }
+
+        // Board info
+        result.push(self.max_shots);
+        result.push_bool(self.is_dark);
+        result.push(self.board_n);
+        result.push(self.board_s);
+        result.push(self.board_w);
+        result.push(self.board_e);
+        result.push_bool(self.reenter_when_zapped);
+        result.push_string(58, &self.message)?;
+        result.push(self.enter_x);
+        result.push(self.enter_y);
+        result.push_i16(self.time_limit);
+        result.push_padding(16);
+
+        // Stats
+        let num_stats: i16 = (self.stats.len() - 1)
+            .try_into()
+            .map_err(|_| "invalid length for stats")?;
+        result.push_i16(num_stats);
+        for stat in &self.stats {
+            result.extend_from_slice(&stat.to_bytes());
+        }
+
+        // Fix up board size
+        let size: u16 = (result.len() - 2)
+            .try_into()
+            .map_err(|_| "too many bytes of board data")?;
+        result.splice(0..2, size.to_le_bytes());
+
+        Ok(result)
+    }
+}
+
+impl Stat {
+    pub fn from_bytes(input: &[u8]) -> IResult<&[u8], Self, LoadError> {
+        let (input, (x, y, x_step, y_step)) = tuple((le_u8, le_u8, le_i16, le_i16))(input)?;
+        let (input, (cycle, p1, p2, p3)) = tuple((le_i16, le_u8, le_u8, le_u8))(input)?;
+        let (input, (follower, leader)) = tuple((le_i16, le_i16))(input)?;
+        let (input, (under_element, under_color)) = tuple((le_u8, le_u8))(input)?;
+        let (input, _) = take(4usize)(input)?;
+        let (input, (instruction_pointer, length)) = tuple((le_i16, le_i16))(input)?;
+        let (input, _) = take(8usize)(input)?;
+        let (input, code) = take(0.max(length) as usize)(input)?;
+        Ok((
+            input,
+            Stat {
+                x,
+                y,
+                x_step,
+                y_step,
+                follower,
+                leader,
+                cycle,
+                p1,
+                p2,
+                p3,
+                under_element,
+                under_color,
+                instruction_pointer,
+                bind_index: 0.min(length),
+                code: Vec::from(code),
+            },
+        ))
+    }
+
+    pub fn to_bytes(&self) -> Vec<u8> {
+        let mut result = vec![];
+        result.push(self.x);
+        result.push(self.y);
+        result.push_i16(self.x_step);
+        result.push_i16(self.y_step);
+        result.push_i16(self.cycle);
+        result.push(self.p1);
+        result.push(self.p2);
+        result.push(self.p3);
+        result.push_i16(self.follower);
+        result.push_i16(self.leader);
+        result.push(self.under_element);
+        result.push(self.under_color);
+        result.push_padding(4);
+        result.push_i16(self.instruction_pointer);
+        // TODO: more safety around valid bind-indexes (positive? negative?)
+        result.push_i16(if self.bind_index < 0 {
+            self.bind_index
+        } else {
+            self.code.len() as i16
+        });
+        result.push_padding(8);
+        if self.bind_index >= 0 {
+            // TODO: more safety around bind-index XOR code
+            result.extend_from_slice(&self.code);
+        }
+        result
+    }
+}
+
+fn bool_u8(input: &[u8]) -> IResult<&[u8], bool, LoadError> {
+    let (input, byte) = le_u8(input)?;
+    Ok((input, byte != 0))
+}
+
+fn pstring(cap: u8) -> impl Fn(&[u8]) -> IResult<&[u8], Vec<u8>, LoadError> {
+    move |input: &[u8]| -> IResult<&[u8], Vec<u8>, LoadError> {
+        let (input, len) = le_u8(input)?;
+        if len >= cap {
+            return fail(input);
+        }
+        let (input, data) = take(len)(input)?;
+        let (input, _) = take(cap - len)(input)?;
+        Ok((input, data.to_vec()))
+    }
+}
+
+trait SerializationHelpers {
+    fn push_bool(&mut self, value: bool);
+    fn push_i16(&mut self, value: i16);
+    fn push_u16(&mut self, value: u16);
+    fn push_string(&mut self, cap: u8, value: &[u8]) -> Result<(), &'static str>;
+    fn push_padding(&mut self, size: usize);
+}
+
+impl SerializationHelpers for Vec<u8> {
+    fn push_bool(&mut self, value: bool) {
+        self.push(if value { 1 } else { 0 });
+    }
+
+    fn push_i16(&mut self, value: i16) {
+        self.extend(value.to_le_bytes());
+    }
+
+    fn push_u16(&mut self, value: u16) {
+        self.extend(value.to_le_bytes());
+    }
+
+    fn push_string(&mut self, cap: u8, value: &[u8]) -> Result<(), &'static str> {
+        if value.len() > cap as usize {
+            return Err("string too long");
+        }
+        self.push(value.len() as u8);
+        self.extend_from_slice(value);
+        self.push_padding(cap as usize - value.len());
+        Ok(())
+    }
+
+    fn push_padding(&mut self, size: usize) {
+        self.resize(self.len() + size, 0);
+    }
+}