Clean up docs for Program enum, text functions

src/text.rs

@@ -3,7 +3,8 @@ use codepage_437::CP437_WINGDINGS;
 use super::errors::EncodeError;
 
 /// Serialize a multi-line code string for an object or a scroll.
-/// Uses ZZT's convention of CR-terminated lines.
+///
+/// Characters that do not have a CP437 equivalent will result in an [`EncodeError::EncodingError`].
 pub fn encode_multiline(input: &str) -> Result<Vec<u8>, EncodeError> {
     input
         .chars()
@@ -33,8 +34,11 @@ pub fn decode_multiline(input: &[u8]) -> String {
         .collect()
 }
 
-/// Serialize a single-line string for a board title.
-/// Newlines cannot be encoded because ZZT interprets them as dingbats (♪).
+/// Serialize a single-line string, such as a board title.
+///
+/// Newlines cannot be encoded because in single-line contexts, ZZT interprets that byte as a
+/// dingbat, specifically the '♪' character. Passing a newline, or any non-CP437 character, will
+/// result in an [`EncodeError::EncodingError`].
 pub fn encode_oneline(input: &str) -> Result<Vec<u8>, EncodeError> {
     input
         .chars()
@@ -46,7 +50,7 @@ pub fn encode_oneline(input: &str) -> Result<Vec<u8>, EncodeError> {
         .collect()
 }
 
-/// Deserialize a board title.
+/// Deserialize a single-line string, such as a board title.
 pub fn decode_oneline(input: &[u8]) -> String {
     input.iter().map(|&x| CP437_WINGDINGS.decode(x)).collect()
 }
@@ -60,7 +64,7 @@ mod tests {
         encode_multiline(input).expect("Error in test")
     }
 
-    fn serialize_title(input: &str) -> Vec<u8> {
+    fn serialize_line(input: &str) -> Vec<u8> {
         encode_oneline(input).expect("Error in test")
     }
 
@@ -73,7 +77,7 @@ mod tests {
     #[test]
     fn roundtrip_oneline() {
         let bytes: Vec<u8> = (0..=255).collect();
-        assert_eq!(bytes, serialize_title(&decode_oneline(&bytes)))
+        assert_eq!(bytes, serialize_line(&decode_oneline(&bytes)))
     }
 
     #[test]

src/world.rs

@@ -351,15 +351,36 @@ pub struct Tile {
     pub color: u8,
 }
 
-/// A stat's ZZT-OOP program.
+/// A stat's ZZT-OOP program text.
 ///
-/// In ZZT, stats can either have their own code or bind to another stat's code.
-/// This enum prevents invalid states where both are set.
+/// Stats must either have their own program or bind to another stat's program. This is either-or:
+/// in ZZT, it is impossible to represent a stat that has both (or neither). This enum models that
+/// constraint.
+///
+/// Internally, this is because ZZT uses a single `i16` like an enum discriminant. Zero or more
+/// indicates the length of the program text, i.e., how many bytes follow the stat on disk. Negative
+/// values are interpreted as negated bind indexes: -123 means no program text follows, and that the
+/// stat instead shares code with the 123rd stat on the board.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum Program {
-    /// The stat owns its own code.
+    /// The stat has its own program code.
+    ///
+    /// This is the typical default for most stats. For stats that don't have or need a program, an
+    /// empty `String` is used. The text has a maximum of 32767 characters, though in ZZT 3.2 you
+    /// will encounter other limits before that.
+    ///
+    /// Despite the name, this has more to do with ZZT's behavior than with Rust ownership.
     Own(String),
-    /// The stat is bound to another stat (index into the stats list).
+    /// The stat is bound to the code at the given stat index.
+    ///
+    /// Bind indexes are 0-based, with the asterisk that zero is not representable due to how ZZT
+    /// encodes bind indexes. The smallest possible value is 1, which represents being bound to the
+    /// stat that comes immediately after the player.
+    ///
+    /// The largest representable bind index is 32768. zztff will successfully serialize this value,
+    /// though it's only useful as a curiosity and will likely crash ZZT: the index will definitely
+    /// be out of bounds because it is impossible to serialize a stat list that long without hitting
+    /// other limits in the file format. Indexes larger than that will result in an [`EncodeError`].
     Bound(NonZero<u16>),
 }