Give information on BLT syntax errors

src/election.rs

@@ -44,8 +44,10 @@ impl<N: Number> Election<N> {
 	/// Parse the given BLT file and return an [Election]
 	pub fn from_blt<I: Iterator<Item=char>>(input: Peekable<I>) -> Self {
 		let mut parser = BLTParser::new(input);
-		parser.parse_blt().expect("Syntax Error");
-		return parser.as_election();
+		match parser.parse_blt() {
+			Ok(_) => { return parser.as_election(); }
+			Err(e) => { panic!("Syntax Error: {}", e); }
+		}
 	}
 	
 	/// Convert ballots with weight >1 to multiple ballots of weight 1

src/parser/blt.rs

@@ -18,6 +18,7 @@
 use crate::election::{Ballot, Candidate, Election};
 use crate::numbers::Number;
 
+use std::fmt;
 use std::iter::Peekable;
 
 /// Utility for parsing a BLT file
@@ -26,7 +27,11 @@ pub struct BLTParser<N: Number, I: Iterator<Item=char>> {
 	chars: Peekable<I>,
 
 	/// Temporary buffer for parsing ballot values
-	buf_ballot_value: String,
+	ballot_value_buf: String,
+	/// Current line number
+	line_no: u32,
+	/// Current column number
+	col_no: u32,
 	
 	/// Number of candidates
 	num_candidates: usize,
@@ -35,10 +40,25 @@ pub struct BLTParser<N: Number, I: Iterator<Item=char>> {
 }
 
 /// An error when parsing a BLT file
-#[derive(Debug)]
 pub enum ParseError {
-	/// Invalid syntax
-	Invalid
+	/// Unexpected character
+	Unexpected(u32, u32, char),
+	/// Unexpected character, expected ...
+	Expected(u32, u32, char, &'static str),
+}
+
+impl fmt::Display for ParseError {
+	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+		match self {
+			ParseError::Unexpected(line_no, col_no, char) => {
+				f.write_fmt(format_args!("Line {} col {}, unexpected '{}'", line_no, col_no, char))?;
+			}
+			ParseError::Expected(line_no, col_no, char, expected) => {
+				f.write_fmt(format_args!("Line {} col {}, unexpected '{}', expected {}", line_no, col_no, char, expected))?;
+			}
+		}
+		return Ok(());
+	}
 }
 
 impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
@@ -85,8 +105,8 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 				
 				if self.lookahead() == '.' {
 					// Decimal
-					self.buf_ballot_value.clear();
-					self.buf_ballot_value.push('0');
+					self.ballot_value_buf.clear();
+					self.ballot_value_buf.push('0');
 					self.ballot()?;
 				} else {
 					// End of ballots
@@ -94,7 +114,7 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 					break;
 				}
 			} else {
-				self.buf_ballot_value.clear();
+				self.ballot_value_buf.clear();
 				self.ballot()?;
 			}
 		}
@@ -123,7 +143,7 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 		self.delimiter();
 		
 		let ballot = Ballot {
-			orig_value: N::parse(&self.buf_ballot_value),
+			orig_value: N::parse(&self.ballot_value_buf),
 			preferences: preferences,
 		};
 		self.election.ballots.push(ballot);
@@ -168,7 +188,7 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 		loop {
 			match self.ballot_value_element() {
 				Err(_) => { break; }
-				Ok(d) => { self.buf_ballot_value.push(d); }
+				Ok(d) => { self.ballot_value_buf.push(d); }
 			}
 		}
 		return Ok(());
@@ -184,7 +204,7 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 			Ok(s) => { return Ok(s); }
 			Err(_) => {}
 		}
-		return Err(ParseError::Invalid);
+		return Err(ParseError::Expected(self.line_no, self.col_no, self.lookahead(), "string"));
 	}
 	
 	/// Parse a quoted string
@@ -202,7 +222,7 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 			}
 			return Ok(result);
 		} else {
-			return Err(ParseError::Invalid);
+			return Err(ParseError::Expected(self.line_no, self.col_no, self.lookahead(), "'\"'"));
 		}
 	}
 	
@@ -268,7 +288,7 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 		if self.lookahead() >= '1' && self.lookahead() <= '9' {
 			return Ok(self.accept());
 		} else {
-			return Err(ParseError::Invalid);
+			return Err(ParseError::Expected(self.line_no, self.col_no, self.lookahead(), "nonzero digit"));
 		}
 	}
 	
@@ -277,7 +297,7 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 		if self.lookahead() >= '0' && self.lookahead() <= '9' {
 			return Ok(self.accept());
 		} else {
-			return Err(ParseError::Invalid);
+			return Err(ParseError::Expected(self.line_no, self.col_no, self.lookahead(), "digit"));
 		}
 	}
 	
@@ -286,7 +306,7 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 		if (self.lookahead() >= '0' && self.lookahead() <= '9') || self.lookahead() == '.' || self.lookahead() == '/' {
 			return Ok(self.accept());
 		} else {
-			return Err(ParseError::Invalid);
+			return Err(ParseError::Expected(self.line_no, self.col_no, self.lookahead(), "number"));
 		}
 	}
 	
@@ -305,7 +325,14 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 	
 	/// Read and return one character from the stream
 	fn accept(&mut self) -> char {
-		return self.chars.next().expect("Unexpected EOF");
+		let result = self.chars.next().expect("Unexpected EOF");
+		if result == '\n' {
+			self.line_no += 1;
+			self.col_no = 1;
+		} else {
+			self.col_no += 1;
+		}
+		return result;
 	}
 	
 	// PUBLIC API
@@ -314,7 +341,9 @@ impl<N: Number, I: Iterator<Item=char>> BLTParser<N, I> {
 	pub fn new(chars: Peekable<I>) -> Self {
 		Self {
 			chars,
-			buf_ballot_value: String::new(),
+			ballot_value_buf: String::new(),
+			line_no: 1,
+			col_no: 1,
 			num_candidates: 0,
 			election: Election {
 				name: String::new(),