3 files changed, 553 insertions, 0 deletions

diff --git a/compiler/src/lexer/error.rs b/compiler/src/lexer/error.rs
new file mode 100644
index 0000000..f251167
--- /dev/null
+++ b/compiler/src/lexer/error.rs
@@ -0,0 +1,18 @@
+use std::{error, fmt, result};
+
+pub type Result<T> = result::Result<T, Error>;
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum Error {
+    UnclosedString,
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Error::UnclosedString => write!(f, "unclosed string literal"),
+        }
+    }
+}
+
+impl error::Error for Error {}
diff --git a/compiler/src/lexer/mod.rs b/compiler/src/lexer/mod.rs
new file mode 100644
index 0000000..2ef4922
--- /dev/null
+++ b/compiler/src/lexer/mod.rs
@@ -0,0 +1,163 @@
+mod error;
+
+use crate::span::{Pos, Span};
+pub use error::{Error, Result};
+
+#[cfg(test)]
+mod tests;
+
+fn is_terminator(ch: char) -> bool {
+    ch.is_whitespace() || matches!(ch, '(' | ')' | '\'' | '"' | ';')
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum Token<'a> {
+    LeftPar,
+    RightPar,
+    Quote,
+    Number(&'a str),
+    String(&'a str),
+    Symbol(&'a str),
+}
+
+pub struct Lexer<'a> {
+    input: &'a str,
+    cursor: usize,
+
+    line: usize,
+    column: usize,
+}
+
+impl<'a> Lexer<'a> {
+    pub fn new(input: &'a str) -> Self {
+        Self {
+            input,
+            cursor: 0,
+
+            line: 1,
+            column: 0,
+        }
+    }
+
+    fn rest(&self) -> &str {
+        &self.input[self.cursor..]
+    }
+
+    fn peek(&self) -> Option<char> {
+        self.rest().chars().next()
+    }
+
+    fn peek_nth(&self, n: usize) -> Option<char> {
+        self.rest().chars().nth(n)
+    }
+
+    fn consume(&mut self) -> Option<char> {
+        let ch = self.peek()?;
+
+        self.cursor += ch.len_utf8();
+        if ch == '\n' {
+            self.line += 1;
+            self.column = 0;
+        } else {
+            self.column += 1;
+        }
+
+        Some(ch)
+    }
+
+    fn next_while(&mut self, mut predicate: impl FnMut(char) -> bool) -> &'a str {
+        let start = self.cursor;
+
+        while let Some(ch) = self.peek() {
+            if !predicate(ch) {
+                break;
+            }
+            self.consume();
+        }
+
+        &self.input[start..self.cursor]
+    }
+
+    fn next_atom(&mut self) -> &'a str {
+        self.next_while(|ch| !is_terminator(ch))
+    }
+
+    fn next_string(&mut self) -> Result<&'a str> {
+        debug_assert_eq!(self.peek(), Some('"'));
+        self.consume();
+
+        let start = self.cursor;
+
+        while let Some(ch) = self.peek() {
+            match ch {
+                '"' => {
+                    let string = &self.input[start..self.cursor];
+                    self.consume();
+                    return Ok(string);
+                }
+                '\n' => return Err(Error::UnclosedString),
+                '\\' => {
+                    self.consume();
+                    self.consume();
+                }
+                _ => {
+                    self.consume();
+                }
+            }
+        }
+
+        Err(Error::UnclosedString)
+    }
+}
+
+impl<'a> Iterator for Lexer<'a> {
+    type Item = Span<Result<Token<'a>>>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        loop {
+            match self.peek()? {
+                ch if ch.is_whitespace() => {
+                    self.next_while(char::is_whitespace);
+                }
+                ';' => {
+                    self.next_while(|ch| ch != '\n');
+                }
+                _ => break,
+            }
+        }
+
+        let start = Pos::new(self.line, self.column, self.cursor);
+
+        let token = match self.peek()? {
+            '(' => {
+                self.consume();
+                Ok(Token::LeftPar)
+            }
+            ')' => {
+                self.consume();
+                Ok(Token::RightPar)
+            }
+            '\'' => {
+                self.consume();
+                Ok(Token::Quote)
+            }
+
+            // Number
+            ch if ch.is_ascii_digit()
+                || matches!(ch, '+' | '-' | '.')
+                    && self.peek_nth(1).is_some_and(|ch| ch.is_ascii_digit()) =>
+            {
+                Ok(Token::Number(self.next_atom()))
+            }
+
+            // String
+            '"' => self.next_string().map(Token::String),
+
+            // Symbol
+            _ => Ok(Token::Symbol(self.next_atom())),
+        };
+
+        let end = Pos::new(self.line, self.column, self.cursor);
+        Some(Span::new(token, start, end))
+    }
+}
diff --git a/compiler/src/lexer/tests.rs b/compiler/src/lexer/tests.rs
new file mode 100644
index 0000000..65dd2f2
--- /dev/null
+++ b/compiler/src/lexer/tests.rs
@@ -0,0 +1,372 @@
+use crate::span::Pos;
+
+use super::Token::*;
+use super::*;
+
+fn tokenize<'a>(input: &'a str) -> Vec<Token<'a>> {
+    Lexer::new(input).map(|s| s.into_inner().unwrap()).collect()
+}
+
+#[test]
+fn test_spaces() {
+    let cases = vec![
+        ("", vec![]),
+        ("        ", vec![]),
+        ("\n", vec![]),
+        ("\t\n  \r\n", vec![]),
+    ];
+    for (code, tokens) in cases {
+        assert_eq!(tokenize(code), tokens);
+    }
+}
+
+#[test]
+fn test_parens() {
+    let cases = vec![
+        ("()", vec![LeftPar, RightPar]),
+        ("( )", vec![LeftPar, RightPar]),
+        ("(())", vec![LeftPar, LeftPar, RightPar, RightPar]),
+        (
+            "((()))",
+            vec![LeftPar, LeftPar, LeftPar, RightPar, RightPar, RightPar],
+        ),
+        (")(", vec![RightPar, LeftPar]),
+    ];
+    for (code, tokens) in cases {
+        assert_eq!(tokenize(code), tokens);
+    }
+}
+
+#[test]
+fn test_quote() {
+    let cases = vec![
+        ("'", vec![Quote]),
+        ("'a", vec![Quote, Symbol("a")]),
+        ("''a", vec![Quote, Quote, Symbol("a")]),
+        ("'()", vec![Quote, LeftPar, RightPar]),
+        (
+            "'(1 2)",
+            vec![Quote, LeftPar, Number("1"), Number("2"), RightPar],
+        ),
+        ("(' )", vec![LeftPar, Quote, RightPar]),
+    ];
+    for (code, tokens) in cases {
+        assert_eq!(tokenize(code), tokens);
+    }
+}
+
+#[test]
+fn test_numbers() {
+    let cases = vec![
+        ("0", vec![Number("0")]),
+        ("42", vec![Number("42")]),
+        ("3.14", vec![Number("3.14")]),
+        ("-7", vec![Number("-7")]),
+        ("+5", vec![Number("+5")]),
+        ("-0.5", vec![Number("-0.5")]),
+        ("1e10", vec![Number("1e10")]),
+        ("1.5e-3", vec![Number("1.5e-3")]),
+        (".5", vec![Number(".5")]),
+    ];
+    for (code, tokens) in cases {
+        assert_eq!(tokenize(code), tokens);
+    }
+}
+
+#[test]
+fn test_strings() {
+    let cases = vec![
+        (r#""""#, vec![String("")]),
+        (r#""hello""#, vec![String("hello")]),
+        (r#""hello world""#, vec![String("hello world")]),
+        (r#""(not a list)""#, vec![String("(not a list)")]),
+        (r#""'not a quote""#, vec![String("'not a quote")]),
+        (r#""; not a comment""#, vec![String("; not a comment")]),
+        (r#""  spaces  ""#, vec![String("  spaces  ")]),
+    ];
+    for (code, tokens) in cases {
+        assert_eq!(tokenize(code), tokens);
+    }
+}
+
+#[test]
+fn test_string_escapes() {
+    let cases = vec![
+        (r#""line\nbreak""#, vec![String(r"line\nbreak")]),
+        (r#""with \"quotes\"""#, vec![String(r#"with \"quotes\""#)]),
+        (r#""\\""#, vec![String(r"\\")]),
+        ("\"single\\\nline\"", vec![String("single\\\nline")]),
+    ];
+    for (code, tokens) in cases {
+        assert_eq!(tokenize(code), tokens);
+    }
+}
+
+#[test]
+fn test_symbols() {
+    let cases = vec![
+        ("foo", vec![Symbol("foo")]),
+        ("foo-bar", vec![Symbol("foo-bar")]),
+        ("foo!", vec![Symbol("foo!")]),
+        ("empty?", vec![Symbol("empty?")]),
+        ("set!", vec![Symbol("set!")]),
+        ("->", vec![Symbol("->")]),
+        ("+", vec![Symbol("+")]),
+        ("-", vec![Symbol("-")]),
+        ("*", vec![Symbol("*")]),
+        ("/", vec![Symbol("/")]),
+        ("=", vec![Symbol("=")]),
+        ("<=", vec![Symbol("<=")]),
+        (">=", vec![Symbol(">=")]),
+        ("a1b2", vec![Symbol("a1b2")]),
+        ("x", vec![Symbol("x")]),
+    ];
+    for (code, tokens) in cases {
+        assert_eq!(tokenize(code), tokens);
+    }
+}
+
+#[test]
+fn test_ambiguous() {
+    let cases = vec![
+        ("-x", vec![Symbol("-x")]),
+        ("+foo", vec![Symbol("+foo")]),
+        ("...", vec![Symbol("...")]),
+        (".foo", vec![Symbol(".foo")]),
+    ];
+    for (code, tokens) in cases {
+        assert_eq!(tokenize(code), tokens);
+    }
+}
+
+#[test]
+fn test_no_separators() {
+    let cases = vec![
+        ("(foo)", vec![LeftPar, Symbol("foo"), RightPar]),
+        ("(1)", vec![LeftPar, Number("1"), RightPar]),
+        ("(a)b", vec![LeftPar, Symbol("a"), RightPar, Symbol("b")]),
+        ("'(a)", vec![Quote, LeftPar, Symbol("a"), RightPar]),
+        (r#"("s")"#, vec![LeftPar, String("s"), RightPar]),
+    ];
+    for (code, tokens) in cases {
+        assert_eq!(tokenize(code), tokens);
+    }
+}
+
+#[test]
+fn test_whitespace_separators() {
+    let cases = vec![
+        (
+            "(\n  foo\n  bar\n)",
+            vec![LeftPar, Symbol("foo"), Symbol("bar"), RightPar],
+        ),
+        (
+            "(\tfoo\tbar\t)",
+            vec![LeftPar, Symbol("foo"), Symbol("bar"), RightPar],
+        ),
+    ];
+    for (code, tokens) in cases {
+        assert_eq!(tokenize(code), tokens);
+    }
+}
+
+#[test]
+fn test_expressions() {
+    let cases = vec![
+        (
+            "(define x 42)",
+            vec![
+                LeftPar,
+                Symbol("define"),
+                Symbol("x"),
+                Number("42"),
+                RightPar,
+            ],
+        ),
+        (
+            "(+ 1 2)",
+            vec![LeftPar, Symbol("+"), Number("1"), Number("2"), RightPar],
+        ),
+        (
+            "(if (= x 0) 'zero 'nonzero)",
+            vec![
+                LeftPar,
+                Symbol("if"),
+                LeftPar,
+                Symbol("="),
+                Symbol("x"),
+                Number("0"),
+                RightPar,
+                Quote,
+                Symbol("zero"),
+                Quote,
+                Symbol("nonzero"),
+                RightPar,
+            ],
+        ),
+        (
+            r#"(print "hello, world")"#,
+            vec![LeftPar, Symbol("print"), String("hello, world"), RightPar],
+        ),
+        (
+            "(lambda (x) (* x x))",
+            vec![
+                LeftPar,
+                Symbol("lambda"),
+                LeftPar,
+                Symbol("x"),
+                RightPar,
+                LeftPar,
+                Symbol("*"),
+                Symbol("x"),
+                Symbol("x"),
+                RightPar,
+                RightPar,
+            ],
+        ),
+        (
+            "'(1 2 3)",
+            vec![
+                Quote,
+                LeftPar,
+                Number("1"),
+                Number("2"),
+                Number("3"),
+                RightPar,
+            ],
+        ),
+    ];
+    for (code, tokens) in cases {
+        assert_eq!(tokenize(code), tokens);
+    }
+}
+
+#[test]
+fn test_comments() {
+    let cases = vec![
+        (";", vec![]),
+        (";\n", vec![]),
+        ("; comment", vec![]),
+        ("; comment\n", vec![]),
+        ("; comment\n42", vec![Number("42")]),
+        ("42 ; comment", vec![Number("42")]),
+        ("42; comment", vec![Number("42")]),
+        (
+            "(+ 1 2) ; calc\n(- 3 4)",
+            vec![
+                LeftPar,
+                Symbol("+"),
+                Number("1"),
+                Number("2"),
+                RightPar,
+                LeftPar,
+                Symbol("-"),
+                Number("3"),
+                Number("4"),
+                RightPar,
+            ],
+        ),
+    ];
+    for (code, tokens) in cases {
+        assert_eq!(tokenize(code), tokens);
+    }
+}
+
+fn first_error(input: &str) -> Error {
+    Lexer::new(input)
+        .find_map(|s| s.into_inner().err())
+        .expect("error expected")
+}
+
+#[test]
+fn test_unclosed_string_at_eof() {
+    assert_eq!(first_error(r#""abc"#), Error::UnclosedString);
+    assert_eq!(first_error(r#"""#), Error::UnclosedString);
+}
+
+#[test]
+fn test_unclosed_string_with_trailing_escape() {
+    assert_eq!(first_error("\"abc\\"), Error::UnclosedString);
+}
+
+#[test]
+fn test_unclosed_string_with_newline() {
+    assert_eq!(first_error("\"abc\ndef\""), Error::UnclosedString);
+}
+
+#[test]
+fn test_lexer_stops_after_string_error() {
+    let mut lex = Lexer::new(r#""abc"#);
+    assert!(lex.next().unwrap().into_inner().is_err());
+    assert!(lex.next().is_none());
+}
+
+fn spans(input: &str) -> Vec<(Pos, Pos)> {
+    Lexer::new(input).map(|s| (s.start(), s.end())).collect()
+}
+
+#[test]
+fn test_span_single_char() {
+    let s = spans("(");
+    assert_eq!(s, vec![(Pos::new(1, 0, 0), Pos::new(1, 1, 1))]);
+}
+
+#[test]
+fn test_span_after_leading_whitespace() {
+    let s = spans("   (");
+    assert_eq!(s, vec![(Pos::new(1, 3, 3), Pos::new(1, 4, 4))]);
+}
+
+#[test]
+fn test_span_after_newline() {
+    let s = spans("\n(");
+    assert_eq!(s, vec![(Pos::new(2, 0, 1), Pos::new(2, 1, 2))]);
+}
+
+#[test]
+fn test_span_multi_char_() {
+    let s = spans("foo");
+    assert_eq!(s, vec![(Pos::new(1, 0, 0), Pos::new(1, 3, 3))]);
+}
+
+#[test]
+fn test_span_string() {
+    let s = spans(r#""hi""#);
+    assert_eq!(s, vec![(Pos::new(1, 0, 0), Pos::new(1, 4, 4))]);
+}
+
+#[test]
+fn test_span_sequence() {
+    // (foo 42)
+    // 012345678
+    let s = spans("(foo 42)");
+    assert_eq!(
+        s,
+        vec![
+            (Pos::new(1, 0, 0), Pos::new(1, 1, 1)), // (
+            (Pos::new(1, 1, 1), Pos::new(1, 4, 4)), // foo
+            (Pos::new(1, 5, 5), Pos::new(1, 7, 7)), // 42
+            (Pos::new(1, 7, 7), Pos::new(1, 8, 8)), // )
+        ],
+    );
+}
+
+#[test]
+fn test_span_lines() {
+    let s = spans("foo\nbar");
+    assert_eq!(
+        s,
+        vec![
+            (Pos::new(1, 0, 0), Pos::new(1, 3, 3)),
+            (Pos::new(2, 0, 4), Pos::new(2, 3, 7)),
+        ],
+    );
+}
+
+#[test]
+fn test_span_after_comment() {
+    // ; cm\nfoo
+    // 01234 5678
+    let s = spans("; cm\nfoo");
+    assert_eq!(s, vec![(Pos::new(2, 0, 5), Pos::new(2, 3, 8))]);
+}