1 files changed, 163 insertions, 0 deletions

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))
+    }
+}