diff --git a/SLS_Rust/sls/src/lexer.rs b/SLS_Rust/sls/src/lexer.rs
index d4a4abe..9dbcf0d 100644
--- a/SLS_Rust/sls/src/lexer.rs
+++ b/SLS_Rust/sls/src/lexer.rs
@@ -1,17 +1,232 @@
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum TokenType {
+    Illegal,
+    Eof,
+
+    Ident,
+    Int,
+    Float,
+    Str,
+
+    // Operators
+    Assign,
+    Plus,
+    Minus,
+    Asterisk,
+    Slash,
+    Bang,
+    Lt,
+    Gt,
+    Eq,
+    NotEq,
+
+    // Delimiters
+    Comma,
+    Semicolon,
+    Colon,
+    LParen,
+    RParen,
+    LBrace,
+    RBrace,
+    LBracket,
+    RBracket,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Token {
+    pub ttype: TokenType,
+    pub lexeme: String,
+    pub line: usize,
+    pub column: usize,
+}
+
 pub struct Lexer {
-    pub input: String,
-    pub pos: usize,
+    input: Vec<char>,
+    pos: usize,      // current position in input (points to current char)
+    read_pos: usize, // current reading position in input (after current char)
+    ch: Option<char>,
+    line: usize,
+    column: usize,
 }
 
 impl Lexer {
     pub fn new(input: impl Into<String>) -> Self {
-        Lexer { input: input.into(), pos: 0 }
+        let s = input.into();
+        let mut lexer = Lexer {
+            input: s.chars().collect(),
+            pos: 0,
+            read_pos: 0,
+            ch: None,
+            line: 1,
+            column: 0,
+        };
+        lexer.read_char();
+        lexer
     }
 
-    /// Stub: return next token as a string or None at EOF.
-    pub fn next_token(&mut self) -> Option<String> {
-        // placeholder for a real lexer implementation
-        None
+    fn read_char(&mut self) {
+        if self.read_pos >= self.input.len() {
+            self.ch = None;
+        } else {
+            self.ch = Some(self.input[self.read_pos]);
+        }
+        self.pos = self.read_pos;
+        self.read_pos += 1;
+
+        if let Some(c) = self.ch {
+            if c == '\n' {
+                self.line += 1;
+                self.column = 0;
+            } else {
+                self.column += 1;
+            }
+        }
+    }
+
+    fn peek_char(&self) -> Option<char> {
+        if self.read_pos >= self.input.len() {
+            None
+        } else {
+            Some(self.input[self.read_pos])
+        }
+    }
+
+    fn skip_whitespace(&mut self) {
+        while let Some(c) = self.ch {
+            if c.is_whitespace() {
+                self.read_char();
+            } else {
+                break;
+            }
+        }
+    }
+
+    fn read_identifier(&mut self) -> String {
+        let start = self.pos;
+        while let Some(c) = self.ch {
+            if is_identifier_char(c) {
+                self.read_char();
+            } else {
+                break;
+            }
+        }
+        self.input[start..self.pos].iter().collect()
+    }
+
+    fn read_number(&mut self) -> String {
+        let start = self.pos;
+        let mut seen_dot = false;
+        while let Some(c) = self.ch {
+            if c == '.' {
+                if seen_dot {
+                    break;
+                }
+                seen_dot = true;
+                self.read_char();
+            } else if c.is_ascii_digit() {
+                self.read_char();
+            } else {
+                break;
+            }
+        }
+        self.input[start..self.pos].iter().collect()
+    }
+
+    fn read_string(&mut self) -> String {
+        // consume opening quote
+        self.read_char();
+        let start = self.pos;
+        while let Some(c) = self.ch {
+            if c == '"' {
+                break;
+            }
+            // support escape handling later
+            self.read_char();
+        }
+        let s: String = self.input[start..self.pos].iter().collect();
+        // consume closing quote
+        self.read_char();
+        s
+    }
+
+    pub fn next_token(&mut self) -> Token {
+        self.skip_whitespace();
+
+        let token = match self.ch {
+            Some('=') => {
+                if self.peek_char() == Some('=') {
+                    self.read_char();
+                    let lex = "==".to_string();
+                    Token { ttype: TokenType::Eq, lexeme: lex, line: self.line, column: self.column }
+                } else {
+                    Token { ttype: TokenType::Assign, lexeme: "=".to_string(), line: self.line, column: self.column }
+                }
+            }
+            Some('+') => { Token { ttype: TokenType::Plus, lexeme: "+".to_string(), line: self.line, column: self.column } }
+            Some('-') => { Token { ttype: TokenType::Minus, lexeme: "-".to_string(), line: self.line, column: self.column } }
+            Some('*') => { Token { ttype: TokenType::Asterisk, lexeme: "*".to_string(), line: self.line, column: self.column } }
+            Some('/') => { Token { ttype: TokenType::Slash, lexeme: "/".to_string(), line: self.line, column: self.column } }
+            Some('!') => {
+                if self.peek_char() == Some('=') {
+                    self.read_char();
+                    Token { ttype: TokenType::NotEq, lexeme: "!=".to_string(), line: self.line, column: self.column }
+                } else {
+                    Token { ttype: TokenType::Bang, lexeme: "!".to_string(), line: self.line, column: self.column }
+                }
+            }
+            Some('<') => { Token { ttype: TokenType::Lt, lexeme: "<".to_string(), line: self.line, column: self.column } }
+            Some('>') => { Token { ttype: TokenType::Gt, lexeme: ">".to_string(), line: self.line, column: self.column } }
+            Some(',') => { Token { ttype: TokenType::Comma, lexeme: ",".to_string(), line: self.line, column: self.column } }
+            Some(';') => { Token { ttype: TokenType::Semicolon, lexeme: ";".to_string(), line: self.line, column: self.column } }
+            Some(':') => { Token { ttype: TokenType::Colon, lexeme: ":".to_string(), line: self.line, column: self.column } }
+            Some('(') => { Token { ttype: TokenType::LParen, lexeme: "(".to_string(), line: self.line, column: self.column } }
+            Some(')') => { Token { ttype: TokenType::RParen, lexeme: ")".to_string(), line: self.line, column: self.column } }
+            Some('{') => { Token { ttype: TokenType::LBrace, lexeme: "{".to_string(), line: self.line, column: self.column } }
+            Some('}') => { Token { ttype: TokenType::RBrace, lexeme: "}".to_string(), line: self.line, column: self.column } }
+            Some('[') => { Token { ttype: TokenType::LBracket, lexeme: "[".to_string(), line: self.line, column: self.column } }
+            Some(']') => { Token { ttype: TokenType::RBracket, lexeme: "]".to_string(), line: self.line, column: self.column } }
+            Some('"') => {
+                let s = self.read_string();
+                Token { ttype: TokenType::Str, lexeme: s, line: self.line, column: self.column }
+            }
+            Some(c) if is_letter(c) => {
+                let ident = self.read_identifier();
+                Token { ttype: TokenType::Ident, lexeme: ident, line: self.line, column: self.column }
+            }
+            Some(c) if c.is_ascii_digit() => {
+                let num = self.read_number();
+                if num.contains('.') {
+                    Token { ttype: TokenType::Float, lexeme: num, line: self.line, column: self.column }
+                } else {
+                    Token { ttype: TokenType::Int, lexeme: num, line: self.line, column: self.column }
+                }
+            }
+            None => { Token { ttype: TokenType::Eof, lexeme: "".to_string(), line: self.line, column: self.column } }
+            Some(_) => { Token { ttype: TokenType::Illegal, lexeme: self.ch.unwrap().to_string(), line: self.line, column: self.column } }
+        };
+
+        // advance to next char if not EOF and we didn't already advance inside readers
+        if self.ch.is_some() {
+            // For cases where read_identifier/read_number/read_string already moved position
+            // we don't want to skip an extra char. The read_* helpers leave `ch` at the
+            // character after the token. To keep behavior consistent, only call read_char
+            // when the token was produced from a single-char branch.
+            match token.ttype {
+                TokenType::Ident | TokenType::Int | TokenType::Float | TokenType::Str => {}
+                TokenType::Eof => {}
+                _ => { self.read_char(); }
+            }
+        }
+
+        token
     }
 }
+
+fn is_letter(c: char) -> bool {
+    c.is_ascii_alphabetic() || c == '_'
+}
+
+fn is_identifier_char(c: char) -> bool {
+    c.is_ascii_alphanumeric() || c == '_'
+}
+