28 files changed, 866 insertions, 446 deletions
diff --git a/server/CMakeLists.txt b/server/CMakeLists.txt
index 7bf356a..4339da2 100644
--- a/server/CMakeLists.txt
+++ b/server/CMakeLists.txt
@@ -1,11 +1,4 @@
-find_package(Boost REQUIRED COMPONENTS filesystem program_options)
-
-option(DEBUG_ASIO "enable debug output for Boost.Asio" OFF)
-
-add_executable(server main.cpp server.cpp session.cpp session_manager.cpp)
-target_include_directories(server SYSTEM PRIVATE ${Boost_INCLUDE_DIRS})
-target_link_libraries(server PRIVATE ${Boost_LIBRARIES})
-
-if(DEBUG_ASIO)
-    target_compile_definitions(server PRIVATE BOOST_ASIO_ENABLE_HANDLER_TRACKING)
-endif()
+add_subdirectory(common)
+add_subdirectory(lexer)
+add_subdirectory(parser)
+add_subdirectory(main)
diff --git a/server/common/CMakeLists.txt b/server/common/CMakeLists.txt
new file mode 100644
index 0000000..5655861
--- /dev/null
+++ b/server/common/CMakeLists.txt
@@ -0,0 +1,6 @@
+find_package(Boost REQUIRED)
+
+add_library(common INTERFACE)
+
+target_include_directories(common SYSTEM INTERFACE ${Boost_INCLUDE_DIRS})
+target_link_libraries(common INTERFACE ${Boost_LIBRARIES})
diff --git a/server/error.hpp b/server/common/error.hpp
index cbfbb1e..cbfbb1e 100644
--- a/server/error.hpp
+++ b/server/common/error.hpp
diff --git a/server/log.hpp b/server/common/log.hpp
index ca0fafd..ca0fafd 100644
--- a/server/log.hpp
+++ b/server/common/log.hpp
diff --git a/server/lexer.hpp b/server/lexer.hpp
deleted file mode 100644
index 8afe15c..0000000
--- a/server/lexer.hpp
+++ /dev/null
@@ -1,261 +0,0 @@
-#pragma once
-
-#include "error.hpp"
-
-#include <cmath>
-
-#include <exception>
-#include <functional>
-#include <limits>
-#include <optional>
-#include <regex>
-#include <string>
-#include <string_view>
-#include <unordered_map>
-#include <vector>
-
-namespace math::server {
-namespace lexer {
-
-class Error : public server::Error {
-public:
-    explicit Error(const std::string &what)
-        : server::Error{"lexer error: " + what}
-    { }
-};
-
-class Token {
-public:
-    enum class Type {
-        LEFT_PAREN,
-        RIGHT_PAREN,
-        PLUS,
-        MINUS,
-        ASTERISK,
-        SLASH,
-        NUMBER,
-    };
-
-    explicit Token(Type type)
-        : m_type{type}, m_number_value{nan()}
-    { }
-
-    explicit Token(double number_value)
-        : m_type{Type::NUMBER}, m_number_value{number_value}
-    { }
-
-    bool operator==(const Token& other) const {
-        return m_type == other.m_type
-            && ((is_nan(m_number_value) && is_nan(other.m_number_value))
-                || m_number_value == other.m_number_value);
-    }
-
-    bool operator!=(const Token& other) const { return !(*this == other); }
-
-    Type get_type() const { return m_type; }
-
-    double get_number_value() const {
-        if (get_type() != Type::NUMBER) {
-            throw Error{"token must be a number to query its value"};
-        }
-        return m_number_value;
-    }
-
-private:
-    static constexpr double nan() { return std::numeric_limits<double>::quiet_NaN(); }
-
-    static bool is_nan(double x) { return std::isnan(x); }
-
-    Type m_type;
-    double m_number_value;
-};
-
-namespace details {
-
-inline std::string_view match_number(const std::string_view& input) {
-    static constexpr std::regex::flag_type flags =
-        std::regex_constants::ECMAScript |
-        std::regex_constants::icase;
-    // This is a hacky attempt to describe a C-like grammar for floating-point
-    // numbers using a regex (the tests seem to pass though).
-    // A proper NFA would be better, I guess.
-    static const std::regex number_regex{R"REGEX(^(?:\d+(?:\.\d*)?|\.\d+)(e[+-]?(\d*))?)REGEX", flags};
-
-    std::cmatch match;
-    if (!std::regex_search(input.cbegin(), input.cend(), match, number_regex)) {
-        return {};
-    }
-    // If we have the numeric part of a number followed by 'e' and no digits,
-    // 1) that 'e' definitely belongs to this number token,
-    // 2) the user forgot to type in the required digits.
-    const auto& exponent = match[1];
-    const auto& abs_power = match[2];
-    if (exponent.matched && abs_power.matched && abs_power.length() == 0) {
-        throw lexer::Error{"exponent has no digits: " + match[0].str()};
-    }
-    return {match[0].first, match[0].length()};
-}
-
-inline std::optional<double> parse_number(const std::string_view& input, std::string_view& token) {
-    const auto view = match_number(input);
-    if (!view.data()) {
-        return {};
-    }
-    try {
-        const auto result = std::stod(std::string{view});
-        token = view;
-        return result;
-    } catch (const std::exception& e) {
-        throw lexer::Error{"couldn't parse number from: " + std::string{view}};
-    }
-    return {};
-}
-
-inline std::optional<double> parse_number(const std::string_view& input) {
-    std::string_view token;
-    return parse_number(input, token);
-}
-
-inline bool starts_with(const std::string_view& a, const std::string_view& b) noexcept {
-    return a.length() >= b.length()
-        && a.compare(0, b.length(), b) == 0;
-}
-
-inline std::optional<Token::Type> parse_const_token(const std::string_view& input, std::string_view& token) {
-    // FIXME: Potentially error-prone if there's const token A which is a
-    // prefix of token B (if the map is not sorted, we'd parse token A, when it
-    // could've been token B).
-    // Can be solved by sorting the keys accordingly.
-
-    static const std::unordered_map<std::string_view, Token::Type> const_tokens{
-        {"(", Token::Type::LEFT_PAREN},
-        {")", Token::Type::RIGHT_PAREN},
-        {"+", Token::Type::PLUS},
-        {"-", Token::Type::MINUS},
-        {"*", Token::Type::ASTERISK},
-        {"/", Token::Type::SLASH},
-    };
-
-    for (const auto& it : const_tokens) {
-        const auto& str = it.first;
-        const auto& type = it.second;
-
-        if (starts_with(input, str)) {
-            token = input.substr(0, str.length());
-            return type;
-        }
-    }
-
-    return {};
-}
-
-inline std::optional<Token::Type> parse_const_token(const std::string_view& input) {
-    std::string_view token;
-    return parse_const_token(input, token);
-}
-
-inline std::string_view parse_whitespace(const std::string_view& input) {
-    static const std::regex ws_regex{R"(\s*)"};
-
-    std::cmatch match;
-    if (std::regex_search(input.cbegin(), input.cend(), match, ws_regex)) {
-        return {match[0].first, match[0].length()};
-    }
-    return {};
-}
-
-}
-
-}
-
-class Lexer {
-public:
-    explicit Lexer(const std::string_view& input)
-        : m_input{input} {
-    }
-
-    using TokenProcessor = std::function<bool (const lexer::Token&)>;
-
-    bool for_each_token(const TokenProcessor& process) {
-        parse_token();
-        for (auto token = peek_token(); token.has_value(); drop_token(), token = peek_token()) {
-            if (!process(*token)) {
-                return false;
-            }
-        }
-        return true;
-    }
-
-    std::vector<lexer::Token> get_tokens() {
-        std::vector<lexer::Token> tokens;
-        for_each_token([&tokens] (const lexer::Token& token) {
-            tokens.emplace_back(token);
-            return true;
-        });
-        return tokens;
-    }
-
-    void parse_token() {
-        if (m_input.length() == 0) {
-            return;
-        }
-        std::string_view token_view;
-        m_token_buffer = parse_token(token_view);
-        if (m_token_buffer.has_value()) {
-            m_input.remove_prefix(token_view.length());
-        }
-    }
-
-    bool has_token() const {
-        return peek_token().has_value();
-    }
-
-    std::optional<lexer::Token> peek_token() const {
-        return m_token_buffer;
-    }
-
-    void drop_token() {
-        if (!has_token()) {
-            throw lexer::Error{"internal: no tokens to drop"};
-        }
-        m_token_buffer = {};
-        parse_token();
-    }
-
-    std::optional<lexer::Token> drop_token_if(lexer::Token::Type type) {
-        if (!has_token()) {
-            throw lexer::Error{"internal: no tokens to drop"};
-        }
-        if (m_token_buffer.value().get_type() != type) {
-            return {};
-        }
-        const auto result = m_token_buffer;
-        drop_token();
-        return result;
-    }
-
-private:
-    void consume_whitespace() {
-        const auto ws = lexer::details::parse_whitespace(m_input);
-        m_input.remove_prefix(ws.length());
-    }
-
-    std::optional<lexer::Token> parse_token(std::string_view& token_view) {
-        consume_whitespace();
-        if (m_input.length() == 0) {
-            return {};
-        }
-        if (const auto const_token = lexer::details::parse_const_token(m_input, token_view); const_token.has_value()) {
-            return lexer::Token{*const_token};
-        }
-        if (const auto number = lexer::details::parse_number(m_input, token_view); number.has_value()) {
-            return lexer::Token{*number};
-        }
-        throw lexer::Error{"invalid input at: " + std::string{m_input}};
-    }
-
-    std::string_view m_input;
-    std::optional<lexer::Token> m_token_buffer;
-};
-
-}
diff --git a/server/lexer/CMakeLists.txt b/server/lexer/CMakeLists.txt
new file mode 100644
index 0000000..65523a7
--- /dev/null
+++ b/server/lexer/CMakeLists.txt
@@ -0,0 +1,2 @@
+add_library(lexer lexer.cpp token.cpp token_type.cpp)
+target_link_libraries(lexer PUBLIC common)
diff --git a/server/lexer/error.hpp b/server/lexer/error.hpp
new file mode 100644
index 0000000..99944c7
--- /dev/null
+++ b/server/lexer/error.hpp
@@ -0,0 +1,16 @@
+#pragma once
+
+#include "../common/error.hpp"
+
+#include <string>
+
+namespace math::server {
+
+class LexerError : public Error {
+public:
+    explicit LexerError(const std::string &what)
+        : Error{"lexer error: " + what}
+    { }
+};
+
+}
diff --git a/server/lexer/input.hpp b/server/lexer/input.hpp
new file mode 100644
index 0000000..1104a4b
--- /dev/null
+++ b/server/lexer/input.hpp
@@ -0,0 +1,42 @@
+#pragma once
+
+#include "error.hpp"
+
+#include <cstddef>
+
+#include <string_view>
+
+namespace math::server::lexer {
+
+class Input {
+public:
+    explicit Input(const std::string_view& input)
+        : m_pos{0}, m_input{input}
+    { }
+
+    const std::string_view& get_input() const { return m_input; }
+
+    std::size_t get_pos() const { return m_pos; }
+
+    std::size_t get_length() const { return m_input.length(); }
+
+    bool empty() const { return m_input.empty(); }
+
+    void consume(std::size_t len) {
+        if (m_input.length() < len) {
+            throw LexerError{"internal: not enough input to consume"};
+        }
+        m_pos += len;
+        m_input.remove_prefix(len);
+    }
+
+    void consume(const std::string_view& sub) {
+        consume(sub.length());
+    }
+
+private:
+    std::size_t m_pos;
+    std::string_view m_input;
+};
+
+}
diff --git a/server/lexer/lexer.cpp b/server/lexer/lexer.cpp
new file mode 100644
index 0000000..c7eea6d
--- /dev/null
+++ b/server/lexer/lexer.cpp
@@ -0,0 +1,206 @@
+#include "error.hpp"
+#include "lexer.hpp"
+#include "token.hpp"
+#include "token_type.hpp"
+
+#include <exception>
+#include <optional>
+#include <regex>
+#include <string_view>
+#include <string>
+#include <vector>
+
+namespace math::server {
+namespace lexer {
+namespace {
+
+std::string_view match_number(const std::string_view& input) {
+    static constexpr std::regex::flag_type flags =
+        std::regex_constants::ECMAScript |
+        std::regex_constants::icase;
+    // This is a hacky attempt to describe a C-like grammar for floating-point
+    // numbers using a regex (the tests seem to pass though).
+    // A proper NFA would be better, I guess.
+    static const std::regex number_regex{R"REGEX(^(?:\d+(?:\.\d*)?|\.\d+)(e[+-]?(\d*))?)REGEX", flags};
+
+    std::cmatch match;
+    if (!std::regex_search(input.cbegin(), input.cend(), match, number_regex)) {
+        return {};
+    }
+    {
+        // If we have the numeric part of a number followed by 'e' and no digits,
+        // 1) that 'e' definitely belongs to this number token,
+        // 2) the user forgot to type in the required digits.
+        const auto& exponent = match[1];
+        const auto& abs_power = match[2];
+        if (exponent.matched && abs_power.matched && abs_power.length() == 0) {
+            throw LexerError{"exponent has no digits: " + match[0].str()};
+        }
+    }
+    return {match[0].first, match[0].length()};
+}
+
+std::optional<double> parse_number(const std::string_view& input, std::string_view& token) {
+    const auto view = match_number(input);
+    if (!view.data()) {
+        return {};
+    }
+    try {
+        const auto result = std::stod(std::string{view});
+        token = view;
+        return result;
+    } catch (const std::exception& e) {
+        throw LexerError{"internal: couldn't parse number from: " + std::string{view}};
+    }
+    return {};
+}
+
+bool starts_with(const std::string_view& a, const std::string_view& b) noexcept {
+    return a.length() >= b.length()
+        && a.compare(0, b.length(), b) == 0;
+}
+
+std::optional<token::Type> parse_const_token(const std::string_view& input, std::string_view& token) {
+    for (const auto type : token::const_tokens()) {
+        const auto str = token::type_to_string(type);
+        if (starts_with(input, str)) {
+            token = {input.cbegin(), str.length()};
+            return {type};
+        }
+    }
+    return {};
+}
+
+}
+
+namespace details {
+
+std::optional<double> parse_number(const std::string_view& input) {
+    std::string_view token;
+    return lexer::parse_number(input, token);
+}
+
+std::optional<token::Type> parse_const_token(const std::string_view& input) {
+    std::string_view token;
+    return lexer::parse_const_token(input, token);
+}
+
+std::string_view parse_whitespace(const std::string_view& input) {
+    static const std::regex ws_regex{R"(^\s+)"};
+
+    std::cmatch match;
+    if (std::regex_search(input.cbegin(), input.cend(), match, ws_regex)) {
+        return {match[0].first, match[0].length()};
+    }
+    return {};
+}
+
+}
+}
+
+Lexer::Lexer(const std::string_view& input)
+    : Lexer{lexer::Input{input}} {
+}
+
+Lexer::Lexer(const lexer::Input& input)
+    : m_input{input} {
+
+    consume_token();
+}
+
+bool Lexer::for_each_token(const TokenProcessor& process) {
+    for (auto token = peek_token(); token.has_value(); drop_token(), token = peek_token()) {
+        if (!process(*token)) {
+            return false;
+        }
+    }
+    return true;
+}
+
+std::vector<Lexer::ParsedToken> Lexer::get_tokens() {
+    std::vector<ParsedToken> tokens;
+    for_each_token([&tokens] (const ParsedToken& token) {
+        tokens.emplace_back(token);
+        return true;
+    });
+    return tokens;
+}
+
+void Lexer::drop_token() {
+    if (!has_token()) {
+        throw LexerError{"internal: no tokens to drop"};
+    }
+    m_token_buffer = {};
+    consume_token();
+}
+
+std::optional<Lexer::ParsedToken> Lexer::drop_token_of_type(Type type) {
+    if (!has_token()) {
+        throw LexerError{"internal: no tokens to drop"};
+    }
+    if (m_token_buffer.value().get_type() != type) {
+        return {};
+    }
+    const auto result = m_token_buffer;
+    drop_token();
+    return result;
+}
+
+void Lexer::consume_whitespace() {
+    const auto ws = parse_whitespace();
+    if (!ws.has_value()) {
+        return;
+    }
+    m_input.consume(ws->get_length());
+}
+
+void Lexer::consume_token() {
+    if (m_input.empty()) {
+        return;
+    }
+    consume_whitespace();
+    if (m_input.empty()) {
+        return;
+    }
+    auto token{parse_token()};
+    m_input.consume(token.get_length());
+    m_token_buffer = std::move(token);
+}
+
+std::optional<Lexer::ParsedToken> Lexer::parse_whitespace() const {
+    const auto token_view = lexer::details::parse_whitespace(m_input.get_input());
+    if (token_view.empty()) {
+        return {};
+    }
+    return ParsedToken{Token{Token::Type::WHITESPACE}, m_input.get_pos(), token_view};
+}
+
+std::optional<Lexer::ParsedToken> Lexer::parse_const_token() const {
+    std::string_view token_view;
+    const auto type = lexer::parse_const_token(m_input.get_input(), token_view);
+    if (!type.has_value()) {
+        return {};
+    }
+    return ParsedToken{Token{*type}, m_input.get_pos(), token_view};
+}
+
+std::optional<Lexer::ParsedToken> Lexer::parse_number() const {
+    std::string_view token_view;
+    const auto number = lexer::parse_number(m_input.get_input(), token_view);
+    if (!number.has_value()) {
+        return {};
+    }
+    return ParsedToken{Token{*number}, m_input.get_pos(), token_view};
+}
+
+Lexer::ParsedToken Lexer::parse_token() const {
+    if (const auto const_token = parse_const_token(); const_token.has_value()) {
+        return *const_token;
+    }
+    if (const auto number = parse_number(); number.has_value()) {
+        return *number;
+    }
+    throw LexerError{"invalid input at: " + std::string{m_input.get_input()}};
+}
+
+}
diff --git a/server/lexer/lexer.hpp b/server/lexer/lexer.hpp
new file mode 100644
index 0000000..d08a2df
--- /dev/null
+++ b/server/lexer/lexer.hpp
@@ -0,0 +1,61 @@
+#pragma once
+
+#include "input.hpp"
+#include "token.hpp"
+#include "token_type.hpp"
+
+#include <functional>
+#include <optional>
+#include <string_view>
+#include <vector>
+
+namespace math::server {
+namespace lexer::details {
+
+// Exposed for testing:
+std::string_view parse_whitespace(const std::string_view&);
+std::optional<double> parse_number(const std::string_view&);
+std::optional<token::Type> parse_const_token(const std::string_view&);
+
+}
+
+class Lexer {
+public:
+    explicit Lexer(const std::string_view& input);
+    explicit Lexer(const lexer::Input& input);
+
+    using Token = lexer::Token;
+    using ParsedToken = lexer::ParsedToken;
+    using Type = Token::Type;
+    using TokenProcessor = std::function<bool (const ParsedToken&)>;
+
+    bool for_each_token(const TokenProcessor& process);
+
+    std::vector<ParsedToken> get_tokens();
+
+    bool has_token() const {
+        return peek_token().has_value();
+    }
+
+    std::optional<ParsedToken> peek_token() const {
+        return m_token_buffer;
+    }
+
+    void drop_token();
+    std::optional<ParsedToken> drop_token_of_type(Type type);
+
+private:
+    std::optional<ParsedToken> parse_whitespace() const;
+    std::optional<ParsedToken> parse_const_token() const;
+    std::optional<ParsedToken> parse_number() const;
+
+    ParsedToken parse_token() const;
+
+    void consume_whitespace();
+    void consume_token();
+
+    lexer::Input m_input;
+    std::optional<ParsedToken> m_token_buffer;
+};
+
+}
diff --git a/server/lexer/token.cpp b/server/lexer/token.cpp
new file mode 100644
index 0000000..6ffb721
--- /dev/null
+++ b/server/lexer/token.cpp
@@ -0,0 +1,71 @@
+#include "error.hpp"
+#include "token.hpp"
+#include "token_type.hpp"
+
+#include <cmath>
+
+#include <limits>
+#include <variant>
+
+namespace math::server::lexer {
+namespace {
+
+static constexpr double nan() { return std::numeric_limits<double>::quiet_NaN(); }
+
+static bool is_nan(double x) { return std::isnan(x); }
+
+static bool numbers_equal(double x, double y) {
+    if (is_nan(x) && is_nan(y)) {
+        return true;
+    }
+    return x == y;
+}
+
+}
+
+Token::Token(Type type)
+    : m_type{type} {
+
+    if (token::token_has_value(type)) {
+        throw LexerError{"internal: must have a value: " + token::type_to_int_string(type)};
+    }
+}
+
+Token::Token(double value)
+    : m_type{Type::NUMBER}, m_value{value}
+{ }
+
+bool Token::operator==(const Token& other) const {
+    if (m_type != other.m_type) {
+        return false;
+    }
+    if (token::is_const_token(m_type)) {
+        return true;
+    }
+    if (m_type == Type::NUMBER) {
+        return numbers_equal(as_number(), other.as_number());
+    }
+    throw LexerError{"internal: can't compare tokens of type: " + token::type_to_int_string(m_type)};
+}
+
+double Token::as_number() const {
+    const auto type = get_type();
+    if (type != Type::NUMBER) {
+        throw LexerError{"internal: not a number: " + token::type_to_int_string(type)};
+    }
+    return std::get<double>(m_value);
+}
+
+std::ostream& operator<<(std::ostream& os, const Token& token) {
+    switch (token.m_type) {
+        case token::Type::NUMBER:
+            os << token.as_number();
+            break;
+        default:
+            os << token::type_to_string(token.m_type);
+            break;
+    }
+    return os;
+}
+
+}
diff --git a/server/lexer/token.hpp b/server/lexer/token.hpp
new file mode 100644
index 0000000..6f98383
--- /dev/null
+++ b/server/lexer/token.hpp
@@ -0,0 +1,49 @@
+#pragma once
+
+#include "token_type.hpp"
+
+#include <cstddef>
+
+#include <string_view>
+#include <utility>
+#include <variant>
+
+namespace math::server::lexer {
+
+class Token {
+public:
+    using Type = token::Type;
+
+    explicit Token(token::Type type);
+    explicit Token(double value);
+
+    bool operator==(const Token& other) const;
+    bool operator!=(const Token& other) const { return !(*this == other); }
+
+    Type get_type() const { return m_type; }
+
+    double as_number() const;
+
+private:
+    token::Type m_type;
+    std::variant<double> m_value;
+
+    friend std::ostream& operator<<(std::ostream&, const Token&);
+};
+
+class ParsedToken : public Token {
+public:
+    ParsedToken(Token&& token, std::size_t pos, const std::string_view& view)
+        : Token{std::move(token)}, m_pos{pos}, m_view{view} {
+    }
+
+    std::size_t get_pos() const { return m_pos; }
+
+    std::size_t get_length() const { return m_view.length(); }
+
+private:
+    std::size_t m_pos;
+    std::string_view m_view;
+};
+
+}
diff --git a/server/lexer/token_type.cpp b/server/lexer/token_type.cpp
new file mode 100644
index 0000000..9a69ba1
--- /dev/null
+++ b/server/lexer/token_type.cpp
@@ -0,0 +1,164 @@
+#include "error.hpp"
+#include "token_type.hpp"
+
+#include <functional>
+#include <map>
+#include <ostream>
+#include <stdexcept>
+#include <string>
+#include <unordered_map>
+
+namespace math::server::lexer::token {
+namespace {
+
+using ToStringMap = std::unordered_map<Type, std::string>;
+using FromStringMap = std::map<std::string, Type, std::greater<std::string>>;
+
+class ToStringConverter {
+public:
+    ToStringConverter() : m_map{to_string_map()} {
+        validate();
+    }
+
+    const ToStringMap& map() const { return m_map; }
+
+private:
+    static const ToStringMap& to_string_map() {
+        static const ToStringMap map{
+            {Type::WHITESPACE, "whitespace"},
+            {Type::PLUS, "+"},
+            {Type::MINUS, "-"},
+            {Type::ASTERISK, "*"},
+            {Type::SLASH, "/"},
+            {Type::LEFT_PAREN, "("},
+            {Type::RIGHT_PAREN, ")"},
+            {Type::NUMBER, "number"},
+        };
+        return map;
+    }
+
+    void validate() const {
+        check_for_duplicates();
+    }
+
+    void check_for_duplicates() const {
+        std::unordered_set<std::string> strings;
+        for (const auto& [type, str] : m_map) {
+            const auto [_, inserted] = strings.emplace(str);
+            if (!inserted) {
+                throw std::logic_error{"multiple tokens have the same string representation: " + str};
+            }
+        }
+    }
+
+    const ToStringMap& m_map;
+};
+
+const ToStringMap& to_string_map() {
+    static const ToStringConverter converter;
+    return converter.map();
+}
+
+class FromStringConverter {
+public:
+    FromStringConverter(const ToStringMap& to_string)
+        : m_map{build_map(to_string)} {
+    }
+
+    const FromStringMap& map() const { return m_map; }
+
+private:
+    static FromStringMap build_map(const ToStringMap& to_string) {
+        FromStringMap from_string;
+        for (const auto& [type, str] : to_string) {
+            const auto [_, inserted] = from_string.emplace(str, type);
+            if (!inserted) {
+                throw std::logic_error{"multiple tokens have the same string representation: " + str};
+            }
+        }
+        return from_string;
+    }
+
+    FromStringMap m_map;
+};
+
+const FromStringMap& from_string_map() {
+    static const FromStringConverter converter{to_string_map()};
+    return converter.map();
+}
+
+class ConstTokens {
+public:
+    ConstTokens() {
+        const auto& map = to_string_map();
+        for (const auto& [type, _] : map) {
+            if (is_const_token(type)) {
+                m_set.emplace(type);
+            }
+        }
+    }
+
+    const TypeSet& set() const { return m_set; }
+
+private:
+    TypeSet m_set;
+};
+
+}
+
+TypeInt type_to_int(Type type) {
+    return static_cast<TypeInt>(type);
+}
+
+std::string type_to_int_string(Type type) {
+    return std::to_string(type_to_int(type));
+}
+
+bool is_const_token(Type type) {
+    switch (type) {
+        case Type::WHITESPACE:
+        case Type::NUMBER:
+            return false;
+        default:
+            return true;
+    }
+}
+
+const TypeSet& const_tokens() {
+    static const ConstTokens tokens;
+    return tokens.set();
+}
+
+bool token_has_value(Type type) {
+    switch (type) {
+        case Type::NUMBER:
+            return true;
+        default:
+            return false;
+    }
+}
+
+std::string type_to_string(Type type) {
+    const auto& map = to_string_map();
+    const auto it = map.find(type);
+    if (it == map.cend()) {
+        throw LexerError{"type_to_string: unsupported token type: " + type_to_int_string(type)};
+    }
+    return it->second;
+}
+
+Type type_from_string(const std::string& src) {
+    const auto& map = from_string_map();
+    const auto it = map.find(src);
+    if (it == map.cend()) {
+        throw LexerError{"type_from_string: unsupported token: " + std::string{src}};
+    }
+    return it->second;
+}
+
+std::ostream& operator<<(std::ostream& os, const Type& type) {
+    os << type_to_int(type);
+    return os;
+}
+
+}
diff --git a/server/lexer/token_type.hpp b/server/lexer/token_type.hpp
new file mode 100644
index 0000000..9489915
--- /dev/null
+++ b/server/lexer/token_type.hpp
@@ -0,0 +1,37 @@
+#pragma once
+
+#include <ostream>
+#include <string>
+#include <type_traits>
+#include <unordered_set>
+
+namespace math::server::lexer::token {
+
+enum class Type {
+    WHITESPACE,
+    PLUS,
+    MINUS,
+    ASTERISK,
+    SLASH,
+    LEFT_PAREN,
+    RIGHT_PAREN,
+    NUMBER,
+};
+
+using TypeInt = std::underlying_type<Type>::type;
+using TypeSet = std::unordered_set<Type>;
+
+TypeInt type_to_int(Type);
+std::string type_to_int_string(Type);
+
+bool is_const_token(Type);
+const TypeSet& const_tokens();
+
+bool token_has_value(Type);
+
+std::string type_to_string(Type);
+Type type_from_string(const std::string&);
+
+std::ostream& operator<<(std::ostream&, const Type&);
+
+}
diff --git a/server/main/CMakeLists.txt b/server/main/CMakeLists.txt
new file mode 100644
index 0000000..b322390
--- /dev/null
+++ b/server/main/CMakeLists.txt
@@ -0,0 +1,13 @@
+find_package(Boost REQUIRED COMPONENTS filesystem program_options)
+
+option(DEBUG_ASIO "enable debug output for Boost.Asio" OFF)
+
+add_executable(server main.cpp server.cpp session.cpp session_manager.cpp)
+target_link_libraries(server PRIVATE common parser)
+
+target_include_directories(server SYSTEM PRIVATE ${Boost_INCLUDE_DIRS})
+target_link_libraries(server PRIVATE ${Boost_LIBRARIES})
+
+if(DEBUG_ASIO)
+    target_compile_definitions(server PRIVATE BOOST_ASIO_ENABLE_HANDLER_TRACKING)
+endif()
diff --git a/server/main.cpp b/server/main/main.cpp
index 2cf6d35..2cf6d35 100644
--- a/server/main.cpp
+++ b/server/main/main.cpp
diff --git a/server/server.cpp b/server/main/server.cpp
index 4dc672c..3800144 100644
--- a/server/server.cpp
+++ b/server/main/server.cpp
@@ -1,10 +1,11 @@
-#include "error.hpp"
-#include "log.hpp"
 #include "server.hpp"
 #include "session.hpp"
 #include "session_manager.hpp"
 #include "settings.hpp"
 
+#include "../common/error.hpp"
+#include "../common/log.hpp"
+
 #include <boost/asio.hpp>
 #include <boost/system/error_code.hpp>
 #include <boost/system/system_error.hpp>
diff --git a/server/server.hpp b/server/main/server.hpp
index 5524f88..5524f88 100644
--- a/server/server.hpp
+++ b/server/main/server.hpp
diff --git a/server/session.cpp b/server/main/session.cpp
index 409ca5a..0ee7f75 100644
--- a/server/session.cpp
+++ b/server/main/session.cpp
@@ -1,9 +1,10 @@
-#include "error.hpp"
-#include "log.hpp"
-#include "parser.hpp"
 #include "session.hpp"
 #include "session_manager.hpp"
 
+#include "../common/error.hpp"
+#include "../common/log.hpp"
+#include "../parser/parser.hpp"
+
 #include <boost/asio.hpp>
 #include <boost/lexical_cast.hpp>
 #include <boost/system/error_code.hpp>
diff --git a/server/session.hpp b/server/main/session.hpp
index ace3755..ace3755 100644
--- a/server/session.hpp
+++ b/server/main/session.hpp
diff --git a/server/session_manager.cpp b/server/main/session_manager.cpp
index a42fca4..c2aef6d 100644
--- a/server/session_manager.cpp
+++ b/server/main/session_manager.cpp
@@ -1,7 +1,8 @@
-#include "log.hpp"
 #include "session.hpp"
 #include "session_manager.hpp"
 
+#include "../common/log.hpp"
+
 #include <memory>
 #include <mutex>
 
diff --git a/server/session_manager.hpp b/server/main/session_manager.hpp
index f0bec0b..f0bec0b 100644
--- a/server/session_manager.hpp
+++ b/server/main/session_manager.hpp
diff --git a/server/settings.hpp b/server/main/settings.hpp
index 310163f..310163f 100644
--- a/server/settings.hpp
+++ b/server/main/settings.hpp
diff --git a/server/parser.hpp b/server/parser.hpp
deleted file mode 100644
index a9e5f54..0000000
--- a/server/parser.hpp
+++ /dev/null
@@ -1,168 +0,0 @@
-#pragma once
-
-#include "error.hpp"
-#include "lexer.hpp"
-
-#include <optional>
-#include <string>
-#include <string_view>
-
-namespace math::server {
-namespace parser {
-
-class Error : public server::Error {
-public:
-    explicit Error(const std::string& what)
-        : server::Error{"parser error: " + what}
-    { }
-};
-
-class BinaryOp {
-public:
-    static bool is(const lexer::Token& token) {
-        using Type = lexer::Token::Type;
-        switch (token.get_type()) {
-            case Type::PLUS:
-            case Type::MINUS:
-            case Type::ASTERISK:
-            case Type::SLASH:
-                return true;
-
-            default:
-                return false;
-        }
-    }
-
-    static BinaryOp from_token(const lexer::Token& token) {
-        if (!is(token)) {
-            throw Error{"internal: token is not a binary operator"};
-        }
-        return BinaryOp{token};
-    }
-
-    static constexpr unsigned min_precedence() { return 0; }
-
-    unsigned get_precedence() const {
-        using Type = lexer::Token::Type;
-        switch (m_type) {
-            case Type::PLUS:
-            case Type::MINUS:
-                return min_precedence();
-
-            case Type::ASTERISK:
-            case Type::SLASH:
-                return min_precedence() + 1;
-
-            default:
-                throw Error{"internal: undefined operator precedence"};
-        }
-    }
-
-    double exec(double lhs, double rhs) const {
-        using Type = lexer::Token::Type;
-        switch (m_type) {
-            case Type::PLUS:
-                return lhs + rhs;
-            case Type::MINUS:
-                return lhs - rhs;
-            case Type::ASTERISK:
-                return lhs * rhs;
-            case Type::SLASH:
-                // Trapping the CPU would be better?
-                if (rhs == 0.) {
-                    throw Error{"division by zero"};
-                }
-                return lhs / rhs;
-            default:
-                throw Error{"internal: unsupported operator"};
-        }
-    }
-
-private:
-    explicit BinaryOp(const lexer::Token& token)
-        : m_type{token.get_type()}
-    { }
-
-    lexer::Token::Type m_type;
-};
-
-}
-
-class Parser {
-public:
-    // I did simple recursive descent parsing a long time ago (see
-    // https://github.com/egor-tensin/simple-interpreter), this appears to be
-    // a finer algorithm for parsing arithmetic expressions.
-    // Reference: https://en.wikipedia.org/wiki/Operator-precedence_parser
-
-    explicit Parser(const std::string_view& input)
-        : m_lexer{input}
-    { }
-
-    double exec() {
-        m_lexer.parse_token();
-        const auto result = exec_expr();
-        if (m_lexer.has_token()) {
-            throw parser::Error{"expected a binary operator"};
-        }
-        return result;
-    }
-
-private:
-    double exec_expr() {
-        return exec_expr(exec_primary(), parser::BinaryOp::min_precedence());
-    }
-
-    double exec_expr(double lhs, unsigned min_prec) {
-        for (auto op = peek_operator(); op.has_value() && op->get_precedence() >= min_prec;) {
-            const auto lhs_op = *op;
-            m_lexer.drop_token();
-            auto rhs = exec_primary();
-
-            for (op = peek_operator(); op.has_value() && op->get_precedence() > lhs_op.get_precedence(); op = peek_operator()) {
-                rhs = exec_expr(rhs, op->get_precedence());
-            }
-
-            lhs = lhs_op.exec(lhs, rhs);
-        }
-        return lhs;
-    }
-
-    std::optional<parser::BinaryOp> peek_operator() {
-        const auto token = m_lexer.peek_token();
-        if (!token.has_value() || !parser::BinaryOp::is(*token)) {
-            return {};
-        }
-        return parser::BinaryOp::from_token(*token);
-    }
-
-    double exec_primary() {
-        if (!m_lexer.has_token()) {
-            throw parser::Error{"expected '-', '(' or a number"};
-        }
-
-        using Type = lexer::Token::Type;
-
-        if (m_lexer.drop_token_if(Type::MINUS).has_value()) {
-            return -exec_primary();
-        }
-
-        if (m_lexer.drop_token_if(Type::LEFT_PAREN).has_value()) {
-            const auto inner = exec_expr();
-            if (!m_lexer.has_token() || !m_lexer.drop_token_if(Type::RIGHT_PAREN).has_value()) {
-                throw parser::Error{"missing closing ')'"};
-            }
-            return inner;
-        }
-
-        if (const auto token = m_lexer.drop_token_if(Type::NUMBER); token.has_value()) {
-            return token.value().get_number_value();
-        }
-
-        throw parser::Error{"expected '-', '(' or a number"};
-    }
-
-    Lexer m_lexer;
-};
-
-}
diff --git a/server/parser/CMakeLists.txt b/server/parser/CMakeLists.txt
new file mode 100644
index 0000000..2490e57
--- /dev/null
+++ b/server/parser/CMakeLists.txt
@@ -0,0 +1,2 @@
+add_library(parser INTERFACE)
+target_link_libraries(parser INTERFACE common lexer)
diff --git a/server/parser/error.hpp b/server/parser/error.hpp
new file mode 100644
index 0000000..1ba29ed
--- /dev/null
+++ b/server/parser/error.hpp
@@ -0,0 +1,16 @@
+#pragma once
+
+#include "../common/error.hpp"
+
+#include <string>
+
+namespace math::server {
+
+class ParserError : public Error {
+public:
+    explicit ParserError(const std::string& what)
+        : Error{"parser error: " + what}
+    { }
+};
+
+}
diff --git a/server/parser/operator.hpp b/server/parser/operator.hpp
new file mode 100644
index 0000000..4226078
--- /dev/null
+++ b/server/parser/operator.hpp
@@ -0,0 +1,79 @@
+#pragma once
+
+#include "error.hpp"
+
+#include "../lexer/token.hpp"
+#include "../lexer/token_type.hpp"
+
+namespace math::server::parser {
+
+class BinaryOp {
+public:
+    using Token = lexer::Token;
+    using Type = Token::Type;
+
+    static bool is(const Token& token) {
+        switch (token.get_type()) {
+            case Type::PLUS:
+            case Type::MINUS:
+            case Type::ASTERISK:
+            case Type::SLASH:
+                return true;
+
+            default:
+                return false;
+        }
+    }
+
+    static BinaryOp from_token(const Token& token) {
+        if (!is(token)) {
+            throw ParserError{"internal: token is not a binary operator"};
+        }
+        return BinaryOp{token};
+    }
+
+    static constexpr unsigned min_precedence() { return 0; }
+
+    unsigned get_precedence() const {
+        switch (m_type) {
+            case Type::PLUS:
+            case Type::MINUS:
+                return min_precedence();
+
+            case Type::ASTERISK:
+            case Type::SLASH:
+                return min_precedence() + 1;
+
+            default:
+                throw ParserError{"internal: undefined operator precedence"};
+        }
+    }
+
+    double exec(double lhs, double rhs) const {
+        switch (m_type) {
+            case Type::PLUS:
+                return lhs + rhs;
+            case Type::MINUS:
+                return lhs - rhs;
+            case Type::ASTERISK:
+                return lhs * rhs;
+            case Type::SLASH:
+                // Trapping the CPU would be better?
+                if (rhs == 0.) {
+                    throw ParserError{"division by zero"};
+                }
+                return lhs / rhs;
+            default:
+                throw ParserError{"internal: unsupported operator"};
+        }
+    }
+
+private:
+    explicit BinaryOp(const Token& token)
+        : m_type{token.get_type()}
+    { }
+
+    Type m_type;
+};
+
+}
diff --git a/server/parser/parser.hpp b/server/parser/parser.hpp
new file mode 100644
index 0000000..1197c31
--- /dev/null
+++ b/server/parser/parser.hpp
@@ -0,0 +1,89 @@
+#pragma once
+
+#include "error.hpp"
+#include "operator.hpp"
+
+#include "../lexer/lexer.hpp"
+
+#include <optional>
+#include <string_view>
+
+namespace math::server {
+
+class Parser {
+public:
+    // I did simple recursive descent parsing a long time ago (see
+    // https://github.com/egor-tensin/simple-interpreter), this appears to be
+    // a finer algorithm for parsing arithmetic expressions.
+    // Reference: https://en.wikipedia.org/wiki/Operator-precedence_parser
+
+    explicit Parser(const std::string_view& input)
+        : m_lexer{input}
+    { }
+
+    double exec() {
+        const auto result = exec_expr();
+        if (m_lexer.has_token()) {
+            throw ParserError{"expected a binary operator"};
+        }
+        return result;
+    }
+
+private:
+    double exec_expr() {
+        return exec_expr(exec_primary(), parser::BinaryOp::min_precedence());
+    }
+
+    double exec_expr(double lhs, unsigned min_prec) {
+        for (auto op = peek_operator(); op.has_value() && op->get_precedence() >= min_prec;) {
+            const auto lhs_op = *op;
+            m_lexer.drop_token();
+            auto rhs = exec_primary();
+
+            for (op = peek_operator(); op.has_value() && op->get_precedence() > lhs_op.get_precedence(); op = peek_operator()) {
+                rhs = exec_expr(rhs, op->get_precedence());
+            }
+
+            lhs = lhs_op.exec(lhs, rhs);
+        }
+        return lhs;
+    }
+
+    std::optional<parser::BinaryOp> peek_operator() {
+        const auto token = m_lexer.peek_token();
+        if (!token.has_value() || !parser::BinaryOp::is(*token)) {
+            return {};
+        }
+        return parser::BinaryOp::from_token(*token);
+    }
+
+    double exec_primary() {
+        if (!m_lexer.has_token()) {
+            throw ParserError{"expected '-', '(' or a number"};
+        }
+
+        using Type = lexer::Token::Type;
+
+        if (m_lexer.drop_token_of_type(Type::MINUS).has_value()) {
+            return -exec_primary();
+        }
+
+        if (m_lexer.drop_token_of_type(Type::LEFT_PAREN).has_value()) {
+            const auto inner = exec_expr();
+            if (!m_lexer.has_token() || !m_lexer.drop_token_of_type(Type::RIGHT_PAREN).has_value()) {
+                throw ParserError{"missing closing ')'"};
+            }
+            return inner;
+        }
+
+        if (const auto token = m_lexer.drop_token_of_type(Type::NUMBER); token.has_value()) {
+            return token.value().as_number();
+        }
+
+        throw ParserError{"expected '-', '(' or a number"};
+    }
+
+    Lexer m_lexer;
+};
+
+}