2 files changed, 129 insertions, 0 deletions
diff --git a/server/lexer/details/parse.cpp b/server/lexer/details/parse.cpp
new file mode 100644
index 0000000..464dfb2
--- /dev/null
+++ b/server/lexer/details/parse.cpp
@@ -0,0 +1,107 @@
+// Copyright (c) 2020 Egor Tensin <Egor.Tensin@gmail.com>
+// This file is part of the "math-server" project.
+// For details, see https://github.com/egor-tensin/math-server.
+// Distributed under the MIT License.
+
+#include "../error.hpp"
+#include "../token_type.hpp"
+
+#include <cstddef>
+
+#include <exception>
+#include <optional>
+#include <regex>
+#include <string>
+#include <string_view>
+
+namespace math::server::lexer::details {
+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;
+    {
+        const auto begin = input.data();
+        const auto end = begin + input.length();
+        if (!std::regex_search(begin, end, 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, static_cast<std::size_t>(match[0].length())};
+}
+
+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<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&) {
+        throw LexerError{"internal: couldn't parse number from: " + std::string{view}};
+    }
+    return {};
+}
+
+std::optional<double> parse_number(const std::string_view& input) {
+    std::string_view token;
+    return parse_number(input, token);
+}
+
+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 = std::string_view(input.data(), str.length());
+            return {type};
+        }
+    }
+    return {};
+}
+
+std::optional<token::Type> parse_const_token(const std::string_view& input) {
+    std::string_view token;
+    return 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;
+    {
+        const auto begin = input.data();
+        const auto end = begin + input.length();
+        if (std::regex_search(begin, end, match, ws_regex)) {
+            return std::string_view(match[0].first, match[0].length());
+        }
+    }
+    return {};
+}
+
+}
diff --git a/server/lexer/details/parse.hpp b/server/lexer/details/parse.hpp
new file mode 100644
index 0000000..72da234
--- /dev/null
+++ b/server/lexer/details/parse.hpp
@@ -0,0 +1,22 @@
+// Copyright (c) 2020 Egor Tensin <Egor.Tensin@gmail.com>
+// This file is part of the "math-server" project.
+// For details, see https://github.com/egor-tensin/math-server.
+// Distributed under the MIT License.
+
+#pragma once
+
+#include "../token_type.hpp"
+
+#include <optional>
+#include <string_view>
+
+namespace math::server::lexer::details {
+
+// Exposed for testing:
+std::string_view parse_whitespace(const std::string_view&);
+std::optional<double> parse_number(const std::string_view&, std::string_view&);
+std::optional<double> parse_number(const std::string_view&);
+std::optional<token::Type> parse_const_token(const std::string_view&, std::string_view&);
+std::optional<token::Type> parse_const_token(const std::string_view&);
+
+}