Compiler.cpp

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#include <Ark/Compiler/Compiler.hpp>
 
#include <chrono>
#include <limits>
#include <filesystem>
#include <picosha2.h>
#include <Ark/Constants.hpp>
#include <termcolor/proxy.hpp>
#include <fmt/core.h>
 
#include <Ark/Literals.hpp>
#include <Ark/Utils.hpp>
#include <Ark/Builtins/Builtins.hpp>
#include <Ark/Compiler/Macros/Processor.hpp>
 
namespace Ark
{
    using namespace internal;
    using namespace literals;
 
    Compiler::Compiler(const unsigned debug) :
        m_debug(debug)
    {}
 
    void Compiler::process(const Node& ast)
    {
        pushFileHeader();
 
        m_code_pages.emplace_back();  // create empty page
 
        // gather symbols, values, and start to create code segments
        compileExpression(ast, /* current_page */ 0, /* is_result_unused */ false, /* is_terminal */ false);
        // throw an error on undefined symbol uses
        checkForUndefinedSymbol();
 
        pushSymAndValTables();
 
        // push the different code segments
        for (std::size_t i = 0, end = m_code_pages.size(); i < end; ++i)
        {
            std::vector<Word>& page = m_code_pages[i];
            // just in case we got too far, always add a HALT to be sure the
            // VM won't do anything crazy
            page.emplace_back(Instruction::HALT);
 
            // push number of elements
            const std::size_t page_size = page.size();
            if (page_size > std::numeric_limits<uint16_t>::max())
                throw std::overflow_error("Size of page " + std::to_string(i) + " exceeds the maximum size of 2^16 - 1");
 
            m_bytecode.push_back(Instruction::CODE_SEGMENT_START);
            m_bytecode.push_back(static_cast<uint16_t>((page_size & 0xff00) >> 8));
            m_bytecode.push_back(static_cast<uint16_t>(page_size & 0x00ff));
 
            for (auto inst : page)
            {
                m_bytecode.push_back(inst.padding);
                m_bytecode.push_back(inst.opcode);
                m_bytecode.push_back(inst.bytes.first);
                m_bytecode.push_back(inst.bytes.second);
            }
        }
 
        if (m_code_pages.empty())
        {
            // code segment with a single instruction
            m_bytecode.push_back(Instruction::CODE_SEGMENT_START);
            m_bytecode.push_back(0_u8);
            m_bytecode.push_back(1_u8);
 
            m_bytecode.push_back(0_u8);
            m_bytecode.push_back(Instruction::HALT);
            m_bytecode.push_back(0_u8);
            m_bytecode.push_back(0_u8);
        }
 
        constexpr std::size_t header_size = 18;
 
        // generate a hash of the tables + bytecode
        std::vector<unsigned char> hash_out(picosha2::k_digest_size);
        picosha2::hash256(m_bytecode.begin() + header_size, m_bytecode.end(), hash_out);
        m_bytecode.insert(m_bytecode.begin() + header_size, hash_out.begin(), hash_out.end());
    }
 
    const bytecode_t& Compiler::bytecode() const noexcept
    {
        return m_bytecode;
    }
 
    void Compiler::pushFileHeader() noexcept
    {
        /*
            Generating headers:
                - lang name (to be sure we are executing an ArkScript file)
                    on 4 bytes (ark + padding)
                - version (major: 2 bytes, minor: 2 bytes, patch: 2 bytes)
                - timestamp (8 bytes, unix format)
        */
 
        m_bytecode.push_back('a');
        m_bytecode.push_back('r');
        m_bytecode.push_back('k');
        m_bytecode.push_back(0_u8);
 
        // push version
        for (const int n : std::array { ARK_VERSION_MAJOR, ARK_VERSION_MINOR, ARK_VERSION_PATCH })
        {
            m_bytecode.push_back(static_cast<uint16_t>((n & 0xff00) >> 8));
            m_bytecode.push_back(static_cast<uint16_t>(n & 0x00ff));
        }
 
        // push timestamp
        const unsigned long long timestamp = std::chrono::duration_cast<std::chrono::seconds>(
                                                 std::chrono::system_clock::now().time_since_epoch())
                                                 .count();
        for (std::size_t i = 0; i < 8; ++i)
        {
            const unsigned shift = 8 * (7 - i);
            uint8_t ts_byte = (timestamp & (0xffULL << shift)) >> shift;
            m_bytecode.push_back(ts_byte);
        }
    }
 
    void Compiler::pushSymAndValTables()
    {
        const std::size_t symbol_size = m_symbols.size();
        if (symbol_size > std::numeric_limits<uint16_t>::max())
            throw std::overflow_error("Too many symbols: " + std::to_string(symbol_size) + ", exceeds the maximum size of 2^16 - 1");
 
        m_bytecode.push_back(SYM_TABLE_START);
        m_bytecode.push_back(static_cast<uint16_t>((symbol_size & 0xff00) >> 8));
        m_bytecode.push_back(static_cast<uint16_t>(symbol_size & 0x00ff));
 
        for (const auto& sym : m_symbols)
        {
            // push the string, null terminated
            std::string s = sym.string();
            for (const char i : s)
                m_bytecode.push_back(i);
            m_bytecode.push_back(0_u8);
        }
 
        const std::size_t value_size = m_values.size();
        if (value_size > std::numeric_limits<uint16_t>::max())
            throw std::overflow_error("Too many values: " + std::to_string(value_size) + ", exceeds the maximum size of 2^16 - 1");
 
        m_bytecode.push_back(VAL_TABLE_START);
        m_bytecode.push_back(static_cast<uint16_t>((value_size & 0xff00) >> 8));
        m_bytecode.push_back(static_cast<uint16_t>(value_size & 0x00ff));
 
        for (const ValTableElem& val : m_values)
        {
            if (val.type == ValTableElemType::Number)
            {
                m_bytecode.push_back(NUMBER_TYPE);
                const auto n = std::get<double>(val.value);
                std::string t = std::to_string(n);
                for (const char i : t)
                    m_bytecode.push_back(i);
            }
            else if (val.type == ValTableElemType::String)
            {
                m_bytecode.push_back(STRING_TYPE);
                auto t = std::get<std::string>(val.value);
                for (const char i : t)
                    m_bytecode.push_back(i);
            }
            else if (val.type == ValTableElemType::PageAddr)
            {
                m_bytecode.push_back(FUNC_TYPE);
                const std::size_t addr = std::get<std::size_t>(val.value);
                m_bytecode.push_back(static_cast<uint16_t>((addr & 0xff00) >> 8));
                m_bytecode.push_back(static_cast<uint16_t>(addr & 0x00ff));
            }
            else
                throw Error("The compiler is trying to put a value in the value table, but the type isn't handled.\nCertainly a logic problem in the compiler source code");
 
            m_bytecode.push_back(0_u8);
        }
    }
 
    std::optional<std::size_t> Compiler::getOperator(const std::string& name) noexcept
    {
        const auto it = std::ranges::find(internal::operators, name);
        if (it != internal::operators.end())
            return std::distance(internal::operators.begin(), it);
        return std::nullopt;
    }
 
    std::optional<std::size_t> Compiler::getBuiltin(const std::string& name) noexcept
    {
        const auto it = std::ranges::find_if(Builtins::builtins,
                                             [&name](const std::pair<std::string, Value>& element) -> bool {
                                                 return name == element.first;
                                             });
        if (it != Builtins::builtins.end())
            return std::distance(Builtins::builtins.begin(), it);
        return std::nullopt;
    }
 
    bool Compiler::isUnaryInst(const Instruction inst) noexcept
    {
        switch (inst)
        {
            case NOT: [[fallthrough]];
            case LEN: [[fallthrough]];
            case EMPTY: [[fallthrough]];
            case TAIL: [[fallthrough]];
            case HEAD: [[fallthrough]];
            case ISNIL: [[fallthrough]];
            case TO_NUM: [[fallthrough]];
            case TO_STR: [[fallthrough]];
            case TYPE: [[fallthrough]];
            case HASFIELD:
                return true;
 
            default:
                return false;
        }
    }
 
    uint16_t Compiler::computeSpecificInstArgc(const Instruction inst, const uint16_t previous) noexcept
    {
        switch (inst)
        {
            case LIST:
                return previous;
 
            case APPEND:
            case APPEND_IN_PLACE:
            case CONCAT:
            case CONCAT_IN_PLACE:
                return previous - 1;
 
            default:
                return 0;
        }
    }
 
    bool Compiler::mayBeFromPlugin(const std::string& name) noexcept
    {
        std::string splitted = Utils::splitString(name, ':')[0];
        const auto it = std::ranges::find_if(m_plugins,
                                             [&splitted](const std::string& plugin) -> bool {
                                                 return std::filesystem::path(plugin).stem().string() == splitted;
                                             });
        return it != m_plugins.end();
    }
 
    void Compiler::compilerWarning(const std::string& message, const Node& node)
    {
        std::cout << termcolor::yellow << "Warning " << termcolor::reset << Diagnostics::makeContextWithNode(message, node) << "\n";
    }
 
    void Compiler::throwCompilerError(const std::string& message, const Node& node)
    {
        throw CodeError(message, node.filename(), node.line(), node.col(), node.repr());
    }
 
    void Compiler::compileExpression(const Node& x, const int p, const bool is_result_unused, const bool is_terminal, const std::string& var_name)
    {
        // register symbols
        if (x.nodeType() == NodeType::Symbol)
            compileSymbol(x, p, is_result_unused);
        else if (x.nodeType() == NodeType::Field)
        {
            // the parser guarantees us that there is at least 2 elements (eg: a.b)
            compileSymbol(x.constList()[0], p, is_result_unused);
            for (auto it = x.constList().begin() + 1, end = x.constList().end(); it != end; ++it)
            {
                uint16_t i = addSymbol(*it);
                page(p).emplace_back(GET_FIELD, i);
            }
        }
        // register values
        else if (x.nodeType() == NodeType::String || x.nodeType() == NodeType::Number)
        {
            uint16_t i = addValue(x);
 
            if (!is_result_unused)
                page(p).emplace_back(LOAD_CONST, i);
        }
        // empty code block should be nil
        else if (x.constList().empty())
        {
            if (!is_result_unused)
            {
                static const std::optional<std::size_t> nil = getBuiltin("nil");
                page(p).emplace_back(BUILTIN, static_cast<uint16_t>(nil.value()));
            }
        }
        // specific instructions
        else if (const auto c0 = x.constList()[0]; c0.nodeType() == NodeType::Symbol && getSpecific(c0.string()).has_value())
            compileSpecific(c0, x, p, is_result_unused);
        // registering structures
        else if (x.constList()[0].nodeType() == NodeType::Keyword)
        {
            switch (const Keyword keyword = x.constList()[0].keyword())
            {
                case Keyword::If:
                    compileIf(x, p, is_result_unused, is_terminal, var_name);
                    break;
 
                case Keyword::Set:
                    [[fallthrough]];
                case Keyword::Let:
                    [[fallthrough]];
                case Keyword::Mut:
                    compileLetMutSet(keyword, x, p);
                    break;
 
                case Keyword::Fun:
                    compileFunction(x, p, is_result_unused, var_name);
                    break;
 
                case Keyword::Begin:
                {
                    for (std::size_t i = 1, size = x.constList().size(); i < size; ++i)
                        compileExpression(
                            x.constList()[i],
                            p,
                            // All the nodes in a begin (except for the last one) are producing a result that we want to drop.
                            (i != size - 1) || is_result_unused,
                            // If the begin is a terminal node, only its last node is terminal.
                            is_terminal && (i == size - 1),
                            var_name);
                    break;
                }
 
                case Keyword::While:
                    compileWhile(x, p);
                    break;
 
                case Keyword::Import:
                    compilePluginImport(x, p);
                    break;
 
                case Keyword::Del:
                    page(p).emplace_back(DEL, addSymbol(x.constList()[1]));
                    break;
            }
        }
        else
        {
            // if we are here, we should have a function name
            // push arguments first, then function name, then call it
            handleCalls(x, p, is_result_unused, is_terminal, var_name);
        }
    }
 
    void Compiler::compileSymbol(const Node& x, const int p, const bool is_result_unused)
    {
        const std::string& name = x.string();
 
        if (const auto it_builtin = getBuiltin(name))
            page(p).emplace_back(Instruction::BUILTIN, static_cast<uint16_t>(it_builtin.value()));
        else if (const auto it_operator = getOperator(name))
            page(p).emplace_back(static_cast<uint8_t>(FIRST_OPERATOR + it_operator.value()));
        else
            page(p).emplace_back(LOAD_SYMBOL, addSymbol(x));  // using the variable
 
        if (is_result_unused)
        {
            compilerWarning("Statement has no effect", x);
            page(p).emplace_back(POP);
        }
    }
 
    void Compiler::compileSpecific(const Node& c0, const Node& x, const int p, const bool is_result_unused)
    {
        std::string name = c0.string();
        Instruction inst = getSpecific(name).value();
 
        // length of at least 1 since we got a symbol name
        const uint16_t argc = x.constList().size() - 1;
        // error, can not use append/concat/pop (and their in place versions) with a <2 length argument list
        if (argc < 2 && inst != LIST)
            throwCompilerError(fmt::format("Can not use {} with less than 2 arguments", name), c0);
 
        // compile arguments in reverse order
        for (uint16_t i = x.constList().size() - 1; i > 0; --i)
            compileExpression(x.constList()[i], p, false, false);
 
        // put inst and number of arguments
        page(p).emplace_back(inst, computeSpecificInstArgc(inst, argc));
 
        if (is_result_unused && name.back() != '!')  // in-place functions never push a value
        {
            compilerWarning("Ignoring return value of function", x);
            page(p).emplace_back(POP);
        }
    }
 
    void Compiler::compileIf(const Node& x, const int p, const bool is_result_unused, const bool is_terminal, const std::string& var_name)
    {
        // compile condition
        compileExpression(x.constList()[1], p, false, false);
 
        // jump only if needed to the if
        const std::size_t jump_to_if_pos = page(p).size();
        page(p).emplace_back(Instruction::POP_JUMP_IF_TRUE);
 
        // else code
        if (x.constList().size() == 4)  // we have an else clause
            compileExpression(x.constList()[3], p, is_result_unused, is_terminal, var_name);
 
        // when else is finished, jump to end
        const std::size_t jump_to_end_pos = page(p).size();
        page(p).emplace_back(Instruction::JUMP);
 
        // absolute address to jump to if condition is true
        page(p)[jump_to_if_pos].data = static_cast<uint16_t>(page(p).size());
        // if code
        compileExpression(x.constList()[2], p, is_result_unused, is_terminal, var_name);
        // set jump to end pos
        page(p)[jump_to_end_pos].data = static_cast<uint16_t>(page(p).size());
    }
 
    void Compiler::compileFunction(const Node& x, const int p, const bool is_result_unused, const std::string& var_name)
    {
        // capture, if needed
        for (const auto& node : x.constList()[1].constList())
        {
            if (node.nodeType() == NodeType::Capture)
            {
                // first check that the capture is a defined symbol
                if (std::ranges::find(m_defined_symbols, node.string()) == m_defined_symbols.end())
                {
                    // we didn't find node in the defined symbol list, thus we can't capture node
                    throwCompilerError("Can not capture " + node.string() + " because node is referencing an unbound variable.", node);
                }
 
                addDefinedSymbol(node.string());
                page(p).emplace_back(CAPTURE, addSymbol(node));
            }
        }
 
        // create new page for function body
        m_code_pages.emplace_back();
        const std::size_t page_id = m_code_pages.size() - 1;
        // save page_id into the constants table as PageAddr and load the const
        page(p).emplace_back(LOAD_CONST, addValue(page_id, x));
 
        // pushing arguments from the stack into variables in the new scope
        for (const auto& node : x.constList()[1].constList())
        {
            if (node.nodeType() == NodeType::Symbol)
            {
                addDefinedSymbol(node.string());
                page(page_id).emplace_back(MUT, addSymbol(node));
            }
        }
 
        // push body of the function
        compileExpression(x.constList()[2], page_id, false, true, var_name);
 
        // return last value on the stack
        page(page_id).emplace_back(RET);
 
        // if the computed function is unused, pop it
        if (is_result_unused)
        {
            compilerWarning("Unused declared function", x);
            page(p).emplace_back(POP);
        }
    }
 
    void Compiler::compileLetMutSet(const Keyword n, const Node& x, const int p)
    {
        const std::string name = x.constList()[1].string();
        uint16_t i = addSymbol(x.constList()[1]);
        if (n != Keyword::Set)
            addDefinedSymbol(name);
 
        // put value before symbol id
        // starting at index = 2 because x is a (let|mut|set variable ...) node
        for (std::size_t idx = 2, end = x.constList().size(); idx < end; ++idx)
            compileExpression(x.constList()[idx], p, false, false, name);
 
        if (n == Keyword::Let)
            page(p).emplace_back(LET, i);
        else if (n == Keyword::Mut)
            page(p).emplace_back(MUT, i);
        else
            page(p).emplace_back(STORE, i);
    }
 
    void Compiler::compileWhile(const Node& x, const int p)
    {
        // save current position to jump there at the end of the loop
        std::size_t current = page(p).size();
        // push condition
        compileExpression(x.constList()[1], p, false, false);
        // absolute jump to end of block if condition is false
        const std::size_t jump_to_end_pos = page(p).size();
        page(p).emplace_back(POP_JUMP_IF_FALSE);
        // push code to page
        compileExpression(x.constList()[2], p, true, false);
 
        // loop, jump to the condition
        page(p).emplace_back(JUMP, current);
 
        // absolute address to jump to if condition is false
        page(p)[jump_to_end_pos].data = static_cast<uint16_t>(page(p).size());
    }
 
    void Compiler::compilePluginImport(const Node& x, const int p)
    {
        std::string path;
        const Node package_node = x.constList()[1];
        for (std::size_t i = 0, end = package_node.constList().size(); i < end; ++i)
        {
            path += package_node.constList()[i].string();
            if (i + 1 != end)
                path += "/";
        }
        path += ".arkm";
 
        // register plugin path in the constants table
        uint16_t id = addValue(Node(NodeType::String, path));
        // save plugin name to use it later
        m_plugins.push_back(path);
        // add plugin instruction + id of the constant referring to the plugin path
        page(p).emplace_back(PLUGIN, id);
    }
 
    void Compiler::handleCalls(const Node& x, const int p, bool is_result_unused, const bool is_terminal, const std::string& var_name)
    {
        m_temp_pages.emplace_back();
        const int proc_page = -static_cast<int>(m_temp_pages.size());
        constexpr std::size_t start_index = 1;
 
        compileExpression(x.constList()[0], proc_page, false, false);  // storing proc
        // closure chains have been handled: closure.field.field.function
 
        // it's a builtin/function
        if (m_temp_pages.back()[0].opcode < FIRST_OPERATOR)
        {
            if (is_terminal && x.constList()[0].nodeType() == NodeType::Symbol && var_name == x.constList()[0].string())
            {
                // we can drop the temp page as we won't be using it
                m_temp_pages.pop_back();
 
                // push the arguments in reverse order
                for (std::size_t i = x.constList().size() - 1; i >= start_index; --i)
                    compileExpression(x.constList()[i], p, false, false);
 
                // jump to the top of the function
                page(p).emplace_back(JUMP, 0_u16);
                return;  // skip the possible Instruction::POP at the end
            }
            else
            {
                // push arguments on current page
                for (auto exp = x.constList().begin() + start_index, exp_end = x.constList().end(); exp != exp_end; ++exp)
                    compileExpression(*exp, p, false, false);
                // push proc from temp page
                for (const Word& word : m_temp_pages.back())
                    page(p).push_back(word);
                m_temp_pages.pop_back();
 
                // number of arguments
                std::size_t args_count = 0;
                for (auto it = x.constList().begin() + 1, it_end = x.constList().end(); it != it_end; ++it)
                {
                    if (it->nodeType() != NodeType::Capture)
                        args_count++;
                }
                // call the procedure
                page(p).emplace_back(CALL, args_count);
            }
        }
        else  // operator
        {
            // retrieve operator
            auto op = m_temp_pages.back()[0];
            m_temp_pages.pop_back();
 
            if (op.opcode == ASSERT)
                is_result_unused = false;
 
            // push arguments on current page
            std::size_t exp_count = 0;
            for (std::size_t index = start_index, size = x.constList().size(); index < size; ++index)
            {
                compileExpression(x.constList()[index], p, false, false);
 
                if ((index + 1 < size && x.constList()[index + 1].nodeType() != NodeType::Capture) || index + 1 == size)
                    exp_count++;
 
                // in order to be able to handle things like (op A B C D...)
                // which should be transformed into A B op C op D op...
                if (exp_count >= 2)
                    page(p).emplace_back(op.opcode, 2);  // TODO generalize to n arguments (n >= 2)
            }
 
            if (exp_count == 1)
            {
                if (isUnaryInst(static_cast<Instruction>(op.opcode)))
                    page(p).emplace_back(op.opcode);
                else
                    throwCompilerError("Operator needs two arguments, but was called with only one", x.constList()[0]);
            }
 
            // need to check we didn't push the (op A B C D...) things for operators not supporting it
            if (exp_count > 2)
            {
                switch (op.opcode)
                {
                    // authorized instructions
                    case ADD: [[fallthrough]];
                    case SUB: [[fallthrough]];
                    case MUL: [[fallthrough]];
                    case DIV: [[fallthrough]];
                    case AND_: [[fallthrough]];
                    case OR_: [[fallthrough]];
                    case MOD:
                        break;
 
                    default:
                        throwCompilerError(
                            "can not create a chained expression (of length " + std::to_string(exp_count) +
                                ") for operator `" + std::string(operators[static_cast<std::size_t>(op.opcode - FIRST_OPERATOR)]) +
                                "'. You most likely forgot a `)'.",
                            x);
                }
            }
        }
 
        if (is_result_unused)
            page(p).emplace_back(POP);
    }
 
    uint16_t Compiler::addSymbol(const Node& sym)
    {
        // otherwise, add the symbol, and return its id in the table
        auto it = std::ranges::find_if(m_symbols, [&sym](const Node& sym_node) -> bool {
            return sym_node.string() == sym.string();
        });
        if (it == m_symbols.end())
        {
            m_symbols.push_back(sym);
            it = m_symbols.begin() + m_symbols.size() - 1;
        }
 
        const auto distance = std::distance(m_symbols.begin(), it);
        if (distance < std::numeric_limits<uint16_t>::max())
            return static_cast<uint16_t>(distance);
        throwCompilerError("Too many symbols (exceeds 65'536), aborting compilation.", sym);
    }
 
    uint16_t Compiler::addValue(const Node& x)
    {
        const ValTableElem v(x);
        auto it = std::ranges::find(m_values, v);
        if (it == m_values.end())
        {
            m_values.push_back(v);
            it = m_values.begin() + m_values.size() - 1;
        }
 
        const auto distance = std::distance(m_values.begin(), it);
        if (distance < std::numeric_limits<uint16_t>::max())
            return static_cast<uint16_t>(distance);
        throwCompilerError("Too many values (exceeds 65'536), aborting compilation.", x);
    }
 
    uint16_t Compiler::addValue(const std::size_t page_id, const Node& current)
    {
        const ValTableElem v(page_id);
        auto it = std::ranges::find(m_values, v);
        if (it == m_values.end())
        {
            m_values.push_back(v);
            it = m_values.begin() + m_values.size() - 1;
        }
 
        const auto distance = std::distance(m_values.begin(), it);
        if (distance < std::numeric_limits<uint16_t>::max())
            return static_cast<uint16_t>(distance);
        throwCompilerError("Too many values (exceeds 65'536), aborting compilation.", current);
    }
 
    void Compiler::addDefinedSymbol(const std::string& sym)
    {
        // otherwise, add the symbol, and return its id in the table
        if (std::ranges::find(m_defined_symbols, sym) == m_defined_symbols.end())
            m_defined_symbols.push_back(sym);
    }
 
    void Compiler::checkForUndefinedSymbol()
    {
        for (const Node& sym : m_symbols)
        {
            const std::string& str = sym.string();
            const bool is_plugin = mayBeFromPlugin(str);
 
            if (auto it = std::ranges::find(m_defined_symbols, str); it == m_defined_symbols.end() && !is_plugin)
            {
                const std::string suggestion = offerSuggestion(str);
                if (suggestion.empty())
                    throwCompilerError("Unbound variable error \"" + str + "\" (variable is used but not defined)", sym);
 
                throwCompilerError("Unbound variable error \"" + str + "\" (did you mean \"" + suggestion + "\"?)", sym);
            }
        }
    }
 
    std::string Compiler::offerSuggestion(const std::string& str) const
    {
        std::string suggestion;
        // our suggestion shouldn't require more than half the string to change
        std::size_t suggestion_distance = str.size() / 2;
 
        for (const std::string& symbol : m_defined_symbols)
        {
            const std::size_t current_distance = Utils::levenshteinDistance(str, symbol);
            if (current_distance <= suggestion_distance)
            {
                suggestion_distance = current_distance;
                suggestion = symbol;
            }
        }
 
        return suggestion;
    }
}