diff --git a/src/compiler.cpp b/src/compiler.cpp
new file mode 100644
index 0000000..bafb2d0
--- /dev/null
+++ b/src/compiler.cpp
@@ -0,0 +1,390 @@
+#include "compiler.hpp"
+#include <cctype>
+#include <algorithm>
+#include <sstream>
+
+function_call::function_call(operation f, const std::string& s) : func {f}, arg {s} {}
+
+ref::ref(const std::string& n, size_t i, size_t l) : name {n}, prog_index {i}, lineno {l} {}
+
+compiler::compiler() {
+    // register operations
+    _ops["add"] = add;
+    _ops["sub"] = sub;
+    _ops["mul"] = mul;
+    _ops["div"] = div;
+    _ops["mod"] = mod;
+    _ops["abs"] = abs;
+    _ops["neg"] = neg;
+    _ops["dup"] = dup;
+    _ops["inc"] = inc;
+    _ops["dec"] = dec;
+    _ops["rev"] = rev;
+    _ops["slc"] = slc;
+    _ops["idx"] = idx;
+    _ops["cat"] = cat;
+    _ops["len"] = len;
+    _ops["rot"] = rot;
+    _ops["gto"] = gto;
+    _ops["geq"] = geq;
+    _ops["gne"] = gne;
+    _ops["glt"] = glt;
+    _ops["gle"] = gle;
+    _ops["ggt"] = ggt;
+    _ops["gge"] = gge;
+    _ops["fun"] = fun;
+    _ops["ret"] = ret;
+    _ops["enl"] = enl;
+    _ops["inp"] = inp;
+    _ops["out"] = out;
+    _ops["err"] = err;
+    _ops["end"] = end;
+}
+
+void compiler::reset() {
+    _prog.clear();
+    _values.clear();
+    _labels.clear();
+    _label_refs.clear();
+    _vars.clear();
+    _var_refs.clear();
+    _calls.clear();
+    _pc = 0;
+    _done = false;
+}
+
+std::string compiler::eval(std::istream& in, bool& done) {
+    std::string result;
+    reset();
+
+    // compile, phase 1
+    size_t lineno = 1;
+    for (std::string line; std::getline(in, line); ++lineno) {
+        std::string val {line.substr(1)};
+        if (line[0] == ':') { // label definition
+            _labels.emplace(val, _prog.size());
+        } else if (line[0] == '#') { // comment
+            continue;
+        } else if (std::isdigit(line[0])) { // literal int
+            _prog.emplace_back(push_literal, line);
+        } else if (line[0] == '\\') { // literal string
+            _prog.emplace_back(push_literal, val);
+        } else if (line[0] == '>') { // label ref
+            _label_refs.emplace_back(val, _prog.size(), lineno);
+            _prog.emplace_back(push_literal, "");
+        } else if (line[0] == '=') { // var assignment
+            _vars[val] = "";
+            _prog.emplace_back(assign, val);
+        } else if (line[0] == '$') { // var ref
+            _var_refs.emplace_back(val, _prog.size(), lineno);
+            _prog.emplace_back(push_variable, val);
+        } else { // operation
+            auto it = _ops.find(line);
+            if (it != _ops.end()) {
+                _prog.emplace_back(it->second, "");
+            } else {
+                throw syntax_error {"undefined operation", lineno};
+            }
+        }
+    }
+
+    // compile, phase 2: resolve references
+    for (const ref& lr : _label_refs) {
+        auto it = _labels.find(lr.name);
+        if (it != _labels.end()) {
+            _prog[lr.prog_index].arg = std::to_string(it->second);
+        } else {
+            throw syntax_error {"undefined label", lr.lineno};
+        }
+    }
+    for (const ref& vr : _var_refs) {
+        auto it = _vars.find(vr.name);
+        if (it == _vars.end()) {
+            throw syntax_error {"undefined variable", vr.lineno};
+        }
+    }
+
+    // run
+    while (_pc < _prog.size() && !_done) {
+        function_call info {_prog[_pc]}; // fetch next operation
+        (*info.func)(*this, info.arg); // call operation
+    }
+    done = _done;
+
+    if (_values.size()) {
+        result = _values.back();
+    }
+    return result;
+}
+
+inline int compiler::pop_int() {
+    std::istringstream iss {pop_str()};
+    int result;
+    iss >> result;
+    return result;
+}
+
+inline std::string compiler::pop_str() {
+    auto val = _values.back();
+    _values.pop_back();
+    return val;
+}
+
+// stack operations -------------------------------------------------------
+
+void compiler::push_literal(compiler& cmp, const std::string& val) {
+    cmp._values.push_back(val);
+    cmp._pc++;
+}
+
+void compiler::assign(compiler& cmp, const std::string& varname) {
+    cmp._vars[varname] = cmp._values.back();
+    cmp._values.pop_back();
+    cmp._pc++;
+}
+
+void compiler::push_variable(compiler& cmp, const std::string& varname) {
+    cmp._values.push_back(cmp._vars[varname]);
+    cmp._pc++;
+}
+
+// integer operations -----------------------------------------------------
+
+void compiler::add(compiler& cmp, const std::string&) {
+    auto val2 = cmp.pop_int();
+    auto val1 = cmp.pop_int();
+    cmp._values.push_back(std::to_string(val1 + val2));
+    cmp._pc++;
+}
+
+void compiler::sub(compiler& cmp, const std::string&) {
+    auto val2 = cmp.pop_int();
+    auto val1 = cmp.pop_int();
+    cmp._values.push_back(std::to_string(val1 - val2));
+    cmp._pc++;
+}
+
+void compiler::mul(compiler& cmp, const std::string&) {
+    auto val2 = cmp.pop_int();
+    auto val1 = cmp.pop_int();
+    cmp._values.push_back(std::to_string(val1 * val2));
+    cmp._pc++;
+}
+
+void compiler::div(compiler& cmp, const std::string&) {
+    auto val2 = cmp.pop_int();
+    auto val1 = cmp.pop_int();
+    cmp._values.push_back(std::to_string(val1 / val2));
+    cmp._pc++;
+}
+
+void compiler::mod(compiler& cmp, const std::string&) {
+    auto val2 = cmp.pop_int();
+    auto val1 = cmp.pop_int();
+    cmp._values.push_back(std::to_string(val1 % val2));
+    cmp._pc++;
+}
+
+void compiler::abs(compiler& cmp, const std::string&) {
+    auto val = cmp.pop_int();
+    cmp._values.push_back(std::to_string(val < 0 ? -val : val));
+    cmp._pc++;
+}
+
+void compiler::neg(compiler& cmp, const std::string&) {
+    auto val = cmp.pop_int();
+    cmp._values.push_back(std::to_string(-val));
+    cmp._pc++;
+}
+
+void compiler::inc(compiler& cmp, const std::string&) {
+    auto val = cmp.pop_int();
+    cmp._values.push_back(std::to_string(val + 1));
+    cmp._pc++;
+}
+
+void compiler::dec(compiler& cmp, const std::string&) {
+    auto val = cmp.pop_int();
+    cmp._values.push_back(std::to_string(val - 1));
+    cmp._pc++;
+}
+
+// string operations ------------------------------------------------------
+
+void compiler::dup(compiler& cmp, const std::string&) {
+    cmp._values.push_back(cmp._values.back());
+    cmp._pc++;
+}
+
+void compiler::rev(compiler& cmp, const std::string&) {
+    auto val = cmp.pop_str();
+    std::string result;
+    for (auto it = val.rbegin(); it != val.rend(); ++it) {
+        result += *it;
+    }
+    cmp._values.push_back(result);
+    cmp._pc++;
+}
+
+void compiler::slc(compiler& cmp, const std::string&) {
+    auto to = cmp.pop_int();
+    auto from = cmp.pop_int();
+    auto val = cmp.pop_str();
+    cmp._values.push_back(val.substr(from, to - from));
+    cmp._pc++;
+}
+
+void compiler::idx(compiler& cmp, const std::string&) {
+    auto idx = cmp.pop_int();
+    auto val = cmp.pop_str();
+    val = val[idx];
+    cmp._values.emplace_back(val);
+    cmp._pc++;
+}
+
+void compiler::cat(compiler& cmp, const std::string&) {
+    auto val2 = cmp.pop_str();
+    auto val1 = cmp.pop_str();
+    cmp._values.emplace_back(val1 + val2);
+    cmp._pc++;
+}
+
+void compiler::len(compiler& cmp, const std::string&) {
+    auto val = cmp.pop_str();
+    cmp._values.push_back(std::to_string(val.size()));
+    cmp._pc++;
+}
+
+void compiler::rot(compiler& cmp, const std::string&) {
+    auto val = cmp.pop_str();
+    std::transform(val.begin(), val.end(), val.begin(), [](char ch) -> char {
+        if (ch >= 'a' && ch <= 'm') {
+            return ch + 13;
+        } else if (ch >= 'n' && ch <= 'z') {
+            return ch - 13;
+        } else if (ch >= 'A' && ch <= 'M') {
+            return ch + 13;
+        } else if (ch >= 'N' && ch <= 'Z') {
+            return ch - 13;
+        }
+        return ch;
+    });
+    cmp._values.push_back(val);
+    cmp._pc++;
+}
+
+void compiler::enl(compiler& cmp, const std::string&) {
+    auto val = cmp.pop_str();
+    cmp._values.emplace_back(val + '\n');
+    cmp._pc++;
+}
+
+// tests & jumps ----------------------------------------------------------
+
+void compiler::gto(compiler& cmp, const std::string&) {
+    cmp._pc = cmp.pop_int();
+}
+
+void compiler::geq(compiler& cmp, const std::string&) {
+    auto label = cmp.pop_int();
+    auto val2 = cmp.pop_str();
+    auto val1 = cmp.pop_str();
+    if (val1 == val2) {
+        cmp._pc = label;
+    } else {
+        cmp._pc++;
+    }
+}
+
+void compiler::gne(compiler& cmp, const std::string&) {
+    auto label = cmp.pop_int();
+    auto val2 = cmp.pop_str();
+    auto val1 = cmp.pop_str();
+    if (val1 != val2) {
+        cmp._pc = label;
+    } else {
+        cmp._pc++;
+    }
+}
+
+void compiler::glt(compiler& cmp, const std::string&) {
+    auto label = cmp.pop_int();
+    auto val2 = cmp.pop_int();
+    auto val1 = cmp.pop_int();
+    if (val1 < val2) {
+        cmp._pc = label;
+    } else {
+        cmp._pc++;
+    }
+}
+
+void compiler::gle(compiler& cmp, const std::string&) {
+    auto label = cmp.pop_int();
+    auto val2 = cmp.pop_int();
+    auto val1 = cmp.pop_int();
+    if (val1 <= val2) {
+        cmp._pc = label;
+    } else {
+        cmp._pc++;
+    }
+}
+
+void compiler::ggt(compiler& cmp, const std::string&) {
+    auto label = cmp.pop_int();
+    auto val2 = cmp.pop_int();
+    auto val1 = cmp.pop_int();
+    if (val1 > val2) {
+        cmp._pc = label;
+    } else {
+        cmp._pc++;
+    }
+}
+
+void compiler::gge(compiler& cmp, const std::string&) {
+    auto label = cmp.pop_int();
+    auto val2 = cmp.pop_int();
+    auto val1 = cmp.pop_int();
+    if (val1 >= val2) {
+        cmp._pc = label;
+    } else {
+        cmp._pc++;
+    }
+}
+
+// subroutines ------------------------------------------------------------
+
+void compiler::fun(compiler& cmp, const std::string&) {
+    cmp._calls.push_back(cmp._pc + 1); // push return address
+    cmp.gto(cmp, "");
+}
+
+void compiler::ret(compiler& cmp, const std::string&) {
+    cmp._pc = cmp._calls.back();
+    cmp._calls.pop_back();
+}
+
+// I/O --------------------------------------------------------------------
+
+void compiler::inp(compiler& cmp, const std::string&) {
+    std::string val;
+    std::cin >> val;
+    cmp._values.push_back(val);
+    cmp._pc++;
+}
+
+void compiler::out(compiler& cmp, const std::string&) {
+    std::cout << cmp._values.back() << '\n';
+    cmp._pc++;
+}
+
+void compiler::err(compiler& cmp, const std::string&) {
+    std::cerr << cmp._values.back() << '\n';
+    cmp._pc++;
+}
+
+// exit -------------------------------------------------------------------
+
+void compiler::end(compiler& cmp, const std::string&) {
+    cmp._done = true;
+}
+
diff --git a/src/compiler.hpp b/src/compiler.hpp
new file mode 100644
index 0000000..3c68040
--- /dev/null
+++ b/src/compiler.hpp
@@ -0,0 +1,109 @@
+#ifndef _compiler_H_
+#define _compiler_H_
+
+#include <iostream>
+#include <string>
+#include <vector>
+#include <map>
+#include <stdexcept>
+#include <set>
+
+class syntax_error : public std::runtime_error {
+public:
+    syntax_error(const std::string& what_arg, size_t lineno)
+        : std::runtime_error {what_arg}, _lineno {lineno} {}
+    size_t lineno() const { return _lineno; }
+private:
+    size_t _lineno {0};
+};
+
+class compiler;
+
+using operation = void(*)(compiler&, const std::string&);
+struct function_call {
+    operation func;
+    std::string arg;
+
+    function_call(operation, const std::string&);
+};
+
+struct ref {
+    std::string name;
+    size_t prog_index;
+    size_t lineno;
+
+    ref(const std::string& n, size_t i, size_t l);
+};
+
+class compiler {
+public:
+    compiler();
+
+    std::string eval(std::istream& in, bool& done);
+
+private:
+    std::map<std::string, operation> _ops; // operation registry
+    std::vector<function_call> _prog; // compiled program
+    std::map<std::string, size_t> _labels; // label name => _prog index
+    std::vector<ref> _label_refs; // used during compilation to resolve label references
+    std::vector<std::string> _values; // value stack
+    std::vector<size_t> _calls; // call stack
+    std::map<std::string, std::string> _vars; // variable name => string value
+    std::vector<ref> _var_refs; // used during compilation to resolve variable references
+    size_t _pc {0}; // current program counter (index into _prog)
+    bool _done {false};
+
+    void reset();
+
+    int pop_int();
+    std::string pop_str();
+
+    // stack operations
+    static void push_literal(compiler&, const std::string& val);
+    static void assign(compiler&, const std::string& varname);
+    static void push_variable(compiler&, const std::string& varname);
+
+    // integer operations
+    static void add(compiler&, const std::string&);
+    static void sub(compiler&, const std::string&);
+    static void mul(compiler&, const std::string&);
+    static void div(compiler&, const std::string&);
+    static void mod(compiler&, const std::string&);
+    static void abs(compiler&, const std::string&);
+    static void neg(compiler&, const std::string&);
+    static void inc(compiler&, const std::string&);
+    static void dec(compiler&, const std::string&);
+
+    // string operations
+    static void dup(compiler&, const std::string&);
+    static void rev(compiler&, const std::string&);
+    static void slc(compiler&, const std::string&);
+    static void idx(compiler&, const std::string&);
+    static void cat(compiler&, const std::string&);
+    static void len(compiler&, const std::string&);
+    static void rot(compiler&, const std::string&);
+    static void enl(compiler&, const std::string&);
+
+    // tests & jumps
+    static void gto(compiler&, const std::string&);
+    static void geq(compiler&, const std::string&);
+    static void gne(compiler&, const std::string&);
+    static void glt(compiler&, const std::string&);
+    static void gle(compiler&, const std::string&);
+    static void ggt(compiler&, const std::string&);
+    static void gge(compiler&, const std::string&);
+
+    // subroutines
+    static void fun(compiler&, const std::string&);
+    static void ret(compiler&, const std::string&);
+
+    // I/O
+    static void inp(compiler&, const std::string&);
+    static void out(compiler&, const std::string&);
+    static void err(compiler&, const std::string&);
+
+    // exit
+    static void end(compiler&, const std::string&);
+};
+
+#endif // _compiler_H_