diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1f1a2ce..2ee01c6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,7 +1,9 @@
 cmake_minimum_required(VERSION 3.13)
 
+add_subdirectory(util)
 add_subdirectory(w6300)
 add_subdirectory(debug_log)
+add_subdirectory(msgpack)
 
 add_library(limen STATIC
     src/arp.cpp
@@ -23,8 +25,10 @@ target_include_directories(limen PUBLIC
 target_compile_options(limen PRIVATE -Wall -Wextra -Wno-unused-parameter)
 
 target_link_libraries(limen PUBLIC
+    util
     w6300
     debug_log
+    msgpack
     pico_stdlib
     pico_sha256
     pico_unique_id
diff --git a/msgpack/CMakeLists.txt b/msgpack/CMakeLists.txt
new file mode 100644
index 0000000..280924e
--- /dev/null
+++ b/msgpack/CMakeLists.txt
@@ -0,0 +1,7 @@
+add_library(msgpack STATIC msgpack.cpp)
+
+target_include_directories(msgpack PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
+
+target_compile_options(msgpack PRIVATE -Wall -Wextra -Wno-unused-parameter)
+
+target_link_libraries(msgpack PUBLIC util)
diff --git a/msgpack/msgpack.cpp b/msgpack/msgpack.cpp
new file mode 100644
index 0000000..424a443
--- /dev/null
+++ b/msgpack/msgpack.cpp
@@ -0,0 +1,419 @@
+#include "msgpack.h"
+
+namespace msgpack {
+
+result<body_info> get_body_info(const uint8_t *p, int size) {
+  if (size < 1) return std::unexpected(error_code::empty);
+  uint8_t b = p[0];
+
+  using namespace format;
+
+  if (is_positive_fixint(b))  return body_info{1, 0};
+  if (is_negative_fixint(b))  return body_info{1, 0};
+  if (is_fixmap(b))           return body_info{1, 0};
+  if (is_fixarray(b))         return body_info{1, 0};
+  if (is_fixstr(b))           return body_info{1, static_cast<uint32_t>(b & 0x1F)};
+
+  switch (b) {
+  case NIL: case FALSE: case TRUE:
+    return body_info{1, 0};
+  case NEVER_USED:
+    return std::unexpected(error_code::invalid);
+
+  case BIN8:  { auto n = body_number<uint8_t>(p, size);  if (!n) return std::unexpected(n.error()); return body_info{1+1, *n}; }
+  case BIN16: { auto n = body_number<uint16_t>(p, size); if (!n) return std::unexpected(n.error()); return body_info{1+2, *n}; }
+  case BIN32: { auto n = body_number<uint32_t>(p, size); if (!n) return std::unexpected(n.error()); return body_info{1+4, *n}; }
+
+  case EXT8:  { auto n = body_number<uint8_t>(p, size);  if (!n) return std::unexpected(n.error()); return body_info{1+1+1, *n}; }
+  case EXT16: { auto n = body_number<uint16_t>(p, size); if (!n) return std::unexpected(n.error()); return body_info{1+2+1, *n}; }
+  case EXT32: { auto n = body_number<uint32_t>(p, size); if (!n) return std::unexpected(n.error()); return body_info{1+4+1, *n}; }
+
+  case FLOAT32: return body_info{1, 4};
+  case FLOAT64: return body_info{1, 8};
+  case UINT8:   return body_info{1, 1};
+  case UINT16:  return body_info{1, 2};
+  case UINT32:  return body_info{1, 4};
+  case UINT64:  return body_info{1, 8};
+  case INT8:    return body_info{1, 1};
+  case INT16:   return body_info{1, 2};
+  case INT32:   return body_info{1, 4};
+  case INT64:   return body_info{1, 8};
+
+  case FIXEXT1:  return body_info{1+1, 1};
+  case FIXEXT2:  return body_info{1+1, 2};
+  case FIXEXT4:  return body_info{1+1, 4};
+  case FIXEXT8:  return body_info{1+1, 8};
+  case FIXEXT16: return body_info{1+1, 16};
+
+  case STR8:  { auto n = body_number<uint8_t>(p, size);  if (!n) return std::unexpected(n.error()); return body_info{1+1, *n}; }
+  case STR16: { auto n = body_number<uint16_t>(p, size); if (!n) return std::unexpected(n.error()); return body_info{1+2, *n}; }
+  case STR32: { auto n = body_number<uint32_t>(p, size); if (!n) return std::unexpected(n.error()); return body_info{1+4, *n}; }
+
+  case ARRAY16: case ARRAY32:
+  case MAP16: case MAP32:
+    return body_info{1 + (b == ARRAY16 || b == MAP16 ? 2 : 4), 0};
+
+  default:
+    return std::unexpected(error_code::invalid);
+  }
+}
+
+packer::pack_result packer::pack_nil() {
+  m_buf.push_back(format::NIL);
+  return *this;
+}
+
+packer::pack_result packer::pack_bool(bool v) {
+  m_buf.push_back(v ? format::TRUE : format::FALSE);
+  return *this;
+}
+
+packer::pack_result packer::pack_uint32_fixed(uint32_t n) {
+  m_buf.push_back(format::UINT32);
+  push_big_endian(n);
+  return *this;
+}
+
+packer::pack_result packer::pack_float(float n) {
+  m_buf.push_back(format::FLOAT32);
+  push_big_endian(n);
+  return *this;
+}
+
+packer::pack_result packer::pack_double(double n) {
+  m_buf.push_back(format::FLOAT64);
+  push_big_endian(n);
+  return *this;
+}
+
+packer::pack_result packer::pack_str(const char *s) {
+  return pack_str(std::string_view(s));
+}
+
+packer::pack_result packer::pack_array(size_t n) {
+  if (n <= 15) {
+    m_buf.push_back(format::FIXARRAY_MIN | static_cast<uint8_t>(n));
+  } else if (n <= std::numeric_limits<uint16_t>::max()) {
+    m_buf.push_back(format::ARRAY16);
+    push_big_endian(static_cast<uint16_t>(n));
+  } else if (n <= std::numeric_limits<uint32_t>::max()) {
+    m_buf.push_back(format::ARRAY32);
+    push_big_endian(static_cast<uint32_t>(n));
+  } else {
+    return std::unexpected(error_code::overflow);
+  }
+  return *this;
+}
+
+packer::pack_result packer::pack_map(size_t n) {
+  if (n <= 15) {
+    m_buf.push_back(format::FIXMAP_MIN | static_cast<uint8_t>(n));
+  } else if (n <= std::numeric_limits<uint16_t>::max()) {
+    m_buf.push_back(format::MAP16);
+    push_big_endian(static_cast<uint16_t>(n));
+  } else if (n <= std::numeric_limits<uint32_t>::max()) {
+    m_buf.push_back(format::MAP32);
+    push_big_endian(static_cast<uint32_t>(n));
+  } else {
+    return std::unexpected(error_code::overflow);
+  }
+  return *this;
+}
+
+packer::pack_result packer::pack_ext16_header(char type, uint16_t len) {
+  m_buf.push_back(format::EXT16);
+  push_big_endian(len);
+  m_buf.push_back(static_cast<uint8_t>(type));
+  return *this;
+}
+
+packer::pack_result packer::pack_bin16_header(uint16_t len) {
+  m_buf.push_back(format::BIN16);
+  push_big_endian(len);
+  return *this;
+}
+
+packer::pack_result packer::pack(bool v)        { return pack_bool(v); }
+packer::pack_result packer::pack(float v)       { return pack_float(v); }
+packer::pack_result packer::pack(double v)      { return pack_double(v); }
+packer::pack_result packer::pack(const char *v) { return pack_str(v); }
+packer::pack_result packer::pack(std::string_view v) { return pack_str(v); }
+packer::pack_result packer::pack(const std::string &v) { return pack_str(v); }
+packer::pack_result packer::pack(const std::vector<uint8_t> &v) { return pack_bin(v); }
+
+result<parser> parser::next() const {
+  auto hdr = header_byte();
+  if (!hdr) return std::unexpected(hdr.error());
+
+  if (is_array()) {
+    auto info = get_body_info(m_p, m_size);
+    if (!info) return std::unexpected(info.error());
+    auto cnt = count();
+    if (!cnt) return std::unexpected(cnt.error());
+    auto cur = advance(info->header);
+    if (!cur) return std::unexpected(cur.error());
+    for (uint32_t i = 0; i < *cnt; ++i) {
+      auto n = cur->next();
+      if (!n) return std::unexpected(n.error());
+      cur = *n;
+    }
+    return *cur;
+  } else if (is_map()) {
+    auto info = get_body_info(m_p, m_size);
+    if (!info) return std::unexpected(info.error());
+    auto cnt = count();
+    if (!cnt) return std::unexpected(cnt.error());
+    auto cur = advance(info->header);
+    if (!cur) return std::unexpected(cur.error());
+    for (uint32_t i = 0; i < *cnt; ++i) {
+      auto k = cur->next();
+      if (!k) return std::unexpected(k.error());
+      cur = *k;
+      auto v = cur->next();
+      if (!v) return std::unexpected(v.error());
+      cur = *v;
+    }
+    return *cur;
+  } else {
+    auto info = get_body_info(m_p, m_size);
+    if (!info) return std::unexpected(info.error());
+    auto total = info->header + static_cast<int>(info->body);
+    return advance(total);
+  }
+}
+
+bool parser::is_nil() const {
+  auto h = header_byte();
+  return h && *h == format::NIL;
+}
+
+bool parser::is_bool() const {
+  auto h = header_byte();
+  return h && (*h == format::TRUE || *h == format::FALSE);
+}
+
+bool parser::is_number() const {
+  auto h = header_byte();
+  if (!h) return false;
+  uint8_t b = *h;
+  if (format::is_positive_fixint(b)) return true;
+  if (format::is_negative_fixint(b)) return true;
+  return b >= format::FLOAT32 && b <= format::INT64;
+}
+
+bool parser::is_string() const {
+  auto h = header_byte();
+  if (!h) return false;
+  uint8_t b = *h;
+  if (format::is_fixstr(b)) return true;
+  return b == format::STR8 || b == format::STR16 || b == format::STR32;
+}
+
+bool parser::is_binary() const {
+  auto h = header_byte();
+  if (!h) return false;
+  uint8_t b = *h;
+  return b == format::BIN8 || b == format::BIN16 || b == format::BIN32;
+}
+
+bool parser::is_ext() const {
+  auto h = header_byte();
+  if (!h) return false;
+  uint8_t b = *h;
+  return (b >= format::FIXEXT1 && b <= format::FIXEXT16) ||
+         b == format::EXT8 || b == format::EXT16 || b == format::EXT32;
+}
+
+bool parser::is_array() const {
+  auto h = header_byte();
+  if (!h) return false;
+  uint8_t b = *h;
+  if (format::is_fixarray(b)) return true;
+  return b == format::ARRAY16 || b == format::ARRAY32;
+}
+
+bool parser::is_map() const {
+  auto h = header_byte();
+  if (!h) return false;
+  uint8_t b = *h;
+  if (format::is_fixmap(b)) return true;
+  return b == format::MAP16 || b == format::MAP32;
+}
+
+result<bool> parser::get_bool() const {
+  auto h = header_byte();
+  if (!h) return std::unexpected(h.error());
+  if (*h == format::TRUE) return true;
+  if (*h == format::FALSE) return false;
+  return std::unexpected(error_code::type_error);
+}
+
+result<std::string_view> parser::get_string() const {
+  auto h = header_byte();
+  if (!h) return std::unexpected(h.error());
+  uint8_t b = *h;
+  size_t offset, len;
+  if (format::is_fixstr(b)) {
+    len = b & 0x1F;
+    offset = 1;
+  } else if (b == format::STR8) {
+    auto n = body_number<uint8_t>(m_p, m_size);
+    if (!n) return std::unexpected(n.error());
+    len = *n; offset = 1 + 1;
+  } else if (b == format::STR16) {
+    auto n = body_number<uint16_t>(m_p, m_size);
+    if (!n) return std::unexpected(n.error());
+    len = *n; offset = 1 + 2;
+  } else if (b == format::STR32) {
+    auto n = body_number<uint32_t>(m_p, m_size);
+    if (!n) return std::unexpected(n.error());
+    len = *n; offset = 1 + 4;
+  } else {
+    return std::unexpected(error_code::type_error);
+  }
+  if (static_cast<int>(offset + len) > m_size) {
+    return std::unexpected(error_code::lack);
+  }
+  return std::string_view(reinterpret_cast<const char *>(m_p + offset), len);
+}
+
+result<std::string_view> parser::get_binary_view() const {
+  auto h = header_byte();
+  if (!h) return std::unexpected(h.error());
+  uint8_t b = *h;
+  size_t offset, len;
+
+  if (b == format::BIN8) {
+    auto n = body_number<uint8_t>(m_p, m_size);
+    if (!n) return std::unexpected(n.error());
+    len = *n; offset = 1 + 1;
+  } else if (b == format::BIN16) {
+    auto n = body_number<uint16_t>(m_p, m_size);
+    if (!n) return std::unexpected(n.error());
+    len = *n; offset = 1 + 2;
+  } else if (b == format::BIN32) {
+    auto n = body_number<uint32_t>(m_p, m_size);
+    if (!n) return std::unexpected(n.error());
+    len = *n; offset = 1 + 4;
+  } else {
+    return std::unexpected(error_code::type_error);
+  }
+  if (static_cast<int>(offset + len) > m_size) {
+    return std::unexpected(error_code::lack);
+  }
+  return std::string_view(reinterpret_cast<const char *>(m_p + offset), len);
+}
+
+result<std::tuple<int8_t, std::string_view>> parser::get_ext() const {
+  auto h = header_byte();
+  if (!h) return std::unexpected(h.error());
+  uint8_t b = *h;
+  int8_t ext_type;
+  size_t data_offset, data_len;
+
+  switch (b) {
+  case format::FIXEXT1:  ext_type = m_p[1]; data_offset = 2; data_len = 1;  break;
+  case format::FIXEXT2:  ext_type = m_p[1]; data_offset = 2; data_len = 2;  break;
+  case format::FIXEXT4:  ext_type = m_p[1]; data_offset = 2; data_len = 4;  break;
+  case format::FIXEXT8:  ext_type = m_p[1]; data_offset = 2; data_len = 8;  break;
+  case format::FIXEXT16: ext_type = m_p[1]; data_offset = 2; data_len = 16; break;
+  case format::EXT8: {
+    auto n = body_number<uint8_t>(m_p, m_size);
+    if (!n) return std::unexpected(n.error());
+    ext_type = m_p[2]; data_offset = 3; data_len = *n;
+    break;
+  }
+  case format::EXT16: {
+    auto n = body_number<uint16_t>(m_p, m_size);
+    if (!n) return std::unexpected(n.error());
+    ext_type = m_p[3]; data_offset = 4; data_len = *n;
+    break;
+  }
+  case format::EXT32: {
+    auto n = body_number<uint32_t>(m_p, m_size);
+    if (!n) return std::unexpected(n.error());
+    ext_type = m_p[5]; data_offset = 6; data_len = *n;
+    break;
+  }
+  default:
+    return std::unexpected(error_code::type_error);
+  }
+  if (static_cast<int>(data_offset + data_len) > m_size) {
+    return std::unexpected(error_code::lack);
+  }
+  return std::tuple{ext_type,
+                    std::string_view(reinterpret_cast<const char *>(m_p + data_offset), data_len)};
+}
+
+result<uint32_t> parser::count() const {
+  auto h = header_byte();
+  if (!h) return std::unexpected(h.error());
+  uint8_t b = *h;
+
+  if (format::is_fixarray(b)) return static_cast<uint32_t>(b & 0x0F);
+  if (format::is_fixmap(b))   return static_cast<uint32_t>(b & 0x0F);
+
+  switch (b) {
+  case format::ARRAY16: { auto n = body_number<uint16_t>(m_p, m_size); if (!n) return std::unexpected(n.error()); return static_cast<uint32_t>(*n); }
+  case format::ARRAY32: { auto n = body_number<uint32_t>(m_p, m_size); if (!n) return std::unexpected(n.error()); return *n; }
+  case format::MAP16:   { auto n = body_number<uint16_t>(m_p, m_size); if (!n) return std::unexpected(n.error()); return static_cast<uint32_t>(*n); }
+  case format::MAP32:   { auto n = body_number<uint32_t>(m_p, m_size); if (!n) return std::unexpected(n.error()); return *n; }
+  default:
+    return std::unexpected(error_code::type_error);
+  }
+}
+
+result<parser> parser::first_item() const {
+  if (!is_array() && !is_map()) return std::unexpected(error_code::type_error);
+  auto info = get_body_info(m_p, m_size);
+  if (!info) return std::unexpected(info.error());
+  return advance(info->header);
+}
+
+parser parser::operator[](int index) const {
+  auto cur = first_item();
+  if (!cur) return {};
+  for (int i = 0; i < index; ++i) {
+    auto n = cur->next();
+    if (!n) return {};
+    cur = *n;
+  }
+  return *cur;
+}
+
+result<parser> unpack(const parser &p, bool &out) {
+  auto v = p.get_bool();
+  if (!v) return std::unexpected(v.error());
+  out = *v;
+  return p.next();
+}
+
+result<parser> unpack(const parser &p, std::string_view &out) {
+  auto v = p.get_string();
+  if (!v) return std::unexpected(v.error());
+  out = *v;
+  return p.next();
+}
+
+result<parser> unpack(const parser &p, std::string &out) {
+  auto v = p.get_string();
+  if (!v) return std::unexpected(v.error());
+  out = std::string(v->data(), v->size());
+  return p.next();
+}
+
+result<parser> unpack(const parser &p, std::vector<uint8_t> &out) {
+  auto v = p.get_binary_view();
+  if (!v) return std::unexpected(v.error());
+  out.assign(v->begin(), v->end());
+  return p.next();
+}
+
+result<parser> unpack(const parser &p, std::span<const uint8_t> &out) {
+  auto v = p.get_binary_view();
+  if (!v) return std::unexpected(v.error());
+  out = std::span<const uint8_t>(reinterpret_cast<const uint8_t*>(v->data()), v->size());
+  return p.next();
+}
+
+} // namespace msgpack
diff --git a/include/msgpack.h b/msgpack/msgpack.h
similarity index 51%
rename from include/msgpack.h
rename to msgpack/msgpack.h
index ae3e331..1a05491 100644
--- a/include/msgpack.h
+++ b/msgpack/msgpack.h
@@ -1,14 +1,16 @@
 #pragma once
 #include <algorithm>
 #include <array>
-#include <cassert>
 #include <cstdint>
 #include <expected>
 #include <iterator>
 #include <limits>
+#include <span>
+#include <string>
 #include <string_view>
 #include <tuple>
 #include <type_traits>
+#include <utility>
 #include <vector>
 #include "span_writer.h"
 
@@ -102,65 +104,11 @@ result<T> body_number(const uint8_t *p, int size) {
 }
 
 struct body_info {
-  int header;     // bytes before the body (includes format byte + length fields + ext type byte)
-  uint32_t body;  // body size in bytes (0 for containers, computed for variable-length)
+  int header;
+  uint32_t body;
 };
 
-inline result<body_info> get_body_info(const uint8_t *p, int size) {
-  if (size < 1) return std::unexpected(error_code::empty);
-  uint8_t b = p[0];
-
-  using namespace format;
-
-  if (is_positive_fixint(b))  return body_info{1, 0};
-  if (is_negative_fixint(b))  return body_info{1, 0};
-  if (is_fixmap(b))           return body_info{1, 0}; // container
-  if (is_fixarray(b))         return body_info{1, 0}; // container
-  if (is_fixstr(b))           return body_info{1, static_cast<uint32_t>(b & 0x1F)};
-
-  switch (b) {
-  case NIL: case FALSE: case TRUE:
-    return body_info{1, 0};
-  case NEVER_USED:
-    return std::unexpected(error_code::invalid);
-
-  case BIN8:  { auto n = body_number<uint8_t>(p, size);  if (!n) return std::unexpected(n.error()); return body_info{1+1, *n}; }
-  case BIN16: { auto n = body_number<uint16_t>(p, size); if (!n) return std::unexpected(n.error()); return body_info{1+2, *n}; }
-  case BIN32: { auto n = body_number<uint32_t>(p, size); if (!n) return std::unexpected(n.error()); return body_info{1+4, *n}; }
-
-  case EXT8:  { auto n = body_number<uint8_t>(p, size);  if (!n) return std::unexpected(n.error()); return body_info{1+1+1, *n}; }
-  case EXT16: { auto n = body_number<uint16_t>(p, size); if (!n) return std::unexpected(n.error()); return body_info{1+2+1, *n}; }
-  case EXT32: { auto n = body_number<uint32_t>(p, size); if (!n) return std::unexpected(n.error()); return body_info{1+4+1, *n}; }
-
-  case FLOAT32: return body_info{1, 4};
-  case FLOAT64: return body_info{1, 8};
-  case UINT8:   return body_info{1, 1};
-  case UINT16:  return body_info{1, 2};
-  case UINT32:  return body_info{1, 4};
-  case UINT64:  return body_info{1, 8};
-  case INT8:    return body_info{1, 1};
-  case INT16:   return body_info{1, 2};
-  case INT32:   return body_info{1, 4};
-  case INT64:   return body_info{1, 8};
-
-  case FIXEXT1:  return body_info{1+1, 1};
-  case FIXEXT2:  return body_info{1+1, 2};
-  case FIXEXT4:  return body_info{1+1, 4};
-  case FIXEXT8:  return body_info{1+1, 8};
-  case FIXEXT16: return body_info{1+1, 16};
-
-  case STR8:  { auto n = body_number<uint8_t>(p, size);  if (!n) return std::unexpected(n.error()); return body_info{1+1, *n}; }
-  case STR16: { auto n = body_number<uint16_t>(p, size); if (!n) return std::unexpected(n.error()); return body_info{1+2, *n}; }
-  case STR32: { auto n = body_number<uint32_t>(p, size); if (!n) return std::unexpected(n.error()); return body_info{1+4, *n}; }
-
-  case ARRAY16: case ARRAY32:
-  case MAP16: case MAP32:
-    return body_info{1 + (b == ARRAY16 || b == MAP16 ? 2 : 4), 0}; // container
-
-  default:
-    return std::unexpected(error_code::invalid);
-  }
-}
+result<body_info> get_body_info(const uint8_t *p, int size);
 
 class packer {
 private:
@@ -185,15 +133,8 @@ public:
 
   using pack_result = result<std::reference_wrapper<packer>>;
 
-  pack_result pack_nil() {
-    m_buf.push_back(format::NIL);
-    return *this;
-  }
-
-  pack_result pack_bool(bool v) {
-    m_buf.push_back(v ? format::TRUE : format::FALSE);
-    return *this;
-  }
+  pack_result pack_nil();
+  pack_result pack_bool(bool v);
 
   template <typename T>
   pack_result pack_integer(T n) {
@@ -201,7 +142,7 @@ public:
       if (n >= 0 && n <= 0x7F) {
         m_buf.push_back(static_cast<uint8_t>(n));
       } else if (n >= -32 && n < 0) {
-        m_buf.push_back(static_cast<uint8_t>(n));  // negative fixint
+        m_buf.push_back(static_cast<uint8_t>(n));
       } else if (n >= std::numeric_limits<int8_t>::min() && n <= std::numeric_limits<int8_t>::max()) {
         m_buf.push_back(format::INT8);
         m_buf.push_back(static_cast<uint8_t>(n));
@@ -235,23 +176,9 @@ public:
     return *this;
   }
 
-  pack_result pack_uint32_fixed(uint32_t n) {
-    m_buf.push_back(format::UINT32);
-    push_big_endian(n);
-    return *this;
-  }
-
-  pack_result pack_float(float n) {
-    m_buf.push_back(format::FLOAT32);
-    push_big_endian(n);
-    return *this;
-  }
-
-  pack_result pack_double(double n) {
-    m_buf.push_back(format::FLOAT64);
-    push_big_endian(n);
-    return *this;
-  }
+  pack_result pack_uint32_fixed(uint32_t n);
+  pack_result pack_float(float n);
+  pack_result pack_double(double n);
 
   template <class Range>
   pack_result pack_str(const Range &r) {
@@ -274,9 +201,7 @@ public:
     return *this;
   }
 
-  pack_result pack_str(const char *s) {
-    return pack_str(std::string_view(s));
-  }
+  pack_result pack_str(const char *s);
 
   template <class Range>
   pack_result pack_bin(const Range &r) {
@@ -297,48 +222,10 @@ public:
     return *this;
   }
 
-  pack_result pack_array(size_t n) {
-    if (n <= 15) {
-      m_buf.push_back(format::FIXARRAY_MIN | static_cast<uint8_t>(n));
-    } else if (n <= std::numeric_limits<uint16_t>::max()) {
-      m_buf.push_back(format::ARRAY16);
-      push_big_endian(static_cast<uint16_t>(n));
-    } else if (n <= std::numeric_limits<uint32_t>::max()) {
-      m_buf.push_back(format::ARRAY32);
-      push_big_endian(static_cast<uint32_t>(n));
-    } else {
-      return std::unexpected(error_code::overflow);
-    }
-    return *this;
-  }
-
-  pack_result pack_map(size_t n) {
-    if (n <= 15) {
-      m_buf.push_back(format::FIXMAP_MIN | static_cast<uint8_t>(n));
-    } else if (n <= std::numeric_limits<uint16_t>::max()) {
-      m_buf.push_back(format::MAP16);
-      push_big_endian(static_cast<uint16_t>(n));
-    } else if (n <= std::numeric_limits<uint32_t>::max()) {
-      m_buf.push_back(format::MAP32);
-      push_big_endian(static_cast<uint32_t>(n));
-    } else {
-      return std::unexpected(error_code::overflow);
-    }
-    return *this;
-  }
-
-  pack_result pack_ext16_header(char type, uint16_t len) {
-    m_buf.push_back(format::EXT16);
-    push_big_endian(len);
-    m_buf.push_back(static_cast<uint8_t>(type));
-    return *this;
-  }
-
-  pack_result pack_bin16_header(uint16_t len) {
-    m_buf.push_back(format::BIN16);
-    push_big_endian(len);
-    return *this;
-  }
+  pack_result pack_array(size_t n);
+  pack_result pack_map(size_t n);
+  pack_result pack_ext16_header(char type, uint16_t len);
+  pack_result pack_bin16_header(uint16_t len);
 
   template <class Range>
   pack_result pack_ext(char type, const Range &r) {
@@ -377,14 +264,13 @@ public:
     requires std::is_enum_v<T>
   pack_result pack(T v) { return pack_integer(static_cast<std::underlying_type_t<T>>(v)); }
 
-  pack_result pack(bool v)        { return pack_bool(v); }
-  pack_result pack(float v)       { return pack_float(v); }
-  pack_result pack(double v)      { return pack_double(v); }
-  pack_result pack(const char *v) { return pack_str(v); }
-  pack_result pack(std::string_view v) { return pack_str(v); }
-  pack_result pack(const std::string &v) { return pack_str(v); }
-
-  pack_result pack(const std::vector<uint8_t> &v) { return pack_bin(v); }
+  pack_result pack(bool v);
+  pack_result pack(float v);
+  pack_result pack(double v);
+  pack_result pack(const char *v);
+  pack_result pack(std::string_view v);
+  pack_result pack(const std::string &v);
+  pack_result pack(const std::vector<uint8_t> &v);
 
   template <typename T>
     requires (!std::is_same_v<T, uint8_t>)
@@ -464,210 +350,21 @@ public:
     return parser(m_p + n, m_size - n);
   }
 
-  result<parser> next() const {
-    auto hdr = header_byte();
-    if (!hdr) return std::unexpected(hdr.error());
+  result<parser> next() const;
 
-    if (is_array()) {
-      auto info = get_body_info(m_p, m_size);
-      if (!info) return std::unexpected(info.error());
-      auto cnt = count();
-      if (!cnt) return std::unexpected(cnt.error());
-      auto cur = advance(info->header);
-      if (!cur) return std::unexpected(cur.error());
-      for (uint32_t i = 0; i < *cnt; ++i) {
-        auto n = cur->next();
-        if (!n) return std::unexpected(n.error());
-        cur = *n;
-      }
-      return *cur;
-    } else if (is_map()) {
-      auto info = get_body_info(m_p, m_size);
-      if (!info) return std::unexpected(info.error());
-      auto cnt = count();
-      if (!cnt) return std::unexpected(cnt.error());
-      auto cur = advance(info->header);
-      if (!cur) return std::unexpected(cur.error());
-      for (uint32_t i = 0; i < *cnt; ++i) {
-        auto k = cur->next();
-        if (!k) return std::unexpected(k.error());
-        cur = *k;
-        auto v = cur->next();
-        if (!v) return std::unexpected(v.error());
-        cur = *v;
-      }
-      return *cur;
-    } else {
-      auto info = get_body_info(m_p, m_size);
-      if (!info) return std::unexpected(info.error());
-      auto total = info->header + static_cast<int>(info->body);
-      return advance(total);
-    }
-  }
+  bool is_nil() const;
+  bool is_bool() const;
+  bool is_number() const;
+  bool is_string() const;
+  bool is_binary() const;
+  bool is_ext() const;
+  bool is_array() const;
+  bool is_map() const;
 
-  bool is_nil() const {
-    auto h = header_byte();
-    return h && *h == format::NIL;
-  }
-
-  bool is_bool() const {
-    auto h = header_byte();
-    return h && (*h == format::TRUE || *h == format::FALSE);
-  }
-
-  bool is_number() const {
-    auto h = header_byte();
-    if (!h) return false;
-    uint8_t b = *h;
-    if (format::is_positive_fixint(b)) return true;
-    if (format::is_negative_fixint(b)) return true;
-    return b >= format::FLOAT32 && b <= format::INT64;
-  }
-
-  bool is_string() const {
-    auto h = header_byte();
-    if (!h) return false;
-    uint8_t b = *h;
-    if (format::is_fixstr(b)) return true;
-    return b == format::STR8 || b == format::STR16 || b == format::STR32;
-  }
-
-  bool is_binary() const {
-    auto h = header_byte();
-    if (!h) return false;
-    uint8_t b = *h;
-    return b == format::BIN8 || b == format::BIN16 || b == format::BIN32;
-  }
-
-  bool is_ext() const {
-    auto h = header_byte();
-    if (!h) return false;
-    uint8_t b = *h;
-    return (b >= format::FIXEXT1 && b <= format::FIXEXT16) ||
-           b == format::EXT8 || b == format::EXT16 || b == format::EXT32;
-  }
-
-  bool is_array() const {
-    auto h = header_byte();
-    if (!h) return false;
-    uint8_t b = *h;
-    if (format::is_fixarray(b)) return true;
-    return b == format::ARRAY16 || b == format::ARRAY32;
-  }
-
-  bool is_map() const {
-    auto h = header_byte();
-    if (!h) return false;
-    uint8_t b = *h;
-    if (format::is_fixmap(b)) return true;
-    return b == format::MAP16 || b == format::MAP32;
-  }
-
-
-  result<bool> get_bool() const {
-    auto h = header_byte();
-    if (!h) return std::unexpected(h.error());
-    if (*h == format::TRUE) return true;
-    if (*h == format::FALSE) return false;
-    return std::unexpected(error_code::type_error);
-  }
-
-  result<std::string_view> get_string() const {
-    auto h = header_byte();
-    if (!h) return std::unexpected(h.error());
-    uint8_t b = *h;
-    size_t offset, len;
-    if (format::is_fixstr(b)) {
-      len = b & 0x1F;
-      offset = 1;
-    } else if (b == format::STR8) {
-      auto n = body_number<uint8_t>(m_p, m_size);
-      if (!n) return std::unexpected(n.error());
-      len = *n; offset = 1 + 1;
-    } else if (b == format::STR16) {
-      auto n = body_number<uint16_t>(m_p, m_size);
-      if (!n) return std::unexpected(n.error());
-      len = *n; offset = 1 + 2;
-    } else if (b == format::STR32) {
-      auto n = body_number<uint32_t>(m_p, m_size);
-      if (!n) return std::unexpected(n.error());
-      len = *n; offset = 1 + 4;
-    } else {
-      return std::unexpected(error_code::type_error);
-    }
-    if (static_cast<int>(offset + len) > m_size) {
-      return std::unexpected(error_code::lack);
-    }
-    return std::string_view(reinterpret_cast<const char *>(m_p + offset), len);
-  }
-
-  result<std::string_view> get_binary_view() const {
-    auto h = header_byte();
-    if (!h) return std::unexpected(h.error());
-    uint8_t b = *h;
-    size_t offset, len;
-
-    if (b == format::BIN8) {
-      auto n = body_number<uint8_t>(m_p, m_size);
-      if (!n) return std::unexpected(n.error());
-      len = *n; offset = 1 + 1;
-    } else if (b == format::BIN16) {
-      auto n = body_number<uint16_t>(m_p, m_size);
-      if (!n) return std::unexpected(n.error());
-      len = *n; offset = 1 + 2;
-    } else if (b == format::BIN32) {
-      auto n = body_number<uint32_t>(m_p, m_size);
-      if (!n) return std::unexpected(n.error());
-      len = *n; offset = 1 + 4;
-    } else {
-      return std::unexpected(error_code::type_error);
-    }
-    if (static_cast<int>(offset + len) > m_size) {
-      return std::unexpected(error_code::lack);
-    }
-    return std::string_view(reinterpret_cast<const char *>(m_p + offset), len);
-  }
-
-  result<std::tuple<int8_t, std::string_view>> get_ext() const {
-    auto h = header_byte();
-    if (!h) return std::unexpected(h.error());
-    uint8_t b = *h;
-    int8_t ext_type;
-    size_t data_offset, data_len;
-
-    switch (b) {
-    case format::FIXEXT1:  ext_type = m_p[1]; data_offset = 2; data_len = 1;  break;
-    case format::FIXEXT2:  ext_type = m_p[1]; data_offset = 2; data_len = 2;  break;
-    case format::FIXEXT4:  ext_type = m_p[1]; data_offset = 2; data_len = 4;  break;
-    case format::FIXEXT8:  ext_type = m_p[1]; data_offset = 2; data_len = 8;  break;
-    case format::FIXEXT16: ext_type = m_p[1]; data_offset = 2; data_len = 16; break;
-    case format::EXT8: {
-      auto n = body_number<uint8_t>(m_p, m_size);
-      if (!n) return std::unexpected(n.error());
-      ext_type = m_p[2]; data_offset = 3; data_len = *n;
-      break;
-    }
-    case format::EXT16: {
-      auto n = body_number<uint16_t>(m_p, m_size);
-      if (!n) return std::unexpected(n.error());
-      ext_type = m_p[3]; data_offset = 4; data_len = *n;
-      break;
-    }
-    case format::EXT32: {
-      auto n = body_number<uint32_t>(m_p, m_size);
-      if (!n) return std::unexpected(n.error());
-      ext_type = m_p[5]; data_offset = 6; data_len = *n;
-      break;
-    }
-    default:
-      return std::unexpected(error_code::type_error);
-    }
-    if (static_cast<int>(data_offset + data_len) > m_size) {
-      return std::unexpected(error_code::lack);
-    }
-    return std::tuple{ext_type,
-                      std::string_view(reinterpret_cast<const char *>(m_p + data_offset), data_len)};
-  }
+  result<bool> get_bool() const;
+  result<std::string_view> get_string() const;
+  result<std::string_view> get_binary_view() const;
+  result<std::tuple<int8_t, std::string_view>> get_ext() const;
 
   template <typename T>
   result<T> get_number() const {
@@ -694,41 +391,9 @@ public:
     }
   }
 
-  result<uint32_t> count() const {
-    auto h = header_byte();
-    if (!h) return std::unexpected(h.error());
-    uint8_t b = *h;
-
-    if (format::is_fixarray(b)) return static_cast<uint32_t>(b & 0x0F);
-    if (format::is_fixmap(b))   return static_cast<uint32_t>(b & 0x0F);
-
-    switch (b) {
-    case format::ARRAY16: { auto n = body_number<uint16_t>(m_p, m_size); if (!n) return std::unexpected(n.error()); return static_cast<uint32_t>(*n); }
-    case format::ARRAY32: { auto n = body_number<uint32_t>(m_p, m_size); if (!n) return std::unexpected(n.error()); return *n; }
-    case format::MAP16:   { auto n = body_number<uint16_t>(m_p, m_size); if (!n) return std::unexpected(n.error()); return static_cast<uint32_t>(*n); }
-    case format::MAP32:   { auto n = body_number<uint32_t>(m_p, m_size); if (!n) return std::unexpected(n.error()); return *n; }
-    default:
-      return std::unexpected(error_code::type_error);
-    }
-  }
-
-  result<parser> first_item() const {
-    if (!is_array() && !is_map()) return std::unexpected(error_code::type_error);
-    auto info = get_body_info(m_p, m_size);
-    if (!info) return std::unexpected(info.error());
-    return advance(info->header);
-  }
-
-  parser operator[](int index) const {
-    auto cur = first_item();
-    if (!cur) return {};
-    for (int i = 0; i < index; ++i) {
-      auto n = cur->next();
-      if (!n) return {};
-      cur = *n;
-    }
-    return *cur;
-  }
+  result<uint32_t> count() const;
+  result<parser> first_item() const;
+  parser operator[](int index) const;
 };
 
 template <typename T>
@@ -750,26 +415,11 @@ result<parser> unpack(const parser &p, T &out) {
   return p.next();
 }
 
-inline result<parser> unpack(const parser &p, bool &out) {
-  auto v = p.get_bool();
-  if (!v) return std::unexpected(v.error());
-  out = *v;
-  return p.next();
-}
-
-inline result<parser> unpack(const parser &p, std::string_view &out) {
-  auto v = p.get_string();
-  if (!v) return std::unexpected(v.error());
-  out = *v;
-  return p.next();
-}
-
-inline result<parser> unpack(const parser &p, std::string &out) {
-  auto v = p.get_string();
-  if (!v) return std::unexpected(v.error());
-  out = std::string(v->data(), v->size());
-  return p.next();
-}
+result<parser> unpack(const parser &p, bool &out);
+result<parser> unpack(const parser &p, std::string_view &out);
+result<parser> unpack(const parser &p, std::string &out);
+result<parser> unpack(const parser &p, std::vector<uint8_t> &out);
+result<parser> unpack(const parser &p, std::span<const uint8_t> &out);
 
 template <size_t N>
 result<parser> unpack(const parser &p, std::array<uint8_t, N> &out) {
@@ -780,20 +430,6 @@ result<parser> unpack(const parser &p, std::array<uint8_t, N> &out) {
   return p.next();
 }
 
-inline result<parser> unpack(const parser &p, std::vector<uint8_t> &out) {
-  auto v = p.get_binary_view();
-  if (!v) return std::unexpected(v.error());
-  out.assign(v->begin(), v->end());
-  return p.next();
-}
-
-inline result<parser> unpack(const parser &p, std::span<const uint8_t> &out) {
-  auto v = p.get_binary_view();
-  if (!v) return std::unexpected(v.error());
-  out = std::span<const uint8_t>(reinterpret_cast<const uint8_t*>(v->data()), v->size());
-  return p.next();
-}
-
 template <typename T>
   requires (!std::is_same_v<T, uint8_t>)
 result<parser> unpack(const parser &p, std::vector<T> &out) {
diff --git a/util/CMakeLists.txt b/util/CMakeLists.txt
new file mode 100644
index 0000000..df3bb83
--- /dev/null
+++ b/util/CMakeLists.txt
@@ -0,0 +1,2 @@
+add_library(util INTERFACE)
+target_include_directories(util INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
diff --git a/include/span_writer.h b/util/span_writer.h
similarity index 100%
rename from include/span_writer.h
rename to util/span_writer.h