libc4

C/C++ reference implementation of the C4 ID system (SMPTE ST 2114:2017).

#include <c4/c4.hpp>

auto id = c4::ID::Identify("hello world");
std::cout << id << std::endl;
// output: c4YGsLNdHGW3vnRB... (90 characters)

What is C4?

C4 IDs are universally unique, consistent identifiers derived from file content using SHA-512. The same data always produces the same 90-character ID, regardless of filename, location, or time. C4 IDs are URL-safe, filename-safe, and double-click selectable.

This library provides:

• C4 ID computation — identify any data by content
• C4 ID encoding — base58 encoding with c4 prefix (90 chars)
• Tree IDs — compute a single ID for a set of IDs
• C4M format — parse and generate .c4m files
• C4M operations — diff, merge, intersect manifests
• Dual API — C API (c4.h) for maximum portability, C++ API (c4.hpp) for ergonomics

Building

cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build
ctest --test-dir build

Requires:

• CMake 3.16+
• C++17 compiler
• OpenSSL (for SHA-512)

API

C++ API

#include <c4/c4.hpp>
#include <c4/c4m.hpp>

// Identify data
auto id = c4::ID::Identify("some data");
auto file_id = c4::ID::IdentifyFile("/path/to/file");

// Parse/encode
auto parsed = c4::ID::Parse("c43inc2q...");
std::string str = id.String();

// Tree ID (set of IDs -> single ID)
c4::IDs ids;
ids.Append(c4::ID::Identify("alfa"));
ids.Append(c4::ID::Identify("bravo"));
auto tree_id = ids.TreeID();  // order-independent

// Parse a c4m file
auto manifest = c4m::Manifest::ParseFile("project.c4m");
auto entries = manifest.Entries();

// Diff two manifests
auto result = c4m::Diff(old_manifest, new_manifest);

C API

#include <c4/c4.h>

c4_id_t *id = c4_id_new();
c4_identify("some data", 9, id);

char buf[91];
c4_id_string(id, buf, sizeof(buf));
printf("%s\n", buf);

c4_id_free(id);

Cross-language compatibility

This implementation produces identical output to the Go reference implementation. Test vectors are shared to ensure all implementations agree on ID computation.

Design Decisions

See the FAQ for design decisions including SHA-512 permanence, the c4m format, and content store scaling.

License

Apache 2.0 — see LICENSE and NOTICE.