Add tile-backed proof generation - #54

This PR adds tile-backed proof generation infrastructure for merklecpp’s tiled-storage stack, including abstract subtree-root sources (memory, tiles, and combined), a proof engine that produces inclusion/consistency proofs compatible with existing TreeT paths, and test/CI wiring to validate large tile hierarchies.

Changes:

• Add TreeT::subtree_root() to expose resident perfect-subtree roots without changing hashing semantics.
• Introduce HashSourceT implementations and ProofEngineT for roots, inclusion proofs, RFC 6962 consistency proofs, and verification.
• Add new proof and level-2 tile end-to-end tests plus LONG_TESTS gating and CI configuration to run long tests selectively.

• Files reviewed: 10/10 changed files
• Comments generated: 3
• Review effort level: Lite

Suppressed comments (3)

merklecpp_tiles.h:1025

• roll_up() allocates a temporary std::vector slice for every subtree roll-up, which is a hot path during proof generation and defeats the goal of avoiding redundant copies. Since span is always a power-of-two <= TILE_WIDTH, this can be rolled up in-place using a fixed buffer and HASH_FUNCTION without heap allocation.

        }
        return perfect_root<HASH_SIZE, HASH_FUNCTION>(std::vector<Hash>(
          tile.begin() + (std::ptrdiff_t)off,
          tile.begin() + (std::ptrdiff_t)(off + span)));

test/tiles_proofs.cpp:15

• This test throws std::runtime_error but does not include directly. Relying on transitive includes from merklecpp headers is not guaranteed and can break with different standard library implementations or include order.

#include <iostream>
#include <limits>
#include <merklecpp.h>
#include <merklecpp_tiles.h>
#include <string>

test/tiles_level2.cpp:18

• This test throws std::runtime_error but does not include directly. Relying on transitive includes from other headers is not guaranteed and can break with different standard library implementations or include order.

#include <ctime>
#include <filesystem>
#include <iostream>
#include <merklecpp.h>
#include <merklecpp_tiles.h>

• Files reviewed: 10/10 changed files
• Comments generated: 0 new
• Review effort level: Lite

Suppressed comments (3)

merklecpp_tiles.h:1025

• TileHashSourceT::roll_up() allocates/copies a temporary std::vector<Hash> for every span>1 roll-up. This can become a noticeable hotspot during proof generation (many small roll-ups). You can compute the perfect root directly over the tile slice without allocating.

        }
        return perfect_root<HASH_SIZE, HASH_FUNCTION>(std::vector<Hash>(
          tile.begin() + (std::ptrdiff_t)off,
          tile.begin() + (std::ptrdiff_t)(off + span)));
      }

test/tiles_proofs.cpp:281

• The temp directory name is derived from time() + rand() seeded with time(). With ctest --parallel enabled in CI, multiple test binaries can start in the same second and produce the same directory name, causing flaky collisions. Use a nonce that includes the process id and a monotonic/sequence component.

  const fs::path base = fs::temp_directory_path() /
    ("merklecpp_tiles_proofs_" + std::to_string((unsigned long long)seed) +
     "_" + std::to_string(std::rand()));

test/tiles_level2.cpp:50

• The temp directory name is based only on time(nullptr), which can collide when tests run in parallel (CI now uses ctest --parallel). Use a name that incorporates process id and a monotonic/sequence nonce to avoid flaky collisions.

  const fs::path dir = fs::temp_directory_path() /
    ("merklecpp_tiles_level2_" +
     std::to_string((unsigned long long)std::time(nullptr)));

• Files reviewed: 10/10 changed files
• Comments generated: 1
• Review effort level: Lite

Suppressed comments (1)

CMakeLists.txt:10

• The build option for enabling/disabling tests switched from the project-specific TESTS cache variable to CTest's BUILD_TESTING. As written, -DTESTS=ON/OFF is now silently ignored (and BUILD_TESTING defaults to ON via include(CTest)), which can break existing scripts and make it hard to turn tests off in downstream builds. Consider adding a small compatibility shim that maps TESTS to BUILD_TESTING (with a deprecation warning) before including CTest.

include(GNUInstallDirs)
include(CTest)

• Files reviewed: 10/10 changed files
• Comments generated: 0 new
• Review effort level: Lite

Suppressed comments (1)

CMakeLists.txt:49

• include(CTest) makes BUILD_TESTING default to ON, so this condition will add the test/ subtree (and FetchContent-download doctest) even when merklecpp is consumed as a dependency via add_subdirectory/FetchContent. That can unexpectedly pull network dependencies and slow or break downstream builds; consider only adding tests when merklecpp is the top-level project.

if(BUILD_TESTING)

• Files reviewed: 10/10 changed files
• Comments generated: 0 new
• Review effort level: Lite

• Files reviewed: 10/10 changed files
• Comments generated: 0 new
• Review effort level: Lite

• Files reviewed: 10/10 changed files
• Comments generated: 0 new
• Review effort level: Lite