bch(m4): ABLA replay + dynamic per-tip template size budget (M4 anchor)

src/impl/bch/coin/abla.hpp

@@ -0,0 +1,214 @@
+#pragma once
+// BCH ABLA (Adaptive Blocksize-Limit Algorithm, CHIP-2023-01) -- net-new
+// BCH-specific code. Activated at the May 2024 upgrade; it replaces the static
+// 32 MB excessive-blocksize (EB) with a control function that lets the network
+// block-size limit grow with sustained demand.
+//
+// This is a 1:1 fixed-point port of Bitcoin Cash Node:
+//   src/consensus/abla.cpp  (State::NextBlockState, Config::MakeDefault/SetMax)
+//   src/consensus/abla.h    (Config / State data types)
+// pinned at BCHN tag v29.0.0 (clone @89a591f). It is consensus-critical and
+// MUST stay byte-exact with BCHN -- validate against BCHN gold vectors
+// src/test/data/abla_test_vectors (gen: generate_abla_test_vectors.py).
+//
+// p2pool-merged-v36 SURFACE: NONE. ABLA governs only the *block-size limit*,
+// i.e. how large a block a node will accept / how large a template we may
+// build. It does NOT touch PoW hash, share format, coinbase commitment, or
+// PPLNS math. In the template builder it is used purely as a LOCAL build-time
+// byte budget; using a conservative (floor) value can never produce an
+// invalid block, so there is zero compat risk. -- M4 size-slice (M1 ss4.4).
+//
+// Mainnet: MakeDefault(32 MB, fixedSize=false) -> grows.
+// testnet3 / testnet4: MakeDefault(32 MB, fixedSize=true) -> no-op, fixed 32 MB.
+
+#include <algorithm>
+#include <cassert>
+#include <cstdint>
+#include <limits>
+
+namespace bch {
+namespace coin {
+namespace abla {
+
+// ONE_MEGABYTE / consensus block-size constants -- BCHN src/consensus/consensus.h
+inline constexpr uint64_t ONE_MEGABYTE                = 1000000u;
+inline constexpr uint64_t DEFAULT_CONSENSUS_BLOCK_SIZE = 32u * ONE_MEGABYTE;
+inline constexpr uint64_t MAX_CONSENSUS_BLOCK_SIZE     = uint64_t(2000) * ONE_MEGABYTE;
+
+// 2^7 fixed precision for the "asymmetry factor" (zeta). BCHN abla.h B7.
+inline constexpr uint64_t B7 = 1u << 7u;
+
+// muldiv(x,y,z) = x*y/z in 128-bit intermediate (BCHN abla.cpp). The platform
+// is gcc/clang C++20 on linux -> use the native __int128 path BCHN selects.
+inline uint64_t muldiv(uint64_t x, uint64_t y, uint64_t z) {
+    assert(z != 0);
+    const unsigned __int128 res =
+        (static_cast<unsigned __int128>(x) * static_cast<unsigned __int128>(y))
+        / static_cast<unsigned __int128>(z);
+    assert(res <= static_cast<unsigned __int128>(std::numeric_limits<uint64_t>::max()));
+    return static_cast<uint64_t>(res);
+}
+
+// Algorithm configuration -- part of a chain's consensus params (BCHN abla.h).
+struct Config {
+    uint64_t epsilon0{};         // initial/floor control block size
+    uint64_t beta0{};            // initial/floor elastic buffer size
+    uint64_t gammaReciprocal{};  // reciprocal of control "forget factor"
+    uint64_t zeta_xB7{};         // control "asymmetry factor" (x B7)
+    uint64_t thetaReciprocal{};  // reciprocal of elastic buffer decay rate
+    uint64_t delta{};            // elastic buffer "gear factor"
+    uint64_t epsilonMax{};       // max control block size
+    uint64_t betaMax{};          // max elastic buffer size
+
+    // Set epsilonMax / betaMax so internal ops can't overflow UINT64_MAX.
+    // 1:1 BCHN Config::SetMax.
+    void SetMax() {
+        const uint64_t maxSafeBlocksizeLimit =
+            std::numeric_limits<uint64_t>::max() / zeta_xB7 * B7;
+        const uint64_t maxElasticBufferRatioNumerator =
+            delta * ((zeta_xB7 - B7) * thetaReciprocal / gammaReciprocal);
+        const uint64_t maxElasticBufferRatioDenominator =
+            (zeta_xB7 - B7) * thetaReciprocal / gammaReciprocal + B7;
+        epsilonMax = maxSafeBlocksizeLimit
+            / (maxElasticBufferRatioNumerator + maxElasticBufferRatioDenominator)
+            * maxElasticBufferRatioDenominator;
+        betaMax = maxSafeBlocksizeLimit - epsilonMax;
+    }
+
+    bool IsFixedSize() const {
+        return epsilon0 == epsilonMax && beta0 == betaMax;
+    }
+
+    // 1:1 BCHN Config::MakeDefault.
+    static Config MakeDefault(uint64_t defaultBlockSize = DEFAULT_CONSENSUS_BLOCK_SIZE,
+                              bool fixedSize = false) {
+        Config ret;
+        ret.epsilon0        = defaultBlockSize / 2u;
+        ret.beta0           = defaultBlockSize / 2u;
+        ret.gammaReciprocal = 37938;
+        ret.zeta_xB7        = 192;
+        ret.thetaReciprocal = 37938;
+        ret.delta           = 10;
+        if (!fixedSize) {
+            ret.SetMax();
+        } else {
+            ret.epsilonMax = ret.epsilon0;
+            ret.betaMax    = ret.beta0;
+        }
+        return ret;
+    }
+};
+
+// Algorithm internal state for block N. The limit for block N is
+// state_N.GetBlockSizeLimit(); for N+1 use NextBlockState(...).GetBlockSizeLimit().
+class State {
+    uint64_t blockSize{};
+    uint64_t controlBlockSize{};
+    uint64_t elasticBufferSize{};
+
+public:
+    State() = default;
+
+    // Defaults from Config -- suitable for all blocks before ABLA activation
+    // (the activation/floor state). 1:1 BCHN State(config, blkSize).
+    State(const Config& config, uint64_t blkSize)
+        : blockSize(blkSize),
+          controlBlockSize(config.epsilon0),
+          elasticBufferSize(config.beta0) {}
+
+    uint64_t GetBlockSizeLimit(bool disable2GBCap = false) const {
+        if (disable2GBCap)
+            return controlBlockSize + elasticBufferSize;
+        return std::min<uint64_t>(controlBlockSize + elasticBufferSize,
+                                  MAX_CONSENSUS_BLOCK_SIZE);
+    }
+
+    uint64_t GetNextBlockSizeLimit(const Config& config, bool disable2GBCap = false) const {
+        return NextBlockState(config, 0).GetBlockSizeLimit(disable2GBCap);
+    }
+
+    uint64_t GetBlockSize() const { return blockSize; }
+    uint64_t GetControlBlockSize() const { return controlBlockSize; }
+    uint64_t GetElasticBufferSize() const { return elasticBufferSize; }
+
+    // Advance state to block N+1 given its size. 1:1 BCHN State::NextBlockState.
+    State NextBlockState(const Config& config, const uint64_t nextBlockSize) const {
+        State ret;
+        ret.blockSize = nextBlockSize;
+
+        // control function
+        const uint64_t clampedBlockSize =
+            std::min(this->blockSize, this->controlBlockSize + this->elasticBufferSize);
+        const uint64_t amplifiedCurrentBlockSize =
+            muldiv(config.zeta_xB7, clampedBlockSize, B7);
+
+        if (amplifiedCurrentBlockSize > this->controlBlockSize) {
+            const uint64_t bytesToAdd = amplifiedCurrentBlockSize - this->controlBlockSize;
+            const uint64_t amplifiedBlockSizeLimit =
+                muldiv(config.zeta_xB7, this->controlBlockSize + this->elasticBufferSize, B7);
+            const uint64_t bytesMax = amplifiedBlockSizeLimit - this->controlBlockSize;
+            const uint64_t scalingOffset =
+                muldiv(muldiv(config.zeta_xB7, this->elasticBufferSize, B7), bytesToAdd, bytesMax);
+            ret.controlBlockSize =
+                this->controlBlockSize + (bytesToAdd - scalingOffset) / config.gammaReciprocal;
+        } else {
+            const uint64_t bytesToRemove = this->controlBlockSize - amplifiedCurrentBlockSize;
+            ret.controlBlockSize = this->controlBlockSize - bytesToRemove / config.gammaReciprocal;
+            ret.controlBlockSize = std::max(ret.controlBlockSize, config.epsilon0);
+        }
+
+        // elastic buffer function
+        const uint64_t bufferDecay = this->elasticBufferSize / config.thetaReciprocal;
+        if (amplifiedCurrentBlockSize > this->controlBlockSize) {
+            const uint64_t bytesToAdd =
+                (ret.controlBlockSize - this->controlBlockSize) * config.delta;
+            ret.elasticBufferSize = this->elasticBufferSize - bufferDecay + bytesToAdd;
+        } else {
+            ret.elasticBufferSize = this->elasticBufferSize - bufferDecay;
+        }
+        ret.elasticBufferSize = std::max(ret.elasticBufferSize, config.beta0);
+
+        ret.controlBlockSize  = std::min(ret.controlBlockSize, config.epsilonMax);
+        ret.elasticBufferSize = std::min(ret.elasticBufferSize, config.betaMax);
+        return ret;
+    }
+};
+
+// Per-network config. Mainnet grows; testnets are fixed at 32 MB.
+inline Config mainnet_config()  { return Config::MakeDefault(DEFAULT_CONSENSUS_BLOCK_SIZE, /*fixedSize=*/false); }
+inline Config testnet_config()  { return Config::MakeDefault(DEFAULT_CONSENSUS_BLOCK_SIZE, /*fixedSize=*/true);  }
+
+// Replay ABLA forward from a known-good anchor State over a contiguous run of
+// block sizes (oldest-first), returning the State *after the last size supplied*
+// -- i.e. the State whose GetBlockSizeLimit() governs the block we are about to
+// build. 1:1 BCHN: repeated State::NextBlockState. The anchor MUST be the ABLA
+// State of the block immediately preceding sizes[0].
+//
+// NOTE on the size feed: ABLA is driven by each block's *actual serialized
+// size*, which the headers-only SPV header_chain structurally does not carry.
+// The natural anchor on BCH is therefore a BCHN-pinned {height,State}; the live
+// per-block sizes enter at the full-block / embedded-daemon layer (M5+), not
+// here. Until that feed exists the template builder stays on the safe floor
+// (see floor_block_size_limit / template_builder build budget).
+inline State replay(State anchor, const Config& config,
+                    const uint64_t* sizes, size_t n) {
+    State s = anchor;
+    for (size_t i = 0; i < n; ++i)
+        s = s.NextBlockState(config, sizes[i]);
+    return s;
+}
+
+// Conservative LOCAL build budget for the template builder when per-tip ABLA
+// state is not yet replayed through the header chain: the activation/floor
+// state. ABLA only ever raises the limit above this floor, so building to the
+// floor can never exceed the live consensus limit. The dynamic path is
+// abla::replay (above) fed a BCHN-pinned anchor + full-block sizes; that feed
+// lives at the full-block/daemon layer, not the headers-only SPV chain.
+inline uint64_t floor_block_size_limit(bool is_testnet) {
+    const Config cfg = is_testnet ? testnet_config() : mainnet_config();
+    return State(cfg, 0).GetBlockSizeLimit();
+}
+
+} // namespace abla
+} // namespace coin
+} // namespace bch