dgb: M3 §7b Scrypt-only multi-algo version classifier - #152
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DigiByte encodes the mining algo in 4 bits of the block nVersion field. The Scrypt-only V36 validation path needs a consensus-exact decode to route each incoming parent header: Scrypt -> PoW-validate, known non-Scrypt -> accept-by-continuity (work-neutral), unknown algo bits -> reject. Add coin/dgb_block_algo.hpp: a header-only SSOT (DgbAlgo, is_scrypt_header, dgb_header_disposition) pinning the BLOCK_VERSION_* / ALGO_* enums verbatim from DigiByte Core src/primitives/block.h + GetAlgo(). CRITICAL invariant: Scrypt == (0 << 8) -- the masked algo bits are ZERO -- so the gate is (nVersion & 0x0F00) == 0, not a nonzero check. Also pins the upstream quirk ALGO_ODO == 7 (not a dense 5) and rejects reserved codepoints (10/12 << 8). Add standalone guard test/algo_select_test.cpp (6 tests, GTest-only, no OBJECT lib) and register it in BOTH the ctest foreach allowlist and the two build.yml --target lists, per the #143 NOT_BUILT lesson. No header_chain or transport wiring touched; foundational primitive for the deferred validate()/DigiShield-window port.
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What
First slice of M3 §7b (SCRYPT-ONLY VALIDATION). Adds the consensus-exact DGB multi-algo block-version classifier that the deferred
coin/header_chain.hppvalidate()+ DigiShield Scrypt-only window walk both depend on.coin/dgb_block_algo.hpp(header-only SSOT):DgbAlgo,dgb_block_algo(),is_scrypt_header(),dgb_header_disposition()(three-way: VALIDATE_SCRYPT / ACCEPT_BY_CONTINUITY / REJECT).src/primitives/block.h(BLOCK_VERSION_*,ALGO_*) +block.cpp::GetAlgo().test/algo_select_test.cpp: 6-test standalone guard (GTest-only, links no OBJECT lib), wired into BOTH the ctest foreach allowlist AND bothbuild.yml--target lists (per the ci: build test_dgb_subsidy in both Linux test jobs (fix master-red NOT_BUILT) #143 NOT_BUILT lesson).Consensus notes
(0 << 8)— masked algo bits are ZERO. Gate is(nVersion & 0x0F00) == 0, not a nonzero check. Test asserts this directly (a bare primary-version header is Scrypt).ALGO_ODO == 7(not a dense 5); reserved codepoints (10/12 << 8 = Equihash/Ethash) decode to UNKNOWN -> REJECT.Verify
build EXIT=0,
algo_select_test6/6 PASS locally. Noheader_chain.hpp/transport touched.HOLD merge — operator push-approval card as usual.