Demo — parallel hashing / mini proof-of-work

• Type: feature — example app. Track B/E (build with $(PXX_STABLE); a compiler/frontend gap it hits → file under the owning lane).
• Opened: 2026-07-17 (parallel-demo candidate).
• Relation: integer/compute companion to examples/parallel/collatz.pas and membw.pas. Uses parallel(P) for + reduction ([[feature-parallel-for-scheduling-policy]]). Can reuse the crypto in lib/** (sha256 already exists — see the lib-test roster).

Goal

A relatable, compute-bound integer demo: mini proof-of-work. Over nonces [0..N-1], hash (prefix || nonce) and count / find those whose digest has ≥ K leading zero bits.

var found, bestZeros: Integer;
found := 0; bestZeros := 0;
parallel(pdChunked) for nonce := 0 to N-1
  reduction(+: found)
  reduction(max: bestZeros)
do begin
  z := LeadingZeroBits(Hash(prefix, nonce));   { Hash = function, private scratch }
  if z >= K then found := found + 1;
  if z > bestZeros then bestZeros := z;
end;

• Compute-bound (hashing is ALU-heavy, little memory traffic) → scales close to linear, the clean counterpoint to membw's memory wall.
• Deterministic for a fixed prefix/N → serial == parallel exact (count + best-zeros), a real correctness oracle.
• Relatable ("mining") and shows two reduction ops (+ and max) in one pass.
• Hash is a function so its state is private per worker (safe pattern).

Hash choice

• Simplest: a small integer mixing hash (splitmix64 / xorshift / FNV) — no dependencies, plenty of ALU, fully in registers. Best for the pure-compute story.
• Realistic: SHA-256 from lib/** (already in the lib-test roster) — heavier, "real crypto", but pulls the lib in. Offer both via a --hash fast|sha256 flag.

Extensions

• Throughput report — hashes/sec serial vs parallel (the headline number for a "miner").
• Actually find a nonce — stop-early once a target difficulty is hit (needs a shared found-flag; note that early-exit across workers is a separate concern — the current parallel-for has no cancellation, so v1 scans the whole range and reduces, no early stop).

Constraints

• Build with $(PXX_STABLE); never rebuild the compiler.
• No automated multithread test without explicit permission (core-pegging) — manual-validation, compile-smoke at most. A single-threaded fixed-seed digest check is fine to automate and bounded.

Acceptance

• A PoW demo: for a fixed prefix + N + K, prints found-count, best-leading-zeros, and hashes/sec (serial vs parallel); asserts parallel == serial. Compiles + runs x86-64; compiles cross. --hash fast default; --hash sha256 optional.

Log

• 2026-07-17 — Filed as a parallel-demo candidate (compute-bound counterpoint to the memory-bound membw demo; fast integer hash default, sha256 optional; two reductions +/max; note the no-cancellation limitation for early-exit).
• 2026-07-20 — resolved, commit HEAD.