Lite Blue 1.2.1

Lite Blue 1.2.1 — a timing bugfix.

• Fix a 32-bit overflow in elapsed_ms(): searches longer than about two
  seconds on AIX reported a negative time and a bogus nps, and the same
  value feeds time management, so a long time-controlled search could overrun.

Found on obsidian while checking the site's node-rate figure — which turned out
to be stale anyway: the 1.1.0/1.2.0 evaluation terms dropped the rate from about
47,000 to about 32,000 nodes/second.

Engine by DeepSeek (deepseek-v4.1-flash).

Lite Blue 1.2.0

Lite Blue 1.2.0 — pawn-structure evaluation.

Engine

• Add a pawn-structure term to the evaluation: passed-pawn bonuses by how close
  they are to promoting, and penalties for doubled and isolated pawns.

Measured with the tools/match harness over 512 openings (1024 games),
colours swapped, at depth 6: +33 Elo versus 1.1.0 (about 2.2 sigma).

Move generation is unchanged (perft still passes). The version is reported as
Lite Blue 1.2.0 over UCI.

Engine by DeepSeek (deepseek-v4.1-flash).

Lite Blue 1.1.0

Lite Blue 1.1.0 — the first strength improvement.

Engine

• Add a pawn-shield king-safety term to the evaluation, scaled by the
  material phase. The engine now castles instead of walking its king up the
  board in the middlegame.

Measured with the new tools/match harness (32 openings, 64 games, colours
swapped): +38 Elo at depth 6 and +89 Elo at depth 8 against 1.0.0. Error
bars are wide at this sample size, so this is consistent evidence rather than
proof.

Move generation is unchanged (perft still passes). The version is reported as
Lite Blue 1.1.0 over UCI.

Engine by DeepSeek (deepseek-v4.1-flash).

Lite Blue 1.0.0

Lite Blue 1.0.0 — a correct, playable, portable C89 chess engine for AIX and
other Unix systems.

Engine

• 0x88 board; negamax alpha-beta with iterative deepening, a transposition
  table, quiescence search, null-move pruning, late-move reductions,
  principal-variation search, and killer/history move ordering.
• Hand-tuned evaluation: material, piece-square tables, and a tapered king.
• Three front ends over one library: UCI (authoritative), an interactive TUI,
  and a self-play arena.

Correctness

• Move generation perft-verified against the standard suite, with the Zobrist
  key recomputed and compared at every internal node.
• Deterministic across XL C 5.0.2 and GCC 2.95.3 (byte-identical node counts).
• make check runs a repetition regression test plus the perft suite.

Builds with xlc, gcc or cc:

cd src && make
# or: make CC=/usr/vac/bin/xlc CFLAGS="-O2 -qlanglvl=ansi"

Fixed (review by Claude Opus 5.5): repetition-history bounds; zeroed-limit
time handling; always return a legal move; transposition-table re-init leak.

Engine by DeepSeek (deepseek-v4.1-flash).