Horizon Technical Description

General Features

• Winboard-compatible engine supporting all commands (ponder is disabled internally) including: move, take-back, analysis, and time control
• Opening book with opening move 'style' configurable
• Resignation can be turned off or on
• Adjustable size for hash, pawn hash, and egtb cache
• Uses available Nalimov egtb tables (up to 5-man)
• ICS kibbitz support
• Adjustable Elo weakening (by wasting time and by disabling search options)

Search

Just like everyone else:

• Aspirated PVS
• Quiescent search with check extensions
• Null move
• Iterative deepening
• Internal iterative deepening
• Move ordering is ordinary except for possible "threat moves" which get boosted
• Two killer moves for each ply, 1 mate killer
• Extensions for: checkmate threat, single reply, check, revealed check, recaptures, 7th rank pawn pushes
• Adaptive razoring reductions for non-dangerous moves
• Search uses lazy eval for pruning decisions

Unique search features:

• Up to 3 null moves per search line
• Null-move reduction range: 2->4.5 plies
• Null-move verification search
• No history reductions
• No attack board
• Search aspiration is asymmetric based on side-to-move
• Thinking time extended for "interesting" positions