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)


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