Fifty-Move Rule Counter

Update halfmove ledger after supplied pawn-move/capture flags; 100/150 plies indicate claim/automatic-draw candidates only, with checkmate precedence and claim procedure not verified. Independent of repetition.

Description

The chess halfmove clock resets on a pawn move or capture.

The chess halfmove clock resets on a pawn move or capture. Move-count thresholds provide draw candidates rather than final adjudication.^[1]

Enter nonnegative safe-integer starting clock and binary reset flags in move order,1 for a reset. Return clock history and final threshold candidate flags.

When to use Fifty-Move Rule Counter

Use Fifty-Move Rule Counter when you need to evaluate the supplied records under the calculation and evidence rules below. No board, legal move history, claim procedure or checkmate precedence is checked.

Prepare the supplied game state, observations and explicit variant settings. Follow the card, board, coordinate and state conventions in the calculation rules.

How to use Fifty-Move Rule Counter and understand its results

Prepare the supplied game state, observations and explicit variant settings. Follow the card, board, coordinate and state conventions in the calculation rules.
Enter startingHalfmoveClock, pawnMoveOrCaptureFlags using the field table and calculation rules, then run the tool.
Read finalHalfmoveClock, threefoldIndependentFiftyMoveClaimCandidate, seventyFiveMoveAutomaticDrawCandidate. The explanation below defines the units, signs, ordering and any flags or placeholder values.
If the tool reports an input issue, correct the values and run it again. Apply the stated assumptions when interpreting a successfully returned result.

Input field	Meaning	Value type; see rules below	Requirement
startingHalfmoveClock	Starting halfmove clock	number	Required
pawnMoveOrCaptureFlags	Pawn move or capture flags	list of number	Required

Enter nonnegative safe-integer starting clock and binary reset flags in move order,1 for a reset. Return clock history and final threshold candidate flags.

clock=0 on reset else clock+1; thresholds100 and150 halfmoves

Units, indexing, boundary conditions and exceptional cases are defined in the calculation rules above.

Output field	Meaning	Value type
finalHalfmoveClock	Final halfmove clock	number
threefoldIndependentFiftyMoveClaimCandidate	Threefold independent fifty move claim candidate	boolean
seventyFiveMoveAutomaticDrawCandidate	Seventy five move automatic draw candidate	boolean

json

{
  "input": {
    "startingHalfmoveClock": 99,
    "pawnMoveOrCaptureFlags": [
      0,
      0
    ]
  },
  "output": {
    "finalHalfmoveClock": 101,
    "threefoldIndependentFiftyMoveClaimCandidate": true,
    "seventyFiveMoveAutomaticDrawCandidate": false
  }
}

Limitations and assumptions

No board, legal move history, claim procedure or checkmate precedence is checked.

Numeric inputs, where present, must be finite. Follow the calculation-specific rules for overflow, underflow and exceptional cases. Numeric lists are limited to 10,000 entries; string lists to 128 strings of at most 128 characters each. The rules above may impose smaller limits. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors. A successfully returned false flag is an evaluated outcome; its meaning is explained above.

Alternative or Complementary methods

Use Threefold Repetition Counter: Chess repetition compares position identity while ignoring move clocks. An en-passant target changes identity only when a legal capture exists. For a complementary task, use Insufficient Material Draw Check: A conservative dead-material screen recognizes a limited set of mating-impossible inventories. An unrecognized position remains unclassified.

References

Fifty-Move Rule Counter — tool.io

Similar or alternative tools

Threefold Repetition Counter
Chess repetition compares position identity while ignoring move clocks. An en-passant target changes identity only when a legal capture exists.
Insufficient Material Draw Check
A conservative dead-material screen recognizes a limited set of mating-impossible inventories. An unrecognized position remains unclassified.