Feen.rb

FEEN (Forsyth–Edwards Enhanced Notation) implementation for the Ruby language.

What is FEEN?

FEEN (Forsyth–Edwards Enhanced Notation) is a universal, rule-agnostic notation for representing board game positions. It extends traditional FEN to support:

Multiple game systems (Chess, Shōgi, Xiangqi, and more)
Cross-style games where players use different piece sets
Multi-dimensional boards (2D, 3D, and beyond)
Captured pieces (pieces-in-hand for drop mechanics)
Arbitrarily large boards with efficient empty square encoding
Completely irregular structures (any valid combination of ranks and separators)
Board-less positions (positions without piece placement, useful for pure style/turn tracking)

This gem implements the FEEN Specification v1.0.0 as a pure functional library with immutable data structures.

Installation

gem "sashite-feen"

Or install manually:

gem install sashite-feen

Quick Start

require "sashite/feen"

# Parse a FEEN string into an immutable position object
position = Sashite::Feen.parse("+rnbq+kbn+r/+p+p+p+p+p+p+p+p/8/8/8/8/+P+P+P+P+P+P+P+P/+RNBQ+KBN+R / C/c")

# Access position components
position.placement  # Board configuration
position.hands      # Captured pieces
position.styles     # Game styles and active player

# Convert placement to array based on dimensionality
position.placement.to_a # => [[pieces...], [pieces...], ...] for 2D boards

# Convert back to canonical FEEN string
feen_string = Sashite::Feen.dump(position) # or position.to_s

FEEN Format

A FEEN string consists of three space-separated fields:

<piece-placement> <pieces-in-hand> <style-turn>

Example:

+rnbq+kbn+r/+p+p+p+p+p+p+p+p/8/8/8/8/+P+P+P+P+P+P+P+P/+RNBQ+KBN+R / C/c

Piece placement: Board configuration using EPIN notation with / separators (can be empty for board-less positions)
Pieces in hand: Captured pieces for each player (format: first/second)
Style-turn: Game styles and active player (format: active/inactive)

See the FEEN Specification for complete format details.

API Reference

Module Methods

`Sashite::Feen.parse(string)`

Parses a FEEN string into an immutable Position object.

Parameter: string (String) - FEEN notation string
Returns: Position - Immutable position object
Raises: Sashite::Feen::Error subclasses on invalid input

position = Sashite::Feen.parse("8/8/8/8/8/8/8/8 / C/c")

# Board-less position (empty piece placement)
position = Sashite::Feen.parse(" / C/c")

`Sashite::Feen.dump(position)`

Converts a position object into its canonical FEEN string.

Parameter: position (Position) - Position object
Returns: String - Canonical FEEN string
Guarantees: Deterministic output (same position always produces same string)

feen_string = Sashite::Feen.dump(position)

Position Object

The Position object is immutable and provides read-only access to three components:

position.placement  # => Placement (board configuration)
position.hands      # => Hands (pieces in hand)
position.styles     # => Styles (style-turn information)
position.to_s       # => String (canonical FEEN)

Equality and hashing:

position1 == position2  # Component-wise equality
position1.hash          # Consistent hash for same positions

Placement Object

Represents the board configuration as a flat array of ranks with explicit separators.

placement.ranks         # => Array<Array> - Flat array of all ranks
placement.separators    # => Array<String> - Separators between ranks (e.g., ["/", "//"])
placement.dimension     # => Integer - Board dimensionality (1 + max consecutive slashes)
placement.rank_count    # => Integer - Total number of ranks
placement.one_dimensional? # => Boolean - True if dimension is 1
placement.all_pieces    # => Array - All pieces (nils excluded)
placement.total_squares # => Integer - Total square count
placement.to_s          # => String - Piece placement field
placement.to_a          # => Array - Array representation (dimension-aware)

`to_a` - Dimension-Aware Array Conversion

The to_a method returns an array representation that adapts to the board's dimensionality:

1D boards: Returns a single rank array (or empty array if no ranks)
2D+ boards: Returns array of ranks

# 1D board - Returns flat array
feen = "K2P3k / C/c"
position = Sashite::Feen.parse(feen)
position.placement.to_a
# => [K, nil, nil, P, nil, nil, nil, k]

# 2D board - Returns array of arrays
feen = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR / C/c"
position = Sashite::Feen.parse(feen)
position.placement.to_a
# => [[r,n,b,q,k,b,n,r], [p,p,p,p,p,p,p,p], [nil×8], ...]

# 3D board - Returns array of ranks (to be structured by application)
feen = "5/5//5/5 / R/r"
position = Sashite::Feen.parse(feen)
position.placement.to_a
# => [[nil×5], [nil×5], [nil×5], [nil×5]]

# Empty board
placement = Sashite::Feen::Placement.new([], [], 1)
placement.to_a
# => []

Other methods:

# Access specific positions
first_rank = placement.ranks[0]
piece_at_a1 = first_rank[0] # Piece object or nil

# Check dimensionality
placement.dimension # => 2 (2D board)

# Inspect separator structure
placement.separators # => ["/", "/", "/", "/", "/", "/", "/"]

Hands Object

Represents captured pieces held by each player.

hands.first_player   # => Array - Pieces held by first player
hands.second_player  # => Array - Pieces held by second player
hands.empty?         # => Boolean - True if both hands are empty
hands.to_s           # => String - Pieces-in-hand field

Example:

# Count pieces in hand
first_player_pawns = hands.first_player.count { |p| p.to_s == "P" }

# Check if any captures
hands.empty? # => false

Styles Object

Represents game styles and indicates the active player.

styles.active    # => SIN identifier - Active player's style
styles.inactive  # => SIN identifier - Inactive player's style
styles.to_s      # => String - Style-turn field

Example:

# Determine active player
styles.active.to_s    # => "C" (first player Chess)
styles.inactive.to_s  # => "c" (second player Chess)

# Check if cross-style
styles.active.to_s.upcase != styles.inactive.to_s.upcase

Examples

Chess Positions

# Starting position
chess_start = Sashite::Feen.parse(
  "+rnbq+kbn+r/+p+p+p+p+p+p+p+p/8/8/8/8/+P+P+P+P+P+P+P+P/+RNBQ+KBN+R / C/c"
)

# After 1.e4
after_e4 = Sashite::Feen.parse(
  "+rnbq+kbn+r/+p+p+p+p+p+p+p+p/8/8/4P3/8/+P+P+P+P1+P+P+P/+RNBQ+KBN+R / c/C"
)

# Ruy Lopez opening
ruy_lopez = Sashite::Feen.parse(
  "r1bqkbnr/+p+p+p+p1+p+p+p/2n5/1B2p3/4P3/5N2/+P+P+P+P1+P+P+P/RNBQK2R / c/C"
)

Shōgi with Captured Pieces

# Starting position
shogi_start = Sashite::Feen.parse(
  "lnsgkgsnl/1r5b1/ppppppppp/9/9/9/PPPPPPPPP/1B5R1/LNSGKGSNL / S/s"
)

# Position with pieces in hand
shogi_midgame = Sashite::Feen.parse(
  "lnsgkgsnl/1r5b1/pppp1pppp/9/4p4/9/PPPP1PPPP/1B5R1/LNSGKGSNL P/p s/S"
)

# Access captured pieces
position = shogi_midgame
position.hands.first_player   # => [P] (one pawn)
position.hands.second_player  # => [p] (one pawn)

# Count specific pieces in hand
position.hands.first_player.count { |p| p.to_s == "P" } # => 1

Cross-Style Games

# Chess vs Makruk
chess_vs_makruk = Sashite::Feen.parse(
  "rnsmksnr/8/pppppppp/8/8/8/+P+P+P+P+P+P+P+P/+RNBQ+KBN+R / C/m"
)

# Chess vs Shōgi
chess_vs_shogi = Sashite::Feen.parse(
  "lnsgkgsnl/1r5b1/pppppppp/9/9/9/+P+P+P+P+P+P+P+P/+RNBQ+KBN+R / C/s"
)

# Check styles
position = chess_vs_makruk
position.styles.active.to_s    # => "C" (Chess, first player)
position.styles.inactive.to_s  # => "m" (Makruk, second player)

Multi-Dimensional Boards

# 3D Chess (Raumschach)
raumschach = Sashite::Feen.parse(
  "rnknr/+p+p+p+p+p/5/5/5//buqbu/+p+p+p+p+p/5/5/5//5/5/5/5/5//5/5/5/+P+P+P+P+P/BUQBU//5/5/5/+P+P+P+P+P/RNKNR / R/r"
)

# Check dimensionality
raumschach.placement.dimension  # => 3 (3D board)
raumschach.placement.ranks.size # => 25 (total ranks)

# Inspect separator structure
level_seps = raumschach.placement.separators.count { |s| s == "//" }
rank_seps = raumschach.placement.separators.count { |s| s == "/" }
# level_seps => 4 (separates 5 levels)
# rank_seps => 20 (separates ranks within levels)

Irregular Boards

# Diamond-shaped board
diamond = Sashite::Feen.parse("3/4/5/4/3 / G/g")

# Check structure
diamond.placement.ranks.map(&:size) # => [3, 4, 5, 4, 3]

# Very large board
large_board = Sashite::Feen.parse("100/100/100 / G/g")
large_board.placement.total_squares # => 300

# Single square
single = Sashite::Feen.parse("K / C/c")
single.placement.rank_count # => 1

Completely Irregular Structures

FEEN supports any valid combination of ranks and separators:

# Extreme irregularity with variable separators
feen = "99999/3///K/k//r / G/g"
position = Sashite::Feen.parse(feen)

# Access the structure
position.placement.ranks.size      # => 5 ranks
position.placement.separators      # => ["/", "///", "/", "//"]
position.placement.dimension       # => 4 (max separator is "///")

# Each rank can have different sizes
position.placement.ranks[0].size   # => 99999
position.placement.ranks[1].size   # => 3
position.placement.ranks[2].size   # => 1
position.placement.ranks[3].size   # => 1
position.placement.ranks[4].size   # => 1

# Round-trip preservation
Sashite::Feen.dump(position) == feen # => true

Empty Ranks

FEEN supports empty ranks (ranks with no pieces):

# Trailing separator creates empty rank
feen = "K/// / C/c"
position = Sashite::Feen.parse(feen)

position.placement.ranks.size  # => 2
position.placement.ranks[0]    # => [K]
position.placement.ranks[1]    # => [] (empty rank)
position.placement.separators  # => ["///"]

# Round-trip preserves structure
Sashite::Feen.dump(position) == feen # => true

Board-less Positions

FEEN supports positions without piece placement, useful for tracking only style and turn information:

# Position with empty board (no piece placement)
board_less = Sashite::Feen.parse(" / C/c")

board_less.placement.ranks.size     # => 1
board_less.placement.dimension      # => 1
board_less.placement.to_a           # => []

# Convert back to FEEN
Sashite::Feen.dump(board_less) # => " / C/c"

Working with Positions

# Compare positions
position1 = Sashite::Feen.parse("8/8/8/8/8/8/8/8 / C/c")
position2 = Sashite::Feen.parse("8/8/8/8/8/8/8/8 / C/c")
position1 == position2 # => true

# Round-trip parsing
original = "+rnbq+kbn+r/+p+p+p+p+p+p+p+p/8/8/8/8/+P+P+P+P+P+P+P+P/+RNBQ+KBN+R / C/c"
position = Sashite::Feen.parse(original)
Sashite::Feen.dump(position) == original # => true

# Extract specific information
position.placement.ranks[0] # First rank (array of pieces/nils)
position.hands.first_player.size # Number of captured pieces

State Modifiers and Derivation

# Enhanced pieces (promoted, with special rights)
enhanced = Sashite::Feen.parse("+K+Q+R+B/8/8/8/8/8/8/8 / C/c")

# Diminished pieces (weakened, vulnerable)
diminished = Sashite::Feen.parse("-K-Q-R-B/8/8/8/8/8/8/8 / C/c")

# Foreign pieces (using opponent's style)
foreign = Sashite::Feen.parse("K'Q'R'B'/k'q'r'b'/8/8/8/8/8/8 / C/s")

Error Handling

FEEN defines specific error classes for different validation failures:

begin
  position = Sashite::Feen.parse("invalid feen")
rescue Sashite::Feen::Error => e
  # Base error class catches all FEEN errors
  warn "FEEN error: #{e.message}"
end

Error Hierarchy

Sashite::Feen::Error             # Base error class
├── Error::Syntax                # Malformed FEEN structure
├── Error::Piece                 # Invalid EPIN notation
├── Error::Style                 # Invalid SIN notation
├── Error::Count                 # Invalid piece counts
└── Error::Validation            # Other semantic violations

Common Errors

# Syntax error - wrong field count
Sashite::Feen.parse("8/8/8/8/8/8/8/8 /")
# => Error::Syntax: "FEEN must have exactly 3 space-separated fields, got 2"

# Style error - invalid SIN
Sashite::Feen.parse("8/8/8/8/8/8/8/8 / 1/2")
# => Error::Style: "failed to parse SIN '1': invalid SIN notation: '1' (must be a single letter A-Z or a-z)"

# Count error - invalid quantity
Sashite::Feen.parse("8/8/8/8/8/8/8/8 0P/ C/c")
# => Error::Count: "piece count must be at least 1, got 0"

Properties

Purely functional: Immutable data structures, no side effects
Canonical output: Deterministic string generation (same position → same string)
Specification compliant: Strict adherence to FEEN v1.0.0
Minimal API: Two methods (parse and dump) for complete functionality
Universal: Supports any abstract strategy board game
Completely flexible: Accepts any valid combination of ranks and separators
Perfect round-trip: parse(dump(position)) == position guaranteed
Dimension-aware: Intelligent array conversion based on board structure
Composable: Built on EPIN and SIN specifications

Dependencies

sashite-epin — Extended Piece Identifier Notation
sashite-sin — Style Identifier Notation

Documentation

FEEN Specification v1.0.0 — Complete technical specification
FEEN Examples — Comprehensive examples
API Documentation — Full API reference
GitHub Wiki — Advanced usage and patterns

Development

# Clone the repository
git clone https://github.com/sashite/feen.rb.git
cd feen.rb

# Install dependencies
bundle install

# Run tests
ruby test.rb

# Generate documentation
yard doc

Contributing

Fork the repository
Create a feature branch (git checkout -b feature/new-feature)
Add tests for your changes
Ensure all tests pass (ruby test.rb)
Commit your changes (git commit -am 'Add new feature')
Push to the branch (git push origin feature/new-feature)
Create a Pull Request

License

Available as open source under the MIT License.

About

Maintained by Sashité — promoting chess variants and sharing the beauty of board game cultures.

Feen.rb

What is FEEN?

Installation

Quick Start

FEEN Format

API Reference

Module Methods

Sashite::Feen.parse(string)

Sashite::Feen.dump(position)

Position Object

Placement Object

to_a - Dimension-Aware Array Conversion

Hands Object

Styles Object

Examples

Chess Positions

Shōgi with Captured Pieces

Cross-Style Games

Multi-Dimensional Boards

Irregular Boards

Completely Irregular Structures

Empty Ranks

Board-less Positions

Working with Positions

State Modifiers and Derivation

Error Handling

Error Hierarchy

Common Errors

Properties

Dependencies

Documentation

Development

Contributing

License

About

`Sashite::Feen.parse(string)`

`Sashite::Feen.dump(position)`

`to_a` - Dimension-Aware Array Conversion