FEEN Examples

Field Expression Encoding Notation (FEEN) v1.0.0
Author: Cyril Kato
License: Open Web Foundation Agreement 1.0

Overview

This document provides comprehensive examples of FEEN (Field Expression Encoding Notation) usage across various game types, scenarios, and configurations. These examples illustrate practical applications of the FEEN specification while remaining rule-agnostic.

Note: All examples are for illustration purposes only. FEEN does not define or assume any game-specific rules, movement patterns, or win conditions.

Quick Reference Examples

Traditional Game Starting Positions

# Chinese Xiangqi (2D: 9×10 = 90 squares, 32 pieces)
rheag^aehr/9/1c5c1/p1p1p1p1p/9/9/P1P1P1P1P/1C5C1/9/RHEAG^AEHR / X/x

# Japanese Shōgi (2D: 9×9 = 81 squares, 40 pieces)
lnsgk^gsnl/1r5b1/ppppppppp/9/9/9/PPPPPPPPP/1B5R1/LNSGK^GSNL / S/s

# Japanese Shōgi - Gote has the Jeweled King
lnsgk^'gsnl/1r5b1/ppppppppp/9/9/9/PPPPPPPPP/1B5R1/LNSGK^GSNL / S/s

# Japanese Shōgi - Sente has the Jeweled King
lnsgk^gsnl/1r5b1/ppppppppp/9/9/9/PPPPPPPPP/1B5R1/LNSGK^'GSNL / S/s

# Thai Makruk (2D: 8×8 = 64 squares, 32 pieces)
rnsmk^snr/8/pppppppp/8/8/PPPPPPPP/8/RNSK^MSNR / M/m

# Western Chess (2D: 8×8 = 64 squares, 32 pieces)
-rnbqk^bn-r/+p+p+p+p+p+p+p+p/8/8/8/8/+P+P+P+P+P+P+P+P/-RNBQK^BN-R / C/c

Games After Opening Moves

# Chess after 1.e4
-rnbqk^bn-r/+p+p+p+p+p+p+p+p/8/8/4P3/8/+P+P+P+P1+P+P+P/-RNBQK^BN-R / c/C

# Chess after 1.e4 c5 (Sicilian Defense)
-rnbqk^bn-r/+p+p1+p+p+p+p+p/8/2p5/4P3/8/+P+P+P+P1+P+P+P/-RNBQK^BN-R / C/c

# Shōgi after 1.P-7f
lnsgk^gsnl/1r5b1/ppppppppp/9/9/2P6/PP1PPPPPP/1B5R1/LNSGK^GSNL / s/S

Dimensional Examples

FEEN supports boards of any dimensionality through a hierarchical separator system.

1D Boards

1D boards have no separators — just a sequence of placement tokens.

# 1D board with 8 squares, 2 pieces
k^+p4+PK^ / C/c

# 1D board with 4 squares, 4 pieces
K^QqK^ / C/c

# Minimal 1D board: 1 square, 1 piece
k^ / S/s

# Minimal 1D board: 1 square, 0 pieces (empty)
1 / G/g

2D Boards

2D boards use single slashes (/) to separate ranks.

# Standard chess starting position (8 ranks × 8 files)
-rnbqk^bn-r/+p+p+p+p+p+p+p+p/8/8/8/8/+P+P+P+P+P+P+P+P/-RNBQK^BN-R / C/c

# Shōgi starting position (9 ranks × 9 files)
lnsgk^gsnl/1r5b1/ppppppppp/9/9/9/PPPPPPPPP/1B5R1/LNSGK^GSNL / S/s

# Irregular 2D board (ranks of different sizes: 3, 2, 4)
rkr/pp/PPPP / G/g

# Empty 8×8 board (64 squares, 0 pieces)
8/8/8/8/8/8/8/8 / C/c

# Empty 3×3 board (9 squares, 0 pieces)
3/3/3 / G/g

3D Boards

3D boards use double slashes (//) to separate layers, and single slashes (/) to separate ranks within each layer.

# 3D Raumschach starting position (5 layers × 5 ranks × 5 files = 125 squares)
-rnk^n-r/+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/-RNK^N-R / R/r

# Simple 3D board (2 layers × 2 ranks × 2 files = 8 squares)
ab/cd//AB/CD / G/g

# Empty 3D board (2 layers × 3 ranks × 3 files = 18 squares)
3/3/3//3/3/3 / G/g

4D Boards

4D boards use triple slashes (///) to separate cubes, double slashes (//) to separate layers, and single slashes (/) to separate ranks.

# 4D board (2 cubes × 2 layers × 2 ranks × 2 files = 16 squares)
ab/cd//ef/gh///AB/CD//EF/GH / G/g

Dimensional Coherence

FEEN enforces dimensional coherence: if a separator group of length N appears, each segment it separates must contain separators of length N-1.

Valid examples:

Piece Placement	Structure	Why valid
`rkr`	1D (3 squares)	No separators needed
`rkr/PPPP`	2D (2 ranks)	`/` separates segments
`a/b//c/d`	3D (2 layers × 2 ranks)	`//` separates structures containing `/`
`a/b//c/d//e/f`	3D (3 layers × 2 ranks)	`//` separates structures containing `/`
`a/b//c/d///e/f//g/h`	4D	`///` separates structures containing `//`

Invalid examples:

Piece Placement	Why invalid
`rkr//PPPP`	`//` used but segments lack `/` (skips dimension)
`a//b//c`	`//` used but segments lack `/`
`a/b///c/d`	`///` used but segments lack `//`
`a///b`	`///` used but segments lack `//` and `/`

Cardinality Constraints

FEEN enforces cardinality constraints inherited from the Game Protocol:

n ≥ 1: The board must have at least one square.
p ≤ n: The number of pieces cannot exceed the number of squares.

Where:

n = total squares (sum of empty-counts + piece tokens in Field 1)
p = total pieces (piece tokens in Field 1 + sum of multiplicities in Field 2)

Valid Examples

FEEN	n	p	Validity
`k^ / S/s`	1	1	✓ (1 ≤ 1)
`1 / G/g`	1	0	✓ (0 ≤ 1)
`8/8/8/8/8/8/8/8 / C/c`	64	0	✓ (0 ≤ 64)
`4/4/4/4 2P2p/ C/c`	16	4	✓ (4 ≤ 16)
`-rnbqk^bn-r/+p+p+p+p+p+p+p+p/8/8/8/8/+P+P+P+P+P+P+P+P/-RNBQK^BN-R / C/c`	64	32	✓ (32 ≤ 64)

Invalid Examples

FEEN	n	p	Why invalid
`K^k^ 2K^/2k^ S/s`	2	6	p > n (6 > 2)
`k^K^ 10k^/ S/s`	2	12	p > n (12 > 2)

Empty Board Positions

FEEN supports positions with zero pieces, useful for placement games like Go.

# Empty 19×19 Go board (361 squares, 0 pieces)
19/19/19/19/19/19/19/19/19/19/19/19/19/19/19/19/19/19/19 / G/g

# Empty 9×9 board (81 squares, 0 pieces)
9/9/9/9/9/9/9/9/9 / S/s

# Empty 8×8 board (64 squares, 0 pieces)
8/8/8/8/8/8/8/8 / C/c

# Minimal empty board (1 square, 0 pieces)
1 / G/g

Same-Style Games

Chess Positions

Opening Phase

# Ruy Lopez Opening: 1.e4 e5 2.Nf3 Nc6 3.Bb5
r1bqk^bnr/+p+p+p+p1+p+p+p/2n5/1B2p3/4P3/5N2/+P+P+P+P1+P+P+P/RNBQK^2R / c/C

# Queen's Gambit: 1.d4 d5 2.c4
-rnbqk^bn-r/+p+p+p1+p+p+p+p/8/3p4/2PP4/8/+P+P2+P+P+P+P/-RNBQK^BN-R / C/c

Cross-Style Games

Chess vs Alternative Styles

Chess vs Makruk

# Initial hybrid position (First player: Chess, Second player: Makruk)
rnsmk^snr/8/pppppppp/8/8/8/+P+P+P+P+P+P+P+P/-RNBQK^BN-R / C/m

Custom Style Combinations

# Fantasy styles
abcde^fgh/hgfedcba/8/8/8/8/ABCDEFGH/HGFED^CBA / A/b

# Numeric style identifiers
xyz123xy/8y^6/8/8/8/8/6Y^8/YX321ZYX / S/t

Positions with Captured Pieces

Pieces in Hand

The second field encodes pieces located in each player’s Hand.

# Position with captures (First player has: 1 pawn; Second player has: 1 bishop)
r1bq1b1r/+p+p+p+p1k^+p+p/2n2n2/4p3/4P3/5N2/+P+P+P+P1+P+P+P/-RNBQK^2+R p/B C/c

# Shōgi position with multiple pieces in hand
ln1gk^g1nl/1r2s2b1/pppp1pppp/9/4p4/9/PPPP1PPPP/1B5R1/LNSGK^GSNL P/p S/s

# Multiple pieces of same type in hand (using multiplicity)
8/8/8/8/8/8/8/8 3P2B/3p2b C/c

Hand Attribution

The Piece Side encoded in an EPIN token is independent of the Hand’s associated Side.

# First player's hand contains a piece with Side = second (lowercase 'p')
8/8/8/3K^4/8/8/8/8 p/ C/c

# Second player's hand contains a piece with Side = first (uppercase 'P')
8/8/8/3k^4/8/8/8/8 /P c/C

This reflects the Game Protocol’s rule that captured pieces go to the Active Player’s Hand regardless of their Piece Side.

Piece State Modifiers

EPIN tokens support state modifiers (+ for enhanced, - for diminished).

# Chess with castling rights encoded via state modifiers
# Rooks with '+' indicate castling is available
-rnbqk^bn-r/+p+p+p+p+p+p+p+p/8/8/8/8/+P+P+P+P+P+P+P+P/-RNBQK^BN-R / C/c

# Shōgi with promoted pieces ('+' prefix)
ln1gk^gsnl/1r5b1/p1pppp+Bpp/9/9/1p7/P1PPPPPPP/7R1/LNSGK^GSNL / S/s

Terminal Pieces

The terminal marker (^) indicates pieces critical to match continuation.

# Chess: Kings are terminal
-rnbqk^bn-r/+p+p+p+p+p+p+p+p/8/8/8/8/+P+P+P+P+P+P+P+P/-RNBQK^BN-R / C/c

# Xiangqi: Generals are terminal
rheag^aehr/9/1c5c1/p1p1p1p1p/9/9/P1P1P1P1P/1C5C1/9/RHEAG^AEHR / X/x

# Multiple terminal pieces
k^4K^/8/8/8/8/8/8/q^4Q^ / C/c

Canonical Form

FEEN requires canonical form for valid strings.

Empty-Count Merging

Consecutive empty squares must be merged into a single empty-count.

# Canonical (correct)
8/8/8/8/8/8/8/8 / C/c

# Non-canonical (invalid): "4" + "4" should be "8"
# 44/44/44/44/44/44/44/44 / C/c  ← INVALID

Hand Ordering

Hand items must be ordered by: multiplicity (desc), letter (asc), case (upper first), state modifier (- < + < none), terminal marker (absent < present), derivation marker (absent < present).

# Canonical ordering: 3P comes before 2B (higher count first)
8/8/8/8/8/8/8/8 3P2B/ C/c

# Canonical ordering: B before P (alphabetical), then uppercase before lowercase
8/8/8/8/8/8/8/8 BbPp/ C/c

QPI Inference Examples

These examples demonstrate how FEEN positions map to Qualified Piece Identifier (QPI) identifiers:

Simple QPI Conversion

Chess Position

Given this FEEN position:

-rnbqk^bn-r/+p+p+p1+p+p+p+p/8/8/4p3/8/+P+P+P+P1+P+P+P/-RNBQK^BN-R /P C/c

Board QPI Mappings:

-r (a8) → c:-r
-R (a1) → C:-R
p (e4) → c:p

Hand QPI Mappings:

P (second player’s hand) → C:P

Summary Table

Feature	Example	Notes
1D board	`k^+p4+PK^`	No separators
2D board	`8/8/8/8/8/8/8/8`	Single `/` separators
3D board	`a/b//c/d`	`//` separates layers
4D board	`a/b//c/d///e/f//g/h`	`///` separates cubes
Empty board	`8/8/8/8/8/8/8/8 / C/c`	p = 0 is valid
Pieces in hand	`8/8 3Pp/ C/c`	Multiplicity + EPIN tokens
State modifier	`+P`, `-R`	Enhanced/diminished states
Terminal marker	`k^`, `K^`	Critical pieces
Derivation marker	`k^'`, `K^'`	Derived style