Task 221: .FEN File Format
Task 221: .FEN File Format
1. List of All Properties of the .FEN File Format Intrinsic to Its File System
The .FEN (Forsyth-Edwards Notation) file format is a plain text (ASCII) file with the .fen extension, typically containing one or more FEN strings, one per line. It is designed to describe chess board positions and game state. The intrinsic properties are derived from the standard FEN structure, which consists of exactly 6 fields separated by single spaces in a single line. These properties are fixed and define the format's core structure:
- Piece Placement: A string representing the 8x8 board, starting from rank 8 to rank 1, with ranks separated by '/'. Each rank is a sequence of piece symbols (uppercase for white: R=rook, N=knight, B=bishop, Q=queen, K=king, P=pawn; lowercase for black) and digits 1-8 for consecutive empty squares. Total symbols per rank sum to 8.
- Active Color: A single character indicating whose turn it is: 'w' for white or 'b' for black.
- Castling Availability: A string indicating available castling rights: 'K' (white kingside), 'Q' (white queenside), 'k' (black kingside), 'q' (black queenside), or '-' if none.
- En Passant Target Square: A string in algebraic notation (e.g., 'e3') for the square vulnerable to en passant, or '-' if none.
- Halfmove Clock: An integer (0 or greater) counting plies since the last pawn advance or capture (used for 50-move rule).
- Fullmove Number: An integer (starting at 1) indicating the number of completed full moves.
These properties are mandatory for a valid FEN string. Files may contain multiple lines (positions), but each line must conform to this structure. No headers, footers, or binary elements are intrinsic; it's purely line-delimited text.
2. Two Direct Download Links for Files of Format .FEN
Based on searches, .FEN files are often compressed or embedded in datasets. Here are two direct download links to files containing FEN data (the lichess_db_puzzle.csv.zst is a compressed CSV with one FEN per puzzle position; lichess_db_eval.jsonl.zst is a compressed JSONL with one FEN per line per evaluation):
- https://database.lichess.org/lichess_db_puzzle.csv.zst (Lichess puzzle database, ~1.5 million FEN positions in CSV format, compressed with Zstandard)
- https://database.lichess.org/lichess_db_eval.jsonl.zst (Lichess evaluation database, millions of FEN positions with evaluations, one per JSONL line, compressed with Zstandard)
These can be decompressed (e.g., using zstd tool) to access the FEN strings.
3. Ghost Blog Embedded HTML JavaScript for Drag-and-Drop .FEN File Parsing
This is a self-contained HTML snippet with embedded JavaScript for embedding in a Ghost blog post (use the HTML card in Ghost editor). It supports drag-and-drop of a .FEN file, reads it as text, parses the first FEN string (assuming one position per file for simplicity), extracts the 6 properties, and dumps them to a <pre> block on screen. Handles multiple lines by parsing the first valid FEN.
Drag and drop a .FEN file here to parse its properties.
4. Python Class for .FEN File Handling
This class uses the built-in chess library (pip install python-chess if needed, but assuming availability). It opens a .FEN file, parses all FEN strings (one per line), prints properties for each, and supports writing a new FEN file from properties.
import chess
class FENHandler:
def __init__(self, filename=None):
self.filename = filename
self.positions = []
def read(self):
if not self.filename:
raise ValueError("Filename required for reading.")
with open(self.filename, 'r') as f:
for line_num, line in enumerate(f, 1):
fen_str = line.strip()
if not fen_str:
continue
try:
board = chess.Board(fen_str)
self.positions.append({
'line': line_num,
'piecePlacement': fen_str.split(' ')[0],
'activeColor': 'w' if board.turn else 'b',
'castlingAvailability': board.fen().split(' ')[2],
'enPassantTarget': board.ep_square if board.ep_square else '-',
'halfmoveClock': board.halfmove_clock,
'fullmoveNumber': board.fullmove_number
})
except ValueError as e:
print(f"Invalid FEN on line {line_num}: {e}")
return self.positions
def print_properties(self):
self.read()
for pos in self.positions:
print(f"Position from line {pos['line']}:")
for key, value in pos.items():
print(f" {key}: {value}")
print()
def write(self, output_filename, positions=None):
if positions is None:
positions = self.positions
with open(output_filename, 'w') as f:
for pos in positions:
fen = f"{pos['piecePlacement']} {pos['activeColor']} {pos['castlingAvailability']} {pos['enPassantTarget']} {pos['halfmoveClock']} {pos['fullmoveNumber']}"
f.write(fen + '\n')
# Example usage:
# handler = FENHandler('sample.fen')
# handler.print_properties()
# handler.write('output.fen')
5. Java Class for .FEN File Handling
This class manually parses FEN (no external libs assumed). It opens a .FEN file, parses lines, prints properties, and supports writing.
import java.io.*;
import java.util.*;
public class FENHandler {
private String filename;
private List<Map<String, Object>> positions = new ArrayList<>();
public FENHandler(String filename) {
this.filename = filename;
}
public void read() throws IOException {
try (BufferedReader reader = new BufferedReader(new FileReader(filename))) {
String line;
int lineNum = 1;
while ((line = reader.readLine()) != null) {
String fen = line.trim();
if (fen.isEmpty()) continue;
String[] fields = fen.split(" ");
if (fields.length != 6) {
System.out.println("Invalid FEN on line " + lineNum + ": Must have 6 fields.");
lineNum++;
continue;
}
try {
Map<String, Object> pos = new HashMap<>();
pos.put("line", lineNum);
pos.put("piecePlacement", fields[0]);
pos.put("activeColor", fields[1]);
pos.put("castlingAvailability", fields[2]);
pos.put("enPassantTarget", fields[3]);
pos.put("halfmoveClock", Integer.parseInt(fields[4]));
pos.put("fullmoveNumber", Integer.parseInt(fields[5]));
positions.add(pos);
} catch (NumberFormatException e) {
System.out.println("Invalid numbers on line " + lineNum + ": " + e.getMessage());
}
lineNum++;
}
}
}
public void printProperties() throws IOException {
read();
for (Map<String, Object> pos : positions) {
System.out.println("Position from line " + pos.get("line") + ":");
for (Map.Entry<String, Object> entry : pos.entrySet()) {
System.out.println(" " + entry.getKey() + ": " + entry.getValue());
}
System.out.println();
}
}
public void write(String outputFilename, List<Map<String, Object>> positionsToWrite) throws IOException {
try (PrintWriter writer = new PrintWriter(new FileWriter(outputFilename))) {
for (Map<String, Object> pos : positionsToWrite) {
String fen = String.format("%s %s %s %s %d %d",
pos.get("piecePlacement"),
pos.get("activeColor"),
pos.get("castlingAvailability"),
pos.get("enPassantTarget"),
pos.get("halfmoveClock"),
pos.get("fullmoveNumber"));
writer.println(fen);
}
}
}
// Example usage:
// FENHandler handler = new FENHandler("sample.fen");
// handler.printProperties();
// handler.write("output.fen", handler.positions);
}
6. JavaScript Class for .FEN File Handling
This Node.js-compatible class (uses fs for file I/O; for browser, adapt with FileReader). Manually parses FEN, prints to console, supports writing.
const fs = require('fs');
class FENHandler {
constructor(filename = null) {
this.filename = filename;
this.positions = [];
}
read() {
if (!this.filename) throw new Error('Filename required for reading.');
const content = fs.readFileSync(this.filename, 'utf8');
const lines = content.trim().split('\n');
lines.forEach((line, index) => {
const fen = line.trim();
if (!fen) return;
const fields = fen.split(' ');
if (fields.length !== 6) {
console.log(`Invalid FEN on line ${index + 1}: Must have 6 fields.`);
return;
}
try {
const pos = {
line: index + 1,
piecePlacement: fields[0],
activeColor: fields[1],
castlingAvailability: fields[2],
enPassantTarget: fields[3],
halfmoveClock: parseInt(fields[4], 10),
fullmoveNumber: parseInt(fields[5], 10)
};
this.positions.push(pos);
} catch (e) {
console.log(`Invalid numbers on line ${index + 1}: ${e.message}`);
}
});
return this.positions;
}
printProperties() {
this.read();
this.positions.forEach(pos => {
console.log(`Position from line ${pos.line}:`);
Object.entries(pos).forEach(([key, value]) => {
console.log(` ${key}: ${value}`);
});
console.log('');
});
}
write(outputFilename, positions = null) {
if (positions === null) positions = this.positions;
const fens = positions.map(pos =>
`${pos.piecePlacement} ${pos.activeColor} ${pos.castlingAvailability} ${pos.enPassantTarget} ${pos.halfmoveClock} ${pos.fullmoveNumber}`
);
fs.writeFileSync(outputFilename, fens.join('\n') + '\n');
}
}
// Example usage:
// const handler = new FENHandler('sample.fen');
// handler.printProperties();
// handler.write('output.fen');
7. C Class (Struct with Functions) for .FEN File Handling
This C implementation uses standard library (stdio, stdlib, string.h). Defines a struct for position, functions to read/parse/print/write. Compile with gcc (e.g., gcc fen_handler.c -o fen_handler).
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct {
int line;
char piece_placement[100];
char active_color[2];
char castling[5];
char en_passant[5];
int halfmove_clock;
int fullmove_number;
} FENPosition;
typedef struct {
FENPosition* positions;
int count;
} FENHandler;
FENHandler* fen_handler_new(const char* filename) {
FENHandler* handler = malloc(sizeof(FENHandler));
handler->positions = NULL;
handler->count = 0;
FILE* file = fopen(filename, "r");
if (!file) {
fprintf(stderr, "Error opening file: %s\n", filename);
free(handler);
return NULL;
}
char line[256];
int line_num = 1;
while (fgets(line, sizeof(line), file)) {
char* fen = line;
while (*fen == ' ' || *fen == '\t') fen++; // Trim start
char* end = fen + strlen(fen) - 1;
while (end > fen && (*end == ' ' || *end == '\t' || *end == '\n')) *end-- = '\0';
if (strlen(fen) == 0) {
line_num++;
continue;
}
char* fields[6];
int field_count = 0;
char* token = strtok(fen, " ");
while (token && field_count < 6) {
fields[field_count++] = token;
token = strtok(NULL, " ");
}
if (field_count != 6) {
fprintf(stderr, "Invalid FEN on line %d: Must have 6 fields.\n", line_num);
line_num++;
continue;
}
FENPosition pos = {line_num, "", "", "", "", 0, 0};
strncpy(pos.piece_placement, fields[0], sizeof(pos.piece_placement) - 1);
strncpy(pos.active_color, fields[1], 1);
strncpy(pos.castling, fields[2], sizeof(pos.castling) - 1);
strncpy(pos.en_passant, fields[3], sizeof(pos.en_passant) - 1);
pos.halfmove_clock = atoi(fields[4]);
pos.fullmove_number = atoi(fields[5]);
handler->positions = realloc(handler->positions, (handler->count + 1) * sizeof(FENPosition));
handler->positions[handler->count++] = pos;
line_num++;
}
fclose(file);
return handler;
}
void print_properties(FENHandler* handler) {
for (int i = 0; i < handler->count; i++) {
FENPosition* pos = &handler->positions[i];
printf("Position from line %d:\n", pos->line);
printf(" piecePlacement: %s\n", pos->piece_placement);
printf(" activeColor: %s\n", pos->active_color);
printf(" castlingAvailability: %s\n", pos->castling);
printf(" enPassantTarget: %s\n", pos->en_passant);
printf(" halfmoveClock: %d\n", pos->halfmove_clock);
printf(" fullmoveNumber: %d\n", pos->fullmove_number);
printf("\n");
}
}
void write_fen(FENHandler* handler, const char* output_filename) {
FILE* out = fopen(output_filename, "w");
if (!out) {
fprintf(stderr, "Error opening output file: %s\n", output_filename);
return;
}
for (int i = 0; i < handler->count; i++) {
FENPosition* pos = &handler->positions[i];
fprintf(out, "%s %s %s %s %d %d\n",
pos->piece_placement, pos->active_color, pos->castling,
pos->en_passant, pos->halfmove_clock, pos->fullmove_number);
}
fclose(out);
}
void fen_handler_free(FENHandler* handler) {
free(handler->positions);
free(handler);
}
// Example usage in main:
// int main() {
// FENHandler* handler = fen_handler_new("sample.fen");
// if (handler) {
// print_properties(handler);
// write_fen(handler, "output.fen");
// fen_handler_free(handler);
// }
// return 0;
// }