Describe a chess position in FEN and get the image

board.go

/*
A simple Go program, http://golang.org, i wrote to demonstrate the
language. It converts chess FEN notation to images or html tables.
Uses the public domain pieces from http://www.crockford.com/chess/pieces.html

The program is small in size but not in functionality. It includes
http client and server, html templates, image processing, strings, io
and demonstrates the expressiveness of Go

See http://blog.golang.org/2011/12/from-zero-to-go-launching-on-google.html
for a more advanced example of the same APIs running on GAE

Usage:
 - go run fen.go
 - open chrome to http://localhost:9000 to see the examples 

by http://www.twitter.com/anastasop
*/

package main

import (
	"bytes"
	"fmt"
	"strings"
	"io"
	"io/ioutil"
	"image"
	"image/gif"
	"image/png"
	"image/draw"
	"html/template"
	"net/http"
)

var piecesImages map[string]image.Image = make(map[string]image.Image)
var piecesGIFs map[string][]byte = make(map[string][]byte)
var boardTemplate *template.Template

func downloadPiece(url string) (image.Image, []byte, error) {
	r, e := http.Get(url)
	if e != nil {
		return nil, nil, fmt.Errorf("failed to download piece %s: %s", url, e)
	}
	defer r.Body.Close()
	pix, e := ioutil.ReadAll(r.Body)
	if e != nil {
		return nil, nil, fmt.Errorf("failed to download piece %s: %s", url, e)
	}
	img, e := gif.Decode(bytes.NewReader(pix))
	if e != nil {
		return nil, nil, fmt.Errorf("failed to decode piece %s: %s", url, e)
	}
	return img, pix, nil
}

/* download the public domain chess pieces from http://www.crockford.com/chess/pieces.html */
func downloadPieces() {
	baseurl := "http://www.crockford.com/chess/"
	for _, square := range "wb" {
		for _, color := range "wb" {
			for _, name := range "prnbqk" {
				key := string([]rune{color, name, square})
				if img, pix, e := downloadPiece(baseurl + key + ".gif"); e != nil {
					panic(e)
				} else {
					piecesImages[key] = img
					piecesGIFs[key] = pix
				}
			}
		}
	}
	if img, pix, e := downloadPiece(baseurl + "w" + ".gif"); e != nil {
		panic(e)
	} else {
		piecesImages["w"] = img
		piecesGIFs["w"] = pix
	}
	if img, pix, e := downloadPiece(baseurl + "b" + ".gif"); e != nil {
		panic(e)
	} else {
		piecesImages["b"] = img
		piecesGIFs["b"] = pix
	}
}

func pieceHandler(w http.ResponseWriter, req *http.Request) {
	parts := strings.Split(req.URL.Path, "/")
	key := parts[len(parts) - 1]
	if v, ok := piecesGIFs[key]; ok {
		w.Header().Set("Content-Type", "image/gif")
		w.Write(v)
	} else {
		http.NotFound(w, req)
	}
}

func fenHandler(w http.ResponseWriter, req *http.Request) {
	fen := req.FormValue("fen")
	if fen == "" {
		fen = "rnbqkbnr/pppppppp/////PPPPPPPP/RNBQKBNR"
	}
	how := req.FormValue("r")
	if how == "" {
		how = "html"
	}
	board := parse(fen)
	switch how {
	default:
		w.Header().Set("Content-Type", "text/html")
		boardTemplate.Execute(w, board)
	case "image":
		w.Header().Set("Content-Type", "image/png")
		w.Header().Set("Cache-Control", "no-store, no-cache")
		img := image.NewRGBA(image.Rect(0, 0, 256, 256))
		for r, rank := range board {
			for f, piece := range rank {
				p := image.Pt(f * 32, r * 32)
				draw.Draw(img, image.Rect(p.X, p.Y, p.X + 32, p.Y + 32),
					piecesImages[piece], image.ZP, draw.Src)
			}
		}
		png.Encode(w, img)
	}
}

func main() {
	downloadPieces()
	t, e := template.New("Board").Parse(
		`<table>{{range .}}<tr>{{range .}}<td><img src="../pieces/{{.}}"/></td>{{end}}</tr>{{end}}</table>`)
	if e != nil {
		panic(e)
	} else {
		boardTemplate = t
	}

	http.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
		io.WriteString(w, indexPage)
	});
	http.HandleFunc("/board", fenHandler)
	http.HandleFunc("/pieces/", pieceHandler)
	e = http.ListenAndServe(":9000", nil)
	if e != nil {
		panic(e)
	}
}

func parse(fen string) [8][8]string  {
	var board [8][8]string
	fen = strings.TrimSpace(fen)
	ranks := strings.Split(fen +"/////////", "/")[:8]
	repl := strings.NewReplacer(
		"1", "*", "2", "**", "3", "***", "4", "****", "5", "*****",
		"6", "******", "7", "*******", "8", "********")
	var colors = [2]string{"w", "b"}
	for r, rank := range ranks {
		s := (repl.Replace(rank) + "********")[:8]
		for f, p := range s {
			var piece string
			switch p {
			case 'P', 'R', 'N', 'B', 'Q', 'K':
				piece = "w" + strings.ToLower(string(p))
			case 'p', 'r', 'n', 'b', 'q', 'k':
				piece = "b" + strings.ToLower(string(p))
			default:
				piece = ""
			}
			board[r][f] = piece + colors[(f + (r % 2)) % 2]
		}
	}
	return board
}

var indexPage string = `<!DOCTYPE html>
<html>
        <head>
                <title>Chess Board Generator</title>
                <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
                <script>
                        $(function() {
				$("#asTable").click(function(e) {
					var fen = $("#fen").val()
                                        $("#board").load('/board', "fen=" + encodeURIComponent(fen) + "&r=html");
                                });
                                $("#asImage").click(function(e) {
					var fen = $("#fen").val()
					var url = "/board?fen=" + encodeURIComponent(fen) + "&r=image";
					$("#board").html('<img src="' + url + '">');
                                });
				$("#asImage").click()
                        });
                </script>
        </head>
        <body>
                <h1>Chess Board Generator</h1>
                Enter a chess board notation in <a href="http://en.wikipedia.org/wiki/Forsyth%E2%80%93Edwards_Notation">FEN</a> and see it either as an html table of individual images or as a single PNG image
                <h2>Examples</h2>
                <ul class="examples">
                        <li>Starting Position - <b>rnbqkbnr/pppppppp/////PPPPPPPP/RNBQKBNR</b></li>
                        <li>Lucena Position - <b>1K1k/1P6///r///2R</b></li>
                        <li>Kasparov vs Topalov 1999 - <b>b2r3r/k4p1p/p2q1np1/NppP4/3p1Q2/P4PPB/1PP4P/1K1RR3</b></li>
                </ul>
                <label for="fen">FEN:</label><input type="text" size="66" id="fen" value="4k///////4K" />
		<button id="asImage">Image</button><button id="asTable">Table</button><br><br>
		<div id="board"></div>
        </body>
</html>
`