Chess

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</style></head><body><main class="app"><section class="board-wrap"><div id="board" class="board" aria-label="Chess board"></div><div id="promo" class="promo" hidden></div><div id="banner" class="banner"></div></section><aside class="panel"><div class="brand"><h1>Walnut Chess</h1><small>classical engine</small></div><div id="status" class="status">White to move</div><div class="trays"><div class="tray"><label>White captured</label><span id="capW"></span></div><div class="tray"><label>Black captured</label><span id="capB"></span></div></div><div id="moves" class="moves"></div><div class="selectrow"><select id="side"><option value="w">Play White</option><option value="b">Play Black</option><option value="both">Analysis / both sides</option></select><select id="depth"><option value="1">Difficulty · I</option><option value="2">Difficulty · II</option><option value="3" selected>Difficulty · III</option></select></div><div class="controls"><button id="undo">↶ Undo turn</button><button id="flip">⇅ Flip board</button><button id="new">New game</button><button id="mute">♬ Sound on</button></div><p class="hint">Click or drag a piece. Legal squares are marked; red rings capture. Your game is saved automatically.</p></aside></main><script>
'use strict';
/* ===== Tunable constants ===== */
const CFG={depths:{1:1,2:2,3:3},AI_DELAY:80,VALUE:{p:100,n:320,b:330,r:500,q:900,k:20000}};
const FILES='abcdefgh', GLYPH={K:'♔',Q:'♕',R:'♖',B:'♗',N:'♘',P:'♙',k:'♚',q:'♛',r:'♜',b:'♝',n:'♞',p:'♟'};
const PST={p:[0,0,0,0,0,0,0,0,5,10,10,-20,-20,10,10,5,5,-5,-10,0,0,-10,-5,5,0,0,0,20,20,0,0,0,5,5,10,25,25,10,5,5,10,10,20,30,30,20,10,10,50,50,50,50,50,50,50,50,0,0,0,0,0,0,0,0],n:[-50,-40,-30,-30,-30,-30,-40,-50,-40,-20,0,5,5,0,-20,-40,-30,5,10,15,15,10,5,-30,-30,0,15,20,20,15,0,-30,-30,5,15,20,20,15,5,-30,-30,0,10,15,15,10,0,-30,-40,-20,0,0,0,0,-20,-40,-50,-40,-30,-30,-30,-30,-40,-50],b:[-20,-10,-10,-10,-10,-10,-10,-20,-10,5,0,0,0,0,5,-10,-10,10,10,10,10,10,10,-10,-10,0,10,10,10,10,0,-10,-10,5,5,10,10,5,5,-10,-10,0,5,10,10,5,0,-10,-10,0,0,0,0,0,0,-10,-20,-10,-10,-10,-10,-10,-10,-20]};
/* ===== Engine: mailbox array, rank 8 at index 0 ===== */
function initial(){return {b:['r','n','b','q','k','b','n','r','p','p','p','p','p','p','p','p',...Array(32).fill(null),'P','P','P','P','P','P','P','P','R','N','B','Q','K','B','N','R'],turn:'w',castle:'KQkq',ep:-1,half:0,full:1,history:[],last:null};}
let S=initial(), selected=-1, targets=[], flipped=false, dragFrom=-1, aiTimer=null, muted=false;
const color=p=>p?(p===p.toUpperCase()?'w':'b'):null, enemy=c=>c==='w'?'b':'w', rc=i=>[Math.floor(i/8),i%8], idx=(r,c)=>r*8+c, alg=i=>FILES[i%8]+(8-Math.floor(i/8)), at=s=>{let f=FILES.indexOf(s[0]),r=8-+s[1];return f<0||r<0||r>7?-1:idx(r,f)};
function clone(s){return {b:s.b.slice(),turn:s.turn,castle:s.castle,ep:s.ep,half:s.half,full:s.full,history:s.history.slice(),last:s.last?{...s.last}:null};}
function attacked(s,sq,by){let [r,c]=rc(sq), b=s.b;
 // pawns attacking target: source rank depends color
 let pr=r+(by==='w'?1:-1);for(let dc of[-1,1])if(pr>=0&&pr<8&&c+dc>=0&&c+dc<8&&b[idx(pr,c+dc)]===(by==='w'?'P':'p'))return true;
 for(let [dr,dc] of [[-2,-1],[-2,1],[-1,-2],[-1,2],[1,-2],[1,2],[2,-1],[2,1]])if(r+dr>=0&&r+dr<8&&c+dc>=0&&c+dc<8&&b[idx(r+dr,c+dc)]===(by==='w'?'N':'n'))return true;
 for(let [dr,dc,types] of [[-1,0,'rq'],[1,0,'rq'],[0,-1,'rq'],[0,1,'rq'],[-1,-1,'bq'],[-1,1,'bq'],[1,-1,'bq'],[1,1,'bq']]){let R=r+dr,C=c+dc;while(R>=0&&R<8&&C>=0&&C<8){let p=b[idx(R,C)];if(p){if(color(p)===by&&types.includes(p.toLowerCase()))return true;break}R+=dr;C+=dc}}
 for(let dr=-1;dr<=1;dr++)for(let dc=-1;dc<=1;dc++)if((dr||dc)&&r+dr>=0&&r+dr<8&&c+dc>=0&&c+dc<8&&b[idx(r+dr,c+dc)]===(by==='w'?'K':'k'))return true;return false;
}
function inCheck(s,c){let k=s.b.indexOf(c==='w'?'K':'k');return k>=0&&attacked(s,k,enemy(c));}
function pseudo(s){let out=[],c=s.turn,b=s.b;const add=(f,t,extra={})=>out.push({from:f,to:t,piece:b[f],capture:b[t]||null,...extra});for(let i=0;i<64;i++){let p=b[i];if(color(p)!==c)continue;let [r,x]=rc(i),t=p.toLowerCase();if(t==='p'){let d=c==='w'?-1:1,start=c==='w'?6:1,prom=c==='w'?0:7,one=r+d;if(one>=0&&!b[idx(one,x)]){if(one===prom)for(let q of['q','r','b','n'])add(i,idx(one,x),{promotion:q});else add(i,idx(one,x));if(r===start&&!b[idx(r+2*d,x)])add(i,idx(r+2*d,x),{double:true})}for(let dc of[-1,1]){let R=r+d,C=x+dc;if(R>=0&&R<8&&C>=0&&C<8){let to=idx(R,C);if(b[to]&&color(b[to])!==c){if(R===prom)for(let q of['q','r','b','n'])add(i,to,{promotion:q});else add(i,to)}if(to===s.ep)add(i,to,{ep:true,capture:c==='w'?'p':'P'})}}}
 else if(t==='n'){for(let [dr,dc] of [[-2,-1],[-2,1],[-1,-2],[-1,2],[1,-2],[1,2],[2,-1],[2,1]]){let R=r+dr,C=x+dc;if(R>=0&&R<8&&C>=0&&C<8&&color(b[idx(R,C)])!==c)add(i,idx(R,C))}}
 else if(t==='k'){for(let dr=-1;dr<=1;dr++)for(let dc=-1;dc<=1;dc++)if(dr||dc){let R=r+dr,C=x+dc;if(R>=0&&R<8&&C>=0&&C<8&&color(b[idx(R,C)])!==c)add(i,idx(R,C))}let rank=c==='w'?7:0, rights=c==='w'?['K','Q']:['k','q'];if(i===idx(rank,4)&&!inCheck(s,c)){if(s.castle.includes(rights[0])&&!b[idx(rank,5)]&&!b[idx(rank,6)]&&b[idx(rank,7)]===(c==='w'?'R':'r')&&!attacked(s,idx(rank,5),enemy(c))&&!attacked(s,idx(rank,6),enemy(c)))add(i,idx(rank,6),{castle:'K'});if(s.castle.includes(rights[1])&&!b[idx(rank,3)]&&!b[idx(rank,2)]&&!b[idx(rank,1)]&&b[idx(rank,0)]===(c==='w'?'R':'r')&&!attacked(s,idx(rank,3),enemy(c))&&!attacked(s,idx(rank,2),enemy(c)))add(i,idx(rank,2),{castle:'Q'})}}
 else {let dirs=t==='r'?[[-1,0],[1,0],[0,-1],[0,1]]:t==='b'?[[-1,-1],[-1,1],[1,-1],[1,1]]:[[-1,0],[1,0],[0,-1],[0,1],[-1,-1],[-1,1],[1,-1],[1,1]];for(let [dr,dc]of dirs){let R=r+dr,C=x+dc;while(R>=0&&R<8&&C>=0&&C<8){let q=idx(R,C);if(!b[q])add(i,q);else{if(color(b[q])!==c)add(i,q);break}R+=dr;C+=dc}}}}
 return out;
}
function apply(s,m){let n=clone(s),p=n.b[m.from],c=color(p);n.b[m.to]=m.promotion?(c==='w'?m.promotion.toUpperCase():m.promotion):p;n.b[m.from]=null;if(m.ep)n.b[m.to+(c==='w'?8:-8)]=null;if(m.castle){let r=c==='w'?7:0;if(m.castle==='K'){n.b[idx(r,5)]=n.b[idx(r,7)];n.b[idx(r,7)]=null}else{n.b[idx(r,3)]=n.b[idx(r,0)];n.b[idx(r,0)]=null}}let rights=n.castle; if(p.toLowerCase()==='k')rights=rights.replace(c==='w'?/K|Q/g:/k|q/g,'');if(p.toLowerCase()==='r'||m.capture){for(let z of [[63,'K'],[56,'Q'],[7,'k'],[0,'q']])if(m.from===z[0]||m.to===z[0])rights=rights.replace(z[1],'')}n.castle=rights;n.ep=m.double?(m.from+m.to)/2:-1;n.half=(p.toLowerCase()==='p'||m.capture||m.ep)?0:n.half+1;n.full+=c==='b'?1:0;n.turn=enemy(c);n.last={from:m.from,to:m.to};return n;}
function legal(s=S){let c=s.turn;return pseudo(s).filter(m=>!inCheck(apply(s,m),c));}
function san(before,m,after){if(m.castle)return 'O-O'+(m.castle==='Q'?'-O':'')+(inCheck(after,after.turn)?(legal(after).length?'+':'#'):'');let p=m.piece.toUpperCase(), cap=!!(m.capture||m.ep),same=legal(before).filter(x=>x!==m&&x.to===m.to&&x.piece===m.piece);let dis='';if(p!=='P'&&same.length){let fs=same.some(x=>x.from%8===m.from%8),rs=same.some(x=>Math.floor(x.from/8)===Math.floor(m.from/8));dis=fs&&!rs?String(8-Math.floor(m.from/8)):FILES[m.from%8]}let txt=(p==='P'?(cap?FILES[m.from%8]:''):p+dis)+(cap?'x':'')+alg(m.to)+(m.promotion?'='+m.promotion.toUpperCase():'')+(m.ep?' e.p.':'');if(inCheck(after,after.turn))txt+=legal(after).length?'+':'#';return txt;}
/* ===== UI and game flow ===== */
const $=id=>document.getElementById(id);function render(){let b=$('board');b.innerHTML='';let order=[0,1,2,3,4,5,6,7];if(flipped)order=order.reverse();for(let rr of order)for(let cc of order){let r=rr,c=cc,i=idx(r,c),d=document.createElement('div'),p=S.b[i];d.className='sq '+((r+c)%2?'dark':'light')+(S.last&&(S.last.from===i||S.last.to===i)?' last':'')+(selected===i?' selected':'')+(inCheck(S,S.turn)&&p===(S.turn==='w'?'K':'k')?' check':'');d.dataset.i=i;if(targets.some(m=>m.to===i))d.classList.add(S.b[i]||targets.find(m=>m.to===i).ep?'capture':'target');d.innerHTML=(c===0?'<span class="coord rank">'+(8-r)+'</span>':'')+(r===7?'<span class="coord file">'+FILES[c]+'</span>':'')+(p?'<span draggable="true" class="piece '+(color(p)==='w'?'white':'black')+'">'+GLYPH[p]+'</span>':'');b.appendChild(d)}renderInfo();}
function renderInfo(){let caps={w:[],b:[]};for(let h of S.history)if(h.capture)caps[color(h.capture)].push(GLYPH[h.capture]);$('capW').textContent=caps.w.join('');$('capB').textContent=caps.b.join('');let html='';for(let i=0;i<S.history.length;i+=2)html+='<div class="move-row"><span class="move-no">'+(i/2+1)+'.</span><span>'+S.history[i].san+'</span><span>'+(S.history[i+1]?.san||'')+'</span></div>';$('moves').innerHTML=html;$('moves').scrollTop=99999;let over=outcome();$('banner').className='banner '+(over?'show':'');$('banner').textContent=over||'';let player=$('side').value, ai=player!=='both'&&S.turn!==player;$('status').textContent=over||((ai?'Computer thinking…':(S.turn==='w'?'White':'Black')+' to move')+(inCheck(S,S.turn)?' — CHECK':''));$('status').classList.toggle('thinking',ai&&!over);}
function outcome(){let L=legal();if(!L.length)return inCheck(S,S.turn)?'Checkmate — '+(S.turn==='w'?'Black':'White')+' wins':'Draw — stalemate';if(S.half>=100)return 'Draw — fifty-move rule';let pcs=S.b.filter(Boolean).filter(x=>x.toLowerCase()!=='k');if(!pcs.length||(pcs.length===1&&['b','n'].includes(pcs[0].toLowerCase())))return 'Draw — insufficient material';return ''}
function select(i){if(outcome()||isAI())return;let p=S.b[i];if(selected>=0){let opts=targets.filter(m=>m.to===i);if(opts.length){if(opts[0].promotion){showPromotion(opts);return}play(opts[0]);return}}if(p&&color(p)===S.turn){selected=i;targets=legal().filter(m=>m.from===i)}else{selected=-1;targets=[]}render()}
function showPromotion(opts){let q=$('promo'), to=opts[0].to,[r,c]=rc(to);q.hidden=false;q.style.left=(c*12.5)+'%';q.style.top=((r>3?r-3:r)*12.5)+'%';q.innerHTML='';for(let x of opts){let bt=document.createElement('button');bt.textContent=GLYPH[S.turn==='w'?x.promotion.toUpperCase():x.promotion];bt.onclick=()=>{q.hidden=true;play(x)};q.appendChild(bt)}}
function sound(type){if(muted)return;try{let ac=new(window.AudioContext||window.webkitAudioContext)(),o=ac.createOscillator(),g=ac.createGain();o.frequency.value=type==='check'?180:type==='capture'?120:360;g.gain.setValueAtTime(.045,ac.currentTime);g.gain.exponentialRampToValueAtTime(.001,ac.currentTime+.12);o.connect(g);g.connect(ac.destination);o.start();o.stop(ac.currentTime+.13)}catch(e){}}
function play(m){let before=S,after=apply(S,m);m.san=san(before,m,after);m.capture=m.capture||(m.ep?(before.turn==='w'?'p':'P'):null);after.history=before.history.concat([{...m}]);S=after;selected=-1;targets=[];save();sound(inCheck(S,S.turn)?'check':m.capture?'capture':'move');render();if(!outcome())scheduleAI()}
function isAI(){let p=$('side').value;return p!=='both'&&S.turn!==p}function scheduleAI(){clearTimeout(aiTimer);if(isAI()&&!outcome())aiTimer=setTimeout(()=>{let m=bestMove(S,+$('depth').value);if(m)play(m)},CFG.AI_DELAY)}
/* ===== AI: alpha-beta with capture-first ordering ===== */
function evaluate(s){let score=0;for(let i=0;i<64;i++){let p=s.b[i];if(!p)continue;let v=CFG.VALUE[p.toLowerCase()]||0,table=PST[p.toLowerCase()],pos=color(p)==='w'?i:63-i;if(table)v+=table[pos];score+=color(p)==='w'?v:-v}return score}
function ordered(s){return legal(s).sort((a,b)=>((b.capture?10*CFG.VALUE[b.capture.toLowerCase()]:0)+(b.promotion?800:0))-((a.capture?10*CFG.VALUE[a.capture.toLowerCase()]:0)+(a.promotion?800:0)))}
function search(s,d,a,b){let L=ordered(s);if(!d||!L.length){if(!L.length)return inCheck(s,s.turn)?(s.turn==='w'?-999999:999999):0;return evaluate(s)}if(s.turn==='w'){let v=-1e9;for(let m of L){v=Math.max(v,search(apply(s,m),d-1,a,b));a=Math.max(a,v);if(a>=b)break}return v}else{let v=1e9;for(let m of L){v=Math.min(v,search(apply(s,m),d-1,a,b));b=Math.min(b,v);if(a>=b)break}return v}}
function bestMove(s,d){let L=ordered(s),best=L[0],val=s.turn==='w'?-1e9:1e9;for(let m of L){let v=search(apply(s,m),d-1,-1e9,1e9);if((s.turn==='w'&&v>val)||(s.turn==='b'&&v<val)){val=v;best=m}}return best}
function save(){localStorage.setItem('walnutChess',JSON.stringify(S))}function restore(){try{let x=JSON.parse(localStorage.getItem('walnutChess'));if(x&&x.b&&x.b.length===64)S=x}catch(e){}}function reset(){clearTimeout(aiTimer);S=initial();selected=-1;targets=[];save();render();scheduleAI()}
$('board').onclick=e=>{let q=e.target.closest('.sq');if(q)select(+q.dataset.i)};$('board').addEventListener('dragstart',e=>{let q=e.target.closest('.sq');if(q){dragFrom=+q.dataset.i;e.dataTransfer.setData('text','x')}});$('board').addEventListener('dragover',e=>e.preventDefault());$('board').addEventListener('drop',e=>{e.preventDefault();let q=e.target.closest('.sq');if(q){selected=dragFrom;targets=legal().filter(m=>m.from===dragFrom);select(+q.dataset.i)}});
$('undo').onclick=()=>{clearTimeout(aiTimer);let n=$('side').value==='both'?1:2;if(S.history.length){let keep=Math.max(0,S.history.length-n),moves=S.history.slice(0,keep),fresh=initial();for(let h of moves){let m=legal(fresh).find(x=>x.from===h.from&&x.to===h.to&&x.promotion===h.promotion);fresh=apply(fresh,m);fresh.history=fresh.history.concat([h])}S=fresh;save();render()}};$('flip').onclick=()=>{flipped=!flipped;render()};$('new').onclick=reset;$('mute').onclick=()=>{muted=!muted;$('mute').textContent=muted?'♬ Sound off':'♬ Sound on'};$('side').onchange=()=>{render();scheduleAI()};$('depth').onchange=save;
/* ===== Debug interface ===== */
window.game={fen:()=>{let rows=[];for(let r=0;r<8;r++){let z='',n=0;for(let c=0;c<8;c++){let p=S.b[idx(r,c)];if(p){if(n)z+=n,n=0;z+=p}else n++}if(n)z+=n;rows.push(z)}return rows.join('/')+' '+S.turn+' '+(S.castle||'-')+' '+(S.ep>=0?alg(S.ep):'-')+' '+S.half+' '+S.full},legalMoves:()=>legal().map(m=>alg(m.from)+alg(m.to)+(m.promotion||'')),move:str=>{let L=legal(),m=L.find(x=>alg(x.from)+alg(x.to)+(x.promotion||'')===str.toLowerCase());if(!m)return false;play(m);return true},newGame:reset};restore();render();scheduleAI();
</script></body></html>
<!-- agent-meta {"model":"openai/gpt-5.6-terra","provider":"openrouter","persona":"full","sessionId":"f6e2aa36-8d98-4169-bc89-80ed667d1c6a","tokensIn":73547,"tokensOut":7374,"tokensTotal":80921,"turns":7,"toolCalls":6,"failedToolCalls":0,"timestamp":"2026-07-31T19:17:05.861Z"} -->