// ==UserScript==
// @name         Paperchan Image Import
// @match        https://paperchan.club/*
// @grant        none
// ==/UserScript==

(function() {
    const PALETTE = [
        [0,0,0], [255,255,255], [255,0,0], [0,0,255],
        [0,255,0], [255,0,255], [0,255,255], [255,255,0]
    ];

    function nearestColour(r, g, b) {
        let best = 0, bestDist = Infinity;
        for (let i = 0; i < PALETTE.length; i++) {
            const [pr, pg, pb] = PALETTE[i];
            const d = (r-pr)**2 + (g-pg)**2 + (b-pb)**2;
            if (d < bestDist) { bestDist = d; best = i; }
        }
        return PALETTE[best];
    }

    function ditherImage(img, canvas) {
        const w = canvas.width, h = canvas.height;
        const tmp = document.createElement('canvas');
        tmp.width = w; tmp.height = h;
        const tctx = tmp.getContext('2d');
        tctx.drawImage(img, 0, 0, w, h);
        const data = tctx.getImageData(0, 0, w, h);
        const px = data.data;

        for (let y = 0; y < h; y++) {
            for (let x = 0; x < w; x++) {
                const i = (y * w + x) * 4;
                const or = px[i], og = px[i+1], ob = px[i+2];
                const [nr, ng, nb] = nearestColour(or, og, ob);
                px[i] = nr; px[i+1] = ng; px[i+2] = nb;
                const er = or - nr, eg = og - ng, eb = ob - nb;
                const spread = [[1,0,7/16],[-1,1,3/16],[0,1,5/16],[1,1,1/16]];
                for (const [dx, dy, f] of spread) {
                    const nx = x+dx, ny = y+dy;
                    if (nx>=0 && nx<w && ny>=0 && ny<h) {
                        const j = (ny*w+nx)*4;
                        px[j]   = Math.max(0,Math.min(255, px[j]   + er*f));
                        px[j+1] = Math.max(0,Math.min(255, px[j+1] + eg*f));
                        px[j+2] = Math.max(0,Math.min(255, px[j+2] + eb*f));
                    }
                }
            }
        }

        const ctx = canvas.getContext('2d');
        ctx.putImageData(data, 0, 0);
    }

    const sendBtn = document.getElementById('sendbtn');
    if (!sendBtn) return;

    const input = document.createElement('input');
    input.type = 'file';
    input.accept = 'image/*';
    input.style.display = 'none';

    const btn = document.createElement('button');
    btn.type = 'button';
    btn.textContent = 'Import Image';
    btn.style.marginLeft = '8px';

    btn.addEventListener('click', () => input.click());

    input.addEventListener('change', () => {
        const file = input.files[0];
        if (!file) return;
        const reader = new FileReader();
        reader.onload = e => {
            const img = new Image();
            img.onload = () => {
                const canvas = document.getElementById('canvas');
                ditherImage(img, canvas);
            };
            img.src = e.target.result;
        };
        reader.readAsDataURL(file);
    });

    sendBtn.parentNode.insertBefore(btn, sendBtn.nextSibling);
    sendBtn.parentNode.insertBefore(input, sendBtn.nextSibling);
})();