function initJwtDecoder() {
    const inputEl = document.getElementById("jwtInput");
    const statusEl = document.getElementById("jwtStatus");
    const claimsMetaEl = document.getElementById("jwtClaimsMeta");
    const headerOutEl = document.getElementById("jwtHeaderOutput");
    const payloadOutEl = document.getElementById("jwtPayloadOutput");
    const signatureEl = document.getElementById("jwtSignature");
    const decodeBtn = document.getElementById("jwtDecodeBtn");
    const copyHeaderBtn = document.getElementById("jwtCopyHeaderBtn");
    const copyPayloadBtn = document.getElementById("jwtCopyPayloadBtn");
    const clearBtn = document.getElementById("jwtClearBtn");

    if (!inputEl || !headerOutEl || !payloadOutEl || !signatureEl) return;

    function setStatus(text, isError) {
        if (!statusEl) return;
        statusEl.textContent = text;
        statusEl.classList.toggle("status-error", !!isError);
        statusEl.classList.toggle("status-success", !isError);
    }

    function setClaimsMeta(text, isError) {
        if (!claimsMetaEl) return;
        claimsMetaEl.textContent = text;
        claimsMetaEl.classList.toggle("status-error", !!isError);
        claimsMetaEl.classList.toggle("status-success", !isError);
    }

    function updateButtons() {
        const hasInput = !!inputEl.value.trim();
        const hasHeader = !!headerOutEl.value.trim();
        const hasPayload = !!payloadOutEl.value.trim();
        if (decodeBtn) decodeBtn.disabled = !hasInput;
        if (copyHeaderBtn) copyHeaderBtn.disabled = !hasHeader;
        if (copyPayloadBtn) copyPayloadBtn.disabled = !hasPayload;
        if (clearBtn) clearBtn.disabled = !hasInput && !hasHeader && !hasPayload;
    }

    function base64UrlToUtf8(segment) {
        const base64 = segment.replace(/-/g, "+").replace(/_/g, "/");
        const padded = base64 + "=".repeat((4 - (base64.length % 4)) % 4);
        const binary = atob(padded);
        const bytes = Uint8Array.from(binary, (char) => char.charCodeAt(0));
        return new TextDecoder().decode(bytes);
    }

    function parseJsonSegment(segment, label) {
        try {
            return JSON.parse(base64UrlToUtf8(segment));
        } catch (err) {
            throw new Error(`Invalid ${label} segment`);
        }
    }

    function formatEpoch(epoch) {
        if (typeof epoch !== "number" || !Number.isFinite(epoch)) return null;
        const date = new Date(epoch * 1000);
        return `${date.toISOString()} (${Math.floor((date.getTime() - Date.now()) / 1000)}s from now)`;
    }

    function buildClaimsSummary(payload) {
        const parts = [];
        const exp = formatEpoch(payload.exp);
        const iat = formatEpoch(payload.iat);
        const nbf = formatEpoch(payload.nbf);

        if (iat) parts.push(`iat: ${iat}`);
        if (nbf) parts.push(`nbf: ${nbf}`);
        if (exp) parts.push(`exp: ${exp}`);
        return parts.length ? parts.join(" | ") : "Decoded without standard time claims.";
    }

    function decodeAction() {
        const token = inputEl.value.trim();
        if (!token) return;

        const parts = token.split(".");
        if (parts.length < 2) {
            setStatus("Invalid JWT format. Expected header.payload.signature", true);
            setClaimsMeta("No token decoded yet.", true);
            headerOutEl.value = "";
            payloadOutEl.value = "";
            signatureEl.value = "";
            updateButtons();
            return;
        }

        try {
            const header = parseJsonSegment(parts[0], "header");
            const payload = parseJsonSegment(parts[1], "payload");
            headerOutEl.value = JSON.stringify(header, null, 4);
            payloadOutEl.value = JSON.stringify(payload, null, 4);
            signatureEl.value = parts[2] || "";

            if (header.alg === "none") {
                setStatus("Decoded. Warning: alg 'none' token.", true);
            } else {
                setStatus("JWT decoded successfully.");
            }
            setClaimsMeta(buildClaimsSummary(payload), false);
        } catch (err) {
            setStatus(err.message || "Failed to decode JWT.", true);
            setClaimsMeta("No token decoded yet.", true);
            headerOutEl.value = "";
            payloadOutEl.value = "";
            signatureEl.value = "";
        }
        updateButtons();
    }

    async function copyText(sourceEl, buttonEl, successMessage) {
        if (!sourceEl.value.trim()) return;
        await navigator.clipboard.writeText(sourceEl.value);
        const original = buttonEl.textContent;
        buttonEl.textContent = "Copied";
        setStatus(successMessage, false);
        setTimeout(() => {
            buttonEl.textContent = original;
        }, 1200);
    }

    function clearAction() {
        inputEl.value = "";
        headerOutEl.value = "";
        payloadOutEl.value = "";
        signatureEl.value = "";
        setStatus("Paste a JWT to decode header and payload.");
        setClaimsMeta("No token decoded yet.");
        updateButtons();
    }

    inputEl.addEventListener("input", updateButtons);
    if (decodeBtn) decodeBtn.addEventListener("click", decodeAction);
    if (copyHeaderBtn) {
        copyHeaderBtn.addEventListener("click", () =>
            copyText(headerOutEl, copyHeaderBtn, "Header copied.")
        );
    }
    if (copyPayloadBtn) {
        copyPayloadBtn.addEventListener("click", () =>
            copyText(payloadOutEl, copyPayloadBtn, "Payload copied.")
        );
    }
    if (clearBtn) clearBtn.addEventListener("click", clearAction);

    updateButtons();
}

if (document.readyState === "loading") {
    document.addEventListener("DOMContentLoaded", initJwtDecoder);
} else {
    initJwtDecoder();
}