/**
 * 解析Uint8Array, 生成相应字符串
 * @param uint8array
 * @param start
 * @param checkLength
 */

function checkContinuation(uint8array: Uint8Array, start: number, checkLength: number) {
    const array = uint8array;
    if(start + checkLength < array.length) {
        while(checkLength--) {
            if((array[++start] & 0xc0) !== 0x80) return false;
        }
        return true;
    }
    return false;
}

function decodeUTF8(uint8array: Uint8Array) {
    const out = [];
    const input = uint8array;
    let i = 0;
    const { length } = uint8array;

    while(i < length) {
        if(input[i] < 0x80) {
            out.push(String.fromCharCode(input[i]));
            ++i;
            continue;
        } else if(input[i] < 0xc0) {
            // fallthrough
        } else if(input[i] < 0xe0) {
            if(checkContinuation(input, i, 1)) {
                const ucs4 = ((input[i] & 0x1f) << 6) | (input[i + 1] & 0x3f);
                if(ucs4 >= 0x80) {
                    out.push(String.fromCharCode(ucs4 & 0xffff));
                    i += 2;
                    continue;
                }
            }
        } else if(input[i] < 0xf0) {
            if(checkContinuation(input, i, 2)) {
                const ucs4 = ((input[i] & 0xf) << 12) | ((input[i + 1] & 0x3f) << 6) | (input[i + 2] & 0x3f);
                if(ucs4 >= 0x800 && (ucs4 & 0xf800) !== 0xd800) {
                    out.push(String.fromCharCode(ucs4 & 0xffff));
                    i += 3;
                    continue;
                }
            }
        } else if(input[i] < 0xf8) {
            if(checkContinuation(input, i, 3)) {
                let ucs4 = ((input[i] & 0x7) << 18)
                    | ((input[i + 1] & 0x3f) << 12)
                    | ((input[i + 2] & 0x3f) << 6)
                    | (input[i + 3] & 0x3f);
                if(ucs4 > 0x10000 && ucs4 < 0x110000) {
                    ucs4 -= 0x10000;
                    out.push(String.fromCharCode((ucs4 >>> 10) | 0xd800));
                    out.push(String.fromCharCode((ucs4 & 0x3ff) | 0xdc00));
                    i += 4;
                    continue;
                }
            }
        }
        out.push(String.fromCharCode(0xfffd));
        ++i;
    }

    return out.join('');
}

export default decodeUTF8;