// 绝对中央居中（如果容器宽度小于该元素，仍然能保持居中）
// 只对块元素或者行内块元素有效，且父容器必须设置为定位元素
.center-x(@outer_width, @location: top auto, @position: absolute) when (isnumber(@outer_width)) {
    .position(auto auto auto 50%, @position);
    .set(extract(@location, 1), extract(@location, 2));
    margin-left: (@outer_width / 2 * -1);
}
.center-x(@location: top auto, @position: absolute) when not (isnumber(@location)) {
    .position(auto auto auto 50%, @position);
    .set(extract(@location, 1), extract(@location, 2));
    .translate(-50%, 0);
}

.center-y(@outer_height, @location: left auto, @position: absolute) when (isnumber(@outer_height)) {

    & when (ispercentage(@outer_height)) {
        // 高度为百分比时专用（但IE6-7不支持）
        .position(0 auto 0 auto, @position);
        margin-top: auto;
        margin-bottom: auto;
    }

    & when not (ispercentage(@outer_height)) {
        .position(50% auto auto auto, @position);
        margin-top: (@outer_height / 2 * -1);
    }

    .set(extract(@location, 1), extract(@location, 2));
}
.center-y(@location: left auto, @position: absolute) when not (isnumber(@location)) {
    .position(50% auto auto auto, @position);
    .set(extract(@location, 1), extract(@location, 2));
    .translate(0, -50%);
}

.center-xy(@outer_width, @outer_height, @position: absolute) when (isnumber(@outer_width)) and (isnumber(@outer_height)) {

    & when (ispercentage(@outer_height)) {
        // 高度为百分比时专用（但IE6-7不支持）
        .position(0 auto 0 50%, @position);
        margin-left: (@outer_width / 2 * -1);
        margin-top: auto;
        margin-bottom: auto;
    }

    & when not (ispercentage(@outer_height)) {
        .position(50% auto auto 50%, @position);
        margin-left: (@outer_width / 2 * -1);
        margin-top: (@outer_height / 2 * -1);
    }
}
.center-xy(@position: absolute) when (iskeyword(@position)) {
    .position(50% auto auto 50%, @position);
    .translate(-50%, -50%);
}

// 块元素水平居中
.center-x-block(@width: auto, @margin: auto) {
    display: block;
    margin-left: auto;
    margin-right: auto;
    .set(width, @width);

    & when (length(@margin) = 1) {
        .set(margin-top, @margin);
        .set(margin-bottom, @margin);
    }

    & when (length(@margin) = 2) {
        .set(margin-top, extract(@margin, 1));
        .set(margin-bottom, extract(@margin, 2));
    }
}

// 表格元素水平居中
.center-x-table(@width: auto, @margin: auto) {
    display: table;
    margin-left: auto;
    margin-right: auto;
    .set(width, @width);

    & when (length(@margin) = 1) {
        .set(margin-top, @margin);
        .set(margin-bottom, @margin);
    }

    & when (length(@margin) = 2) {
        .set(margin-top, extract(@margin, 1));
        .set(margin-bottom, extract(@margin, 2));
    }
}

// img 专用中央居中（IE6-7不支持）
.center-xy-img(@position: absolute) {
    display: block;
    margin: auto;
    .position(0, @position);
}