51 lines
1.5 KiB
JavaScript
51 lines
1.5 KiB
JavaScript
export function scrollTopFor(y) {
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return y - offsetCalculator(y);
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}
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export default function offsetCalculator(y) {
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const $header = $('header');
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const $container = $('.posts-wrapper');
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const containerOffset = $container.offset();
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let titleHeight = 0;
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const scrollTop = y || $(window).scrollTop();
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if (!containerOffset || scrollTop < containerOffset.top) {
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titleHeight = $('#topic-title').height() || 0;
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}
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const rawWinHeight = $(window).height();
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const windowHeight = rawWinHeight - titleHeight;
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const eyeTarget = (windowHeight / 10);
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const headerHeight = $header.outerHeight(true);
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const expectedOffset = titleHeight - ($header.find('.contents').height() || 0) + (eyeTarget * 2);
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const ideal = headerHeight + ((expectedOffset < 0) ? 0 : expectedOffset);
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if ($container.length === 0) { return expectedOffset; }
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const topPos = $container.offset().top;
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const docHeight = $(document).height();
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let scrollPercent = Math.min((scrollTop / (docHeight-rawWinHeight)), 1.0);
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let inter = topPos - scrollTop + ($container.height() * scrollPercent);
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if (inter < headerHeight + eyeTarget) {
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inter = headerHeight + eyeTarget;
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}
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if (inter > ideal) {
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const bottom = $('#topic-bottom').offset().top;
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const switchPos = bottom - rawWinHeight - ideal;
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if (scrollTop > switchPos) {
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const p = Math.max(Math.min((scrollTop + inter - switchPos) / rawWinHeight, 1.0), 0.0);
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return ((1 - p) * ideal) + (p * inter);
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} else {
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return ideal;
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}
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}
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return inter;
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}
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