Next Image/Previous Image with Alignment and Large Image Support

Scroll down/up by aligning images
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// ==UserScript==
// @name         Next Image/Previous Image with Alignment and Large Image Support
// @author       rekt
// @license      MIT
// @namespace    http://tampermonkey.net/
// @version      1.0
// @description  Scroll down/up by aligning images
// @include      *
// @grant        unsafeWindow
// ==/UserScript==

(function() {
  const KEY_W = "w";
  const KEY_S = "s";

  let positions = [];

  function findNonTruncatedPosition(initialPosition, viewportHeight) {
    let currentPosition = initialPosition;
    let found = false;
    while (!found) {
      found = true;
      for (let i = 0; i < positions.length; i++) {
        const [, imgTop, imgBottom] = positions[i];
        if (imgTop < currentPosition && imgBottom > currentPosition) {
          currentPosition = imgTop;
          found = false;
          break;
        }
      }
    }
    return currentPosition;
  }

  function scrollToNextImage(currentScroll, viewportHeight) {
    const nextScrollPosition = currentScroll + viewportHeight;
    const currentBottomEdge = currentScroll + viewportHeight;

    // First check for large images that extend beyond viewport
    for (let i = 0; i < positions.length; i++) {
      if (positions[i][1] <= currentScroll && positions[i][2] > currentBottomEdge) {
        // Large image case: find next image after this one
        for (let j = i + 1; j < positions.length; j++) {
          if (positions[j][1] > positions[i][1]) {
            return findNonTruncatedPosition(positions[j][1], viewportHeight);
          }
        }
        // If no next image found, scroll by viewport height
        return findNonTruncatedPosition(nextScrollPosition, viewportHeight);
      }
    }

    // Normal case: handle truncated images
    for (let i = 0; i < positions.length; i++) {
      if (positions[i][2] > currentBottomEdge && positions[i][1] < nextScrollPosition) {
        return findNonTruncatedPosition(positions[i][1], viewportHeight);
      }
    }

    return findNonTruncatedPosition(nextScrollPosition, viewportHeight);
  }

  function scrollToPreviousImage(currentScroll, viewportHeight) {
    const previousScrollPosition = currentScroll - viewportHeight;
    const newTopCandidate = Math.max(0, previousScrollPosition);

    for (let i = positions.length - 1; i >= 0; i--) {
      if (positions[i][1] < currentScroll && positions[i][2] > newTopCandidate) {
        return findNonTruncatedPosition(positions[i][1], viewportHeight);
      }
    }
    return findNonTruncatedPosition(newTopCandidate, viewportHeight);
  }

  function scrollShiftUp(currentScroll, viewportHeight) {
    const prevViewportTop = Math.max(currentScroll - viewportHeight, 0);
    let newScroll = prevViewportTop;

    for (let i = positions.length - 1; i >= 0; i--) {
      const imgTop = positions[i][1];

      if (imgTop >= prevViewportTop && imgTop <= prevViewportTop + 20) {
        newScroll = imgTop;
        break;
      }

      if (imgTop < prevViewportTop && imgTop >= prevViewportTop - 100) {
        newScroll = imgTop;
        break;
      }
    }

    return findNonTruncatedPosition(newScroll, viewportHeight);
  }

  function getYOffset(node) {
    let offset = 0;
    while (node) {
      offset += node.offsetTop;
      node = node.offsetParent;
    }
    return offset;
  }

  document.addEventListener("keydown", function(event) {
    if (event.ctrlKey || event.altKey) return;

    const tagName = event.target.tagName;
    const contentEditable = event.target.getAttribute("contenteditable");
    if ((tagName && tagName.match(/input|select|textarea/i)) || contentEditable === "true") {
      return;
    }

    positions = [];
    const allImages = document.images;
    for (let idx = 0; idx < allImages.length; idx++) {
      const img = allImages[idx];
      if (img.width * img.height < 80 * 80) continue;
      const ytop = getYOffset(img);
      const ybottom = ytop + img.height;
      positions.push([idx, ytop, ybottom]);
    }
    positions.sort((a, b) => a[1] - b[1]);

    const currentScroll = Math.max(document.documentElement.scrollTop, document.body.scrollTop);
    const viewportHeight = window.innerHeight;

    let newScrollPosition = currentScroll;

    const key = event.key;
    const lowerKey = key.toLowerCase();

    if (lowerKey === KEY_S && !event.shiftKey) {
      newScrollPosition = scrollToNextImage(currentScroll, viewportHeight);
    } else if (lowerKey === KEY_W && !event.shiftKey) {
      newScrollPosition = scrollToPreviousImage(currentScroll, viewportHeight);
    } else if (lowerKey === KEY_W && event.shiftKey) {
      newScrollPosition = scrollShiftUp(currentScroll, viewportHeight);
    } else {
      return;
    }

    if (newScrollPosition !== currentScroll) {
      event.preventDefault();
      document.body.scrollTop = newScrollPosition;
      document.documentElement.scrollTop = newScrollPosition;
    }
  });
})();