DOMTextScanner (#458)

* Create new class for scanning text in a document

* Update test styles

* Add tests

4 files changed, 1121 insertions, 3 deletions

diff --git a/ext/fg/js/dom-text-scanner.js b/ext/fg/js/dom-text-scanner.js
new file mode 100644
index 00000000..2de65041
--- /dev/null
+++ b/ext/fg/js/dom-text-scanner.js
@@ -0,0 +1,538 @@
+/*
+ * Copyright (C) 2020  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/**
+ * A class used to scan text in a document.
+ */
+class DOMTextScanner {
+    /**
+     * Creates a new instance of a DOMTextScanner.
+     * @param node The DOM Node to start at.
+     * @param offset The character offset in to start at when node is a text node.
+     *   Use 0 for non-text nodes.
+     */
+    constructor(node, offset, forcePreserveWhitespace=false, generateLayoutContent=true) {
+        const ruby = DOMTextScanner.getParentRubyElement(node);
+        const resetOffset = (ruby !== null);
+        if (resetOffset) { node = ruby; }
+
+        this._node = node;
+        this._offset = offset;
+        this._content = '';
+        this._remainder = 0;
+        this._resetOffset = resetOffset;
+        this._newlines = 0;
+        this._lineHasWhitespace = false;
+        this._lineHasContent = false;
+        this._forcePreserveWhitespace = forcePreserveWhitespace;
+        this._generateLayoutContent = generateLayoutContent;
+    }
+
+    /**
+     * Gets the current node being scanned.
+     * @returns A DOM Node.
+     */
+    get node() {
+        return this._node;
+    }
+
+    /**
+     * Gets the current offset corresponding to the node being scanned.
+     * This value is only applicable for text nodes.
+     * @returns An integer.
+     */
+    get offset() {
+        return this._offset;
+    }
+
+    /**
+     * Gets the accumulated content string resulting from calls to seek().
+     * @returns A string.
+     */
+    get content() {
+        return this._content;
+    }
+
+    /**
+     * Seeks a given length in the document and accumulates the text content.
+     * @param length A positive or negative integer corresponding to how many characters
+     *   should be added to content. Content is only added to the accumulation string,
+     *   never removed, so mixing seek calls with differently signed length values
+     *   may give unexpected results.
+     * @returns this
+     */
+    seek(length) {
+        const forward = (length >= 0);
+        this._remainder = (forward ? length : -length);
+        if (length === 0) { return this; }
+
+        const TEXT_NODE = Node.TEXT_NODE;
+        const ELEMENT_NODE = Node.ELEMENT_NODE;
+
+        const generateLayoutContent = this._generateLayoutContent;
+        let node = this._node;
+        let resetOffset = this._resetOffset;
+        let newlines = 0;
+        while (node !== null) {
+            let enterable = false;
+            const nodeType = node.nodeType;
+
+            if (nodeType === TEXT_NODE) {
+                if (!(
+                    forward ?
+                    this._seekTextNodeForward(node, resetOffset) :
+                    this._seekTextNodeBackward(node, resetOffset)
+                )) {
+                    // Length reached
+                    break;
+                }
+            } else if (nodeType === ELEMENT_NODE) {
+                [enterable, newlines] = DOMTextScanner.getElementSeekInfo(node);
+                if (newlines > this._newlines && generateLayoutContent) {
+                    this._newlines = newlines;
+                }
+            }
+
+            const exitedNodes = [];
+            node = DOMTextScanner.getNextNode(node, forward, enterable, exitedNodes);
+
+            for (const exitedNode of exitedNodes) {
+                if (exitedNode.nodeType !== ELEMENT_NODE) { continue; }
+                newlines = DOMTextScanner.getElementSeekInfo(exitedNode)[1];
+                if (newlines > this._newlines && generateLayoutContent) {
+                    this._newlines = newlines;
+                }
+            }
+
+            resetOffset = true;
+        }
+
+        this._node = node;
+        this._resetOffset = resetOffset;
+
+        return this;
+    }
+
+    // Private
+
+    /**
+     * Seeks forward in a text node.
+     * @param textNode The text node to use.
+     * @param resetOffset Whether or not the text offset should be reset.
+     * @returns true if scanning should continue, or false if the scan length has been reached.
+     */
+    _seekTextNodeForward(textNode, resetOffset) {
+        const nodeValue = textNode.nodeValue;
+        const nodeValueLength = nodeValue.length;
+        const [preserveNewlines, preserveWhitespace] = (
+            this._forcePreserveWhitespace ?
+            [true, true] :
+            DOMTextScanner.getWhitespaceSettings(textNode)
+        );
+
+        let lineHasWhitespace = this._lineHasWhitespace;
+        let lineHasContent = this._lineHasContent;
+        let content = this._content;
+        let offset = resetOffset ? 0 : this._offset;
+        let remainder = this._remainder;
+        let newlines = this._newlines;
+
+        while (offset < nodeValueLength) {
+            const char = nodeValue[offset];
+            const charAttributes = DOMTextScanner.getCharacterAttributes(char, preserveNewlines, preserveWhitespace);
+            ++offset;
+
+            if (charAttributes === 0) {
+                // Character should be ignored
+                continue;
+            } else if (charAttributes === 1) {
+                // Character is collapsable whitespace
+                lineHasWhitespace = true;
+            } else {
+                // Character should be added to the content
+                if (newlines > 0) {
+                    if (content.length > 0) {
+                        const useNewlineCount = Math.min(remainder, newlines);
+                        content += '\n'.repeat(useNewlineCount);
+                        remainder -= useNewlineCount;
+                        newlines -= useNewlineCount;
+                    } else {
+                        newlines = 0;
+                    }
+                    lineHasContent = false;
+                    lineHasWhitespace = false;
+                    if (remainder <= 0) {
+                        --offset; // Revert character offset
+                        break;
+                    }
+                }
+
+                lineHasContent = (charAttributes === 2); // 3 = character is a newline
+
+                if (lineHasWhitespace) {
+                    if (lineHasContent) {
+                        content += ' ';
+                        lineHasWhitespace = false;
+                        if (--remainder <= 0) {
+                            --offset; // Revert character offset
+                            break;
+                        }
+                    } else {
+                        lineHasWhitespace = false;
+                    }
+                }
+
+                content += char;
+
+                if (--remainder <= 0) { break; }
+            }
+        }
+
+        this._lineHasWhitespace = lineHasWhitespace;
+        this._lineHasContent = lineHasContent;
+        this._content = content;
+        this._offset = offset;
+        this._remainder = remainder;
+        this._newlines = newlines;
+
+        return (remainder > 0);
+    }
+
+    /**
+     * Seeks backward in a text node.
+     * This function is nearly the same as _seekTextNodeForward, with the following differences:
+     * - Iteration condition is reversed to check if offset is greater than 0.
+     * - offset is reset to nodeValueLength instead of 0.
+     * - offset is decremented instead of incremented.
+     * - offset is decremented before getting the character.
+     * - offset is reverted by incrementing instead of decrementing.
+     * - content string is prepended instead of appended.
+     * @param textNode The text node to use.
+     * @param resetOffset Whether or not the text offset should be reset.
+     * @returns true if scanning should continue, or false if the scan length has been reached.
+     */
+    _seekTextNodeBackward(textNode, resetOffset) {
+        const nodeValue = textNode.nodeValue;
+        const nodeValueLength = nodeValue.length;
+        const [preserveNewlines, preserveWhitespace] = (
+            this._forcePreserveWhitespace ?
+            [true, true] :
+            DOMTextScanner.getWhitespaceSettings(textNode)
+        );
+
+        let lineHasWhitespace = this._lineHasWhitespace;
+        let lineHasContent = this._lineHasContent;
+        let content = this._content;
+        let offset = resetOffset ? nodeValueLength : this._offset;
+        let remainder = this._remainder;
+        let newlines = this._newlines;
+
+        while (offset > 0) {
+            --offset;
+            const char = nodeValue[offset];
+            const charAttributes = DOMTextScanner.getCharacterAttributes(char, preserveNewlines, preserveWhitespace);
+
+            if (charAttributes === 0) {
+                // Character should be ignored
+                continue;
+            } else if (charAttributes === 1) {
+                // Character is collapsable whitespace
+                lineHasWhitespace = true;
+            } else {
+                // Character should be added to the content
+                if (newlines > 0) {
+                    if (content.length > 0) {
+                        const useNewlineCount = Math.min(remainder, newlines);
+                        content = '\n'.repeat(useNewlineCount) + content;
+                        remainder -= useNewlineCount;
+                        newlines -= useNewlineCount;
+                    } else {
+                        newlines = 0;
+                    }
+                    lineHasContent = false;
+                    lineHasWhitespace = false;
+                    if (remainder <= 0) {
+                        ++offset; // Revert character offset
+                        break;
+                    }
+                }
+
+                lineHasContent = (charAttributes === 2); // 3 = character is a newline
+
+                if (lineHasWhitespace) {
+                    if (lineHasContent) {
+                        content = ' ' + content;
+                        lineHasWhitespace = false;
+                        if (--remainder <= 0) {
+                            ++offset; // Revert character offset
+                            break;
+                        }
+                    } else {
+                        lineHasWhitespace = false;
+                    }
+                }
+
+                content = char + content;
+
+                if (--remainder <= 0) { break; }
+            }
+        }
+
+        this._lineHasWhitespace = lineHasWhitespace;
+        this._lineHasContent = lineHasContent;
+        this._content = content;
+        this._offset = offset;
+        this._remainder = remainder;
+        this._newlines = newlines;
+
+        return (remainder > 0);
+    }
+
+    // Static helpers
+
+    /**
+     * Gets the next node in the document for a specified scanning direction.
+     * @param node The current DOM Node.
+     * @param forward Whether to scan forward in the document or backward.
+     * @param visitChildren Whether the children of the current node should be visited.
+     * @param exitedNodes An array which stores nodes which were exited.
+     * @returns The next node in the document, or null if there is no next node.
+     */
+    static getNextNode(node, forward, visitChildren, exitedNodes) {
+        let next = visitChildren ? (forward ? node.firstChild : node.lastChild) : null;
+        if (next === null) {
+            while (true) {
+                exitedNodes.push(node);
+
+                next = (forward ? node.nextSibling : node.previousSibling);
+                if (next !== null) { break; }
+
+                next = node.parentNode;
+                if (next === null) { break; }
+
+                node = next;
+            }
+        }
+        return next;
+    }
+
+    /**
+     * Gets the parent element of a given Node.
+     * @param node The node to check.
+     * @returns The parent element if one exists, otherwise null.
+     */
+    static getParentElement(node) {
+        while (node !== null && node.nodeType !== Node.ELEMENT_NODE) {
+            node = node.parentNode;
+        }
+        return node;
+    }
+
+    /**
+     * Gets the parent <ruby> element of a given node, if one exists. For efficiency purposes,
+     * this only checks the immediate parent elements and does not check all ancestors, so
+     * there are cases where the node may be in a ruby element but it is not returned.
+     * @param node The node to check.
+     * @returns A <ruby> node if the input node is contained in one, otherwise null.
+     */
+    static getParentRubyElement(node) {
+        node = DOMTextScanner.getParentElement(node);
+        if (node !== null && node.nodeName.toUpperCase() === 'RT') {
+            node = node.parentNode;
+            if (node !== null && node.nodeName.toUpperCase() === 'RUBY') {
+                return node;
+            }
+        }
+        return null;
+    }
+
+    /**
+     * @returns [enterable: boolean, newlines: integer]
+     *   The enterable value indicates whether the content of this node should be entered.
+     *   The newlines value corresponds to the number of newline characters that should be added.
+     *     1 newline corresponds to a simple new line in the layout.
+     *     2 newlines corresponds to a significant visual distinction since the previous content.
+     */
+    static getElementSeekInfo(element) {
+        let enterable = true;
+        switch (element.nodeName.toUpperCase()) {
+            case 'HEAD':
+            case 'RT':
+            case 'SCRIPT':
+            case 'STYLE':
+                return [false, 0];
+            case 'BR':
+                return [false, 1];
+            case 'TEXTAREA':
+            case 'INPUT':
+            case 'BUTTON':
+                enterable = false;
+                break;
+        }
+
+        const style = window.getComputedStyle(element);
+        const display = style.display;
+
+        const visible = (display !== 'none' && DOMTextScanner.isStyleVisible(style));
+        let newlines = 0;
+
+        if (!visible) {
+            enterable = false;
+        } else {
+            switch (style.position) {
+                case 'absolute':
+                case 'fixed':
+                case 'sticky':
+                    newlines = 2;
+                    break;
+            }
+            if (newlines === 0 && DOMTextScanner.doesCSSDisplayChangeLayout(display)) {
+                newlines = 1;
+            }
+        }
+
+        return [enterable, newlines];
+    }
+
+    /**
+     * Gets information about how whitespace characters are treated.
+     * @param textNode The Text node to check.
+     * @returns [preserveNewlines: boolean, preserveWhitespace: boolean]
+     *   The value of preserveNewlines indicates whether or not newline characters are treated as line breaks.
+     *   The value of preserveWhitespace indicates whether or not sequences of whitespace characters are collapsed.
+     */
+    static getWhitespaceSettings(textNode) {
+        const element = DOMTextScanner.getParentElement(textNode);
+        if (element !== null) {
+            const style = window.getComputedStyle(element);
+            switch (style.whiteSpace) {
+                case 'pre':
+                case 'pre-wrap':
+                case 'break-spaces':
+                    return [true, true];
+                case 'pre-line':
+                    return [true, false];
+            }
+        }
+        return [false, false];
+    }
+
+    /**
+     * Gets attributes for the specified character.
+     * @param character A string containing a single character.
+     * @returns An integer representing the attributes of the character.
+     *   0: Character should be ignored.
+     *   1: Character is collapsable whitespace.
+     *   2: Character should be added to the content.
+     *   3: Character should be added to the content and is a newline.
+     */
+    static getCharacterAttributes(character, preserveNewlines, preserveWhitespace) {
+        switch (character.charCodeAt(0)) {
+            case 0x09: // Tab ('\t')
+            case 0x0c: // Form feed ('\f')
+            case 0x0d: // Carriage return ('\r')
+            case 0x20: // Space (' ')
+                return preserveWhitespace ? 2 : 1;
+            case 0x0a: // Line feed ('\n')
+                return preserveNewlines ? 3 : 1;
+            case 0x200c: // Zero-width non-joiner ('\u200c')
+                return 0;
+            default: // Other
+                return 2;
+        }
+    }
+
+    /**
+     * Checks whether a given style is visible or not.
+     * This function does not check style.display === 'none'.
+     * @param style An object implementing the CSSStyleDeclaration interface.
+     * @returns true if the style should result in an element being visible, otherwise false.
+     */
+    static isStyleVisible(style) {
+        return !(
+            style.visibility === 'hidden' ||
+            parseFloat(style.opacity) <= 0 ||
+            parseFloat(style.fontSize) <= 0 ||
+            (
+                !DOMTextScanner.isStyleSelectable(style) &&
+                (
+                    DOMTextScanner.isCSSColorTransparent(style.color) ||
+                    DOMTextScanner.isCSSColorTransparent(style.webkitTextFillColor)
+                )
+            )
+        );
+    }
+
+    /**
+     * Checks whether a given style is selectable or not.
+     * @param style An object implementing the CSSStyleDeclaration interface.
+     * @returns true if the style is selectable, otherwise false.
+     */
+    static isStyleSelectable(style) {
+        return !(
+            style.userSelect === 'none' ||
+            style.webkitUserSelect === 'none' ||
+            style.MozUserSelect === 'none' ||
+            style.msUserSelect === 'none'
+        );
+    }
+
+    /**
+     * Checks whether a CSS color is transparent or not.
+     * @param cssColor A CSS color string, expected to be encoded in rgb(a) form.
+     * @returns true if the color is transparent, otherwise false.
+     */
+    static isCSSColorTransparent(cssColor) {
+        return (
+            typeof cssColor === 'string' &&
+            cssColor.startsWith('rgba(') &&
+            /,\s*0.?0*\)$/.test(cssColor)
+        );
+    }
+
+    /**
+     * Checks whether a CSS display value will cause a layout change for text.
+     * @param cssDisplay A CSS string corresponding to the value of the display property.
+     * @returns true if the layout is changed by this value, otherwise false.
+     */
+    static doesCSSDisplayChangeLayout(cssDisplay) {
+        let pos = cssDisplay.indexOf(' ');
+        if (pos >= 0) {
+            // Truncate to <display-outside> part
+            cssDisplay = cssDisplay.substring(0, pos);
+        }
+
+        pos = cssDisplay.indexOf('-');
+        if (pos >= 0) {
+            // Truncate to first part of kebab-case value
+            cssDisplay = cssDisplay.substring(0, pos);
+        }
+
+        switch (cssDisplay) {
+            case 'block':
+            case 'flex':
+            case 'grid':
+            case 'list': // list-item
+            case 'table': // table, table-*
+                return true;
+            case 'ruby': // rubt-*
+                return (pos >= 0);
+            default:
+                return false;
+        }
+    }
+}
diff --git a/test/data/html/test-dom-text-scanner.html b/test/data/html/test-dom-text-scanner.html
new file mode 100644
index 00000000..6b78570a
--- /dev/null
+++ b/test/data/html/test-dom-text-scanner.html
@@ -0,0 +1,393 @@
+<!DOCTYPE html>
+<html>
+    <head>
+        <meta charset="UTF-8">
+        <meta name="viewport" content="width=device-width,initial-scale=1" />
+        <title>Yomichan DOMTextScanner Tests</title>
+        <link rel="icon" type="image/gif" href="data:image/gif;base64,R0lGODlhEAAQAKEBAAAAAP///////////yH5BAEKAAIALAAAAAAQABAAAAImFI6Zpt0B4YkS0TCpq07xbmEgcGVRUpLaI46ZG7ppalY0jDCwUAAAOw==" />
+        <link rel="stylesheet" href="test-stylesheet.css" />
+    </head>
+<body>
+
+    <h1>Yomichan DOMTextScanner Tests</h1>
+
+    <y-test
+        data-test-data='{
+            "node": "div:nth-of-type(1)",
+            "offset": 0,
+            "length": 15,
+            "expected": {
+                "node": "div:nth-of-type(2)>div::text",
+                "offset": 3,
+                "content": "小ぢん\nまり1\n小ぢん\nまり2"
+            }
+        }'
+    >
+        <y-description>Layout newlines expected due to entering and exiting display:block nodes.</y-description>
+<div><div>小ぢん</div>まり1</div>
+<div>小ぢん<div>まり2</div></div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div:nth-of-type(1)::text",
+            "offset": 0,
+            "length": 13,
+            "expected": {
+                "node": "div:nth-of-type(2)::text",
+                "offset": 6,
+                "content": "小ぢんまり1\n小ぢんまり2"
+            }
+        }'
+    >
+        <y-description>Layout newline expected due to sequential display:block elements.</y-description>
+<div>小ぢんまり1</div><div>小ぢんまり2</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div:nth-of-type(1)::text",
+            "offset": 0,
+            "length": 13,
+            "expected": {
+                "node": "div:nth-of-type(2)::text",
+                "offset": 6,
+                "content": "小ぢんまり1\n小ぢんまり2"
+            }
+        }'
+    >
+        <y-description>Layout newline expected due to sequential display:block elements separated by a newline.</y-description>
+<div>小ぢんまり1</div>
+<div>小ぢんまり2</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "span:nth-of-type(1)::text",
+            "offset": 0,
+            "length": 12,
+            "expected": {
+                "node": "span:nth-of-type(2)::text",
+                "offset": 6,
+                "content": "小ぢんまり1小ぢんまり2"
+            }
+        }'
+    >
+        <y-description>No newlines expected due to display:inline.</y-description>
+<span>小ぢんまり1</span><span>小ぢんまり2</span>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "span:nth-of-type(1)::text",
+            "offset": 0,
+            "length": 13,
+            "expected": {
+                "node": "span:nth-of-type(2)::text",
+                "offset": 6,
+                "content": "小ぢんまり1\n小ぢんまり2"
+            }
+        }'
+    >
+        <y-description>No newlines expected due to white-space:normal.</y-description>
+<span>小ぢんまり1</span>
+<span>小ぢんまり2</span>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "span:nth-of-type(1)::text",
+            "offset": 0,
+            "length": 13,
+            "expected": {
+                "node": "span:nth-of-type(2)::text",
+                "offset": 6,
+                "content": "小ぢんまり1\n小ぢんまり2"
+            }
+        }'
+    >
+        <y-description>Newline expected due to white-space:pre.</y-description>
+<pre>
+<span>小ぢんまり1</span>
+<span>小ぢんまり2</span>
+</pre>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "span:nth-of-type(1)::text",
+            "offset": 0,
+            "length": 12,
+            "expected": {
+                "node": "span:nth-of-type(2)::text",
+                "offset": 6,
+                "content": "小ぢんまり1小ぢんまり2"
+            }
+        }'
+    >
+        <y-description>No newlines expected due to display:inline-block. Actual layout flow cannot be determined by DOM/CSS alone.</y-description>
+<span style="display: inline-block;">小ぢんまり1</span><span style="display: inline-block;">小ぢんまり2</span>
+    </y-test>
+
+    <y-test
+        style="position: relative;"
+        data-test-data='{
+            "node": "div:nth-of-type(1)::text",
+            "offset": 0,
+            "length": 13,
+            "expected": {
+                "node": "div:nth-of-type(2)::text",
+                "offset": 6,
+                "content": "小ぢんまり1\n小ぢんまり2"
+            }
+        }'
+    >
+        <y-description>Single newline expected due to display:block layout.</y-description>
+<div>小ぢんまり1</div><div style="position: relative;">小ぢんまり2</div>
+    </y-test>
+
+    <y-test
+        style="position: relative; overflow: hidden;"
+        data-test-data='{
+            "node": "div:nth-of-type(1)::text",
+            "offset": 0,
+            "length": 14,
+            "expected": {
+                "node": "div:nth-of-type(2)::text",
+                "offset": 6,
+                "content": "小ぢんまり1\n\n小ぢんまり2"
+            }
+        }'
+    >
+        <y-description>Two newlines expected due to position:absolute causing a significant layout change.</y-description>
+<div>小ぢんまり1</div><div style="position: absolute;">小ぢんまり2</div>
+    </y-test>
+
+    <y-test
+        style="position: relative; overflow: hidden;"
+        data-test-data='{
+            "node": "div:nth-of-type(1)::text",
+            "offset": 0,
+            "length": 14,
+            "expected": {
+                "node": "div:nth-of-type(2)::text",
+                "offset": 6,
+                "content": "小ぢんまり1\n\n小ぢんまり2"
+            }
+        }'
+    >
+        <y-description>Two newlines expected due to position:fixed causing a significant layout change.</y-description>
+<div>小ぢんまり1</div><div style="position: fixed;">小ぢんまり2</div>
+    </y-test>
+
+    <y-test
+        style="position: relative;"
+        data-test-data='{
+            "node": "div:nth-of-type(1)::text",
+            "offset": 0,
+            "length": 14,
+            "expected": {
+                "node": "div:nth-of-type(2)::text",
+                "offset": 6,
+                "content": "小ぢんまり1\n\n小ぢんまり2"
+            }
+        }'
+    >
+        <y-description>Two newlines expected due to position:sticky being able to cause a significant layout change.</y-description>
+<div>小ぢんまり1</div><div style="position: sticky;">小ぢんまり2</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "rt",
+            "offset": 0,
+            "length": 6,
+            "expected": {
+                "node": "div::text",
+                "offset": 5,
+                "content": "小ぢんまり1"
+            }
+        }'
+    >
+        <y-description>Scanning text starting in an &lt;rt&gt; element. Should start scanning at the start of the &lt;ruby&gt; tag instead.</y-description>
+<div><ruby>小<rp>(</rp><rt>こ</rt><rp>)</rp></ruby>ぢんまり1</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div",
+            "offset": 0,
+            "length": 6,
+            "expected": {
+                "node": "div::nth-text(2)",
+                "offset": 3,
+                "content": "小ぢんまり1"
+            }
+        }'
+    >
+        <y-description>Skip &lt;script&gt; content.</y-description>
+<div>小ぢん<script>/*comment*/</script>まり1</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div",
+            "offset": 0,
+            "length": 6,
+            "expected": {
+                "node": "div::nth-text(2)",
+                "offset": 3,
+                "content": "小ぢんまり1"
+            }
+        }'
+    >
+        <y-description>Skip &lt;style&gt; content.</y-description>
+<div>小ぢん<style>/*comment*/</style>まり1</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div",
+            "offset": 0,
+            "length": 6,
+            "expected": {
+                "node": "div::nth-text(2)",
+                "offset": 3,
+                "content": "小ぢんまり1"
+            }
+        }'
+    >
+        <y-description>Skip &lt;textarea&gt; content.</y-description>
+<div>小ぢん<textarea>textarea content</textarea>まり1</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div",
+            "offset": 0,
+            "length": 6,
+            "expected": {
+                "node": "div::nth-text(2)",
+                "offset": 3,
+                "content": "小ぢんまり1"
+            }
+        }'
+    >
+        <y-description>Skip &lt;input&gt; content.</y-description>
+<div>小ぢん<input value="content" />まり1</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div",
+            "offset": 0,
+            "length": 6,
+            "expected": {
+                "node": "div::nth-text(2)",
+                "offset": 3,
+                "content": "小ぢんまり1"
+            }
+        }'
+    >
+        <y-description>Skip &lt;button&gt; content.</y-description>
+<div>小ぢん<button>content</button>まり1</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div",
+            "offset": 0,
+            "length": 6,
+            "expected": {
+                "node": "div::nth-text(2)",
+                "offset": 3,
+                "content": "小ぢんまり1"
+            }
+        }'
+    >
+        <y-description>Skip content with font-size:0.</y-description>
+<div>小ぢん<span style="font-size: 0;">content</span>まり1</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div",
+            "offset": 0,
+            "length": 6,
+            "expected": {
+                "node": "div::nth-text(2)",
+                "offset": 3,
+                "content": "小ぢんまり1"
+            }
+        }'
+    >
+        <y-description>Skip content with opacity:0.</y-description>
+<div>小ぢん<span style="opacity: 0;">content</span>まり1</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div",
+            "offset": 0,
+            "length": 6,
+            "expected": {
+                "node": "div::nth-text(2)",
+                "offset": 3,
+                "content": "小ぢんまり1"
+            }
+        }'
+    >
+        <y-description>Skip content with visibility:hidden.</y-description>
+<div>小ぢん<span style="visibility: hidden;">content</span>まり1</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div",
+            "offset": 0,
+            "length": 6,
+            "expected": {
+                "node": "div::nth-text(2)",
+                "offset": 3,
+                "content": "小ぢんまり1"
+            }
+        }'
+    >
+        <y-description>Skip content with display:none.</y-description>
+<div>小ぢん<span style="display: none;">content</span>まり1</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div",
+            "offset": 0,
+            "length": 6,
+            "expected": {
+                "node": "div::nth-text(2)",
+                "offset": 3,
+                "content": "小ぢんまり1"
+            }
+        }'
+    >
+        <y-description>Don't skip content with user-select:none.</y-description>
+<div>小ぢ<span style="user-select: none;">ん</span>まり1</div>
+    </y-test>
+
+    <y-test
+        data-test-data='{
+            "node": "div",
+            "offset": 0,
+            "length": 6,
+            "expected": {
+                "node": "div::nth-text(2)",
+                "offset": 3,
+                "content": "小ぢんまり1"
+            }
+        }'
+    >
+        <y-description>Skip content with user-select:none <em>and</em> a transparent color.</y-description>
+<div>小ぢん<span style="user-select: none; color: rgba(0, 0, 0, 0);">content</span>まり1</div>
+    </y-test>
+
+</body>
+</html>
\ No newline at end of file
diff --git a/test/data/html/test-stylesheet.css b/test/data/html/test-stylesheet.css
index f63d2481..2e9a2f52 100644
--- a/test/data/html/test-stylesheet.css
+++ b/test/data/html/test-stylesheet.css
@@ -28,7 +28,9 @@ a, a:visited {
     text-decoration: underline;
 }
 
-.test {
+.test,
+y-test {
+    display: block;
     background-color: #ffffff;
     margin: 1em 0;
     padding: 0.5em;
@@ -36,7 +38,8 @@ a, a:visited {
     border-radius: 4px;
 }
 
-.test:before {
+.test:before,
+y-test:before {
     content: "Test " counter(test-id);
     display: block;
     counter-increment: test-id;
@@ -45,7 +48,10 @@ a, a:visited {
     font-weight: bold;
 }
 
-.description {
+.description,
+y-description {
     color: #444444;
     font-style: italic;
+    display: block;
+    padding-bottom: 0.5em;
 }
diff --git a/test/test-dom-text-scanner.js b/test/test-dom-text-scanner.js
new file mode 100644
index 00000000..41d6e307
--- /dev/null
+++ b/test/test-dom-text-scanner.js
@@ -0,0 +1,181 @@
+/*
+ * Copyright (C) 2020  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+const fs = require('fs');
+const path = require('path');
+const assert = require('assert');
+const {JSDOM} = require('jsdom');
+const {VM} = require('./yomichan-vm');
+
+
+function createJSDOM(fileName) {
+    const domSource = fs.readFileSync(fileName, {encoding: 'utf8'});
+    return new JSDOM(domSource);
+}
+
+function querySelectorTextNode(element, selector) {
+    let textIndex = -1;
+    const match = /::text$|::nth-text\((\d+)\)$/.exec(selector);
+    if (match !== null) {
+        textIndex = (match[1] ? parseInt(match[1], 10) - 1 : 0);
+        selector = selector.substring(0, selector.length - match[0].length);
+    }
+    const result = element.querySelector(selector);
+    if (textIndex < 0) {
+        return result;
+    }
+    for (let n = result.firstChild; n !== null; n = n.nextSibling) {
+        if (n.nodeType === n.constructor.TEXT_NODE) {
+            if (textIndex === 0) {
+                return n;
+            }
+            --textIndex;
+        }
+    }
+    return null;
+}
+
+
+function getComputedFontSizeInPixels(window, getComputedStyle, element) {
+    for (; element !== null; element = element.parentNode) {
+        if (element.nodeType === window.Node.ELEMENT_NODE) {
+            const fontSize = getComputedStyle(element).fontSize;
+            if (fontSize.endsWith('px')) {
+                const value = parseFloat(fontSize.substring(0, fontSize.length - 2));
+                return value;
+            }
+        }
+    }
+    const defaultFontSize = 14;
+    return defaultFontSize;
+}
+
+function createAbsoluteGetComputedStyle(window) {
+    // Wrapper to convert em units to px units
+    const getComputedStyleOld = window.getComputedStyle.bind(window);
+    return (element, ...args) => {
+        const style = getComputedStyleOld(element, ...args);
+        return new Proxy(style, {
+            get: (target, property) => {
+                let result = target[property];
+                if (typeof result === 'string') {
+                    result = result.replace(/([-+]?\d(?:\.\d)?(?:[eE][-+]?\d+)?)em/g, (g0, g1) => {
+                        const fontSize = getComputedFontSizeInPixels(window, getComputedStyleOld, element);
+                        return `${parseFloat(g1) * fontSize}px`;
+                    });
+                }
+                return result;
+            }
+        });
+    };
+}
+
+
+async function testDomTextScanner(dom, {DOMTextScanner}) {
+    const document = dom.window.document;
+    for (const testElement of document.querySelectorAll('y-test')) {
+        let testData = JSON.parse(testElement.dataset.testData);
+        if (!Array.isArray(testData)) {
+            testData = [testData];
+        }
+        for (const testDataItem of testData) {
+            let {
+                node,
+                offset,
+                length,
+                forcePreserveWhitespace,
+                generateLayoutContent,
+                reversible,
+                expected: {
+                    node: expectedNode,
+                    offset: expectedOffset,
+                    content: expectedContent
+                }
+            } = testDataItem;
+
+            node = querySelectorTextNode(testElement, node);
+            expectedNode = querySelectorTextNode(testElement, expectedNode);
+
+            // Standard test
+            {
+                const scanner = new DOMTextScanner(node, offset, forcePreserveWhitespace, generateLayoutContent);
+                scanner.seek(length);
+
+                const {node: actualNode1, offset: actualOffset1, content: actualContent1} = scanner;
+                assert.strictEqual(actualContent1, expectedContent);
+                assert.strictEqual(actualOffset1, expectedOffset);
+                assert.strictEqual(actualNode1, expectedNode);
+            }
+
+            // Substring tests
+            for (let i = 1; i <= length; ++i) {
+                const scanner = new DOMTextScanner(node, offset, forcePreserveWhitespace, generateLayoutContent);
+                scanner.seek(length - i);
+
+                const {content: actualContent} = scanner;
+                assert.strictEqual(actualContent, expectedContent.substring(0, expectedContent.length - i));
+            }
+
+            if (reversible === false) { continue; }
+
+            // Reversed test
+            {
+                const scanner = new DOMTextScanner(expectedNode, expectedOffset, forcePreserveWhitespace, generateLayoutContent);
+                scanner.seek(-length);
+
+                const {content: actualContent} = scanner;
+                assert.strictEqual(actualContent, expectedContent);
+            }
+
+            // Reversed substring tests
+            for (let i = 1; i <= length; ++i) {
+                const scanner = new DOMTextScanner(expectedNode, expectedOffset, forcePreserveWhitespace, generateLayoutContent);
+                scanner.seek(-(length - i));
+
+                const {content: actualContent} = scanner;
+                assert.strictEqual(actualContent, expectedContent.substring(i));
+            }
+        }
+    }
+}
+
+
+async function testDocument1() {
+    const dom = createJSDOM(path.join(__dirname, 'data', 'html', 'test-dom-text-scanner.html'));
+    const window = dom.window;
+    try {
+        const {document, Node, Range} = window;
+
+        window.getComputedStyle = createAbsoluteGetComputedStyle(window);
+
+        const vm = new VM({document, window, Range, Node});
+        vm.execute('fg/js/dom-text-scanner.js');
+        const DOMTextScanner = vm.get('DOMTextScanner');
+
+        await testDomTextScanner(dom, {DOMTextScanner});
+    } finally {
+        window.close();
+    }
+}
+
+
+async function main() {
+    await testDocument1();
+}
+
+
+if (require.main === module) { main(); }