151 lines
5.7 KiB
Markdown
151 lines
5.7 KiB
Markdown
# selderee
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[](https://github.com/mxxii/selderee/blob/main/LICENSE)
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[](https://www.npmjs.com/package/selderee)
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**Sel**ectors **de**cision t**ree** - pick matching selectors, fast.
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----
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## What is it for
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The problem statement: there are multiple CSS selectors with attached handlers, and a HTML DOM to process. For each HTML Element a matching handler has to be found and applied.
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The naive approach is to walk through the DOM and test each and every selector against each Element. This means *O(n\*m)* complexity.
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It is pretty clear though that if we have selectors that share something in common then we can reduce the number of checks.
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The main `selderee` package offers the selectors tree structure. Runnable decision functions for specific DOM implementations are built via plugins.
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## Limitations
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- Pseudo-classes and pseudo-elements are not supported by the underlying library [parseley](https://github.com/mxxii/parseley) (yet?);
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- General siblings (`~`), descendants (` `) and same column combinators (`||`) are also not supported.
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## `selderee` vs `css-select`
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[css-select](https://github.com/fb55/css-select) - a CSS selector compiler & engine.
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| Feature | `selderee` | `css-select` |
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| ------------------------------------- | :--------: | :----------: |
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| Support for `htmlparser2` DOM AST | plugin | + |
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| "Compiles" into a function | + | + |
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| Pick selector(s) for a given Element | + | |
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| Query Element(s) for a given selector | | + |
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## Packages
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| Package | Version | Folder | Changelog |
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| --------- | --------- | --------- | --------- |
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| [selderee](https://www.npmjs.com/package/selderee) | [](https://www.npmjs.com/package/selderee) | [/packages/selderee](https://github.com/mxxii/selderee/tree/main/packages/selderee/) | [changelog](https://github.com/mxxii/selderee/blob/main/packages/selderee/CHANGELOG.md) |
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| [@selderee/plugin-htmlparser2](https://www.npmjs.com/package/@selderee/plugin-htmlparser2) | [](https://www.npmjs.com/package/@selderee/plugin-htmlparser2) | [/packages/plugin-htmlparser2](https://github.com/mxxii/selderee/tree/main/packages/plugin-htmlparser2/) | [changelog](https://github.com/mxxii/selderee/blob/main/packages/plugin-htmlparser2/CHANGELOG.md) |
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## Install
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```shell
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> npm i selderee @selderee/plugin-htmlparser2
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```
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## Documentation
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- [API](https://github.com/mxxii/selderee/blob/main/docs/index.md)
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## Usage example
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```js
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const htmlparser2 = require('htmlparser2');
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const util = require('util');
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const { DecisionTree, Treeify } = require('selderee');
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const { hp2Builder } = require('@selderee/plugin-htmlparser2');
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const selectorValuePairs = [
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['p', 'A'],
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['p.foo[bar]', 'B'],
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['p[class~=foo]', 'C'],
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['div.foo', 'D'],
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['div > p.foo', 'E'],
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['div > p', 'F'],
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['#baz', 'G']
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];
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// Make a tree structure from all given selectors.
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const selectorsDecisionTree = new DecisionTree(selectorValuePairs);
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// `treeify` builder produces a string output for testing and debug purposes.
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// `treeify` expects string values attached to each selector.
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const prettyTree = selectorsDecisionTree.build(Treeify.treeify);
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console.log(prettyTree);
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const html = /*html*/`<html><body>
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<div><p class="foo qux">second</p></div>
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</body></html>`;
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const dom = htmlparser2.parseDocument(html);
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const element = dom.children[0].children[0].children[1].children[0];
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// `hp2Builder` produces a picker that can pick values
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// from the selectors tree.
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const picker = selectorsDecisionTree.build(hp2Builder);
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// Get all matches
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const allMatches = picker.pickAll(element);
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console.log(util.inspect(allMatches, { breakLength: 70, depth: null }));
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// or get the value from the most specific match.
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const bestMatch = picker.pick1(element);
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console.log(`Best matched value: ${bestMatch}`);
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```
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<details><summary>Example output</summary>
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```text
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▽
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├─◻ Tag name
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│ ╟─◇ = p
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│ ║ ┠─▣ Attr value: class
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│ ║ ┃ ╙─◈ ~= "foo"
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│ ║ ┃ ┠─◨ Attr presence: bar
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│ ║ ┃ ┃ ┖─◁ #1 [0,2,1] B
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│ ║ ┃ ┠─◁ #2 [0,1,1] C
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│ ║ ┃ ┖─◉ Push element: >
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│ ║ ┃ └─◻ Tag name
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│ ║ ┃ ╙─◇ = div
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│ ║ ┃ ┖─◁ #4 [0,1,2] E
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│ ║ ┠─◁ #0 [0,0,1] A
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│ ║ ┖─◉ Push element: >
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│ ║ └─◻ Tag name
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│ ║ ╙─◇ = div
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│ ║ ┖─◁ #5 [0,0,2] F
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│ ╙─◇ = div
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│ ┖─▣ Attr value: class
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│ ╙─◈ ~= "foo"
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│ ┖─◁ #3 [0,1,1] D
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└─▣ Attr value: id
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╙─◈ = "baz"
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┖─◁ #6 [1,0,0] G
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[ { index: 2, value: 'C', specificity: [ 0, 1, 1 ] },
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{ index: 4, value: 'E', specificity: [ 0, 1, 2 ] },
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{ index: 0, value: 'A', specificity: [ 0, 0, 1 ] },
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{ index: 5, value: 'F', specificity: [ 0, 0, 2 ] } ]
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Best matched value: E
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```
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*Some gotcha: you may notice the check for `#baz` has to be performed every time the decision tree is called. If it happens to be `p#baz` or `div#baz` or even `.foo#baz` - it would be much better to write it like this. Deeper, narrower tree means less checks on average. (in case of `.foo#baz` the class check might finally outweigh the tag name check and rebalance the tree.)*
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</details>
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## Development
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Targeting Node.js version >=14.
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Monorepo uses NPM v7 workspaces (make sure v7 is installed when used with Node.js v14.)
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