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+# pq-distance: Approximate Tree-Edit Distance for Node.js
+
+![CI status](https://github.com/se2p/pq-distance/actions/workflows/ci.yml/badge.svg?branch=master)
+
+Modern TypeScript implementation of pq-gram distance, an efficient approximation for tree-edit distance. Based on the
+academic paper [^1][^2]. Node.js API inspired by jqgram[^3].
+
+## Installation
+
+The package can be installed as `@se2p/pq-distance` from GitHub's npm registry using `npm` or similar. To this,
+add the `--registry=https://npm.pkg.github.com` flag during installation:
+
+```bash
+npm install @se2p/pq-distance --registry=https://npm.pkg.github.com   
+```
+
+To permanently use GitHub's npm registry for all packages of the `@se2p` scope, you may want to update your
+project-level or global `.npmrc`. For example:
+
+```bash
+echo '@se2p:registry=https://npm.pkg.github.com' >> ~/.npmrc
+```
+
+## Usage
+
+The package exports a single function `pqDistance`:
+
+```javascript
+const {pqDistance} = require("@se2p/pq-distance");
+```
+
+TypeScript users can also import the `PQTree` and `PQOpts` types:
+
+```typescript
+import {pqDistance, PQTree, PQOpts} from "@se2p/pq-distance";
+```
+
+Example trees:
+
+```
+        a              a
+       /|\            /|\
+t1:   a b c    t2:   a b c 
+     / \            / \
+    e   b          e   x
+```
+
+Arbitrary tree-representations are supported. Provide two functions `getLabel()` to extract the label as string from a
+node, and `getChildren()` that returns a node's children in an array.
+
+For example:
+
+```typescript
+const t1 = {
+    label: 'a',
+    children: [
+        {
+            label: 'a',
+            children: [
+                {
+                    label: 'e',
+                    children: []
+                },
+                {
+                    label: 'b',
+                    children: []
+                }
+            ]
+        },
+        {
+            label: 'b',
+            children: []
+        },
+        {
+            label: 'c',
+            children: []
+        }
+    ]
+};
+
+const t2 = {
+    node: 'A',
+    child: [
+        {
+            node: 'A',
+            child: [
+                {node: 'E'},
+                {node: 'B'}
+            ]
+        },
+        {node: 'B'},
+        {node: 'X'}
+    ]
+};
+```
+
+To compute the pq-gram distance between `t1` and `t2`, pass them as `PQTree` objects to `pqDistance()`:
+
+```typescript
+const tree1: PQTree = {
+    root: t1,
+    getLabel: ({label}) => label,
+    getChildren: ({children}) => children
+};
+
+const tree2: PQTree = {
+    root: t2,
+    getLabel: (n) => n.node.toLowerCase(),
+    getChildren: (n) => n.child ?? []
+};
+```
+
+Finally:
+
+```typescript
+const opts: PQOpts = {p: 2, q: 3}; // default values 
+pqDistance(tree1, tree2, opts); // 0.3076923076923077
+```
+
+The object `opts` sets the `p` and `q` values for distance computation, and may be omitted to use the default values
+`p=2` and `q=3`. Please refer to the academic paper how they affect the distance value.
+
+## License
+
+pq-distance 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.
+
+pq-distance 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.
+
+
+[^1]: Nikolaus Augsten, Michael H. Böhlen, and Johann Gamper. 2005. Approximate Matching of Hierarchical Data Using
+   pq-Grams. In Proceedings of the 31st International Conference on Very Large Data Bases, Trondheim, Norway, August 30
+   – September 2, 2005
+[^2]: https://github.com/DatabaseGroup/apted
+[^3]: https://github.com/hoonto/jqgram