From ed2e84ee0a9fe4fe002ea0333cf67a20cc46c1e0 Mon Sep 17 00:00:00 2001
From: Geoff Doty <n2geoff@gmail.com>
Date: Sat, 20 Jul 2024 16:22:06 -0400
Subject: [PATCH] dom diffing for more effecent rerendering

---
 README.md    |   9 ++--
 index.js     |   2 +-
 src/app.js   |   6 ++-
 src/emerj.js | 122 +++++++++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 133 insertions(+), 6 deletions(-)
 create mode 100644 src/emerj.js

diff --git a/README.md b/README.md
index b11d707..5ed0e2a 100644
--- a/README.md
+++ b/README.md
@@ -7,11 +7,11 @@ Um, is an experimental composable UI builder that takes ideas from early [hypera
 Um, because you should think about, um, NOT using it.
 
 ## Features
-- No Virtual Dom
+- Real DOM
+- [Non-destructive (re)Rendering](https://github.com/bryhoyt/emerj)
 - No Build System
 - No Over Engineering
-- ~1kb minified
-- Totally INEFFICIENT rendering (at scale)
+- ~2kb minified
 
 ## Install
 
@@ -81,4 +81,5 @@ The `h()` is an **optional** hypertext build utility that weighs in around **~25
 
 ### TODO
 
-- Improve Update
+- Some tag attributes do not work, like rowspan on td
+- Rethink State Management, might be ok
diff --git a/index.js b/index.js
index c164ad9..71f7ae3 100644
--- a/index.js
+++ b/index.js
@@ -1,5 +1,5 @@
-import h from "./src/tag.js";
 import app from "./src/app.js";
+import h from "./src/tag.js";
 
 export {app};
 export {h};
\ No newline at end of file
diff --git a/src/app.js b/src/app.js
index 7e078a4..28d6a2d 100644
--- a/src/app.js
+++ b/src/app.js
@@ -1,3 +1,7 @@
+import diff from "./emerj.js";
+
+/*! Um v0.5.0 | MIT LICENSE | https://github.com/n2geoff/um */
+
 /**
  * App Builder
  *
@@ -54,7 +58,7 @@ export default function app(opts) {
 
     /** update dom */
     const update = () => {
-        document.querySelector(mount).replaceChildren(view(state, actions));
+        diff.merge(document.querySelector(mount), view(state, actions));
     }
 
     // mount view
diff --git a/src/emerj.js b/src/emerj.js
new file mode 100644
index 0000000..378aa7e
--- /dev/null
+++ b/src/emerj.js
@@ -0,0 +1,122 @@
+/*! Emerj v1.0.0 | MIT LICENSE | https://github.com/bryhoyt/emerj */
+export default {
+    attrs(elem) {
+        const attrs = {};
+        for (let i=0; i < elem.attributes.length; i++) {
+            const attr = elem.attributes[i];
+            attrs[attr.name] = attr.value;
+        }
+        return attrs;
+    },
+    nodesByKey(parent, makeKey) {
+        const map = {};
+        for (let j=0; j < parent.childNodes.length; j++) {
+            const key = makeKey(parent.childNodes[j]);
+            if (key) map[key] = parent.childNodes[j];
+        }
+        return map;
+    },
+    merge(base, modified, opts) {
+        /* Merge any differences between base and modified back into base.
+         *
+         * Operates only the children nodes, and does not change the root node or its
+         * attributes.
+         *
+         * Conceptually similar to React's reconciliation algorithm:
+         * https://facebook.github.io/react/docs/reconciliation.html
+         *
+         * I haven't thoroughly tested performance to compare to naive DOM updates (i.e.
+         * just updating the entire DOM from a string using .innerHTML), but some quick
+         * tests on a basic DOMs were twice as fast -- so at least it's not slower in
+         * a simple scenario -- and it's definitely "fast enough" for responsive UI and
+         * even smooth animation.
+         *
+         * The real advantage for me is not so much performance, but that state & identity
+         * of existing elements is preserved -- text typed into an <input>, an open
+         * <select> dropdown, scroll position, ad-hoc attached events, canvas paint, etc,
+         * are preserved as long as an element's identity remains.
+         *
+         * See https://korynunn.wordpress.com/2013/03/19/the-dom-isnt-slow-you-are/
+         */
+        opts = opts || {};
+        opts.key = opts.key || (node => node.id);
+
+        if (typeof modified === 'string') {
+            const html = modified;
+            // Make sure the parent element of the provided HTML is of the same type as
+            // `base`'s parent. This matters when the HTML contains fragments that are
+            // only valid inside certain elements, eg <td>s, which must have a <tr>
+            // parent.
+            modified = document.createElement(base.tagName);
+            modified.innerHTML = html;
+        }
+
+        // Naively recurse into the children, if any, replacing or updating new
+        // elements that are in the same position as old, deleting trailing elements
+        // when the new list contains fewer children, or appending new elements if
+        // it contains more children.
+        //
+        // For re-ordered children, the `id` attribute can be used to preserve identity.
+
+        // Loop through .childNodes, not just .children, so we compare text nodes (and
+        // comment nodes, fwiw) too.
+
+        const nodesByKey = {old: this.nodesByKey(base, opts.key),
+                          new: this.nodesByKey(modified, opts.key)};
+
+        let idx;
+        for (idx=0; modified.firstChild; idx++) {
+            const newNode = modified.removeChild(modified.firstChild);
+            if (idx >= base.childNodes.length) {
+                // It's a new node. Append it.
+                base.appendChild(newNode);
+                continue;
+            }
+
+            let baseNode = base.childNodes[idx];
+
+            // If the children are indexed, then make sure to retain their identity in
+            // the new order.
+            const newKey = opts.key(newNode);
+            if (opts.key(baseNode) || newKey) {
+                // If the new node has a key, then either use its existing match, or insert it.
+                // If not, but the old node has a key, then make sure to leave it untouched and insert the new one instead.
+                // Else neither node has a key. Just overwrite old with new.
+                const match = (newKey && newKey in nodesByKey.old)? nodesByKey.old[newKey]: newNode;
+                if (match !== baseNode) {
+                    baseNode = base.insertBefore(match, baseNode);
+                }
+            }
+
+            if (baseNode.nodeType !== newNode.nodeType || baseNode.tagName !== newNode.tagName) {
+                // Completely different node types. Just update the whole subtree, like React does.
+                base.replaceChild(newNode, baseNode);
+            } else if ([Node.TEXT_NODE, Node.COMMENT_NODE].indexOf(baseNode.nodeType) >= 0) {
+                // This is the terminating case of the merge() recursion.
+                if (baseNode.textContent === newNode.textContent) continue;  // Don't write if we don't need to.
+                baseNode.textContent = newNode.textContent;
+            } else if (baseNode !== newNode) {   // Only need to update if we haven't just inserted the newNode in.
+                // It's an existing node with the same tag name. Update only what's necessary.
+                // First, make dicts of attributes, for fast lookup:
+                const attrs = {base: this.attrs(baseNode), new: this.attrs(newNode)};
+                for (const attr in attrs.base) {
+                    // Remove any missing attributes.
+                    if (attr in attrs.new) continue;
+                    baseNode.removeAttribute(attr);
+                }
+                for (const attr in attrs.new) {
+                    // Add and update any new or modified attributes.
+                    if (attr in attrs.base && attrs.base[attr] === attrs.new[attr]) continue;
+                    baseNode.setAttribute(attr, attrs.new[attr]);
+                }
+                // Now, recurse into the children. If the only children are text, this will
+                // be the final recursion on this node.
+                this.merge(baseNode, newNode);
+            }
+        }
+        while (base.childNodes.length > idx) {
+            // If base has more children than modified, delete the extras.
+            base.removeChild(base.lastChild);
+        }
+    },
+};