From 025e45f9d23297c0909b57f1d3a4aa28de752802 Mon Sep 17 00:00:00 2001
From: Evan Hahn <me@evanhahn.com>
Date: Wed, 16 Aug 2023 10:46:31 -0500
Subject: [PATCH] GIF: parse Logical Screen Descriptor

---
 public/gif/constants.js |  7 ++++++
 public/gif/getNodeUi.js | 51 ++++++++++++++++++++++++++++++++++++++++-
 public/gif/parseGif.js  | 28 +++++++++++++++++++++-
 3 files changed, 84 insertions(+), 2 deletions(-)

diff --git a/public/gif/constants.js b/public/gif/constants.js
index fb45fdd..6339a23 100644
--- a/public/gif/constants.js
+++ b/public/gif/constants.js
@@ -7,6 +7,13 @@ export const GifNodeType = [
   "header",
   "headerSignature",
   "headerVersion",
+
+  "logicalScreenDescriptor",
+  "logicalScreenWidth",
+  "logicalScreenHeight",
+  "logicalScreenDescriptorPackedFields",
+  "logicalScreenBackgroundColorIndex",
+  "logicalScreenPixelAspectRatio",
 ].reduce((result, id) => {
   result[id] = id;
   return result;
diff --git a/public/gif/getNodeUi.js b/public/gif/getNodeUi.js
index 9cebf81..efe5339 100644
--- a/public/gif/getNodeUi.js
+++ b/public/gif/getNodeUi.js
@@ -4,6 +4,7 @@ import { GifNodeType } from "./constants.js";
 import crel, { fragment } from "../common/crel.js";
 import getOwn from "../common/getOwn.js";
 import { areBytesEqual } from "../common/bytes.js";
+import pluralize from "../common/pluralize.js";
 /** @typedef {import("../../types/gif.d.ts").GifNode} GifNode */
 
 const textEncoder = new TextEncoder();
@@ -16,6 +17,15 @@ const VERSION_89 = textEncoder.encode("89a");
  */
 const p = (...children) => crel("p", {}, ...children);
 
+/**
+ * @param {Uint8Array} bytes
+ * @returns {number}
+ */
+const readGifUint = (bytes) => {
+  if (bytes.length !== 2) throw new Error("Expected 2 bytes");
+  return (bytes[1] << 8) | bytes[0];
+};
+
 /**
  * @typedef {object} NodeUi
  * @prop {string} title
@@ -57,6 +67,43 @@ const NODE_UI_FNS = {
       ),
     };
   },
+
+  // Logical screen descriptor
+
+  [GifNodeType.logicalScreenDescriptor]: () => ({
+    title: "Logical Screen Descriptor",
+    description: p(
+      "The logical screen descriptor contains information about the overall GIF, such as its dimensions.",
+    ),
+  }),
+  [GifNodeType.logicalScreenWidth]: ({ bytes }) => ({
+    title: "Logical Screen Width",
+    description: p(
+      `The width of the image stored as a 16-bit little endian unsigned integer. In this case, the width is ${
+        pluralize(readGifUint(bytes), "byte")
+      }.`,
+    ),
+  }),
+  [GifNodeType.logicalScreenHeight]: ({ bytes }) => ({
+    title: "Logical Screen Height",
+    description: p(
+      `The height of the image stored as a 16-bit little endian unsigned integer. In this case, the height is ${
+        pluralize(readGifUint(bytes), "byte")
+      }.`,
+    ),
+  }),
+  [GifNodeType.logicalScreenDescriptorPackedFields]: () => ({
+    title: "Logical Screen Descriptor packed fields",
+    description: p("TODO"),
+  }),
+  [GifNodeType.logicalScreenBackgroundColorIndex]: () => ({
+    title: "Background Color Index",
+    description: p("TODO"),
+  }),
+  [GifNodeType.logicalScreenPixelAspectRatio]: () => ({
+    title: "Pixel Aspect Ratio",
+    description: p("TODO"),
+  }),
 };
 
 /**
@@ -65,6 +112,8 @@ const NODE_UI_FNS = {
  */
 export default (node) => {
   const uiFn = getOwn(NODE_UI_FNS, node.type);
-  if (!uiFn) throw new Error("Found a node with no matching UI function");
+  if (!uiFn) {
+    throw new Error(`Found a node (${node.type}) with no matching UI function`);
+  }
   return uiFn(node);
 };
diff --git a/public/gif/parseGif.js b/public/gif/parseGif.js
index 759eaed..6409e5c 100644
--- a/public/gif/parseGif.js
+++ b/public/gif/parseGif.js
@@ -20,7 +20,33 @@ export default (bytes) => {
         },
         {
           type: GifNodeType.headerVersion,
-          bytes: bytes.subarray(3, 6),
+          bytes: bytes.subarray(3, 3 + 3),
+        },
+      ],
+    },
+    {
+      type: GifNodeType.logicalScreenDescriptor,
+      bytes: bytes.subarray(6, 6 + 6),
+      children: [
+        {
+          type: GifNodeType.logicalScreenWidth,
+          bytes: bytes.subarray(6, 6 + 2),
+        },
+        {
+          type: GifNodeType.logicalScreenHeight,
+          bytes: bytes.subarray(8, 8 + 2),
+        },
+        {
+          type: GifNodeType.logicalScreenDescriptorPackedFields,
+          bytes: bytes.subarray(10, 10 + 1),
+        },
+        {
+          type: GifNodeType.logicalScreenBackgroundColorIndex,
+          bytes: bytes.subarray(11, 11 + 1),
+        },
+        {
+          type: GifNodeType.logicalScreenPixelAspectRatio,
+          bytes: bytes.subarray(12, 12 + 1),
         },
       ],
     },