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main_plugin/dgrm/infrastructure/png-chunk.js

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+/**
+ * @param {Blob} png
+ * @param {string} chunkName 4 symbol string
+ * @returns {Promise<DataView | null>} chunk data
+ */
+export async function pngChunkGet(png, chunkName) {
+	return chunkGet(
+		await png.arrayBuffer(),
+		toUit32(chunkName));
+}
+
+/**
+ * @param {Blob} png
+ * @param {string} chunkName 4 symbol string
+ * @param {Uint8Array} data
+ * @returns {Promise<Blob>} new png
+ */
+export async function pngChunkSet(png, chunkName, data) {
+	return chunkSet(
+		await png.arrayBuffer(),
+		toUit32(chunkName),
+		data
+	);
+}
+
+/**
+ * @param {ArrayBuffer} pngData
+ * @param {number} chunkNameUint32 chunk name as Uint32
+ * @param {Uint8Array} data
+ * @returns {Blob} new png
+ */
+function chunkSet(pngData, chunkNameUint32, data) {
+	/** @type {DataView} */
+	let startPart;
+	/** @type {DataView} */
+	let endPart;
+
+	const existingChunk = chunkGet(pngData, chunkNameUint32);
+	if (existingChunk) {
+		startPart = new DataView(pngData, 0, existingChunk.byteOffset - 8);
+		endPart = new DataView(pngData, existingChunk.byteOffset + existingChunk.byteLength + 4);
+	} else {
+		const endChunkStart = pngData.byteLength - 12; // 12 - end chunk length
+		startPart = new DataView(pngData, 0, endChunkStart);
+		endPart = new DataView(pngData, endChunkStart);
+	}
+
+	const chunkHeader = new DataView(new ArrayBuffer(8));
+	chunkHeader.setUint32(0, data.length);
+	chunkHeader.setUint32(4, chunkNameUint32);
+
+	return new Blob([
+		startPart,
+
+		// new chunk
+		chunkHeader,
+		data,
+		new Uint32Array([0]),	// CRC fake - not calculated
+
+		endPart
+	],
+	{ type: 'image/png' });
+}
+
+/**
+ * @param {ArrayBuffer} pngData
+ * @param {number} chunkNameUint32 chunk name as Uint32
+ * @returns {DataView | null} chunk data
+ */
+function chunkGet(pngData, chunkNameUint32) {
+	const dataView = new DataView(pngData, 8); // 8 byte - png signature
+
+	let chunkPosition = 0;
+	let chunkUint = dataView.getUint32(4);
+	let chunkLenght;
+	while (chunkUint !== 1229278788) { // last chunk 'IEND'
+		chunkLenght = dataView.getUint32(chunkPosition);
+		if (chunkUint === chunkNameUint32) {
+			return new DataView(pngData, chunkPosition + 16, chunkLenght);
+		}
+		chunkPosition = chunkPosition + 12 + chunkLenght;
+		chunkUint = dataView.getUint32(chunkPosition + 4);
+	}
+	return null;
+}
+
+/**
+ * @param {string} chunkName 4 symbol string
+ * @return {number} uit32
+ */
+function toUit32(chunkName) {
+	return new DataView((new TextEncoder()).encode(chunkName).buffer).getUint32(0);
+}