add stream caching ("offline mode")

src/lib/streamContext.svelte.ts

@@ -0,0 +1,47 @@
+import { createContext } from 'svelte';
+import type { Stream, StreamSummary } from './types.ts';
+
+export class StreamContext {
+	streams: StreamSummary[];
+	current = $state<Stream | null>(null);
+	songIndex = $state<number | null>(null);
+	#timestamps: number[] = [];
+
+	constructor(streams: StreamSummary[]) {
+		this.streams = streams;
+	}
+
+	setCurrent(stream: Stream) {
+		this.current = stream;
+		this.#timestamps = [-Infinity, ...stream.tracks.map((t) => t[0]), Infinity];
+		this.songIndex = 0;
+	}
+
+	clearCurrent() {
+		this.current = null;
+		this.songIndex = null;
+	}
+
+	getSongAtTime(time: number): number {
+		return this.#locationOf(time, this.#timestamps) - 1;
+	}
+
+	updateCurrentSong(currentTime: number, songIndex: number) {
+		const ts = this.#timestamps;
+		const recurse = (idx: number): number => {
+			if (currentTime >= ts[idx + 2]) return recurse(idx + 1);
+			if (currentTime < ts[idx + 1]) return recurse(idx - 1);
+			return idx;
+		};
+		this.songIndex = recurse(songIndex);
+	}
+
+	#locationOf(element: number, array: number[], start = 0, end = array.length): number {
+		const pivot = Math.floor(start + (end - start) / 2);
+		if (end - start <= 1 || array[pivot] === element) return pivot;
+		if (array[pivot] < element) return this.#locationOf(element, array, pivot, end);
+		return this.#locationOf(element, array, start, pivot);
+	}
+}
+
+export const [getStreamContext, setStreamContext] = createContext<StreamContext>();