/**
 * @author tint
 * @template Data, Input
 */
export class Game {
	/**
	 * @param {number} history How many frames of history to keep in memory
	 * @param {(data: Data, input: Input, frame: number) => Data} advance The function to apply game logic. For rollback to work properly, this must be a pure function, and can't mutate inputs.
	 */
	constructor(history, advance) {
		this.historysize = history;
		this.advance = advance;
		
		/** @type {{data: Data, input: Input}[]} */
		this.history = [];
		this.historyStart = 0;
		
		// the game may have inputs from the "future"
		// (local input delay to make sure inputs play at the same time on all machines)
		// so the "present" isn't always the latest frame
		// the game loop should set this every frame
		this.currentFrame = 0;
	}
	
	startHistory(frame, data) {
		this.historyStart = frame;
		this.history = [{ data }];
		this.currentFrame = frame;
	}
	
	getHistory(frame) {
		return this.history[frame - this.historyStart];
	}
	
	getFrame() {
		return this.historyStart + this.history.length - 1;
	}
	
	getCurrentData() {
		const entry = this.history[this.history.length - 1];
		return entry && entry.data;
	}
	
	/**
	 * Sets the input at a specific frame. If that frame is in history,
	 * the game will be rewound, the input applied, and then fast-forwarded to the current head.
	 * If the frame is ahead of the current latest frame, the game will be run until that frame.
	 * @param {number} frame The time to apply the input at
	 * @param {Input} input The input
	 */
	setInput(frame, input) {
		this.editFrame(frame, index => {
			let past = this.history[index - 1];
			if(index === 0) {
				past = { data: undefined };
			}
			this.history[index] = {
				input,
				data: this.advance(past ? past.data : undefined, input, frame),
			};
		});
	}
	
	setData(frame, data) {
		this.editFrame(frame, index => {
			this.history[index] = {
				data,
				input: this.history[index] && this.history[index].input,
			}
		});
	}
	
	/**
	 * @param {number} frame
	 * @param {(index: number) => void} edit
	 */
	editFrame(frame, edit) {
		const head = this.historyStart + this.history.length;
		if(frame < head) {
			if(frame < this.historyStart) {
				throw new Error("Tried to edit a past frame not in history:", frame);
			}
			
			edit(frame - this.historyStart);
			// fast forward back to the present with the new data
			for(let i = frame + 1; i < head; i++) {
				const past = this.history[i - this.historyStart - 1];
				this.history[i - this.historyStart].data = this.advance(
					past ? past.data : undefined,
					this.history[i - this.historyStart].input,
					i
				);
			}
		} else {
			// fast forward the inbetween frames with no input
			for(let i = head; i < frame; i++) {
				const entry = this.history[i - this.historyStart - 1];
				this.history[i - this.historyStart] = {
					input: undefined,
					data: this.advance(entry ? entry.data : undefined, undefined, i),
				};
			}
			edit(frame - this.historyStart);
		}
		
		while(this.history.length > this.historysize) {
			this.history.shift();
			this.historyStart++;
		}
	}
}