/**
 * Mouse-wheel input shared by the Usage and Injections views.
 *
 * Pi enables mouse reporting only in fullscreen (alt-screen) mode, where its
 * viewport forwards wheel reports to the focused overlay instead of consuming
 * them. In regular mode the terminal keeps the wheel for its own scrollback and
 * no report ever reaches a view, so the parser simply never matches there.
 */
import type { TUI } from "@earendil-works/pi-tui";

/** Lines one notch scrolls when the TUI exposes no step of its own. */
export const DEFAULT_WHEEL_SCROLL_LINES = 3;

/** SGR wheel report: `ESC [ < button ; column ; row` closed by `M` or `m`. */
const SGR_WHEEL_PATTERN = /^\x1b\[<(\d+);\d+;\d+[Mm]$/;

/** Legacy X10 wheel report: `ESC [ M` plus offset-encoded button, column, and row bytes. */
const X10_PREFIX = "\x1b[M";
const X10_LENGTH = 6;
const X10_BUTTON_INDEX = 3;
const X10_BYTE_OFFSET = 32;

/** Wheel reports set this button bit; the low two bits then carry axis and direction. */
const WHEEL_BUTTON_BIT = 64;
const DIRECTION_MASK = 3;

/** One notch backwards (up) or forwards (down). */
export type WheelDirection = -1 | 1;

/**
 * Vertical wheel direction encoded in one input chunk, or undefined for every
 * other input, including horizontal notches. Modifier bits are ignored, so a
 * modified notch scrolls like a plain one, as it does in pi's own viewport.
 */
export function parseWheelDirection(data: string): WheelDirection | undefined {
	const sgrButton = SGR_WHEEL_PATTERN.exec(data)?.[1];
	if (sgrButton !== undefined) return decodeWheelButton(Number.parseInt(sgrButton, 10));
	if (data.length === X10_LENGTH && data.startsWith(X10_PREFIX)) {
		return decodeWheelButton(data.charCodeAt(X10_BUTTON_INDEX) - X10_BYTE_OFFSET);
	}
	return undefined;
}

/**
 * Lines one notch scrolls in the surrounding TUI.
 *
 * `TuiAltScreen` keeps its `wheelScrollLines` option private and pi never
 * surfaces it, so read it defensively: honoring the real step keeps previews
 * consistent with the transcript viewport, while any pi that hides, renames, or
 * never had the field falls back to the default instead of a contradicting step.
 */
export function readWheelScrollLines(tui: TUI): number {
	const lines = (tui as { readonly wheelScrollLines?: unknown }).wheelScrollLines;
	if (typeof lines !== "number" || !Number.isFinite(lines)) return DEFAULT_WHEEL_SCROLL_LINES;
	return Math.max(1, Math.floor(lines));
}

/** Decode one button code, rejecting non-wheel buttons and the horizontal axis. */
function decodeWheelButton(button: number): WheelDirection | undefined {
	if ((button & WHEEL_BUTTON_BIT) === 0) return undefined;
	const direction = button & DIRECTION_MASK;
	if (direction === 0) return -1;
	if (direction === 1) return 1;
	return undefined;
}