infinite-canvas/web/src/components/ui/dia-text-reveal.tsx

import { useEffect, useRef, useState } from "react";
import { animate, motion, useInView, useMotionValue, useReducedMotion, useTransform, type HTMLMotionProps } from "motion/react";

import { cn } from "@/lib/utils";

const DEFAULT_COLORS = ["#c679c4", "#fa3d1d", "#ffb005", "#e1e1fe", "#0358f7"];
const BAND_HALF = 17;
const SWEEP_START = -BAND_HALF;
const SWEEP_END = 100 + BAND_HALF;

const sweepEase = (t: number) => (t < 0.5 ? 4 * t ** 3 : 1 - (-2 * t + 2) ** 3 / 2);

function buildGradient(pos: number, colors: string[], textColor: string) {
    const bandStart = pos - BAND_HALF;
    const bandEnd = pos + BAND_HALF;

    if (bandStart >= 100) {
        return `linear-gradient(90deg, ${textColor}, ${textColor})`;
    }
    const n = colors.length;
    const parts: string[] = [];

    if (bandStart > 0) parts.push(`${textColor} 0%`, `${textColor} ${bandStart.toFixed(2)}%`);

    colors.forEach((c, i) => {
        const pct = n === 1 ? pos : bandStart + (i / (n - 1)) * BAND_HALF * 2;
        parts.push(`${c} ${pct.toFixed(2)}%`);
    });

    if (bandEnd < 100) parts.push(`transparent ${bandEnd.toFixed(2)}%`, `transparent 100%`);

    return `linear-gradient(90deg, ${parts.join(", ")})`;
}

function measureWidths(el: HTMLElement, texts: string[]) {
    const ghost = el.cloneNode() as HTMLElement;
    Object.assign(ghost.style, {
        position: "absolute",
        visibility: "hidden",
        pointerEvents: "none",
        width: "auto",
        whiteSpace: "nowrap",
    });
    el.parentElement!.appendChild(ghost);
    const widths = texts.map((t) => {
        ghost.textContent = t;
        return ghost.getBoundingClientRect().width;
    });
    ghost.remove();
    return widths;
}

/**
 * Props for {@link DiaTextReveal}.
 */
export interface DiaTextRevealProps extends Omit<HTMLMotionProps<"span">, "ref" | "children" | "style" | "animate" | "transition" | "color"> {
    /**
     * Text to reveal. Pass multiple strings to rotate when {@link DiaTextRevealProps.repeat} is `true`.
     */
    text: string | string[];
    /**
     * Colors sampled across the moving gradient band. Defaults to a built-in palette.
     */
    colors?: string[];
    /**
     * CSS color for revealed text after the sweep and for leading/trailing regions during the animation.
     * @defaultValue `"var(--foreground)"`
     */
    textColor?: string;
    /**
     * Duration of one sweep pass, in seconds.
     * @defaultValue `1.5`
     */
    duration?: number;
    /**
     * Delay before the sweep starts, in seconds.
     * @defaultValue `0`
     */
    delay?: number;
    /**
     * When `text` is an array, replay the sweep and advance to the next string after each completion.
     * @defaultValue `false`
     */
    repeat?: boolean;
    /**
     * Pause between cycles when {@link DiaTextRevealProps.repeat} is `true`, in seconds.
     * @defaultValue `0.5`
     */
    repeatDelay?: number;
    /**
     * If `true`, the animation starts only after the element enters the viewport.
     * @defaultValue `true`
     */
    startOnView?: boolean;
    /**
     * Passed to `useInView`: if `true`, in-view detection fires at most once (no replay on scroll-back).
     * @defaultValue `true`
     */
    once?: boolean;
    /**
     * Additional class names for the animated `span` (e.g. typography utilities).
     */
    className?: string;
    /**
     * When `text` has multiple entries, use the widest string’s width for layout instead of animating width per line.
     * @defaultValue `false`
     */
    fixedWidth?: boolean;
}

export function DiaTextReveal({ text, colors = DEFAULT_COLORS, textColor = "var(--foreground)", duration = 1.5, delay = 0, repeat = false, repeatDelay = 0.5, startOnView = true, once = true, className, fixedWidth = false, ...props }: DiaTextRevealProps) {
    const texts = Array.isArray(text) ? text : [text];
    const isMulti = texts.length > 1;
    const prefersReducedMotion = useReducedMotion();

    const spanRef = useRef<HTMLSpanElement>(null);
    const optsRef = useRef({
        colors,
        textColor,
        duration,
        delay,
        repeat,
        repeatDelay,
        texts,
    });
    optsRef.current = {
        colors,
        textColor,
        duration,
        delay,
        repeat,
        repeatDelay,
        texts,
    };

    const indexRef = useRef(0);
    const hasPlayedRef = useRef(false);
    const timerRef = useRef<ReturnType<typeof setTimeout>>(undefined);
    const playRef = useRef<() => void>(null!);
    const stopRef = useRef<(() => void) | null>(null);

    const [activeIndex, setActiveIndex] = useState(0);
    const [measuredWidths, setMeasuredWidths] = useState<number[]>([]);

    const sweepPos = useMotionValue(SWEEP_START);

    const backgroundImage = useTransform(sweepPos, (pos) => buildGradient(pos, optsRef.current.colors, optsRef.current.textColor));

    const isInView = useInView(spanRef, { once, amount: 0.1 });

    useEffect(() => {
        const el = spanRef.current;
        if (!el || !isMulti) return;
        setMeasuredWidths(measureWidths(el, texts));
    }, [Array.isArray(text) ? text.join("\0") : text]);

    playRef.current = () => {
        const { duration, delay, repeat, repeatDelay, texts } = optsRef.current;

        sweepPos.set(SWEEP_START);

        const controls = animate(sweepPos, SWEEP_END, {
            duration,
            delay,
            ease: sweepEase,
            onComplete() {
                if (!repeat) return;
                timerRef.current = setTimeout(() => {
                    const next = (indexRef.current + 1) % texts.length;
                    indexRef.current = next;
                    setActiveIndex(next);
                    playRef.current();
                }, repeatDelay * 1000);
            },
        });

        stopRef.current = () => controls.stop();
    };

    useEffect(() => {
        if (prefersReducedMotion) {
            sweepPos.set(SWEEP_END);
            return;
        }
        if (startOnView && !isInView) return;
        if (once && hasPlayedRef.current) return;
        hasPlayedRef.current = true;
        playRef.current();

        return () => {
            stopRef.current?.();
            clearTimeout(timerRef.current);
        };
    }, [isInView, startOnView, once, prefersReducedMotion, sweepPos]);

    const fixedW = isMulti && fixedWidth && measuredWidths.length > 0 ? Math.max(...measuredWidths) : undefined;

    const animatedW = isMulti && !fixedWidth && measuredWidths[activeIndex] != null ? measuredWidths[activeIndex] : undefined;

    return (
        <motion.span
            ref={spanRef}
            className={cn("align-bottom leading-[100%] text-inherit", className)}
            style={{
                transform: "translateY(-2px)",
                color: "transparent",
                backgroundClip: "text",
                WebkitBackgroundClip: "text",
                backgroundSize: "100% 100%",
                backgroundImage,
                ...(isMulti && {
                    display: "inline-block",
                    overflow: "hidden",
                    whiteSpace: "nowrap",
                    verticalAlign: "text-center",
                    ...(fixedW != null && { width: fixedW }),
                }),
            }}
            animate={animatedW != null ? { width: animatedW } : undefined}
            transition={{ duration: 0.4, ease: [0.4, 0, 0.2, 1] }}
            {...props}
        >
            {texts[activeIndex]}
        </motion.span>
    );
}