Matrix Rain

/**
 * Matrix Rain — falling katakana + digit characters with fade trails.
 * Category: backgrounds | Animate: true | Interactive: false | Complexity: medium
 */
export default function matrixRain(ctx, state) {
  const CHAR_SIZE = 14;
  const TRAIL_LENGTH = 16;
  const CHARS =
    'アイウエオカキクケコサシスセソタチツテトナニヌネノハヒフヘホマミムメモヤユヨラリルレロワヲン01';

  let columns = [];

  function init(width, height) {
    const count = Math.ceil(width / CHAR_SIZE);
    columns = Array.from({ length: count }, (_, i) => ({
      x: i * CHAR_SIZE + CHAR_SIZE / 2,
      y: -(Math.random() * height * 1.5),
      speed: 0.6 + Math.random() * 1.4,
      charOffset: Math.floor(Math.random() * CHARS.length),
    }));
  }

  init(state.width, state.height);

  return {
    resize({ width, height }) {
      init(width, height);
    },

    frame({ width, height, time, delta, reducedMotion }) {
      ctx.clearRect(0, 0, width, height);

      if (reducedMotion) {
        // Static snapshot — no animation
        ctx.font = `${CHAR_SIZE}px monospace`;
        for (const col of columns) {
          const steps = Math.floor(Math.abs(col.y) / CHAR_SIZE);
          for (let i = 0; i < Math.min(steps, TRAIL_LENGTH); i++) {
            const alpha = (1 - i / TRAIL_LENGTH) * 0.25;
            ctx.fillStyle = `oklch(0.55 0.2 145 / ${alpha})`;
            const charIdx = (col.charOffset + i) % CHARS.length;
            ctx.fillText(CHARS[charIdx], col.x - CHAR_SIZE / 2, Math.abs(col.y) - i * CHAR_SIZE);
          }
        }
        return;
      }

      ctx.font = `${CHAR_SIZE}px monospace`;
      ctx.textBaseline = 'top';

      for (const col of columns) {
        const headY = col.y;

        // Draw trail from head upward
        for (let i = 0; i < TRAIL_LENGTH; i++) {
          const y = headY - i * CHAR_SIZE;
          if (y < -CHAR_SIZE || y > height) continue;

          const charIdx = (col.charOffset + Math.floor(time * col.speed * 8) + i) % CHARS.length;
          const char = CHARS[charIdx];

          if (i === 0) {
            // Head: bright white-green
            ctx.fillStyle = 'oklch(0.96 0.1 145)';
          } else {
            // Trail: fading green
            const alpha = (1 - i / TRAIL_LENGTH) * 0.85;
            ctx.fillStyle = `oklch(0.55 0.22 145 / ${alpha})`;
          }

          ctx.fillText(char, col.x - CHAR_SIZE / 2, y);
        }

        // Advance column
        col.y += col.speed * delta * 80;
        if (col.y > height + CHAR_SIZE * TRAIL_LENGTH) {
          col.y = -(CHAR_SIZE * (3 + Math.random() * 12));
          col.speed = 0.6 + Math.random() * 1.4;
          col.charOffset = Math.floor(Math.random() * CHARS.length);
        }
      }
    },

    destroy() {
      columns = [];
    },
  };
}

<canvas id="canvas-effect" style="width:100%;height:400px;display:block" aria-hidden="true"></canvas>
<script>
/**
 * mountCanvas — Minimal runtime for Canvas effects.
 *
 * Handles HiDPI scaling, ResizeObserver, RAF loop, pointer tracking,
 * reduced-motion, and cleanup. The effect only implements draw logic.
 *
 * @param {HTMLCanvasElement} canvas
 * @param {(ctx: CanvasRenderingContext2D, state: object) => object} createEffect
 * @param {object} [options]
 * @returns {() => void} destroy function
 */
function mountCanvas(canvas, createEffect, options = {}) {
  const ctx = canvas.getContext('2d', { alpha: true });
  if (!ctx) throw new Error('2D context not available');

  const opts = {
    animate: true,
    clear: true,
    maxDpr: 2,
    pauseWhenHidden: true,
    respectReducedMotion: true,
    ...options,
  };

  const reducedMotion =
    opts.respectReducedMotion && window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  const state = {
    canvas,
    ctx,
    width: 0,
    height: 0,
    pixelWidth: 0,
    pixelHeight: 0,
    dpr: 1,
    time: 0,
    delta: 0,
    frame: 0,
    playing: false,
    reducedMotion,
    pointer: { x: 0, y: 0, inside: false, down: false },
  };

  let rafId = 0;
  let lastTime = 0;
  let disposed = false;

  function measure() {
    const rect = canvas.getBoundingClientRect();
    const w = Math.max(1, rect.width);
    const h = Math.max(1, rect.height);
    const dpr = Math.min(window.devicePixelRatio || 1, opts.maxDpr);
    const pw = Math.round(w * dpr);
    const ph = Math.round(h * dpr);

    if (canvas.width !== pw || canvas.height !== ph) {
      canvas.width = pw;
      canvas.height = ph;
    }

    state.width = w;
    state.height = h;
    state.pixelWidth = pw;
    state.pixelHeight = ph;
    state.dpr = dpr;
    ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
  }

  function clear() {
    ctx.clearRect(0, 0, state.width, state.height);
  }

  const effect = createEffect(ctx, state) || {};

  function onResize() {
    measure();
    effect.resize?.(state);
    if (!opts.animate || reducedMotion) {
      if (opts.clear) clear();
      effect.frame?.(state);
    }
  }

  function loop(now) {
    if (disposed) return;
    if (opts.pauseWhenHidden && document.hidden) {
      rafId = requestAnimationFrame(loop);
      return;
    }

    state.delta = lastTime ? (now - lastTime) / 1000 : 0;
    state.time = now / 1000;
    state.frame += 1;
    lastTime = now;

    if (opts.clear) clear();
    effect.frame?.(state);

    rafId = requestAnimationFrame(loop);
  }

  function onPointerMove(e) {
    const rect = canvas.getBoundingClientRect();
    state.pointer.x = e.clientX - rect.left;
    state.pointer.y = e.clientY - rect.top;
    if (!opts.animate || reducedMotion) {
      if (opts.clear) clear();
      effect.frame?.(state);
    }
  }
  function onPointerEnter() {
    state.pointer.inside = true;
  }
  function onPointerLeave() {
    state.pointer.inside = false;
    state.pointer.down = false;
  }
  function onPointerDown() {
    state.pointer.down = true;
  }
  function onPointerUp() {
    state.pointer.down = false;
  }

  const ro = new ResizeObserver(onResize);
  ro.observe(canvas);

  canvas.addEventListener('pointermove', onPointerMove);
  canvas.addEventListener('pointerenter', onPointerEnter);
  canvas.addEventListener('pointerleave', onPointerLeave);
  canvas.addEventListener('pointerdown', onPointerDown);
  window.addEventListener('pointerup', onPointerUp);

  measure();

  if (opts.animate && !reducedMotion) {
    state.playing = true;
    rafId = requestAnimationFrame(loop);
  } else {
    effect.frame?.(state);
  }

  return function destroy() {
    disposed = true;
    state.playing = false;
    cancelAnimationFrame(rafId);
    ro.disconnect();
    canvas.removeEventListener('pointermove', onPointerMove);
    canvas.removeEventListener('pointerenter', onPointerEnter);
    canvas.removeEventListener('pointerleave', onPointerLeave);
    canvas.removeEventListener('pointerdown', onPointerDown);
    window.removeEventListener('pointerup', onPointerUp);
    effect.destroy?.(state);
  };
}

/**
 * Matrix Rain — falling katakana + digit characters with fade trails.
 * Category: backgrounds | Animate: true | Interactive: false | Complexity: medium
 */
const __effect = function matrixRain(ctx, state) {
  const CHAR_SIZE = 14;
  const TRAIL_LENGTH = 16;
  const CHARS =
    'アイウエオカキクケコサシスセソタチツテトナニヌネノハヒフヘホマミムメモヤユヨラリルレロワヲン01';

  let columns = [];

  function init(width, height) {
    const count = Math.ceil(width / CHAR_SIZE);
    columns = Array.from({ length: count }, (_, i) => ({
      x: i * CHAR_SIZE + CHAR_SIZE / 2,
      y: -(Math.random() * height * 1.5),
      speed: 0.6 + Math.random() * 1.4,
      charOffset: Math.floor(Math.random() * CHARS.length),
    }));
  }

  init(state.width, state.height);

  return {
    resize({ width, height }) {
      init(width, height);
    },

    frame({ width, height, time, delta, reducedMotion }) {
      ctx.clearRect(0, 0, width, height);

      if (reducedMotion) {
        // Static snapshot — no animation
        ctx.font = `${CHAR_SIZE}px monospace`;
        for (const col of columns) {
          const steps = Math.floor(Math.abs(col.y) / CHAR_SIZE);
          for (let i = 0; i < Math.min(steps, TRAIL_LENGTH); i++) {
            const alpha = (1 - i / TRAIL_LENGTH) * 0.25;
            ctx.fillStyle = `oklch(0.55 0.2 145 / ${alpha})`;
            const charIdx = (col.charOffset + i) % CHARS.length;
            ctx.fillText(CHARS[charIdx], col.x - CHAR_SIZE / 2, Math.abs(col.y) - i * CHAR_SIZE);
          }
        }
        return;
      }

      ctx.font = `${CHAR_SIZE}px monospace`;
      ctx.textBaseline = 'top';

      for (const col of columns) {
        const headY = col.y;

        // Draw trail from head upward
        for (let i = 0; i < TRAIL_LENGTH; i++) {
          const y = headY - i * CHAR_SIZE;
          if (y < -CHAR_SIZE || y > height) continue;

          const charIdx = (col.charOffset + Math.floor(time * col.speed * 8) + i) % CHARS.length;
          const char = CHARS[charIdx];

          if (i === 0) {
            // Head: bright white-green
            ctx.fillStyle = 'oklch(0.96 0.1 145)';
          } else {
            // Trail: fading green
            const alpha = (1 - i / TRAIL_LENGTH) * 0.85;
            ctx.fillStyle = `oklch(0.55 0.22 145 / ${alpha})`;
          }

          ctx.fillText(char, col.x - CHAR_SIZE / 2, y);
        }

        // Advance column
        col.y += col.speed * delta * 80;
        if (col.y > height + CHAR_SIZE * TRAIL_LENGTH) {
          col.y = -(CHAR_SIZE * (3 + Math.random() * 12));
          col.speed = 0.6 + Math.random() * 1.4;
          col.charOffset = Math.floor(Math.random() * CHARS.length);
        }
      }
    },

    destroy() {
      columns = [];
    },
  };
}

const canvas = document.getElementById('canvas-effect');
mountCanvas(canvas, __effect, { animate: true, clear: true });
</script>