closes #363 and adds default layouts based on number of images in pack and size of the container.

src/components/PhotoPack/PhotoPack.mdx

@@ -8,9 +8,22 @@ import * as PhotoPackStories from './PhotoPack.stories.svelte';
 
 The `PhotoPack` component makes simple photo grids with custom layouts at various breakpoints.
 
-`images` are defined with their src, alt text, captions and an optional `maxHeight`, which ensures that the images are no taller than that height in any layout.
+`images` are defined with their src, alt text, captions and an optional `maxHeight`, which ensures that an image is no taller than that height in any layout.
 
-`layouts` describe how images will be laid out at different breakpoints. The default layout is one photo per row, stacked vertically -- i.e. mobile layout. You can customise the layouts and group images into `rows` above a certain `breakpoint` by specifying the number of images that should go in that row. For example:
+```javascript
+ const images = [
+  {
+    src: 'https://...',
+    altText: 'Alt text',
+    caption: 'Lorem ipsum. REUTERS/Photog',
+    // Optional max-height of images across all layouts
+    maxHeight: 800,
+  },
+  // ...
+];
+```
+
+`layouts` optionally define how images are laid out at different breakpoints. You can customise the layouts and group images into `rows` above a certain `breakpoint` by specifying the number of images that should go in that row. For example:
 
 ```javascript
 const layouts = [
@@ -23,6 +36,8 @@ const layouts = [
 
 ... tells the component that when the `PhotoPack` container is 450 pixels or wider, it should group the 4 images in 3 rows: 1 in the first, 2 in the second and 1 in the last.
 
+If you don't specify any layouts, the component will use a default responsive layout based on the number of images in your pack.
+
 You can define as many layouts for as many images as you like.
 
 ```svelte
@@ -123,3 +138,39 @@ gap: 10 # Optional; must be a number.
 ```
 
 <Canvas of={PhotoPackStories.ArchieML} />
+
+## Smart default layouts
+
+If you don't specify the `layouts` prop, `PhotoPack` will automatically generate responsive layouts based on the number of images and the container width.
+
+**How it works:**
+
+- **Desktop** (1024px+): Number of images per row depends on container width:
+  - `normal`: max 2 per row
+  - `wide` / `wider`: max 3 per row
+  - `widest` / `fluid`: max 4 per row
+- **Tablet** (768px+): Always max 2 per row
+- **Mobile** (below 768px): 1 per row
+
+The smart defaults use a **bottom-heavy distribution**, meaning earlier rows have fewer images (making them larger and more prominent), while later rows have more images.
+
+**Examples:**
+- 5 images, `wide` container, desktop: `[2, 3]` (2 in first row, 3 in second)
+- 7 images, `widest` container, desktop: `[3, 4]` (3 in first row, 4 in second)
+- 4 images, any container, desktop: `[2, 2]` (evenly distributed)
+
+```svelte
+<script>
+  import { PhotoPack } from '@reuters-graphics/graphics-components';
+
+  const images = [
+    { src: `${assets}/image1.jpg`, altText: 'Photo 1', caption: 'Caption 1' },
+    { src: `${assets}/image2.jpg`, altText: 'Photo 2', caption: 'Caption 2' },
+    { src: `${assets}/image3.jpg`, altText: 'Photo 3', caption: 'Caption 3' },
+    { src: `${assets}/image4.jpg`, altText: 'Photo 4', caption: 'Caption 4' },
+  ];
+</script>
+
+<!-- No layouts prop = smart defaults! -->
+<PhotoPack {images} width="wide" />
+```

src/components/PhotoPack/PhotoPack.stories.svelte

@@ -19,6 +19,12 @@
 </script>
 
 <script lang="ts">
+  import type { ComponentProps } from 'svelte';
+
+  type SmartDefaultsArgs = Omit<ComponentProps<typeof PhotoPack>, 'images'> & {
+    imageCount: number;
+  };
+
   const defaultImages = [
     {
       src: 'https://graphics.thomsonreuters.com/cdn/django-tools/media/graphics-gallery/galleries/world-cup-2022/spain-germany-11-27/2022-11-27T194630Z_544493697_UP1E.jpeg',
@@ -51,7 +57,7 @@
     { breakpoint: 750, rows: [1, 3] },
   ];
 
-  const archieMLImages = [
+  const allImages = [
     {
       src: 'https://graphics.thomsonreuters.com/cdn/django-tools/media/graphics-gallery/galleries/world-cup-2022/spain-germany-11-27/2022-11-27T194630Z_544493697_UP1E.jpeg',
       caption:
@@ -88,7 +94,7 @@
     width: 'wide' as const,
     textWidth: 'normal' as const,
     gap: Number('15'),
-    images: archieMLImages,
+    images: allImages.slice(0, 5),
     layouts: [
       { breakpoint: 750, rows: [2, 3] },
       { breakpoint: 450, rows: [1, 2, 2] },
@@ -106,3 +112,29 @@
   }}
 />
 <Story name="ArchieML" args={archieMLBlock} />
+
+<Story
+  name="Smart layouts"
+  args={{
+    width: 'wide',
+    textWidth: 'normal',
+    // @ts-expect-error - imageCount is a custom arg for this story's template
+    imageCount: 4,
+  }}
+  argTypes={{
+    // @ts-expect-error - imageCount is a custom arg for this story's template
+    imageCount: {
+      control: { type: 'range', min: 2, max: 5, step: 1 },
+      description:
+        'Number of images to display (demonstrates smart default layouts)',
+    },
+  }}
+>
+  {#snippet children(args)}
+    {@const { imageCount, ...photoPackProps } = args as SmartDefaultsArgs}
+    <PhotoPack
+      {...photoPackProps}
+      images={allImages.slice(0, imageCount || 4)}
+    />
+  {/snippet}
+</Story>

src/components/PhotoPack/PhotoPack.svelte

@@ -6,7 +6,7 @@
 
   // Utils
   import { random4 } from '../../utils';
-  import { groupRows } from './utils';
+  import { groupRows, generateDefaultLayouts } from './utils';
 
   // Types
   export interface Image {
@@ -33,7 +33,7 @@
     /** Add an ID to target with SCSS. Should be unique from all other elements. */
     id?: string;
     /** Add a class to target with SCSS. */
-    class: string;
+    class?: string;
     /** Width of the component within the text well: 'normal' | 'wide' | 'wider' | 'widest' | 'fluid' */
     width: ContainerWidth;
     /** Set a different width for captions within the text well. For example, "normal" to keep captions inline with the rest of the text well.
@@ -60,10 +60,14 @@
    *
    * @NOTE - We can't use `sort` directly on the array because it mutates the original array; we can't update a state inside a derived expression: https://svelte.dev/docs/svelte/runtime-errors#Client-errors-state_unsafe_mutation
    *
-   * So, we need to use `toSorted` instead.
+   * We avoid `toSorted` because it's not supported on older iPhones. Instead, we create a shallow copy using the spread operator and then sort that copy.
+   *
+   * If no layouts are provided, we generate smart defaults based on the container width and number of images.
    */
   let sortedLayouts = $derived(
-    layouts?.toSorted((a, b) => (a.breakpoint < b.breakpoint ? 1 : -1))
+    layouts ?
+      [...layouts].sort((a, b) => (a.breakpoint < b.breakpoint ? 1 : -1))
+    : generateDefaultLayouts(images.length, width)
   );
 
   let layout = $derived(

src/components/PhotoPack/utils.ts

@@ -1,5 +1,78 @@
 import type { Image, Layout } from './PhotoPack.svelte';
 
+// Breakpoint constants for smart default layouts
+export const DESKTOP_BREAKPOINT = 1024;
+export const TABLET_BREAKPOINT = 768;
+
+/**
+ * Generates a smart layout for a given number of images with bottom-heavy distribution.
+ * Avoids single-image rows by redistributing when necessary.
+ *
+ * @param imageCount - Total number of images
+ * @param maxPerRow - Maximum images per row
+ * @param breakpoint - Breakpoint threshold for this layout
+ * @returns Layout object with rows array
+ */
+export const generateSmartLayout = (
+  imageCount: number,
+  maxPerRow: number,
+  breakpoint: number
+): Layout => {
+  // Handle edge cases
+  if (imageCount === 0) return { breakpoint, rows: [] };
+  if (imageCount === 1) return { breakpoint, rows: [1] };
+
+  const fullRows = Math.floor(imageCount / maxPerRow);
+  const remainder = imageCount % maxPerRow;
+
+  let rows: number[] = [];
+
+  if (remainder === 0) {
+    // Perfect division: all rows have maxPerRow
+    rows = Array(fullRows).fill(maxPerRow);
+  } else {
+    // Bottom-heavy: smaller row at top, larger rows below
+    // This makes early images larger (fewer per row = bigger display size)
+    rows = [remainder, ...Array(fullRows).fill(maxPerRow)];
+  }
+
+  return {
+    breakpoint,
+    rows,
+  };
+};
+
+type ContainerWidth = 'normal' | 'wide' | 'wider' | 'widest' | 'fluid';
+
+/**
+ * Generates smart default layouts for desktop and tablet breakpoints.
+ * Mobile (below TABLET_BREAKPOINT) automatically shows 1 image per row.
+ *
+ * Max images per row by container width:
+ * - normal: 2
+ * - wide/wider: 3
+ * - widest/fluid: 4
+ *
+ * @param imageCount - Total number of images
+ * @param width - Container width setting
+ * @returns Array of 2 layouts [desktop, tablet]
+ */
+export const generateDefaultLayouts = (
+  imageCount: number,
+  width: ContainerWidth
+): Layout[] => {
+  // Map container width to max images per row for desktop
+  const desktopMaxPerRow = width === 'normal' ? 2 : width === 'widest' || width === 'fluid' ? 4 : 3;
+
+  // Tablet always uses max 2 per row
+  const tabletMaxPerRow = 2;
+
+  return [
+    generateSmartLayout(imageCount, desktopMaxPerRow, DESKTOP_BREAKPOINT),
+    generateSmartLayout(imageCount, tabletMaxPerRow, TABLET_BREAKPOINT),
+  ];
+};
+
 export const groupRows = (images: Image[], layout?: Layout) => {
   // Default layout, one img per row
   if (!layout) return images.map((img) => [img]);