Lychee-layouts

Photo layout engine for Lychee, compiled to WebAssembly.

Provides four layout algorithms — justified, masonry, grid, and square — as a pure-logic library with no DOM dependency. It ships in two implementations that produce identical results:

Implementation	Toolchain	Typical output size
Go (/)	go build / TinyGo	~2 MB / ~400 KB
Rust (rust/)	wasm-pack	~80 KB

---

Layout algorithms

All four algorithms take a list of aspect ratios (width ÷ height) and container parameters, and return a flat list of boxes — one per photo — with top, left, width, and height in pixels, plus the required containerHeight.

Justified

Every photo in a row shares the same height and the row fills the container width exactly. The last (partial) row is placed at targetRowHeight without stretching.

┌──────┐ ┌────────────┐ ┌─────┐
│      │ │            │ │     │  ← same height, fills width
└──────┘ └────────────┘ └─────┘
┌───────────┐ ┌──────┐ ┌─────┐
│           │ │      │ │     │  ← same height, fills width
└───────────┘ └──────┘ └─────┘
┌──────┐ ┌──────┐                ← last row: targetRowHeight, not stretched
│      │ │      │
└──────┘ └──────┘

Replaces the justified-layout npm package with a zero-dependency Go/Rust implementation of the same greedy algorithm.

Masonry

Each photo flows into the shortest column (Pinterest-style). Columns are not row-aligned.

┌─────┐  ┌─────┐  ┌─────┐
│     │  │     │  │     │
│     │  └─────┘  │     │
└─────┘  ┌─────┐  └─────┘
┌─────┐  │     │  ┌─────┐
│     │  │     │  │     │
└─────┘  └─────┘  └─────┘

Grid

Fixed columns, variable photo height per cell. All columns are synced to the same top at the start of each row, so rows stay horizontally aligned even though individual photo heights differ.

┌─────┐  ┌─────┐  ┌─────┐  ← row start aligned
│     │  │     │  │     │
│     │  └─────┘  └─────┘
└─────┘
┌─────┐  ┌─────┐  ┌─────┐  ← next row starts at max bottom of previous
│     │  │     │  │     │
└─────┘  └─────┘  │     │
                  └─────┘

Square

Every photo is rendered as a square cell (targetSize × targetSize). Aspect ratios are ignored. Cells are distributed evenly to fill the container width.

┌─────┐  ┌─────┐  ┌─────┐
│     │  │     │  │     │
└─────┘  └─────┘  └─────┘
┌─────┐  ┌─────┐  ┌─────┐
│     │  │     │  │     │
└─────┘  └─────┘  └─────┘

---

API

All four functions share the same signature pattern and return the same shape.

Input

Parameter	Type	Description
ratios	Float64Array	Aspect ratio (width / height) for each photo. Falls back to 1.0 for invalid values.
containerWidth	f64 / number	Usable container width in pixels (exclude padding and scrollbar).
targetRowHeight (justified)	f64 / number	Desired row height in pixels.
targetSize (square)	f64 / number	Desired cell size in pixels.
targetWidth (masonry, grid)	f64 / number	Desired column width in pixels.
gap / spacing	f64 / number	Spacing between photos in pixels (horizontal and vertical).

Computing ratios from photo metadata:

const ratios = photos.map(p => p.width > 0 && p.height > 0 ? p.width / p.height : 1.0);

Output

type Box = { top: number; left: number; width: number; height: number };
type LayoutResult = { containerHeight: number; boxes: Box[] };

boxes[i] corresponds to ratios[i]. Set containerHeight on your container element so that absolutely-positioned children are not clipped.

---

Building

Rust (recommended — smallest output)

Prerequisites:

curl https://sh.rustup.rs -sSf | sh        # install Rust
cargo install wasm-pack                    # install wasm-pack

Build:

make build-rs
# output: dist/rs/lychee_layouts_bg.wasm + JS/TS glue

Or manually from the rust/ directory:

cd rust
wasm-pack build --target web --out-dir ../dist/rs --release

Go

Prerequisites: Go 1.21+

make build
# output: dist/Lychee-layouts.wasm + dist/wasm_exec.js

For a smaller binary (~5× smaller) with TinyGo:

make build-tinygo
# output: dist/Lychee-layouts-tiny.wasm

---

Usage

Rust WASM (wasm-pack output)

wasm-pack generates a JavaScript/TypeScript module alongside the .wasm file. Import it as a standard ES module:

<script type="module">
import init, { justified, square, masonry, grid } from './dist/rs/lychee_layouts.js';

await init(); // load and compile the WASM module once

const ratios = new Float64Array(photos.map(p => p.width / p.height));
const containerWidth = document.getElementById('gallery').clientWidth;

// Justified layout
const { containerHeight, boxes } = justified(ratios, containerWidth, 320, 3);

applyLayout(containerHeight, boxes);
</script>

TypeScript types are generated automatically by wasm-pack in dist/rs/lychee_layouts.d.ts.

Go WASM

The Go WASM module exposes a lycheelayouts global object. It requires wasm_exec.js (shipped with Go) as a loader.

<script src="dist/wasm_exec.js"></script>
<script>
const go = new Go();
WebAssembly.instantiateStreaming(fetch('dist/Lychee-layouts.wasm'), go.importObject)
  .then(({ instance }) => {
    go.run(instance);

    const ratios = photos.map(p => p.width / p.height);
    const containerWidth = document.getElementById('gallery').clientWidth;

    // Justified layout
    const { containerHeight, boxes } = lycheelayouts.justified(
      ratios, containerWidth, 320, 3
    );

    applyLayout(containerHeight, boxes);
  });
</script>

Applying the result

function applyLayout(containerHeight, boxes) {
  const el = document.getElementById('gallery');
  el.style.position = 'relative';
  el.style.height = containerHeight + 'px';

  const items = el.querySelectorAll('.photo-item');
  boxes.forEach((box, i) => {
    const item = items[i];
    item.style.position = 'absolute';
    item.style.top    = box.top    + 'px';
    item.style.left   = box.left   + 'px';
    item.style.width  = box.width  + 'px';
    item.style.height = box.height + 'px';
  });
}

Call applyLayout on page load and whenever the container is resized (debounce as needed):

const resizeObserver = new ResizeObserver(() => {
  const { containerHeight, boxes } = justified(ratios, el.clientWidth, 320, 3);
  applyLayout(containerHeight, boxes);
});
resizeObserver.observe(document.getElementById('gallery'));

---

Function reference

justified(ratios, containerWidth, targetRowHeight, spacing)

ratios          Float64Array   aspect ratios of all photos
containerWidth  number         usable container width in px
targetRowHeight number         desired row height in px  (default: containerWidth / 4)
spacing         number         gap between photos in px

Implements the Flickr justified-layout greedy algorithm without the npm dependency. The last incomplete row is placed at targetRowHeight (not stretched).

---

square(ratios, containerWidth, targetSize, gap)

ratios          Float64Array   aspect ratios — used only for the item count
containerWidth  number         usable container width in px
targetSize      number         desired square cell size in px
gap             number         gap between cells in px

Distributes cells evenly: cellSize = targetSize + ceil(remainingSpace / columns).

---

masonry(ratios, containerWidth, targetWidth, gap)

ratios          Float64Array   aspect ratios of all photos
containerWidth  number         usable container width in px
targetWidth     number         desired column width in px
gap             number         gap between photos in px

Each photo's height is columnWidth / ratio. New photos always go into the column with the smallest accumulated height.

---

grid(ratios, containerWidth, targetWidth, gap)

ratios          Float64Array   aspect ratios of all photos
containerWidth  number         usable container width in px
targetWidth     number         desired column width in px
gap             number         gap between photos in px

Like masonry but row-aligned: at the start of every new row all column heights are synced to the tallest column, so rows begin at a consistent horizontal baseline.

---

Project structure

.
├── Makefile
│
├── layouts/              Go library (pure, no WASM dependency)
│   ├── types.go          Box, LayoutResult
│   ├── justified.go
│   ├── masonry.go
│   ├── grid.go
│   └── square.go
│
├── cmd/wasm/
│   └── main.go           Go WASM entry point (syscall/js)
│
├── rust/                 Rust implementation
│   ├── Cargo.toml
│   └── src/
│       ├── lib.rs        wasm-bindgen exports
│       ├── types.rs      PhotoBox, LayoutResult
│       ├── justified.rs
│       ├── masonry.rs
│       ├── grid.rs
│       └── square.rs
│
└── ref/                  Original TypeScript reference (Lychee source)
    ├── PhotoLayout.ts
    ├── useJustify.ts
    ├── useMasonry.ts
    ├── useGrid.ts
    └── useSquare.ts

---

Make targets

Target	Description
make build	Go WASM (standard toolchain) → dist/
make build-tinygo	Go WASM (TinyGo, smaller) → dist/
make build-rs	Rust WASM (wasm-pack) → dist/rs/
make test	Go unit tests
make test-rs	Rust unit tests (native, no WASM runtime needed)
make clean	Remove dist/ and Rust build artifacts

---

License

Part of the Lychee project.