# CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

## Project Overview

Collection of interactive calculators visualizing how seemingly small differences grow exponentially over time. Norwegian-language UI. Hosted at `forskjeller.naiv.no`.

## Architecture

Static site in `public/` — no build system, no package manager. Serve `public/` as the site root. Each topic gets its own subdirectory with `index.html` and `app.js`.

### Shared CSS

`public/style.css` — Shared stylesheet used by all calculators (referenced as `../style.css`). CSS custom properties in `:root`, CSS Grid layouts, responsive breakpoints at 560px/480px/360px. Calculators can add a supplementary `<style>` block or extra CSS file for page-specific styles if needed.

### Shared pattern (all calculators)

Each calculator directory contains two files following the same pattern:
- `index.html` — Header, range-slider controls, stat cards, chart canvases, explainer section. Links to `../style.css`.
- `app.js` — Self-contained logic with `fmtKr()`, `fmtShort()`, `getData()`, `baseOpts()`, `update()`. Charts update with `'none'` animation mode.

### Calculators

- **`public/lonn/`** — Salary gap: percentage raises amplify absolute differences. Model: `gap = (B₀ − A₀) × (1 + r)ⁿ`
- **`public/sparing/`** — Compound interest: same savings, different returns. Model: `FV = P × ((1+r)^n - 1) / r` (monthly compounding). Stacked bar chart for deposits vs interest.
- **`public/bolig/`** — Housing leverage: same price growth, different starting prices. 85% LTV amplifies equity gap ~6.67×. Model: `Equity = Price × (1+g)^n - Loan`
- **`public/arv/`** — Inheritance gap: lump sum + equal savings, gap never closes. Model: `Gap = Arv × (1+r)^n`. Has percentage y-axis chart.

**External dependencies** (loaded via CDN, no install needed): Chart.js 4.4.1, Google Fonts (Fraunces + DM Sans).

### OpenGraph

All pages must include OG meta tags (`og:type`, `og:locale`, `og:site_name`, `og:title`, `og:description`, `og:url`) and `twitter:card`. No `og:image` for now. Base URL: `https://forskjeller.naiv.no/`.

### Key Design Decisions

- Color palette: Blue (#1a4a8a) = A, Red (#c0392b) = B, Purple (#4a3a8a) = gap
- All charts share `baseOpts()` — modify this for consistent chart styling changes
- Label pattern: "I dag" at year 0, show every 5th year, always show last year
- DOM updates use `textContent` or DOM methods (not innerHTML) for security

## Development

```bash
python3 -m http.server 8000 -d public
# open http://localhost:8000/ for landing page
# open http://localhost:8000/{lonn,sparing,bolig,arv}/ for individual calculators
```

## Language

All user-facing text is in Norwegian Bokmål (nb-NO). Use Norwegian for UI strings and `nb-NO` locale for number formatting.