feat: add local currency display support with auto-fetched exchange rates

[GordonBeeming]

Summary

• Add local currency display support via ~/.goccc.json config file — set "currency": "ZAR" (or any ISO 4217 code) and costs are automatically converted
• Exchange rates auto-fetched from open.er-api.com (free, no API key) and cached for 24 hours in the config file; falls back to cached rate when offline
• Built-in symbol table for 20 common currencies (e.g., ZAR→R, EUR→€, GBP→£, AUD→A$)
• CLI overrides with -currency-symbol and -currency-rate flags for one-off use (both required together)
• Currency info line shown in terminal output (e.g., Costs in AUD (1 USD = 1.4158 AUD))
• JSON output remains in USD for backward compatibility; adds currency metadata object when non-USD currency is active
• Color thresholds unchanged — still evaluated against USD cost internally

Config example

{
  "currency": "AUD"
}

After first run, the file is updated with cached rate:

{
  "currency": "AUD",
  "cached_rate": 1.4158,
  "rate_updated": "2026-03-05T08:44:04Z"
}

[GordonBeeming]

Great app @backstabslash ... a small addition that I think will be awesome ☺️

[Copilot]

Pull request overview

Adds local-currency display support to goccc by introducing a currency config (~/.goccc.json) with auto-fetched USD exchange rates, optional CLI overrides, and optional currency metadata in JSON output (while keeping all cost calculations and JSON cost fields in USD for compatibility).

Changes:

• Add currency config/exchange-rate fetching + caching logic and a symbol table.
• Update terminal formatting to display converted costs when a currency is active and print an informational “Costs in …” line; add currency metadata object to JSON output.
• Add CLI flags for one-off currency overrides and add unit tests + docs.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
main.go	Adds -currency-symbol / -currency-rate flags and initializes currency handling at startup.
format.go	Makes fmtCost currency-aware; adds JSON currency metadata; updates summary output to include currency info.
currency.go	Implements config load/save, symbol lookup, rate fetching, and 24h caching.
currency_test.go	Adds unit tests for config parsing, symbol lookup, formatting, and cached-rate behavior.
README.md	Documents local currency configuration and CLI override flags.
CLAUDE.md	Updates module map and feature notes to include the new currency support.

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[backstabslash]

Nice feature, clean implementation! A few things to address:

1. No HTTP timeout on fetchExchangeRate (high priority)

http.Get uses the default client with no timeout — if the API is slow or unresponsive, the CLI hangs indefinitely. Use a client with a timeout:

var httpClient = &http.Client{Timeout: 5 * time.Second}

func fetchExchangeRate(currency string) (float64, error) {
	resp, err := httpClient.Get("https://open.er-api.com/v6/latest/USD")

2. initCurrency should return error, not string

Every other error path in the codebase returns error. Returning a string message is non-idiomatic Go:

func initCurrency(symbolFlag string, rateFlag float64) error {

3. CLI override sets Code to the symbol value

When using -currency-symbol "€" -currency-rate 0.92, both Code and Symbol get set to "€", which makes the info line print Costs in € (1 USD = 0.9200 €). Code is meant for ISO codes like EUR.

Either leave Code empty and skip the info line for CLI overrides, or accept a separate flag for the ISO code.

4. Config write errors silently swallowed

saveCurrencyConfig ignores the os.WriteFile error. A stderr warning would be more helpful — consistent with how fetch failures are already handled a few lines below:

if err := os.WriteFile(path, append(data, '\n'), 0644); err != nil {
	fmt.Fprintf(os.Stderr, "goccc: warning: failed to save currency config: %v\n", err)
}

[GordonBeeming]

Updated, I used 2s as the timeout as it felt more reasonable to not be blocking for the tool ☺️

[backstabslash]

Thanks for the contribution! Merged and released as v0.1.5 🎉