codex-rs/protocol/src/num_format.rs

use std::sync::OnceLock;

use icu_decimal::DecimalFormatter;
use icu_decimal::input::Decimal;
use icu_decimal::options::DecimalFormatterOptions;
use icu_locale_core::Locale;

fn make_local_formatter() -> Option<DecimalFormatter> {
    let loc: Locale = sys_locale::get_locale()?.parse().ok()?;
    DecimalFormatter::try_new(loc.into(), DecimalFormatterOptions::default()).ok()
}

fn make_en_us_formatter() -> DecimalFormatter {
    #![allow(clippy::expect_used)]
    let loc: Locale = "en-US".parse().expect("en-US wasn't a valid locale");
    DecimalFormatter::try_new(loc.into(), DecimalFormatterOptions::default())
        .expect("en-US wasn't a valid locale")
}

fn formatter() -> &'static DecimalFormatter {
    static FORMATTER: OnceLock<DecimalFormatter> = OnceLock::new();
    FORMATTER.get_or_init(|| make_local_formatter().unwrap_or_else(make_en_us_formatter))
}

/// Format an i64 with locale-aware digit separators (e.g. "12345" -> "12,345"
/// for en-US).
pub fn format_with_separators(n: i64) -> String {
    formatter().format(&Decimal::from(n)).to_string()
}