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const c = @cImport({
    @cInclude("locale.h");
    @cInclude("stdio.h");
});

// ...

pub fn formatNumber(ally: std.mem.Allocator, number: u64) ![]u8 {
    // max is 18,446,744,073,709,551,615 (a 20 digit number with 6 commas)
    // but allow up to two chars per separator for anything non-standard
    var buff = [_]u8{0} ** 33; // 32 + 1 for terminator

    // %'8.2f for floats or lld for signed
    _ = c.setlocale(c.LC_NUMERIC, "");
    const n = c.snprintf(&buff, 33, "%'llu", number);

    const len: usize = @intCast(n);
    const result = buff[0..len];

    return try ally.dupe(u8, result);
}

test "formatNumber should work with the max 64-bit unsigned integer size" {
    const ally = std.testing.allocator;
    const result = try formatNumber(ally, 18_446_744_073_709_551_615);
    defer ally.free(result);

    try std.testing.expect(std.mem.eql(u8, result, "18,446,744,073,709,551,615"));
}