You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: japanese.rs
    path of file: ./repos/icu4x/provider/datagen/src/transform/cldr/calendar
    
    the code of the file until where you have to start completion: // This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).

use crate::transform::cldr::cldr_serde;
use icu_calendar::provider::*;
use icu_locid::langid;
use icu_provider::datagen::IterableDataProvider;
use icu_provider::prelude::*;
use std::collections::BTreeMap;
use std::env;
use std::str::FromStr;
use tinystr::tinystr;
use tinystr::TinyStr16;

const JAPANEXT_FILE: &str =
    include_str!("../../../../data/japanese-golden/calendar/japanext@1/und.json");

impl crate::DatagenProvider {
    fn load_japanese_eras(
        &self,
        japanext
    
pub fn get_era_code_map() -> &'static BTreeMap<String, TinyStr16> {
    static MAP: once_cell::sync::OnceCell<BTreeMap<String, TinyStr16>> =
        once_cell::sync::OnceCell::new();

    MAP.get_or_init(|| {
        let snapshot: JapaneseErasV1 = serde_json::from_str(JAPANEXT_FILE)
            .expect("Failed to parse the precached golden. This is a bug.");
        let mut map: BTreeMap<_, _> = snapshot
            .dates_to_eras
            .iter()
            .enumerate()
            .map(|(i, (_, value))| (i.to_string(), value))
            .collect();
        // Splice in details about gregorian eras for pre-meiji dates
        map.insert("-2".to_string(), tinystr!(16, "bce"));
        map.insert("-1".to_string(), tinystr!(16, "ce"));
        map
    })
}