use chrono::format::StrftimeItems;
use chrono::{DateTime, FixedOffset, LocalResult, TimeZone, Utc};
use jj_lib::backend::Timestamp;
use once_cell::sync::Lazy;

/// Parsed formatting items which should never contain an error.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct FormattingItems<'a> {
    items: Vec<chrono::format::Item<'a>>,
}

impl<'a> FormattingItems<'a> {
    /// Parses strftime-like format string.
    pub fn parse(format: &'a str) -> Option<Self> {
        // If the parsed format contained an error, format().to_string() would panic.
        let items = StrftimeItems::new(format)
            .map(|item| match item {
                chrono::format::Item::Error => None,
                _ => Some(item),
            })
            .collect::<Option<_>>()?;
        Some(FormattingItems { items })
    }

    pub fn into_owned(self) -> FormattingItems<'static> {
        use chrono::format::Item;
        let items = self
            .items
            .into_iter()
            .map(|item| match item {
                Item::Literal(s) => Item::OwnedLiteral(s.into()),
                Item::OwnedLiteral(s) => Item::OwnedLiteral(s),
                Item::Space(s) => Item::OwnedSpace(s.into()),
                Item::OwnedSpace(s) => Item::OwnedSpace(s),
                Item::Numeric(spec, pad) => Item::Numeric(spec, pad),
                Item::Fixed(spec) => Item::Fixed(spec),
                Item::Error => Item::Error, // shouldn't exist, but just copy
            })
            .collect();
        FormattingItems { items }
    }
}

fn datetime_from_timestamp(context: &Timestamp) -> Option<DateTime<FixedOffset>> {
    let utc = match Utc.timestamp_opt(
        context.timestamp.0.div_euclid(1000),
        (context.timestamp.0.rem_euclid(1000)) as u32 * 1000000,
    ) {
        LocalResult::None => {
            return None;
        }
        LocalResult::Single(x) => x,
        LocalResult::Ambiguous(y, _z) => y,
    };

    Some(
        utc.with_timezone(
            &FixedOffset::east_opt(context.tz_offset * 60)
                .unwrap_or_else(|| FixedOffset::east_opt(0).unwrap()),
        ),
    )
}

pub fn format_absolute_timestamp(timestamp: &Timestamp) -> String {
    static DEFAULT_FORMAT: Lazy<FormattingItems> =
        Lazy::new(|| FormattingItems::parse("%Y-%m-%d %H:%M:%S.%3f %:z").unwrap());
    format_absolute_timestamp_with(timestamp, &DEFAULT_FORMAT)
}

pub fn format_absolute_timestamp_with(timestamp: &Timestamp, format: &FormattingItems) -> String {
    match datetime_from_timestamp(timestamp) {
        Some(datetime) => datetime.format_with_items(format.items.iter()).to_string(),
        None => "<out-of-range date>".to_string(),
    }
}

pub fn format_duration(from: &Timestamp, to: &Timestamp, format: &timeago::Formatter) -> String {
    datetime_from_timestamp(from)
        .zip(datetime_from_timestamp(to))
        .and_then(|(from, to)| to.signed_duration_since(from).to_std().ok())
        .map(|duration| format.convert(duration))
        .unwrap_or_else(|| "<out-of-range date>".to_string())
}

pub fn format_timestamp_relative_to_now(timestamp: &Timestamp) -> String {
    format_duration(timestamp, &Timestamp::now(), &timeago::Formatter::new())
}