//! The WebCal (published iCal feed) leg.
//!
//! A feed is a single iCalendar document fetched over HTTP, read-only by
//! protocol: there is nothing to push and no conflict to resolve. That makes the
//! whole leg a download, a split by UID and a vdir write, which is why calcalist
//! does it itself rather than through pimsync — a subprocess, a generated config
//! file and a status database bought nothing that this module does not.
//!
//! The fetch is unconditional. Feeds are small, publishers rarely honour
//! `If-Modified-Since`, and the reconciler already compares content hashes, so a
//! re-read of unchanged events costs a download and changes nothing on disk.

use std::path::Path;

use thiserror::Error;

use crate::ical::{Calendar, IcalError};
use crate::secret::SecretError;
use crate::vdir::{self, VdirError};

#[derive(Debug, Error)]
pub enum WebcalError {
    #[error("endpoint `{0}` has neither url nor url_command")]
    NoUrl(String),
    #[error(transparent)]
    Secret(#[from] SecretError),
    #[error("could not fetch {url}: {source}")]
    Fetch {
        url: String,
        #[source]
        source: Box<ureq::Error>,
    },
    #[error("{url} answered {status}")]
    Status { url: String, status: u16 },
    #[error("{url} did not return a readable calendar: {source}")]
    Malformed {
        url: String,
        #[source]
        source: IcalError,
    },
    #[error(transparent)]
    Vdir(#[from] VdirError),
}

#[derive(Debug, Default)]
pub struct PullReport {
    pub written: usize,
    pub deleted: usize,
}

/// Brings an endpoint's local vdir into step with its feed.
///
/// The feed is the whole truth every time — it is one document describing the
/// calendar in full — so an item held locally that the feed no longer mentions
/// has been withdrawn, and goes.
pub fn pull(url: &str, vdir_dir: &Path) -> Result<PullReport, WebcalError> {
    let feed = fetch(url)?;
    let calendar = Calendar::parse(&feed).map_err(|source| WebcalError::Malformed {
        url: url.to_string(),
        source,
    })?;
    let events = calendar.split_by_uid();

    let held = vdir::read(vdir_dir)?;
    let mut report = PullReport::default();

    for (uid, event) in &events {
        // Unchanged events are left alone: rewriting them would churn every
        // file on every cycle, and the reconciler reads content, not mtimes.
        let current = held.get(uid);
        if current.is_some_and(|item| item.hash == event.content_hash()) {
            continue;
        }
        vdir::write(
            vdir_dir,
            uid,
            event,
            current.map(|item| item.path.as_path()),
        )?;
        report.written += 1;
    }

    for (uid, item) in &held {
        if !events.contains_key(uid) {
            vdir::remove(&item.path)?;
            report.deleted += 1;
        }
    }

    Ok(report)
}

/// The feed's address, which may itself be the credential.
///
/// Google's "secret address in iCal format" grants read access to the whole
/// calendar to anyone holding it, so such a URL is fetched by command and never
/// written into the configuration file.
pub fn resolve_url(
    endpoint: &str,
    url: Option<&str>,
    url_command: Option<&str>,
) -> Result<String, WebcalError> {
    match (url, url_command) {
        (_, Some(command)) => Ok(crate::secret::run(command)?),
        (Some(url), None) => Ok(url.to_string()),
        (None, None) => Err(WebcalError::NoUrl(endpoint.to_string())),
    }
}

fn fetch(url: &str) -> Result<String, WebcalError> {
    let mut response =
        crate::http::agent()
            .get(url)
            .call()
            .map_err(|source| WebcalError::Fetch {
                url: url.to_string(),
                source: Box::new(source),
            })?;
    let status = response.status().as_u16();
    if !(200..300).contains(&status) {
        return Err(WebcalError::Status {
            url: url.to_string(),
            status,
        });
    }
    response
        .body_mut()
        .read_to_string()
        .map_err(|source| WebcalError::Fetch {
            url: url.to_string(),
            source: Box::new(source),
        })
}

#[cfg(test)]
mod tests {
    use super::*;

    const FEED: &str = "BEGIN:VCALENDAR\r\nVERSION:2.0\r\nPRODID:-//Test//EN\r\n\
        BEGIN:VEVENT\r\nUID:one@example.com\r\nDTSTAMP:20260101T000000Z\r\n\
        DTSTART;VALUE=DATE:20260101\r\nSUMMARY:New Year\r\nEND:VEVENT\r\n\
        BEGIN:VEVENT\r\nUID:two@example.com\r\nDTSTAMP:20260101T000000Z\r\n\
        DTSTART;VALUE=DATE:20260501\r\nSUMMARY:May Day\r\nEND:VEVENT\r\n\
        END:VCALENDAR\r\n";

    /// A recurring event and one overridden occurrence share a UID, so they have
    /// to land in one file; splitting them would give the vdir two events where
    /// the calendar has one.
    const SERIES: &str = "BEGIN:VCALENDAR\r\nVERSION:2.0\r\n\
        BEGIN:VTIMEZONE\r\nTZID:Europe/Berlin\r\n\
        BEGIN:STANDARD\r\nDTSTART:19701025T030000\r\nTZOFFSETFROM:+0200\r\n\
        TZOFFSETTO:+0100\r\nEND:STANDARD\r\nEND:VTIMEZONE\r\n\
        BEGIN:VTIMEZONE\r\nTZID:America/New_York\r\n\
        BEGIN:STANDARD\r\nDTSTART:19701101T020000\r\nTZOFFSETFROM:-0400\r\n\
        TZOFFSETTO:-0500\r\nEND:STANDARD\r\nEND:VTIMEZONE\r\n\
        BEGIN:VEVENT\r\nUID:series@example.com\r\nDTSTAMP:20260101T000000Z\r\n\
        DTSTART;TZID=Europe/Berlin:20260105T090000\r\nRRULE:FREQ=WEEKLY\r\n\
        SUMMARY:Standup\r\nEND:VEVENT\r\n\
        BEGIN:VEVENT\r\nUID:series@example.com\r\nDTSTAMP:20260101T000000Z\r\n\
        RECURRENCE-ID;TZID=Europe/Berlin:20260112T090000\r\n\
        DTSTART;TZID=Europe/Berlin:20260112T100000\r\n\
        SUMMARY:Standup (late)\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n";

    fn split(feed: &str) -> std::collections::BTreeMap<String, Calendar> {
        Calendar::parse(feed)
            .expect("the feed should parse")
            .split_by_uid()
    }

    #[test]
    fn each_event_becomes_a_calendar_of_its_own() {
        let events = split(FEED);
        assert_eq!(events.len(), 2);
        let one = events["one@example.com"].to_ics();
        assert!(one.contains("SUMMARY:New Year"), "{one}");
        assert!(!one.contains("May Day"), "{one}");
        // The envelope travels, or the file is a fragment rather than a calendar.
        assert!(one.contains("PRODID:-//Test//EN"), "{one}");
    }

    #[test]
    fn a_series_and_its_override_stay_together() {
        let events = split(SERIES);
        assert_eq!(events.len(), 1);
        let series = events["series@example.com"].to_ics();
        assert!(series.contains("SUMMARY:Standup"), "{series}");
        assert!(series.contains("SUMMARY:Standup (late)"), "{series}");
        assert!(series.contains("RECURRENCE-ID"), "{series}");
    }

    /// A time is meaningless without the zone it names, and carrying every zone
    /// in the feed into every file would bloat each one for nothing.
    #[test]
    fn only_the_timezones_an_event_names_come_with_it() {
        let events = split(SERIES);
        let series = events["series@example.com"].to_ics();
        assert!(series.contains("TZID:Europe/Berlin"), "{series}");
        assert!(!series.contains("America/New_York"), "{series}");
    }

    #[test]
    fn an_event_without_a_uid_is_skipped_rather_than_failing_the_feed() {
        let feed = "BEGIN:VCALENDAR\r\nVERSION:2.0\r\n\
            BEGIN:VEVENT\r\nDTSTAMP:20260101T000000Z\r\nSUMMARY:Nameless\r\nEND:VEVENT\r\n\
            BEGIN:VEVENT\r\nUID:kept@example.com\r\nDTSTAMP:20260101T000000Z\r\n\
            SUMMARY:Kept\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n";
        let events = split(feed);
        assert_eq!(events.keys().collect::<Vec<_>>(), ["kept@example.com"]);
    }

    #[test]
    fn a_url_command_supplies_the_address() {
        let resolved = resolve_url("feed", None, Some("printf https://example.com/f.ics"))
            .expect("the command should supply a url");
        assert_eq!(resolved, "https://example.com/f.ics");
    }

    #[test]
    fn an_endpoint_with_no_address_at_all_is_an_error() {
        let error = resolve_url("feed", None, None).expect_err("there is no url");
        assert!(matches!(error, WebcalError::NoUrl(_)), "{error:?}");
    }
}