//! HTTP transport to a ThoughtSync server.
//!
//! Covers the compatibility handshake (M10.6) and device-token auth (M10.7a). The
//! engine that moves notes — push, pull, cursor — grows on top of the same client,
//! which is why the timeout, identity headers and error vocabulary live here rather
//! than inline at each call site.
//!
//! Nothing here runs unless the user has linked a server; the app is local-first and
//! fully usable with no network at all.

use std::path::Path;
use std::time::Duration;

use reqwest::{RequestBuilder, StatusCode};
use serde::{Deserialize, Serialize};

use super::compat::{self, Compatibility, ServerInfo};
use super::wire;

/// Timeout for the short request/response calls in this module. Kept tight because a
/// user is watching a button while they run, and the most common mistake — a wrong
/// host on a LAN — fails by hanging rather than refusing, so an unbounded wait would
/// just look frozen. The sync engine's bulk transfers will need their own, longer one.
const REQUEST_TIMEOUT: Duration = Duration::from_secs(10);

/// Bulk transfers get much longer: a first full sync can be thousands of notes, and
/// failing one at ten seconds would make a large store impossible to ever pull.
const SYNC_TIMEOUT: Duration = Duration::from_secs(120);

/// Shared by every call that presents a token, so a revoked one reads the same way
/// wherever it surfaces.
const TOKEN_REJECTED: &str = "This server rejected the device token — it may have been \
                              revoked. Unlink and link again to issue a new one.";

/// What the link UI needs after a handshake: where we ended up (the normalized URL,
/// which may differ from what was typed), who answered, and whether we can work
/// with them.
#[derive(Debug, Serialize)]
pub struct ProbeResult {
    pub base_url: String,
    pub server: ServerInfo,
    pub compatibility: Compatibility,
}

/// The account a device token belongs to. Surfaced after linking so the user can
/// confirm they linked the account they meant to — easy to get wrong on a server
/// hosting more than one.
#[derive(Debug, Clone, Deserialize, Serialize)]
pub struct Identity {
    pub id: String,
    pub email: String,
    #[serde(default)]
    pub display_name: String,
}

#[derive(Deserialize)]
struct DeviceLoginResponse {
    token: String,
    user: Identity,
}

/// What became of this device's token on the SERVER when unlinking.
///
/// Not a bool, and not an error: unlinking must never be blocked by the network —
/// wanting to stop syncing is a local decision — so the remote half reports back
/// instead of failing the call, and each outcome needs different advice.
///
/// Serialized tagged, like `Compatibility`, so the frontend can `switch` on `status`.
#[derive(Debug, Clone, PartialEq, Eq, Serialize)]
#[serde(tag = "status", rename_all = "snake_case")]
pub enum RevokeOutcome {
    /// The server confirmed it: this token authenticates nothing now.
    Revoked,
    /// This server has no self-revoke route — it predates one. The token is still
    /// live, and only the web app can retire it.
    Unsupported,
    /// We couldn't reach the server, or it refused. The token is still live.
    Failed { reason: String },
    /// Nothing to revoke; the app wasn't linked.
    Skipped,
}

/// Retire the device token we authenticate with, server-side.
///
/// Identified by the token itself rather than a device id, because a token pasted
/// from the web app never carried one — a route keyed on the id would work for
/// exactly one of the two ways this app can be linked.
pub async fn revoke_self(base_url: &str, token: &str) -> RevokeOutcome {
    let client = match http() {
        Ok(client) => client,
        Err(reason) => return RevokeOutcome::Failed { reason },
    };
    let request = prepare(client.delete(revoke_self_url(base_url)), Some(token));
    let response = match request.send().await {
        Ok(response) => response,
        Err(e) => {
            return RevokeOutcome::Failed {
                reason: describe_transport_error(base_url, &e),
            }
        }
    };

    let status = response.status();
    // 401 counts as revoked: the token already authenticates nothing — retired by
    // another device, or purged server-side — which is the state we were asking for.
    if status.is_success() || status == StatusCode::UNAUTHORIZED {
        return RevokeOutcome::Revoked;
    }
    match status {
        // No such route: a server older than self-revoke. Any other shape of 404
        // (a proxy, a stale base URL) leaves the token live too, so the advice the
        // user needs is the same either way.
        StatusCode::NOT_FOUND | StatusCode::METHOD_NOT_ALLOWED => RevokeOutcome::Unsupported,
        other => RevokeOutcome::Failed {
            reason: unexpected_status(base_url, other),
        },
    }
}

fn http_with(timeout: Duration) -> Result<reqwest::Client, String> {
    reqwest::Client::builder()
        .timeout(timeout)
        .build()
        .map_err(|e| format!("Could not start the network client: {e}"))
}

fn http() -> Result<reqwest::Client, String> {
    http_with(REQUEST_TIMEOUT)
}

/// Attach the client-identity headers every request carries, plus a bearer token
/// when we hold one.
fn prepare(builder: RequestBuilder, token: Option<&str>) -> RequestBuilder {
    let mut builder = builder;
    for (name, value) in compat::client_headers() {
        builder = builder.header(name, value);
    }
    match token {
        Some(t) => builder.bearer_auth(t),
        None => builder,
    }
}

fn unexpected_status(base_url: &str, status: StatusCode) -> String {
    format!(
        "{base_url} answered with HTTP {}. Check the address — a reverse proxy or a \
         different site may be answering there.",
        status.as_u16()
    )
}

/// Ask a server who it is and whether we can sync with it.
///
/// `Err` means we never got a usable answer (bad address, unreachable, not a
/// ThoughtSync server). A server that answers but is *incompatible* comes back `Ok`
/// with a verdict — that distinction matters, because the two need very different
/// messages: one is "check what you typed", the other is "update something".
pub async fn probe(raw_url: &str) -> Result<ProbeResult, String> {
    let base_url = compat::normalize_base_url(raw_url)
        .ok_or("Enter a server address, like https://notes.example.com")?;

    let request = prepare(http()?.get(config_url(&base_url)), None);
    let response = request
        .send()
        .await
        .map_err(|e| describe_transport_error(&base_url, &e))?;

    let status = response.status();
    if !status.is_success() {
        return Err(unexpected_status(&base_url, status));
    }

    // Something answered 200 that isn't a ThoughtSync server (a router login page, a
    // captive portal). Report the address, not the parse error, which would mean
    // nothing to the person reading it.
    let server: ServerInfo = response.json().await.map_err(|_| {
        format!(
            "{base_url} responded, but not with ThoughtSync's configuration. \
             Is that the right address?"
        )
    })?;

    let compatibility = compat::evaluate(&server);
    Ok(ProbeResult {
        base_url,
        server,
        compatibility,
    })
}

/// Exchange email + password for a device bearer token.
///
/// The fresh-install path: it needs no existing session, which is what lets a brand
/// new desktop install link without visiting the web app first.
pub async fn device_login(
    base_url: &str,
    email: &str,
    password: &str,
    device_name: &str,
) -> Result<(String, Identity), String> {
    let body = serde_json::json!({
        "email": email,
        "password": password,
        "name": device_name,
    });
    let request = prepare(http()?.post(device_login_url(base_url)), None).json(&body);
    let response = request
        .send()
        .await
        .map_err(|e| describe_transport_error(base_url, &e))?;

    let status = response.status();
    if status == StatusCode::UNAUTHORIZED {
        return Err("That email and password didn't match an account on this server.".to_string());
    }
    if !status.is_success() {
        return Err(unexpected_status(base_url, status));
    }

    let parsed: DeviceLoginResponse = response
        .json()
        .await
        .map_err(|_| format!("{base_url} signed us in but sent an unexpected reply."))?;
    Ok((parsed.token, parsed.user))
}

/// Validate a token by asking whom it belongs to.
///
/// Used when the user pastes a token issued from the web app. Storing it unverified
/// would turn a copy/paste slip into a failure that only surfaces at the next sync,
/// far from the thing that caused it.
pub async fn fetch_identity(base_url: &str, token: &str) -> Result<Identity, String> {
    let request = prepare(http()?.get(me_url(base_url)), Some(token));
    let response = request
        .send()
        .await
        .map_err(|e| describe_transport_error(base_url, &e))?;

    let status = response.status();
    if status == StatusCode::UNAUTHORIZED {
        let message = "That token isn't valid on this server — it may have been revoked. \
                       Issue a new one from the web app under Account → Linked devices.";
        return Err(message.to_string());
    }
    if !status.is_success() {
        return Err(unexpected_status(base_url, status));
    }

    response
        .json()
        .await
        .map_err(|_| format!("{base_url} accepted the token but sent an unexpected reply."))
}

/// Fetch one page of the change feed, starting after `since`.
///
/// The caller loops until `has_more` is false (see `pull::run`); paging lives there
/// rather than here so the transport stays a single request/response.
pub async fn fetch_changes(
    base_url: &str,
    token: &str,
    since: i64,
) -> Result<wire::ChangesPage, String> {
    let url = format!("{base_url}/api/sync/changes?since={since}");
    let request = prepare(http_with(SYNC_TIMEOUT)?.get(url), Some(token));
    let response = request
        .send()
        .await
        .map_err(|e| describe_transport_error(base_url, &e))?;

    let status = response.status();
    if status == StatusCode::UNAUTHORIZED {
        return Err(TOKEN_REJECTED.to_string());
    }
    if !status.is_success() {
        return Err(unexpected_status(base_url, status));
    }

    response
        .json()
        .await
        .map_err(|e| format!("Couldn't read the change feed from {base_url}: {e}"))
}

/// Download one attachment's bytes.
///
/// Metadata already arrived on the delta feed; this is only the payload, fetched
/// over the same route the web app uses (owner/shared scoped server-side).
pub async fn fetch_attachment(
    base_url: &str,
    token: &str,
    note_id: &str,
    attachment_id: &str,
) -> Result<Vec<u8>, String> {
    let url = format!("{base_url}/api/notes/{note_id}/attachments/{attachment_id}");
    let request = prepare(http_with(SYNC_TIMEOUT)?.get(url), Some(token));
    let response = request
        .send()
        .await
        .map_err(|e| describe_transport_error(base_url, &e))?;

    let status = response.status();
    if status == StatusCode::UNAUTHORIZED {
        return Err(TOKEN_REJECTED.to_string());
    }
    if !status.is_success() {
        return Err(unexpected_status(base_url, status));
    }

    response
        .bytes()
        .await
        .map(|b| b.to_vec())
        .map_err(|e| format!("Couldn't download an attachment from {base_url}: {e}"))
}

/// Send a batch of changes and hand back the raw reply.
///
/// Returns text rather than parsed results so this module stays pure transport —
/// `push::parse_results` owns the result shapes, and keeping them there is what lets
/// the parsing be unit-tested without a server.
pub async fn push_changes<T: Serialize>(
    base_url: &str,
    token: &str,
    changes: &[T],
) -> Result<String, String> {
    let body = serde_json::json!({ "changes": changes });
    let url = format!("{base_url}/api/sync/push");
    let request = prepare(http_with(SYNC_TIMEOUT)?.post(url), Some(token)).json(&body);
    let response = request
        .send()
        .await
        .map_err(|e| describe_transport_error(base_url, &e))?;

    let status = response.status();
    if status == StatusCode::UNAUTHORIZED {
        return Err(TOKEN_REJECTED.to_string());
    }
    if !status.is_success() {
        return Err(unexpected_status(base_url, status));
    }

    response
        .text()
        .await
        .map_err(|e| format!("Couldn't read the push reply from {base_url}: {e}"))
}

/// The public, unauthenticated endpoint carrying the handshake.
fn config_url(base_url: &str) -> String {
    format!("{base_url}/api/config")
}

fn device_login_url(base_url: &str) -> String {
    format!("{base_url}/api/auth/device-login")
}

fn me_url(base_url: &str) -> String {
    format!("{base_url}/api/auth/me")
}

/// `self` rather than a device id: see `revoke_self`.
fn revoke_self_url(base_url: &str) -> String {
    format!("{base_url}/api/auth/devices/self")
}

/// Turn a transport failure into something a person can act on. reqwest's own
/// Display is accurate but reads like a stack trace.
fn describe_transport_error(base_url: &str, err: &reqwest::Error) -> String {
    if err.is_timeout() {
        // No specific duration here: these calls run under two different budgets
        // (interactive vs bulk sync), and naming the wrong one is worse than naming
        // none.
        format!(
            "{base_url} didn't respond in time. It may be offline, or unreachable \
             from this network."
        )
    } else if err.is_connect() {
        format!(
            "Couldn't reach {base_url}. Check the address and that the server is \
             running. If it uses plain HTTP, include http:// explicitly."
        )
    } else {
        format!("Couldn't reach {base_url}: {err}")
    }
}

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

    #[test]
    fn urls_join_without_doubling_slashes() {
        // normalize_base_url has already stripped any trailing slash, so plain
        // concatenation is correct — this pins that assumption.
        assert_eq!(
            config_url("https://notes.example.com"),
            "https://notes.example.com/api/config"
        );
        assert_eq!(
            device_login_url("https://notes.example.com"),
            "https://notes.example.com/api/auth/device-login"
        );
        assert_eq!(
            me_url("https://notes.example.com"),
            "https://notes.example.com/api/auth/me"
        );
        assert_eq!(
            revoke_self_url("https://notes.example.com"),
            "https://notes.example.com/api/auth/devices/self"
        );
    }

    #[test]
    fn revoke_outcome_serializes_tagged_for_the_frontend() {
        // The UI decides between "signed out on the server" and "still valid, go
        // revoke it" by reading this tag, so its shape is part of the contract.
        let json = serde_json::to_string(&RevokeOutcome::Failed {
            reason: "offline".into(),
        })
        .expect("outcome serializes");
        assert!(json.contains("\"status\":\"failed\""), "got {json}");
        let json = serde_json::to_string(&RevokeOutcome::Revoked).expect("outcome serializes");
        assert!(json.contains("\"status\":\"revoked\""), "got {json}");
    }

    #[test]
    fn urls_preserve_a_port_and_subpath() {
        assert_eq!(
            config_url("http://192.168.1.10:8000/thoughtsync"),
            "http://192.168.1.10:8000/thoughtsync/api/config"
        );
    }
}

/// The Android client a linked server can hand out.
///
/// Mirrors `/api/client/android` (see the server's `client_dist.py`). Absent there
/// means the server has no client to offer, which is an ordinary state and not an
/// error — a self-hoster who never touches Android has one.
#[derive(Debug, Clone, Deserialize)]
pub struct ClientRelease {
    pub version: String,
    /// What decides "is this newer". The name is for people and sorts like a string.
    pub version_code: i64,
    pub size: i64,
    pub sha256: String,
    /// Path on the same server, not an absolute URL — the client joins it to the
    /// base it is already linked to, so a compromised or misconfigured server
    /// cannot redirect the download somewhere else.
    pub url: String,
}

/// What Android client the linked server has, if any.
///
/// `Ok(None)` for a server that simply has none — that is the answer to the
/// question, not a failure to answer it.
pub async fn fetch_client_release(
    base_url: &str,
    token: &str,
) -> Result<Option<ClientRelease>, String> {
    let url = format!("{base_url}/api/client/android");
    let response = prepare(http()?.get(url), Some(token))
        .send()
        .await
        .map_err(|e| describe_transport_error(base_url, &e))?;

    let status = response.status();
    if status == StatusCode::NOT_FOUND {
        return Ok(None);
    }
    if status == StatusCode::UNAUTHORIZED {
        return Err(TOKEN_REJECTED.to_string());
    }
    if !status.is_success() {
        return Err(unexpected_status(base_url, status));
    }

    response
        .json::<ClientRelease>()
        .await
        .map(Some)
        .map_err(|e| {
            format!("{base_url} described its Android client in a way this app could not read: {e}")
        })
}

/// Download the client to `dest`, verifying it on the way in.
///
/// Streamed rather than buffered: the APK is ~55 MiB and holding that in memory on
/// a phone, on top of whatever the app is already using, is how an update gets
/// killed by the low-memory killer half way through.
///
/// Written to `dest.part` and renamed only once the digest matches, so an
/// interrupted download can never be mistaken for a finished one. The digest is
/// not a trust anchor — the APK signature is, and Android checks that at install —
/// but it catches a truncated or corrupted transfer before the installer is
/// bothered with it.
pub async fn download_client(
    base_url: &str,
    token: &str,
    release: &ClientRelease,
    dest: &Path,
) -> Result<(), String> {
    use sha2::{Digest, Sha256};
    use std::io::Write;

    // The advertised path is joined to the base we are LINKED to. Taking an
    // absolute URL from the response would let a server point the download at a
    // host the user never agreed to.
    let path = release.url.trim_start_matches('/');
    let url = format!("{base_url}/{path}");

    let mut response = prepare(http_with(SYNC_TIMEOUT)?.get(url), Some(token))
        .send()
        .await
        .map_err(|e| describe_transport_error(base_url, &e))?;

    let status = response.status();
    if status == StatusCode::UNAUTHORIZED {
        return Err(TOKEN_REJECTED.to_string());
    }
    if !status.is_success() {
        return Err(unexpected_status(base_url, status));
    }

    let partial = dest.with_extension("part");
    if let Some(parent) = partial.parent() {
        std::fs::create_dir_all(parent)
            .map_err(|e| format!("Couldn't prepare a place to download to: {e}"))?;
    }
    let mut file = std::fs::File::create(&partial)
        .map_err(|e| format!("Couldn't open the download file: {e}"))?;

    let mut hasher = Sha256::new();
    let mut written: i64 = 0;
    loop {
        let chunk = response
            .chunk()
            .await
            .map_err(|e| describe_transport_error(base_url, &e))?;
        let Some(chunk) = chunk else { break };
        hasher.update(&chunk);
        written += chunk.len() as i64;
        file.write_all(&chunk)
            .map_err(|e| format!("Couldn't write the download: {e}"))?;
    }
    file.flush()
        .map_err(|e| format!("Couldn't finish writing the download: {e}"))?;
    drop(file);

    let digest = format!("{:x}", hasher.finalize());
    let mismatch = if written != release.size {
        Some(format!("expected {} bytes, got {written}", release.size))
    } else if !digest.eq_ignore_ascii_case(&release.sha256) {
        Some("the contents did not match the checksum the server published".to_string())
    } else {
        None
    };
    if let Some(why) = mismatch {
        // The half-file is removed rather than left: a later run finding it would
        // have no way to tell it from a good one.
        let _ = std::fs::remove_file(&partial);
        return Err(format!("The download from {base_url} was damaged — {why}."));
    }

    std::fs::rename(&partial, dest)
        .map_err(|e| format!("Couldn't put the downloaded update in place: {e}"))
}