//! Finding out what a machine can run, rather than being told.
//!
//! The phone adds a machine by giving connection details; this asks the
//! machine itself which of the known programs it has, and the answer
//! becomes its providers. That is a security property, not a convenience:
//! **no route accepts a command from the phone.** If it did, the enrolled
//! token would be able to introduce arbitrary programs to run on every
//! machine a setup names, and the transport already reaches those over
//! ssh. Here the phone's authority is "add this machine", never "run
//! this".
//!
//! It is also the better interface. Nobody wants to type an absolute path
//! on a phone keyboard, and a machine that has moved its binaries answers
//! correctly on the next probe without anyone editing anything.
//!
//! The cost is that a program somewhere unusual is invisible. That is a
//! deliberate trade rather than an oversight: the escape hatch is editing
//! `config.ron` on the backend, which is exactly the authority the phone
//! is not being given.

use anyhow::{Context, Result};

use crate::config::{DriverKind, ProviderConfig};
use crate::session::transport::{Launch, Transport};

/// What is looked for, and what finding it makes.
///
/// Extending this is how a new driver becomes discoverable -- one row, not
/// a branch anywhere. The name is what the provider gets called, so it is
/// what the phone shows and what a session stores.
const PROBES: &[(&str, &str, DriverKind)] = &[
    ("claude-cli", "claude", DriverKind::ClaudeCli),
    ("local-llama", "llama-server", DriverKind::LlamaCpp),
];

/// Models offered for a discovered Claude CLI. A shortcut list for the
/// spawn screen, not a restriction -- the field stays free text.
const CLAUDE_MODELS: &[&str] = &["fable", "opus", "sonnet", "haiku"];

/// Asks `transport`'s machine which of [`PROBES`] it has.
///
/// One round trip rather than one per program: over ssh each would be a
/// separate connection and handshake, and a person waiting on "test this
/// setup" notices. `command -v` is POSIX and a shell builtin, so it works
/// whatever is installed -- and `|| true` keeps a missing program from
/// ending the loop, since the caller wants the whole answer rather than
/// the first failure.
pub async fn discover(transport: &Transport) -> Result<Vec<ProviderConfig>> {
    let wanted: Vec<&str> = PROBES.iter().map(|(_, binary, _)| *binary).collect();
    let script = format!(
        "for p in {}; do command -v \"$p\" || true; done",
        wanted.join(" ")
    );
    let launch = Launch::new("sh", vec!["-c".to_string(), script], None);
    let found = transport.capture(&launch).await.map_err(explain)?;

    let mut providers = Vec::new();
    // Echo runs inside this server, so it exists exactly where this server
    // does and nowhere else. Nothing to probe for, and offering it on a
    // remote machine would be a choice that changes nothing.
    if matches!(transport, Transport::Here) {
        providers.push(ProviderConfig {
            name: crate::config::ECHO_PROVIDER.to_string(),
            kind: DriverKind::Echo,
            command: None,
            models: Vec::new(),
        });
    }
    for (name, binary, kind) in PROBES {
        let path = found
            .lines()
            .map(str::trim)
            .find(|line| line.rsplit('/').next() == Some(*binary));
        let Some(path) = path else {
            continue;
        };
        providers.push(ProviderConfig {
            name: (*name).to_string(),
            kind: *kind,
            // The resolved path rather than the bare name: PATH under a
            // non-interactive ssh session is not the one a person sees
            // when they log in, so "it is on my PATH" is not enough.
            command: Some(path.to_string()),
            models: match kind {
                DriverKind::ClaudeCli => CLAUDE_MODELS.iter().map(|m| (*m).to_string()).collect(),
                _ => Vec::new(),
            },
        });
    }
    Ok(providers)
}

/// Adds what to do to failures whose own wording does not say.
///
/// ssh's messages are written for someone at a terminal on the backend,
/// which is exactly who is not reading this one. Host key verification is
/// the case that matters: **every** machine fails it the first time,
/// because its key is not in `known_hosts` yet -- so without this, adding
/// a machine from the phone looks broken rather than unfinished.
///
/// Deliberately not fixed by relaxing the check. `StrictHostKeyChecking`
/// stays at its default, so a first connection is a decision somebody
/// makes on the backend with the key in front of them, rather than
/// something this app quietly accepts on their behalf.
fn explain(err: anyhow::Error) -> anyhow::Error {
    let message = format!("{err:#}");
    if message.contains("Host key verification failed") {
        return anyhow::anyhow!(
            "{message} This machine has not been connected to before, so its key is not \
             trusted yet. Ssh to it once from the backend -- that is where the decision to \
             trust a key belongs -- and try again.",
        );
    }
    if message.contains("Permission denied") {
        return anyhow::anyhow!(
            "{message} The key named here has to be authorized on that machine, and the path \
             is read on the backend rather than on the phone.",
        );
    }
    err
}

/// A short, stable, filename-safe id derived from a label.
///
/// Derived once when a setup is added and then fixed, so the label stays
/// editable. Collisions are resolved by the caller, which is the only
/// place that knows what already exists.
pub fn id_from(label: &str) -> String {
    let slug: String = label
        .chars()
        .map(|c| {
            if c.is_ascii_alphanumeric() {
                c.to_ascii_lowercase()
            } else {
                '-'
            }
        })
        .collect();
    let slug = slug.trim_matches('-').replace("--", "-");
    if slug.is_empty() {
        crate::session::random_hex()
    } else {
        slug.chars().take(32).collect()
    }
}

/// Normalises what a phone keyboard produced: trims, drops blanks, and
/// expands a leading `~` the way a shell would.
pub fn tidy(value: &str) -> Option<String> {
    let value = value.trim();
    if value.is_empty() {
        return None;
    }
    Some(match value.strip_prefix("~/") {
        Some(rest) => match std::env::home_dir() {
            Some(home) => home.join(rest).to_string_lossy().into_owned(),
            None => value.to_string(),
        },
        None => value.to_string(),
    })
}

/// Runs a launch to completion and returns its stdout.
impl Transport {
    pub async fn capture(&self, launch: &Launch) -> Result<String> {
        // See `CAPTURE_PERMITS`: caps how many of these run their ssh
        // connection at once, so a batch doesn't open more than the remote
        // sshd tolerates before it starts dropping them.
        let _permit = super::session::transport::CAPTURE_PERMITS
            .acquire()
            .await
            .expect("capture semaphore is never closed");
        let child = self.spawn(launch, super::session::transport::Streams::Piped)?;
        let output = child
            .wait_with_output()
            .await
            .context("waiting for the probe to finish")?;
        if !output.status.success() {
            // ssh's own failures land on stderr -- "Permission denied",
            // "Could not resolve hostname" -- and are the useful half of
            // why a setup cannot be reached, so they are what comes back.
            let stderr = String::from_utf8_lossy(&output.stderr).trim().to_string();
            anyhow::bail!(if stderr.is_empty() {
                format!("couldn't reach it ({})", output.status)
            } else {
                stderr
            });
        }
        Ok(String::from_utf8_lossy(&output.stdout).into_owned())
    }
}