//! Which runtime an agent runs, as read from its environment.

use std::collections::BTreeMap;

/// Selects the runtime: `claude` (also when unset) or `acp`.
pub const RUNTIME_ENV: &str = "HIVE_RUNTIME";
/// The ACP agent's program, spawned as-is (a path or a name on `PATH`).
pub const ACP_COMMAND_ENV: &str = "HIVE_ACP_COMMAND";
/// The ACP agent's arguments, as a JSON array of strings. Optional.
pub const ACP_ARGS_ENV: &str = "HIVE_ACP_ARGS";
/// Extra environment for the ACP agent only, as a JSON object of strings.
/// Optional. The agent also inherits the harness's own environment.
pub const ACP_ENV_ENV: &str = "HIVE_ACP_ENV";

/// The runtime an agent is configured with.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum RuntimeSpec {
    Claude,
    Acp(AcpCommand),
}

/// How to spawn an ACP agent.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct AcpCommand {
    pub command: String,
    pub args: Vec<String>,
    pub env: BTreeMap<String, String>,
}

/// A runtime configuration that cannot be acted on.
#[derive(Debug, thiserror::Error)]
#[non_exhaustive]
pub enum SpecError {
    #[error("{RUNTIME_ENV}={0:?} names no runtime (expected \"claude\" or \"acp\")")]
    UnknownRuntime(String),
    #[error("{RUNTIME_ENV}=acp but {ACP_COMMAND_ENV} is unset or empty")]
    MissingCommand,
    #[error("{var} is not valid JSON of the expected shape: {source}")]
    Malformed {
        var: &'static str,
        #[source]
        source: serde_json::Error,
    },
}

impl RuntimeSpec {
    /// Read the spec from the process environment.
    pub fn from_env() -> Result<Self, SpecError> {
        Self::parse(|var| std::env::var(var).ok())
    }

    /// Build the spec from `lookup`, which returns an env var's value.
    pub fn parse(lookup: impl Fn(&str) -> Option<String>) -> Result<Self, SpecError> {
        let runtime = lookup(RUNTIME_ENV).unwrap_or_default();
        match runtime.trim() {
            "" | "claude" => Ok(Self::Claude),
            "acp" => {
                let command = lookup(ACP_COMMAND_ENV).unwrap_or_default();
                if command.trim().is_empty() {
                    return Err(SpecError::MissingCommand);
                }
                Ok(Self::Acp(AcpCommand {
                    command,
                    args: json_or_default(&lookup, ACP_ARGS_ENV)?,
                    env: json_or_default(&lookup, ACP_ENV_ENV)?,
                }))
            }
            other => Err(SpecError::UnknownRuntime(other.to_owned())),
        }
    }
}

fn json_or_default<T: Default + serde::de::DeserializeOwned>(
    lookup: &impl Fn(&str) -> Option<String>,
    var: &'static str,
) -> Result<T, SpecError> {
    match lookup(var) {
        Some(raw) if !raw.trim().is_empty() => {
            serde_json::from_str(&raw).map_err(|source| SpecError::Malformed { var, source })
        }
        _ => Ok(T::default()),
    }
}

#[cfg(test)]
mod tests {
    use super::{AcpCommand, RuntimeSpec, SpecError};
    use std::collections::HashMap;

    fn parse(vars: &[(&str, &str)]) -> Result<RuntimeSpec, SpecError> {
        let vars: HashMap<String, String> = vars
            .iter()
            .map(|(k, v)| ((*k).to_owned(), (*v).to_owned()))
            .collect();
        RuntimeSpec::parse(|k| vars.get(k).cloned())
    }

    #[test]
    fn unset_or_claude_is_the_claude_runtime() {
        assert_eq!(parse(&[]).unwrap(), RuntimeSpec::Claude);
        assert_eq!(
            parse(&[("HIVE_RUNTIME", "claude")]).unwrap(),
            RuntimeSpec::Claude
        );
        // ACP settings alone do not switch the runtime.
        assert_eq!(
            parse(&[("HIVE_ACP_COMMAND", "/bin/agent")]).unwrap(),
            RuntimeSpec::Claude
        );
    }

    #[test]
    fn acp_resolves_command_args_and_env() {
        let spec = parse(&[
            ("HIVE_RUNTIME", "acp"),
            ("HIVE_ACP_COMMAND", "/nix/store/x/bin/agent"),
            ("HIVE_ACP_ARGS", r#"["acp","--flag"]"#),
            (
                "HIVE_ACP_ENV",
                r#"{"AGENT_CONFIG":"/nix/store/y/config.json"}"#,
            ),
        ])
        .unwrap();
        assert_eq!(
            spec,
            RuntimeSpec::Acp(AcpCommand {
                command: "/nix/store/x/bin/agent".into(),
                args: vec!["acp".into(), "--flag".into()],
                env: [("AGENT_CONFIG".into(), "/nix/store/y/config.json".into())].into(),
            })
        );
    }

    #[test]
    fn acp_args_and_env_are_optional() {
        let spec = parse(&[("HIVE_RUNTIME", "acp"), ("HIVE_ACP_COMMAND", "agent")]).unwrap();
        let RuntimeSpec::Acp(cmd) = spec else {
            panic!("expected acp, got {spec:?}");
        };
        assert!(cmd.args.is_empty());
        assert!(cmd.env.is_empty());
    }

    #[test]
    fn acp_without_a_command_is_refused() {
        assert!(matches!(
            parse(&[("HIVE_RUNTIME", "acp")]),
            Err(SpecError::MissingCommand)
        ));
        assert!(matches!(
            parse(&[("HIVE_RUNTIME", "acp"), ("HIVE_ACP_COMMAND", "  ")]),
            Err(SpecError::MissingCommand)
        ));
    }

    #[test]
    fn malformed_json_names_the_variable() {
        let err = parse(&[
            ("HIVE_RUNTIME", "acp"),
            ("HIVE_ACP_COMMAND", "agent"),
            ("HIVE_ACP_ARGS", r#"{"not":"a list"}"#),
        ])
        .unwrap_err();
        assert!(err.to_string().starts_with("HIVE_ACP_ARGS "), "{err}");
    }

    #[test]
    fn an_unknown_runtime_is_refused_rather_than_defaulted() {
        assert!(matches!(
            parse(&[("HIVE_RUNTIME", "bogus")]),
            Err(SpecError::UnknownRuntime(r)) if r == "bogus"
        ));
    }
}