//! The gate policy (spec §15, acceptance-blocking).
//!
//! Tools declare *effects*; the policy maps effects to the six gated
//! categories. Classification is deny-by-default: any externally-visible
//! effect without a more specific category gates as an outbound message,
//! and input tainted by untrusted sources gates every external effect.
//! This module is pure — no I/O — so its invariants are property-tested.

use cm_domain::GatedCategory;
use serde::{Deserialize, Serialize};

/// What a tool can do, declared statically by the tool itself.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum Effect {
    /// Reads data inside the workspace boundary.
    ReadsWorkspaceData,
    /// Writes data inside the workspace boundary.
    WritesWorkspaceData,
    /// Contacts systems outside the workspace without sending user content
    /// (e.g. fetching a public page).
    ReachesExternally,
    /// Sends messages/emails/posts outside the workspace.
    SendsExternally,
    /// Exposes keys, secrets, or credentials.
    SharesSecrets,
    /// Changes access, permissions, or sharing.
    ChangesAccess,
    /// Financial transactions and credit purchases.
    MovesMoney,
    /// Deletes files or records.
    DeletesData,
    /// Grants or extends external-infrastructure access.
    GrantsInfraAccess,
}

impl Effect {
    /// Effects visible outside the workspace boundary.
    pub fn is_external(&self) -> bool {
        !matches!(
            self,
            Effect::ReadsWorkspaceData | Effect::WritesWorkspaceData
        )
    }

    /// The §15 category this effect always gates as, if any.
    fn gated_category(&self) -> Option<GatedCategory> {
        match self {
            Effect::SendsExternally => Some(GatedCategory::OutboundMessage),
            Effect::SharesSecrets => Some(GatedCategory::SecretSharing),
            Effect::ChangesAccess => Some(GatedCategory::AccessChange),
            Effect::MovesMoney => Some(GatedCategory::FinancialTransaction),
            Effect::DeletesData => Some(GatedCategory::FileDeletion),
            Effect::GrantsInfraAccess => Some(GatedCategory::InfraAccessGrant),
            Effect::ReadsWorkspaceData
            | Effect::WritesWorkspaceData
            | Effect::ReachesExternally => None,
        }
    }

    /// Severity order for picking the headline category of a multi-effect
    /// tool (most consequential wins).
    fn severity(&self) -> u8 {
        match self {
            Effect::ReadsWorkspaceData => 0,
            Effect::WritesWorkspaceData => 1,
            Effect::ReachesExternally => 2,
            Effect::SendsExternally => 3,
            Effect::DeletesData => 4,
            Effect::ChangesAccess => 5,
            Effect::SharesSecrets => 6,
            Effect::GrantsInfraAccess => 7,
            Effect::MovesMoney => 8,
        }
    }
}

/// Where a piece of content came from. Anything here marks the content as
/// untrusted-by-default (§15): data, never instructions.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum TaintSource {
    Web,
    Email,
    InterAgent,
    ToolResult,
}

impl std::str::FromStr for TaintSource {
    type Err = String;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s {
            "web" => Ok(TaintSource::Web),
            "email" => Ok(TaintSource::Email),
            "inter_agent" => Ok(TaintSource::InterAgent),
            "tool_result" => Ok(TaintSource::ToolResult),
            other => Err(format!("unknown taint source: {other}")),
        }
    }
}

impl TaintSource {
    pub fn as_str(&self) -> &'static str {
        match self {
            TaintSource::Web => "web",
            TaintSource::Email => "email",
            TaintSource::InterAgent => "inter_agent",
            TaintSource::ToolResult => "tool_result",
        }
    }
}

/// The set of untrusted sources that influenced a tool input.
#[derive(Debug, Clone, PartialEq, Eq, Default, Serialize, Deserialize)]
pub struct TaintSet {
    sources: Vec<TaintSource>,
}

impl TaintSet {
    pub fn clean() -> TaintSet {
        TaintSet::default()
    }

    /// Rebuilds a set from stored strings (checkpoints, step rows);
    /// unknown strings are ignored rather than dropped runs.
    pub fn from_strings(raw: &[String]) -> TaintSet {
        let sources: Vec<TaintSource> = raw.iter().filter_map(|s| s.parse().ok()).collect();
        TaintSet::from_sources(&sources)
    }

    pub fn from_sources(sources: &[TaintSource]) -> TaintSet {
        let mut unique: Vec<TaintSource> = Vec::new();
        for source in sources {
            if !unique.contains(source) {
                unique.push(*source);
            }
        }
        TaintSet { sources: unique }
    }

    pub fn is_clean(&self) -> bool {
        self.sources.is_empty()
    }

    pub fn sources(&self) -> &[TaintSource] {
        &self.sources
    }

    /// Storage form for `steps.taint` / `approvals.taint_sources`.
    pub fn as_strings(&self) -> Vec<String> {
        self.sources.iter().map(|s| s.as_str().to_owned()).collect()
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum GateDecision {
    Allow,
    RequireApproval(GatedCategory),
}

/// Pure classifier from declared effects + input taint to a decision.
#[derive(Debug, Default)]
pub struct GatePolicy;

impl GatePolicy {
    pub fn classify(&self, effects: &[Effect], taint: &TaintSet) -> GateDecision {
        let headline = effects.iter().max_by_key(|e| e.severity()).copied();

        let Some(headline) = headline else {
            return GateDecision::Allow;
        };

        if let Some(category) = effects
            .iter()
            .filter_map(Effect::gated_category)
            .max_by_key(|c| {
                effects
                    .iter()
                    .filter(|e| e.gated_category() == Some(*c))
                    .map(|e| e.severity())
                    .max()
                    .unwrap_or(0)
            })
        {
            return GateDecision::RequireApproval(category);
        }

        // No always-gated effect. External reach gates by default — and
        // unconditionally when the input carries untrusted taint (§15).
        if headline.is_external() {
            return GateDecision::RequireApproval(GatedCategory::OutboundMessage);
        }
        if !taint.is_clean() && effects.iter().any(Effect::is_external) {
            return GateDecision::RequireApproval(GatedCategory::OutboundMessage);
        }
        GateDecision::Allow
    }
}