自律型AIエージェントと「AIの思い込み」を防ぐ3重チェック設計：
〜命令文（プロンプト）の勉強なしで、ベテラン秘書のように安全に働く仕組み〜

#![forbid(unsafe_code)]
use core::marker::PhantomData;

/// Marker types for compile-time Information Flow Control (IFC)
pub struct TaintedExternalData;
pub struct VerifiedSystemPolicy;

#[derive(Clone, Copy, Debug)]
pub struct SecurityContext<Origin> {
    pub tenant_id: [u8; 16],
    pub payload_hash: [u8; 32],
    _origin: PhantomData<Origin>,
}

/// Strictly typed MCP Tool Operations — arbitrary shell/SQL strings cannot be represented.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum SafeMcpAction {
    DraftFreeeInvoicePreview { partner_code: u32, amount_jpy: u64 },
    VerifyNationalTaxInvoiceNumber { reg_digits_13: u64 },
    GenerateScsAuditChecklistPdf { target_star_level: u8 },
}

pub struct AuthorizedToolInvocation {
    pub tenant_id: [u8; 16],
    pub action: SafeMcpAction,
    pub human_approval_token: Option<[u8; 32]>,
}

impl SecurityContext<TaintedExternalData> {
    pub const fn ingest_ocr_or_email(tenant_id: [u8; 16], payload_hash: [u8; 32]) -> Self {
        Self { tenant_id, payload_hash, _origin: PhantomData }
    }
    // NOTE: Intentionally has NO method to construct `AuthorizedToolInvocation` directly!
    // An Indirect Prompt Injection inside an OCR/Email payload cannot escalate to tool execution.
}

impl SecurityContext<VerifiedSystemPolicy> {
    pub const fn from_authenticated_session(tenant_id: [u8; 16], policy_hash: [u8; 32]) -> Self {
        Self { tenant_id, payload_hash: policy_hash, _origin: PhantomData }
    }

    /// Validates that T+13 digit Qualified Invoice Number satisfies National Tax Agency Modulo-9 check digit
    pub const fn verify_nta_invoice_mod9(digits_13: u64) -> bool {
        if digits_13 < 1_000_000_000_000 || digits_13 > 9_999_999_999_999 {
            return false;
        }
        let check_digit = (digits_13 % 10) as u32;
        let mut rem = digits_13 / 10;
        let mut pos = 1u32; — 1..=12 from rightmost body digit
        let mut weighted_sum = 0u32;
        while pos <= 12 {
            let d = (rem % 10) as u32;
            let weight = if pos % 2 == 1 { 2 } else { 1 };
            weighted_sum += d * weight;
            rem /= 10;
            pos += 1;
        }
        let expected = 9 - (weighted_sum % 9);
        check_digit == expected
    }
}