//! Mod validation: two-layer inspection of Forge/Fabric mod JARs.
//!
//! 1. Metadata check — reads `META-INF/mods.toml` (Forge) or `fabric.mod.json` (Fabric).
//! 2. Class scan — walks `.class` entries looking for `net/minecraft/server`
//!    vs `net/minecraft/client` references.
//!
//! **Rejection policy:** only mods that explicitly declare themselves client-only
//! in their metadata (`mods.toml` side=CLIENT, `fabric.mod.json` environment=client)
//! are rejected. The class scan is purely informational — many Forge mods that
//! work on servers reference `net/minecraft/client/…` for rendering/GUI but do
//! their server work through common API classes (`net/minecraft/block/Block`, etc.)
//! without ever referencing `net/minecraft/server/…` directly.

use std::io::{Cursor, Read};
use zip::ZipArchive;

/// The canonical side for a mod, derived from metadata or class scan.
#[derive(Debug, Clone, Copy, PartialEq)]
pub enum ModSide {
    Both,
    Client,
    Server,
    Unknown,
}

impl std::fmt::Display for ModSide {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            ModSide::Both => write!(f, "BOTH"),
            ModSide::Client => write!(f, "CLIENT"),
            ModSide::Server => write!(f, "SERVER"),
            ModSide::Unknown => write!(f, "UNKNOWN"),
        }
    }
}

/// The result of validating a single mod file.
#[derive(Debug)]
pub struct ValidationResult {
    pub accepted: bool,
    pub rejection_reason: Option<String>,
    pub display_name: Option<String>,
    pub side: ModSide,
}

/// Maximum size for a single mod JAR file (500 MB).
const MAX_MOD_SIZE_BYTES: usize = 500 * 1024 * 1024;

/// Validate a mod JAR (raw bytes, filename for error messages).
///
/// Returns a `ValidationResult` indicating whether the mod is safe to install.
///
/// **Rejection policy:**
/// - Only mods that explicitly declare themselves client-only **in their metadata**
///   (`mods.toml` side=CLIENT, `fabric.mod.json` environment=client) are rejected
///   outright. The class-file scan is purely informational — many Forge mods that
///   work perfectly on a server reference `net/minecraft/client/…` classes for
///   rendering/GUI while doing their server logic through common API classes
///   (`net/minecraft/block/Block`, `net/minecraft/item/Item`, …) without ever
///   referencing `net/minecraft/server/…` directly.
pub fn validate_mod(data: &[u8], filename: &str) -> ValidationResult {
    // Reject oversized files early to prevent DoS
    if data.len() > MAX_MOD_SIZE_BYTES {
        return ValidationResult {
            accepted: false,
            rejection_reason: Some(format!(
                "File too large: {} bytes (maximum is {} MB)",
                data.len(),
                MAX_MOD_SIZE_BYTES / (1024 * 1024)
            )),
            display_name: None,
            side: ModSide::Unknown,
        };
    }
    // Phase 1: metadata check
    let (meta_side, meta_display_name) = read_metadata(data);

    // Only reject based on metadata — the mod author's explicit declaration.
    if meta_side == Some(ModSide::Client) {
        return ValidationResult {
            accepted: false,
            rejection_reason: Some(format!(
                "Mod '{}' declares itself as client-only in its metadata. \
                 It would crash the server.",
                meta_display_name.as_deref().unwrap_or(filename)
            )),
            display_name: meta_display_name,
            side: ModSide::Client,
        };
    }

    // Phase 2: class scan (informational only — never causes rejection)
    let class_side = scan_jar_for_side(data);

    let side = if meta_side.is_some() {
        meta_side.unwrap()
    } else {
        match class_side {
            ScanResult::HasServer => ModSide::Both,
            ScanResult::OnlyClient => ModSide::Client,
            ScanResult::NoClearSide => ModSide::Unknown,
        }
    };

    ValidationResult {
        accepted: true,
        rejection_reason: if side == ModSide::Client && meta_side.is_none() {
            Some("Class scan suggests this mod may be client-side only, \
                 but it has been accepted anyway (no metadata declaration).".to_string())
        } else {
            None
        },
        display_name: meta_display_name,
        side,
    }
}

// ── Metadata parsing ──────────────────────────────────────────────

/// Read side and display-name from Forge's mods.toml or Fabric's fabric.mod.json.
/// Returns `(side, display_name)` where side is `None` if no metadata found.
fn read_metadata(data: &[u8]) -> (Option<ModSide>, Option<String>) {
    let cursor = Cursor::new(data);
    let mut archive = match ZipArchive::new(cursor) {
        Ok(a) => a,
        Err(_) => return (None, None),
    };

    // Try Forge mods.toml
    if let Ok(mut file) = archive.by_name("META-INF/mods.toml") {
        let mut content = String::new();
        if file.read_to_string(&mut content).is_ok()
            && let Some((side, name)) = parse_forge_metadata(&content) {
                return (Some(side), name);
            }
    }

    // Try Fabric fabric.mod.json
    if let Ok(mut file) = archive.by_name("fabric.mod.json") {
        let mut content = String::new();
        if file.read_to_string(&mut content).is_ok()
            && let Some((side, name)) = parse_fabric_metadata(&content) {
                return (Some(side), name);
            }
    }

    (None, None)
}

/// Parse `[[mods]]` entries from Forge's mods.toml.
fn parse_forge_metadata(content: &str) -> Option<(ModSide, Option<String>)> {
    let value: toml::Value = content.parse().ok()?;
    let mods = value.get("mods")?.as_array()?;

    let mut has_both = false;
    let mut has_server = false;
    let mut has_client = false;
    let mut display_name = None;

    for entry in mods {
        let entry = entry.as_table()?;
        // Use displayName or modId
        let name = entry
            .get("displayName")
            .and_then(|v| v.as_str())
            .or_else(|| entry.get("modId").and_then(|v| v.as_str()))
            .map(|s| s.to_string());
        if display_name.is_none() {
            display_name = name;
        }

        match entry.get("side").and_then(|v| v.as_str()) {
            Some("BOTH") => has_both = true,
            Some("SERVER") => has_server = true,
            Some("CLIENT") => has_client = true,
            _ => {}
        }
    }

    if has_both || (has_server && has_client) {
        Some((ModSide::Both, display_name))
    } else if has_server {
        Some((ModSide::Server, display_name))
    } else if has_client {
        Some((ModSide::Client, display_name))
    } else {
        None
    }
}

/// Parse Fabric's fabric.mod.json.
fn parse_fabric_metadata(content: &str) -> Option<(ModSide, Option<String>)> {
    let value: serde_json::Value = content.parse().ok()?;
    let env = value.get("environment").and_then(|v| v.as_str());
    let name = value
        .get("name")
        .and_then(|v| v.as_str())
        .or_else(|| value.get("id").and_then(|v| v.as_str()))
        .map(|s| s.to_string());

    match env {
        Some("*") | Some("server") => Some((ModSide::Both, name)),
        Some("client") => Some((ModSide::Client, name)),
        _ => None,
    }
}

// ── Class-file scan ───────────────────────────────────────────────

#[derive(Debug, Clone, Copy, PartialEq)]
enum ScanResult {
    /// At least one class references server paths (mod is safe).
    HasServer,
    /// Every class that references Minecraft packages only references client paths.
    OnlyClient,
    /// No class gave a clear indication either way.
    NoClearSide,
}

/// Maximum number of class files to scan inside a single JAR (prevents DoS).
const MAX_CLASSES_TO_SCAN: usize = 10_000;

/// Walk all `.class` entries in the JAR and look for client vs server references.
///
/// Two independent signal layers:
/// 1. **Server-content**: `net/minecraft/block/`, `net/minecraft/tileentity/`,
///    `net/minecraft/command/`, `net/minecraft/world/gen/`, `net/minecraft/entity/ai/`,
///    `net/minecraft/pathfinding/`, `net/minecraft/server/`.
/// 2. **Client-API**: `net/minecraft/client/`, `net/minecraftforge/client/`,
///    `net/minecraftforge/fml/client/`.
///
/// The `@Mod` annotation (`net/minecraftforge/fml/common/Mod`) is deliberately
/// excluded from both sets — it is mandatory boilerplate for EVERY Forge mod
/// and says nothing about server compatibility.
fn scan_jar_for_side(data: &[u8]) -> ScanResult {
    let cursor = Cursor::new(data);
    let mut archive = match ZipArchive::new(cursor) {
        Ok(a) => a,
        Err(_) => return ScanResult::NoClearSide,
    };

    let mut has_server_content = false;
    let mut has_client_api = false;
    let mut scanned_count = 0_usize;

    for i in 0..archive.len() {
        if scanned_count >= MAX_CLASSES_TO_SCAN {
            tracing::warn!("Reached max class scan limit ({MAX_CLASSES_TO_SCAN}), stopping");
            break;
        }
        let mut file = match archive.by_index(i) {
            Ok(f) => f,
            Err(_) => continue,
        };

        let name = file.name().to_string();
        if !name.ends_with(".class") {
            continue;
        }

        let mut bytes = Vec::with_capacity(file.size() as usize);
        if file.read_to_end(&mut bytes).is_err() {
            continue;
        }

        scanned_count += 1;

        let result = scan_single_class(&bytes);
        match result {
            ClassScan::HasServerContent => has_server_content = true,
            ClassScan::HasClientApi => has_client_api = true,
            ClassScan::Both => {
                has_server_content = true;
                has_client_api = true;
            }
            ClassScan::Neither => {}
        }

        if has_server_content && has_client_api {
            return ScanResult::HasServer;
        }
    }

    if has_server_content {
        ScanResult::HasServer
    } else if has_client_api {
        ScanResult::OnlyClient
    } else {
        ScanResult::NoClearSide
    }
}

#[derive(Debug, PartialEq)]
enum ClassScan {
    /// Found server-side content API refs (`net/minecraft/block/`, `net/minecraft/item/Item`,
    /// `net/minecraft/tileentity/`, etc.) and/or direct `net/minecraft/server/` refs.
    HasServerContent,
    /// Found only client-side API refs (`net/minecraft/client/gui/`,
    /// `net/minecraftforge/client/`, `net/minecraftforge/fml/client/`) —
    /// no server or content API refs.
    HasClientApi,
    /// Found both server-content AND client-api refs in the same class.
    Both,
    /// No Minecraft references at all.
    Neither,
}

/// Server-side content API path prefixes — mods referencing these
/// provide actual world content (blocks, tile entities, commands,
/// worldgen, AI/pathfinding).
///
/// `net/minecraft/item/Item` is deliberately excluded — the Item class is
/// referenced by both client-only mods (item tooltips, inventory rendering)
/// and server mods (item registration), so it's not a reliable indicator.
/// The `ItemStack` substring makes it even worse.
///
/// Forge-specific packages (`net/minecraftforge/common/`, etc.) are also
/// excluded — they are shared client+server APIs referenced by every
/// Forge mod including purely client-side ones.
const SERVER_CONTENT_PREFIXES: &[&[u8]] = &[
    b"net/minecraft/block/",
    b"net/minecraft/tileentity/",
    b"net/minecraft/command/",
    b"net/minecraft/world/gen/",
    b"net/minecraft/entity/ai/",         // entity AI (server-side only)
    b"net/minecraft/pathfinding/",       // pathfinding (server-side only)
];

/// Client-side API path prefixes — mods referencing ONLY these (without
/// any SERVER_CONTENT_PREFIXES) are likely client-only.
///
/// `net/minecraft/client/` covers everything under the client package
/// (Minecraft, gui, renderer, resources, settings, network handlers, etc.)
/// — all are client-only classes that don't exist on the server.
const CLIENT_API_PREFIXES: &[&[u8]] = &[
    b"net/minecraft/client/",
    b"net/minecraftforge/client/",
    b"net/minecraftforge/fml/client/",
];

/// Check a single `.class` file's raw bytes for server-side content API vs
/// client-side API references in the constant pool.
///
/// We parse the constant pool minimally to find `CONSTANT_Utf8` entries,
/// which store the actual string literals (class names, method signatures, etc.)
/// used by the bytecode. This avoids false positives from raw byte scanning.
/// Maximum constant pool entries to scan in a single class file (prevents crafted-file DoS).
const MAX_CP_ENTRIES: usize = 10_000;

fn scan_single_class(data: &[u8]) -> ClassScan {
    if data.len() < 10 {
        return ClassScan::Neither;
    }
    if data[0..4] != [0xCA, 0xFE, 0xBA, 0xBE] {
        return ClassScan::Neither;
    }

    let cp_count = read_u16_be(&data[8..10]) as usize;
    if !(1..=MAX_CP_ENTRIES).contains(&cp_count) {
        return ClassScan::Neither;
    }

    let mut pos: usize = 10;
    let mut has_server = false;
    let mut has_client = false;
    let mut i = 1;

    while i < cp_count && pos < data.len() {
        if pos >= data.len() {
            break;
        }
        let tag = data[pos];
        pos += 1;

        match tag {
            1 => {
                if pos + 2 > data.len() {
                    break;
                }
                let len = read_u16_be(&data[pos..pos + 2]) as usize;
                pos += 2;
                if pos + len > data.len() {
                    break;
                }
                let slice = &data[pos..pos + len];

                if !has_server
                    && (   bytes_contains(slice, b"net/minecraft/server/")
                        || SERVER_CONTENT_PREFIXES.iter().any(|p| bytes_contains(slice, p)))
                {
                    has_server = true;
                }
                if !has_client
                    && CLIENT_API_PREFIXES.iter().any(|p| bytes_contains(slice, p))
                {
                    has_client = true;
                }
                pos += len;

                if has_server && has_client {
                    return ClassScan::Both;
                }
            }
            // CONSTANT_Integer, CONSTANT_Float, CONSTANT_Fieldref,
            // CONSTANT_Methodref, CONSTANT_InterfaceMethodref,
            // CONSTANT_Dynamic, CONSTANT_InvokeDynamic
            3 | 4 | 9 | 10 | 11 | 12 | 17 | 18 => {
                if pos + 4 > data.len() {
                    break;
                }
                pos += 4;
            }
            // CONSTANT_Long, CONSTANT_Double (take 2 cp slots)
            5 | 6 => {
                if pos + 8 > data.len() {
                    break;
                }
                pos += 8;
                i += 1; // extra slot consumed
            }
            // CONSTANT_Class, CONSTANT_String, CONSTANT_MethodType,
            // CONSTANT_Module, CONSTANT_Package
            7 | 8 | 16 | 19 | 20 => {
                if pos + 2 > data.len() {
                    break;
                }
                pos += 2;
            }
            // CONSTANT_MethodHandle
            15 => {
                if pos + 3 > data.len() {
                    break;
                }
                pos += 3;
            }
            // Unknown/invalid tag — bail to avoid misparse
            _ => break,
        }
        i += 1;
    }

    if has_server && has_client {
        ClassScan::Both
    } else if has_server {
        ClassScan::HasServerContent
    } else if has_client {
        ClassScan::HasClientApi
    } else {
        ClassScan::Neither
    }
}

// ── Helpers ───────────────────────────────────────────────────────

#[inline]
fn read_u16_be(bytes: &[u8]) -> u16 {
    u16::from_be_bytes([bytes[0], bytes[1]])
}

/// Check if `haystack` contains `needle` at any position.
#[inline]
fn bytes_contains(haystack: &[u8], needle: &[u8]) -> bool {
    haystack.windows(needle.len()).any(|w| w == needle)
}

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

    #[test]
    fn test_read_u16_be() {
        assert_eq!(read_u16_be(&[0x00, 0x01]), 1);
        assert_eq!(read_u16_be(&[0x01, 0x00]), 256);
        assert_eq!(read_u16_be(&[0xFF, 0xFF]), 65535);
    }

    #[test]
    fn test_bytes_contains() {
        assert!(bytes_contains(b"hello world", b"world"));
        assert!(!bytes_contains(b"hello world", b"foo"));
        assert!(bytes_contains(b"net/minecraft/server/MinecraftServer", b"net/minecraft/server/"));
        assert!(bytes_contains(b"net/minecraft/client/Minecraft", b"net/minecraft/client/"));
    }

    #[test]
    fn test_invalid_class_file() {
        assert_eq!(scan_single_class(&[0x00; 4]), ClassScan::Neither);
        assert_eq!(scan_single_class(&[]), ClassScan::Neither);
    }

    #[test]
    fn test_minimal_class_file() {
        // Valid magic + zero constant pool count — should not crash
        let data = [0xCA, 0xFE, 0xBA, 0xBE, 0x00, 0x00, 0x00, 0x3D, 0x00, 0x00];
        assert_eq!(scan_single_class(&data), ClassScan::Neither);
    }

    #[test]
    fn test_empty_jar() {
        // Build an empty valid zip
        let buf = {
            let mut cursor = Cursor::new(Vec::new());
            let zip = zip::ZipWriter::new(&mut cursor);
            zip.finish().unwrap();
            cursor.into_inner()
        };
        assert_eq!(scan_jar_for_side(&buf), ScanResult::NoClearSide);
    }
}