use serde::Deserialize;
use std::{collections::HashSet, fmt};

const MAX_MANIFEST_BYTES: usize = 2 * 1024 * 1024;
const CURRENT_SCHEMA_VERSION: u32 = 1;

#[derive(Debug, Deserialize)]
#[serde(rename_all = "camelCase")]
pub struct Manifest {
    pub schema_version: u32,
    pub id: String,
    pub display_name: String,
    pub minecraft: Minecraft,
    pub files: Vec<ManagedFile>,
}

#[derive(Debug, Deserialize)]
#[serde(rename_all = "camelCase")]
pub struct Minecraft {
    pub version: String,
    pub loader: Loader,
    pub java_major: u8,
}

#[derive(Debug, Deserialize)]
pub struct Loader {
    pub kind: String,
    pub version: String,
}

#[derive(Debug, Deserialize)]
#[serde(rename_all = "camelCase")]
pub struct ManagedFile {
    pub path: String,
    pub url: String,
    pub sha256: String,
    pub size: u64,
    pub policy: FilePolicy,
}

#[derive(Debug, Deserialize, PartialEq, Eq)]
#[serde(rename_all = "snake_case")]
pub enum FilePolicy {
    Managed,
    Seed,
}

#[derive(Debug)]
pub enum ManifestError {
    TooLarge,
    InvalidJson(serde_json::Error),
    Invalid(String),
}

impl fmt::Display for ManifestError {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::TooLarge => write!(formatter, "Manifest is larger than 2 MiB"),
            Self::InvalidJson(error) => write!(formatter, "Invalid manifest JSON: {error}"),
            Self::Invalid(message) => formatter.write_str(message),
        }
    }
}

pub fn validate_json(source: &str) -> Result<Manifest, ManifestError> {
    if source.len() > MAX_MANIFEST_BYTES {
        return Err(ManifestError::TooLarge);
    }

    let manifest = serde_json::from_str::<Manifest>(source).map_err(ManifestError::InvalidJson)?;
    validate(&manifest)?;
    Ok(manifest)
}

fn validate(manifest: &Manifest) -> Result<(), ManifestError> {
    if manifest.schema_version != CURRENT_SCHEMA_VERSION {
        return Err(ManifestError::Invalid(format!(
            "Unsupported schemaVersion {}; expected {CURRENT_SCHEMA_VERSION}",
            manifest.schema_version
        )));
    }
    if !is_identifier(&manifest.id) {
        return Err(ManifestError::Invalid("Profile id must contain only lowercase letters, numbers and hyphens".into()));
    }
    if manifest.display_name.trim().is_empty() {
        return Err(ManifestError::Invalid("Profile displayName cannot be empty".into()));
    }
    if manifest.minecraft.version.trim().is_empty()
        || manifest.minecraft.loader.kind.trim().is_empty()
        || manifest.minecraft.loader.version.trim().is_empty()
    {
        return Err(ManifestError::Invalid("Minecraft version and loader must be specified".into()));
    }
    if !(8..=25).contains(&manifest.minecraft.java_major) {
        return Err(ManifestError::Invalid("Unsupported Java major version".into()));
    }

    let mut paths = HashSet::new();
    for file in &manifest.files {
        match file.policy {
            FilePolicy::Managed | FilePolicy::Seed => {}
        }
        if !is_safe_relative_path(&file.path) {
            return Err(ManifestError::Invalid(format!("Unsafe file path: {}", file.path)));
        }
        if !paths.insert(&file.path) {
            return Err(ManifestError::Invalid(format!("Duplicate file path: {}", file.path)));
        }
        if !file.url.starts_with("https://") {
            return Err(ManifestError::Invalid(format!("File URL must use HTTPS: {}", file.path)));
        }
        if file.size == 0 {
            return Err(ManifestError::Invalid(format!("File has zero size: {}", file.path)));
        }
        if file.sha256.len() != 64 || !file.sha256.bytes().all(|byte| byte.is_ascii_hexdigit()) {
            return Err(ManifestError::Invalid(format!("Invalid SHA-256 for {}", file.path)));
        }
    }
    Ok(())
}

fn is_identifier(value: &str) -> bool {
    !value.is_empty()
        && value.len() <= 48
        && value.bytes().all(|byte| byte.is_ascii_lowercase() || byte.is_ascii_digit() || byte == b'-')
}

fn is_safe_relative_path(value: &str) -> bool {
    !value.is_empty()
        && !value.starts_with('/')
        && !value.starts_with('\\')
        && !value.contains('\\')
        && !value.split('/').any(|part| part.is_empty() || part == "." || part == "..")
}

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

    const VALID: &str = r#"{
      "schemaVersion": 1,
      "id": "aeronautics",
      "displayName": "All of Create Aeronautics",
      "minecraft": { "version": "1.21.1", "loader": { "kind": "neoforge", "version": "21.1.248" }, "javaMajor": 21 },
      "files": [{
        "path": "mods/example.jar",
        "url": "https://cdn.shacraft.ru/aeronautics/example.jar",
        "sha256": "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef",
        "size": 42,
        "policy": "managed"
      }]
    }"#;

    #[test]
    fn accepts_a_safe_manifest() {
        assert!(validate_json(VALID).is_ok());
    }

    #[test]
    fn rejects_path_traversal() {
        assert!(validate_json(&VALID.replace("mods/example.jar", "../secrets.txt")).is_err());
    }

    #[test]
    fn rejects_insecure_downloads() {
        assert!(validate_json(&VALID.replace("https://", "http://")).is_err());
    }
}