//! Read-only generated files. Each open captures an independent finite snapshot.
use crate::*;
use std::{collections::BTreeMap, io::ErrorKind as E};
type Generator = Arc<dyn Fn() -> Result<Vec<u8>> + Send + Sync>;
/// A static tree of generated files. Parent folders are implicit.
/// Generators must be cheap and must not block. Stat can invoke them as well as open.
#[derive(Default)]
pub struct Generated {
    files: BTreeMap<String, Generator>,
}
impl Generated {
    pub fn insert(
        &mut self,
        path: &str,
        generate: impl Fn() -> Result<Vec<u8>> + Send + Sync + 'static,
    ) {
        self.files.insert(resolve("/", path), Arc::new(generate));
    }
    fn directory(&self, path: &str) -> bool {
        path == "/"
            || self
                .files
                .keys()
                .any(|p| p.starts_with(&format!("{path}/")))
    }
}
struct Snapshot(Vec<u8>);
fn readonly<T>() -> Result<T> {
    Err(error(E::ReadOnlyFilesystem))
}
#[async_trait]
impl Layer for Generated {
    async fn stat(&self, path: &str, _follow: bool) -> Result<Stat> {
        if let Some(f) = self.files.get(path) {
            return Snapshot(f()?).stat().await;
        }
        if !self.directory(path) {
            return Err(error(E::NotFound));
        }
        Ok(Stat {
            directory: true,
            symlink: false,
            size: 0,
            accessed: None,
            modified: None,
        })
    }
    async fn list(&self, path: &str) -> Result<Vec<Entry>> {
        if !self.directory(path) {
            return Err(error(if self.files.contains_key(path) {
                E::NotADirectory
            } else {
                E::NotFound
            }));
        }
        let prefix = if path == "/" {
            "/".into()
        } else {
            format!("{path}/")
        };
        let mut entries = BTreeMap::new();
        for p in self.files.keys() {
            if let Some(rest) = p.strip_prefix(&prefix) {
                let (name, directory) = rest
                    .split_once('/')
                    .map_or((rest, false), |(name, _)| (name, true));
                entries.insert(name.to_string(), directory);
            }
        }
        Ok(entries
            .into_iter()
            .map(|(name, directory)| Entry {
                name,
                directory,
                symlink: false,
            })
            .collect())
    }
    async fn open(&self, path: &str, o: Open) -> Result<Arc<dyn File>> {
        if o.write || o.create || o.truncate {
            return readonly();
        }
        if let Some(f) = self.files.get(path) {
            if o.directory {
                return Err(error(E::NotADirectory));
            }
            return Ok(Arc::new(Snapshot(f()?)));
        }
        if self.directory(path) {
            return Memory::default().open("/", o).await;
        }
        Err(error(E::NotFound))
    }
    async fn mkdir(&self, _: &str) -> Result<()> {
        readonly()
    }
    async fn remove_file(&self, _: &str) -> Result<()> {
        readonly()
    }
    async fn remove_dir(&self, _: &str) -> Result<()> {
        readonly()
    }
    async fn rename(&self, _: &str, _: &str) -> Result<()> {
        readonly()
    }
}
#[async_trait]
impl File for Snapshot {
    async fn stat(&self) -> Result<Stat> {
        Ok(Stat {
            directory: false,
            symlink: false,
            size: self.0.len() as u64,
            accessed: None,
            modified: None,
        })
    }
    async fn read(&self, offset: u64, length: usize) -> Result<Vec<u8>> {
        let start = usize::try_from(offset)
            .unwrap_or(usize::MAX)
            .min(self.0.len());
        Ok(self.0[start..start.saturating_add(length).min(self.0.len())].to_vec())
    }
    async fn write(&self, _: u64, _: Vec<u8>) -> Result<usize> {
        readonly()
    }
    async fn append(&self, _: Vec<u8>) -> Result<usize> {
        readonly()
    }
    async fn set_size(&self, _: u64) -> Result<()> {
        readonly()
    }
    async fn sync(&self) -> Result<()> {
        Ok(())
    }
    async fn set_times(&self, _: Option<SystemTime>, _: Option<SystemTime>) -> Result<()> {
        readonly()
    }
}