//! Incremental build cache.
//!
//! Stores BLAKE3 hashes of source files in `.el/build-cache.json` so the
//! build system can skip recompiling files that haven't changed.

use std::collections::HashMap;
use std::path::{Path, PathBuf};

use serde::{Deserialize, Serialize};

/// The on-disk structure of the build cache.
#[derive(Debug, Default, Clone, Serialize, Deserialize)]
pub struct BuildCache {
    /// Map of file path (relative to workspace root) → BLAKE3 hex hash.
    pub file_hashes: HashMap<String, String>,
}

impl BuildCache {
    /// Load the build cache from `.el/build-cache.json`.
    ///
    /// Returns an empty cache if the file doesn't exist yet.
    pub fn load(workspace_root: &Path) -> Self {
        let path = cache_path(workspace_root);
        if !path.exists() {
            return Self::default();
        }
        let text = std::fs::read_to_string(&path).unwrap_or_default();
        serde_json::from_str(&text).unwrap_or_default()
    }

    /// Persist the cache back to disk.
    pub fn save(&self, workspace_root: &Path) -> Result<(), std::io::Error> {
        let path = cache_path(workspace_root);
        if let Some(parent) = path.parent() {
            std::fs::create_dir_all(parent)?;
        }
        let json = serde_json::to_string_pretty(self)?;
        std::fs::write(&path, json)
    }

    /// Hash a single file and return the BLAKE3 hex string.
    pub fn hash_file(path: &Path) -> Result<String, std::io::Error> {
        let bytes = std::fs::read(path)?;
        Ok(hex_encode(blake3::hash(&bytes).as_bytes()))
    }

    /// Returns `true` if the file's current hash matches the cached hash.
    pub fn is_up_to_date(&self, rel_path: &str, current_hash: &str) -> bool {
        self.file_hashes
            .get(rel_path)
            .map(|cached| cached == current_hash)
            .unwrap_or(false)
    }

    /// Update the cached hash for a file.
    pub fn record(&mut self, rel_path: impl Into<String>, hash: impl Into<String>) {
        self.file_hashes.insert(rel_path.into(), hash.into());
    }

    /// Collect all `.el` source files under a directory recursively.
    pub fn collect_sources(root: &Path) -> Vec<PathBuf> {
        let mut sources = Vec::new();
        collect_el_files(root, &mut sources);
        sources.sort();
        sources
    }
}

fn cache_path(workspace_root: &Path) -> PathBuf {
    workspace_root.join(".el").join("build-cache.json")
}

fn collect_el_files(dir: &Path, out: &mut Vec<PathBuf>) {
    let Ok(entries) = std::fs::read_dir(dir) else {
        return;
    };
    for entry in entries.flatten() {
        let path = entry.path();
        if path.is_dir() {
            // Skip hidden dirs and build outputs
            let name = path.file_name().unwrap_or_default().to_string_lossy();
            if name.starts_with('.') || name == "dist" || name == "target" {
                continue;
            }
            collect_el_files(&path, out);
        } else if path.extension().map(|e| e == "el").unwrap_or(false) {
            out.push(path);
        }
    }
}

fn hex_encode(bytes: &[u8]) -> String {
    bytes.iter().map(|b| format!("{b:02x}")).collect()
}

// ── Tests ─────────────────────────────────────────────────────────────────────

#[cfg(test)]
mod tests {
    use super::*;
    use std::io::Write;
    use tempfile::TempDir;

    fn temp_dir() -> TempDir {
        tempfile::TempDir::new().unwrap()
    }

    #[test]
    fn test_cache_empty_by_default() {
        let dir = temp_dir();
        let cache = BuildCache::load(dir.path());
        assert!(cache.file_hashes.is_empty());
    }

    #[test]
    fn test_cache_save_and_load() {
        let dir = temp_dir();
        let mut cache = BuildCache::default();
        cache.record("src/main.el", "abc123");
        cache.save(dir.path()).unwrap();

        let loaded = BuildCache::load(dir.path());
        assert_eq!(loaded.file_hashes.get("src/main.el").map(|s| s.as_str()), Some("abc123"));
    }

    #[test]
    fn test_is_up_to_date() {
        let mut cache = BuildCache::default();
        cache.record("src/main.el", "deadbeef");

        assert!(cache.is_up_to_date("src/main.el", "deadbeef"));
        assert!(!cache.is_up_to_date("src/main.el", "different"));
        assert!(!cache.is_up_to_date("src/other.el", "deadbeef"));
    }

    #[test]
    fn test_hash_file() {
        let dir = temp_dir();
        let path = dir.path().join("test.el");
        std::fs::write(&path, b"let x = 1").unwrap();
        let hash1 = BuildCache::hash_file(&path).unwrap();
        let hash2 = BuildCache::hash_file(&path).unwrap();
        assert_eq!(hash1, hash2); // deterministic

        std::fs::write(&path, b"let x = 2").unwrap();
        let hash3 = BuildCache::hash_file(&path).unwrap();
        assert_ne!(hash1, hash3); // different content → different hash
    }

    #[test]
    fn test_collect_sources() {
        let dir = temp_dir();
        let src = dir.path().join("src");
        std::fs::create_dir(&src).unwrap();
        std::fs::write(src.join("main.el"), b"fn main() {}").unwrap();
        std::fs::write(src.join("lib.el"), b"fn helper() {}").unwrap();
        std::fs::write(src.join("README.md"), b"# README").unwrap();

        let sources = BuildCache::collect_sources(dir.path());
        assert_eq!(sources.len(), 2);
        assert!(sources.iter().any(|p| p.file_name().unwrap() == "main.el"));
        assert!(sources.iter().any(|p| p.file_name().unwrap() == "lib.el"));
    }
}