1 files changed, 145 insertions, 0 deletions
diff --git a/dhall/src/semantics/resolve/cache.rs b/dhall/src/semantics/resolve/cache.rs
new file mode 100644
index 0000000..3b57cdf
--- /dev/null
+++ b/dhall/src/semantics/resolve/cache.rs
@@ -0,0 +1,145 @@
+use std::env;
+use std::io::Write;
+use std::path::{PathBuf, Path};
+
+use crate::error::{CacheError, Error, ErrorKind};
+use crate::parse::parse_binary_file;
+use crate::semantics::{Import, TypedHir};
+use crate::syntax::Hash;
+use crate::syntax::{binary, Expr};
+use crate::Parsed;
+use std::fs::File;
+
+#[cfg(unix)]
+const ALTERNATE_ENV_VAR: &str = "HOME";
+
+#[cfg(windows)]
+const ALTERNATE_ENV_VAR: &str = "LOCALAPPDATA";
+
+fn alternate_env_var_cache_dir() -> Option<PathBuf> {
+    env::var(ALTERNATE_ENV_VAR)
+        .map(PathBuf::from)
+        .map(|env_dir| env_dir.join(".cache").join("dhall"))
+        .ok()
+}
+
+fn env_var_cache_dir() -> Option<PathBuf> {
+    env::var("XDG_CACHE_HOME")
+        .map(PathBuf::from)
+        .map(|cache_home| cache_home.join("dhall"))
+        .ok()
+}
+
+fn load_cache_dir() -> Result<PathBuf, CacheError> {
+    env_var_cache_dir()
+        .or_else(alternate_env_var_cache_dir)
+        .ok_or(CacheError::MissingConfiguration)
+}
+
+pub struct Cache {
+    cache_dir: Option<PathBuf>,
+}
+
+impl Cache {
+    pub fn new() -> Cache {
+        // Should warn that we can't initialize cache on error
+        let cache_dir = load_cache_dir().and_then(|path| {
+            std::fs::create_dir_all(path.as_path())
+                .map(|_| path)
+                .map_err(|e| CacheError::InitialisationError { cause: e })
+        });
+        Cache {
+            cache_dir: cache_dir.ok(),
+        }
+    }
+}
+
+impl Cache {
+    fn cache_file(&self, import: &Import) -> Option<PathBuf> {
+        self.cache_dir.as_ref()
+            .and_then(|cache_dir| import.hash.as_ref().map(|hash| (cache_dir, hash)))
+            .map(|(cache_dir, hash)| cache_dir.join(cache_filename(hash)))
+    }
+
+    fn search_cache_file(&self, import: &Import) -> Option<PathBuf> {
+        self.cache_file(import)
+            .filter(|cache_file| cache_file.exists())
+    }
+
+    fn search_cache(&self, import: &Import) -> Option<Result<Parsed, Error>> {
+        self.search_cache_file(import)
+            .map(|cache_file| parse_binary_file(cache_file.as_path()))
+    }
+
+    // Side effect since we don't use the result
+    fn delete_cache(&self, import: &Import) {
+        self.search_cache_file(import)
+            .map(|cache_file| std::fs::remove_file(cache_file.as_path()));
+    }
+
+    // Side effect since we don't use the result
+    fn save_expr(&self, import: &Import, expr: &Expr) {
+        self.cache_file(import)
+            .map(|cache_file| {
+                save_expr(cache_file.as_path(), expr)
+            });
+    }
+
+    pub fn caching_import<F, R>(&self, import: &Import, fetcher: F, mut resolver: R) -> Result<TypedHir, Error>
+        where F: FnOnce() -> Result<Parsed, Error>,
+              R: FnMut(Parsed) -> Result<TypedHir, Error> {
+        // Lookup the cache
+        self.search_cache(import)
+            // On cache found
+            .and_then(|cache_result| {
+                // Try to resolve the cache imported content
+                match cache_result.and_then(|parsed| resolver(parsed))
+                    .and_then(|typed_hir| check_hash(import.hash.as_ref().unwrap(), typed_hir))
+                {
+                    // Cache content is invalid (can't be parsed / can't be resolved / content sha invalid )
+                    Err(_) => {
+                        // Delete cache file since it's invalid
+                        self.delete_cache(import);
+                        // Result as there were no cache
+                        None
+                    }
+                    // Cache valid
+                    r => {
+                        Some(r)
+                    }
+                }
+            }).unwrap_or_else(|| {
+            // Fetch and resolve as provided
+            let imported = fetcher().and_then(resolver);
+            // Save in cache the result if ok
+            let _ = imported.as_ref().map(|(hir, _)| self.save_expr(import, &hir.to_expr_noopts()));
+            imported
+        })
+    }
+}
+
+fn save_expr(file_path: &Path, expr: &Expr) -> Result<(), Error> {
+    File::create(file_path)?
+        .write_all(binary::encode(expr)?.as_slice())?;
+    Ok(())
+}
+
+fn check_hash(hash: &Hash, typed_hir: TypedHir) -> Result<TypedHir, Error> {
+    if hash.as_ref()[..] != typed_hir.0.to_expr_alpha().hash()?[..] {
+        Err(Error::new(ErrorKind::Cache(CacheError::CacheHashInvalid)))
+    } else {
+        Ok(typed_hir)
+    }
+}
+
+fn cache_filename<A: AsRef<[u8]>>(v: A) -> String {
+    format!("1220{}", hex::encode(v.as_ref()))
+}
+
+impl AsRef<[u8]> for Hash {
+    fn as_ref(&self) -> &[u8] {
+        match self {
+            Hash::SHA256(sha) => sha.as_slice(),
+        }
+    }
+}