Rust

Large Persistent Hash Table Stored on Filesystem

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Demonstrates how to implement a large, persistent hash table that is stored on the filesystem and allows for quick retrieval using a hash key. Rust

// Let's say we would like to implement a persistent hash table with approximately 200,000 entries.
// We want something simple and straightforward.
// 
// The Chilkat Cache class is a solution that might fit.
// 
// Each hash table entry is a file.  Depending on the number of anticipated hash entries,
// the files can be contained in a single directory, or a collection of 256 directories,
// or a collection of 256x256 directories.

// There are 3 options:
// 
// Level 0: All cache files are in a single directory (the cache root).
// Level 1: Cache files are located in 256 sub-directories numbered 0 .. 255 directly under the cache root.
// Level 2: There are two levels of sub-directories under the cache root. 
//          The 1st level has 256 sub-directories numbered 0 .. 255 directly under the cache root. 
//          The 2nd level allows for up to 256 sub-directories (0..255) under each level-1 directory. 
//          Cache files are stored in the leaf directories. 

// For this example, given that we anticipate a larger number of hash entries, we choose a "level 2" cache.
let cache = chilkat::Cache::new();

// We can also spread the cache among several root directories, but for this example we'll only use one root directory.
// Call AddRoot once for each root directory.
cache.add_root("c:/temp/myCache");
cache.set_level(2);

// Add some key/values to the persisted hash table (i.e. the cache).
// The eTag is optional metadata.
let mut key = "apple".to_string();
let e_tag = String::new();
let mut item_value = "macos".to_string();
let _ = cache.save_text_no_expire(&key, &e_tag, &item_value).is_ok();

// Add more items..
let _ = cache.save_text_no_expire("microsoft", "", "windows").is_ok();
let _ = cache.save_text_no_expire("google", "", "android").is_ok();

// Lookup items:
key = "microsoft".to_string();
item_value = cache.fetch_text(&key).unwrap_or_default();
println!("{}: {}", key, item_value);

key = "apple".to_string();
item_value = cache.fetch_text(&key).unwrap_or_default();
println!("{}: {}", key, item_value);