feat: Added a file cache

storage/file_cache.go

@@ -0,0 +1,62 @@
+package storage
+
+import (
+	"math/rand"
+	"os"
+)
+
+// A `FileCache` stores open file descriptors for all files that have been
+// requested, and returns them when requested
+type FileCache struct {
+	cacheSize      int
+	fileNames      map[string]int
+	indexesOfFiles map[int]string
+	files          []*os.File
+}
+
+func NewFileCache(cacheSize int) FileCache {
+	var c FileCache
+
+	c.cacheSize = cacheSize
+	c.fileNames = make(map[string]int)
+	c.indexesOfFiles = make(map[int]string)
+	c.files = make([]*os.File, cacheSize)
+
+	return c
+}
+
+// `FetchFile` takes in a file's name and returns its file descriptor
+//
+// If the file is not found in the cache, the file is added to the cache
+func (fc *FileCache) FetchFile(fileName string) (*os.File, error) {
+	// The file is already in the cache
+	if index, contains := fc.fileNames[fileName]; contains {
+		return fc.files[index], nil
+	}
+
+	// The file was not in the cache, try and insert it
+
+	// Random cache eviction
+	indexToRemove := rand.Intn(fc.cacheSize)
+
+	// Remove the old value
+	oldName, present := fc.indexesOfFiles[indexToRemove]
+
+	// If there is already a value, delete its mapping
+	if present {
+		delete(fc.fileNames, oldName)
+		delete(fc.indexesOfFiles, indexToRemove)
+	}
+
+	f, err := os.Open(fileName)
+	if err != nil {
+		return nil, err
+	}
+
+	// Insert the new value
+	fc.files[indexToRemove] = f
+	fc.fileNames[fileName] = indexToRemove
+	fc.indexesOfFiles[indexToRemove] = fileName
+
+	return f, nil
+}