DiskImageStore.swift

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

import Shared
import UIKit
import Deferred
import XCGLogger

private var log = XCGLogger.default

private class DiskImageStoreErrorType: MaybeErrorType {
    let description: String
    init(description: String) {
        self.description = description
    }
}

/**
 * Disk-backed key-value image store.
 */
open class DiskImageStore {
    fileprivate let files: FileAccessor
    fileprivate let filesDir: String
    fileprivate let queue = DispatchQueue(label: "DiskImageStore")
    fileprivate let quality: CGFloat
    fileprivate var keys: Set<String>

    required public init(files: FileAccessor, namespace: String, quality: Float) {
        self.files = files
        self.filesDir = try! files.getAndEnsureDirectory(namespace)
        self.quality = CGFloat(quality)

        // Build an in-memory set of keys from the existing images on disk.
        var keys = [String]()
        if let fileEnumerator = FileManager.default.enumerator(atPath: filesDir) {
            for file in fileEnumerator {
                keys.append(file as! String)
            }
        }
        self.keys = Set(keys)
    }

    /// Gets an image for the given key if it is in the store.
    open func get(_ key: String) -> Deferred<Maybe<UIImage>> {
        return deferDispatchAsync(queue) { () -> Deferred<Maybe<UIImage>> in
            if !self.keys.contains(key) {
                return deferMaybe(DiskImageStoreErrorType(description: "Image key not found"))
            }

            let imagePath = URL(fileURLWithPath: self.filesDir).appendingPathComponent(key)
            if let data = try? Data(contentsOf: imagePath),
                   let image = UIImage.imageFromDataThreadSafe(data) {
                return deferMaybe(image)
            }

            return deferMaybe(DiskImageStoreErrorType(description: "Invalid image data"))
        }
    }

    /// Adds an image for the given key.
    /// This put is asynchronous; the image is not recorded in the cache until the write completes.
    /// Does nothing if this key already exists in the store.
    @discardableResult open func put(_ key: String, image: UIImage) -> Success {
        return deferDispatchAsync(queue) { () -> Success in
            if self.keys.contains(key) {
                return deferMaybe(DiskImageStoreErrorType(description: "Key already in store"))
            }

            let imageURL = URL(fileURLWithPath: self.filesDir).appendingPathComponent(key)
            if let data = UIImageJPEGRepresentation(image, self.quality) {
                do {
                    try data.write(to: imageURL, options: .noFileProtection)
                    self.keys.insert(key)
                    return succeed()
                } catch {
                    log.error("Unable to write image to disk: \(error)")
                }
            }

            return deferMaybe(DiskImageStoreErrorType(description: "Could not write image to file"))
        }
    }

    /// Clears all images from the cache, excluding the given set of keys.
    open func clearExcluding(_ keys: Set<String>) -> Success {
        return deferDispatchAsync(queue) { () -> Success in
            let keysToDelete = self.keys.subtracting(keys)

            for key in keysToDelete {
                let url = URL(fileURLWithPath: self.filesDir).appendingPathComponent(key)
                do {
                    try FileManager.default.removeItem(at: url)
                } catch {
                    log.warning("Failed to remove DiskImageStore item at \(url.absoluteString): \(error)")
                }
            }

            self.keys = self.keys.intersection(keys)

            return succeed()
        }
    }
}