RFID: functional tag reading

Add a new class to handle the RFID module and associated data
in order to track active tags.
Update main with RFID initialization and a minimal usage to
check it is working as expected.

include/RFID.h

@@ -0,0 +1,91 @@
+//
+// Created by trotfunky on 10/09/23.
+//
+
+#ifndef JIN_BARBAPAPA_RFID_H
+#define JIN_BARBAPAPA_RFID_H
+
+#include <Arduino.h>
+#include <Wire.h>
+#include <SPI.h>
+#include <SD.h>
+#include <Metro.h>
+
+#include <MFRC522v2.h>
+#include <MFRC522DriverPinSimple.h>
+#include <MFRC522DriverSPI.h>
+
+#include "Pinout.h"
+
+/***
+ * Linked list containing the uids of the tags and the corresponding IDs.
+ */
+struct uidNode {
+	MFRC522Constants::Uid uid{};
+	uidNode* next = nullptr;
+	// IDs should be positive integers < 127 as the sign is used for direction.
+	int8_t tag_ID = 0;
+};
+
+class RFID {
+public:
+	RFID() : comData{}, sd(SD) {};
+	void init();
+	/***
+	 * Check if any new tags are present or have left. Only one event is reported
+	 * at a time, another call would be needed to get eventual concurrent events.
+	 * This updates the respective lists and returns the ID with the direction
+	 * of the change.
+	 * @return 0 if no change, +ID if new tag detected, -ID if the tag left.
+	 */
+	int8_t checkTags();
+private:
+	/* Used for "encrypted" communication with the tags. */
+	static MFRC522Constants::MIFARE_Key defaultKey;
+	static constexpr int maxTags = 2;
+	int currentActiveTags = 0;
+
+	/*
+	 * The linked list tracking active tags.
+	 * It is statically allocated so that the memory footprint is constant.
+	 */
+	uidNode activeTagsArray[maxTags];
+	uidNode* activeTags = nullptr;
+	uidNode* nextFreeTagSlot = activeTagsArray;
+
+	MFRC522DriverPinSimple driverPin{pin_NFC1_CS};
+	MFRC522DriverSPI driverSpi{driverPin};
+	MFRC522 mfrc{driverSpi};
+
+	/*
+	 * Array used to read and write blocks to the tags. Blocks are 16 bytes but
+	 * during reads a two-bytes CRC is added.
+	 */
+	byte comData[18];
+
+	SDClass &sd;
+	Metro checkGoneTimer{250};
+
+	/***
+	 * Add a tag to the static linked list.
+	 * @return The new node, or nullptr if all nodes are allocated.
+	 */
+	uidNode* addActiveTag(const MFRC522Constants::Uid &newTagUid);
+	/***
+	 * Remove goneNode from the list and optionally update its predecessor.
+	 * @param goneNode Node to remove from the list.
+	 * @param previousNode Node to update, can be nullptr if goneNode is the head.
+	 */
+	void removeActiveTag(uidNode* goneNode, uidNode* previousNode);
+
+	/**
+	 * Read block at blockADdr from tagUid and return if the read succeeded or not.
+	 * The read block will be in comData.
+	 * @param tagUid Uid of the tag to be read.
+	 * @param blockAddr Address of the block to be read.
+	 * @return True if read succeeded, false otherwise.
+	 */
+	bool readBlock(MFRC522Constants::Uid &tagUid, byte blockAddr);
+};
+
+#endif //JIN_BARBAPAPA_RFID_H