Add code to consider an empty (zero'd out) save as an invalid checksum

The gen 2 saves contain 2 markers per section (main/backup) to serve as canary values to see whether an actual save is stored.
Because after all: checksum 0 is a valid checksum in some cases. So to distinguish between a valid and a corrupted save, these markers are used.

And now we use it too! This will be necessary to detect that there's no save file in case the save was wiped (or a reproduction cart is used)

examples/Makefile

@@ -8,6 +8,7 @@ all:
 	$(MAKE) -C encodeText_gen2 $(MAKECMDGOALS) $(MAKEOVERRIDES)
 	$(MAKE) -C decodeSprite $(MAKECMDGOALS) $(MAKEOVERRIDES)
 	$(MAKE) -C addDistributionPoke $(MAKECMDGOALS) $(MAKEOVERRIDES)
+	$(MAKE) -C checkSave $(MAKECMDGOALS) $(MAKEOVERRIDES)
 	$(MAKE) -C gen2_gsball $(MAKECMDGOALS) $(MAKEOVERRIDES)
 	$(MAKE) -C gen2_addItem $(MAKECMDGOALS) $(MAKEOVERRIDES)
 	$(MAKE) -C gen2_removeItem $(MAKECMDGOALS) $(MAKEOVERRIDES)
@@ -21,6 +22,7 @@ clean:
 	$(MAKE) -C encodeText_gen2 clean
 	$(MAKE) -C decodeSprite clean
 	$(MAKE) -C addDistributionPoke clean
+	$(MAKE) -C checkSave clean
 	$(MAKE) -C gen2_gsball clean
 	$(MAKE) -C gen2_addItem clean
 	$(MAKE) -C gen2_removeItem clean

examples/checkSave/Makefile

@@ -0,0 +1,48 @@
+# # Compiler flags
+CXXFLAGS := -std=c++11 -fno-rtti -fno-exceptions -fno-unwind-tables -Wall -Wextra -I $(CURDIR) -I $(CURDIR)/../../include -g -Os
+
+# Source files directory
+SRC_DIR := .
+# Build directory
+BUILD_DIR := build
+
+# Source files (add more as needed)
+SRCS := $(shell find $(SRC_DIR) -type f -name '*.cpp')
+# Object files
+OBJS := $(patsubst $(SRC_DIR)/%.cpp,$(BUILD_DIR)/%.o,$(SRCS))
+
+LIBS := -lpokemegb
+ifeq ($(PNG_SUPPORT),1)
+	LIBS += -lpng
+endif
+
+# Ensure necessary directories exist
+# This function ensures the directory for the target exists
+define make_directory
+	@mkdir -p $(dir $@)
+endef
+
+# Target executable
+TARGET := ../../checkSave
+
+# Phony targets
+.PHONY: all clean
+
+# Default target
+all: $(TARGET)
+
+$(TARGET): $(OBJS)
+	$(CXX) $(CXXFLAGS) $^ -o $@ $(LIBS) -L ../../
+
+# Rule to compile source files
+$(BUILD_DIR)/%.o: $(SRC_DIR)/%.cpp | $(BUILD_DIR)
+	$(make_directory)
+	$(CXX) $(CXXFLAGS) -c $< -o $@
+
+# Create the build directory if it doesn't exist
+$(BUILD_DIR):
+	mkdir -p $(BUILD_DIR)
+
+# Clean rule
+clean:
+	rm -rf $(BUILD_DIR) $(TARGET)

examples/checkSave/main.cpp

@@ -0,0 +1,73 @@
+#include "gen1/Gen1GameReader.h"
+#include "gen2/Gen2GameReader.h"
+#include "RomReader.h"
+#include "SaveManager.h"
+#include "utils.h"
+
+#include <cstdio>
+#include <cstdint>
+#include <cstdlib>
+
+static void print_usage()
+{
+    printf("Usage: checkSave <path/to/rom.gbc> <path/to/file.sav>\n");
+}
+
+int main(int argc, char** argv)
+{
+    if(argc != 3)
+    {
+        print_usage();
+        return 1;
+    }
+    uint8_t* romBuffer;
+    uint8_t* savBuffer;
+    uint32_t romFileSize;
+    uint32_t savFileSize;
+
+    printf("rom: %s, save: %s\n", argv[1], argv[2]);
+
+    romBuffer = readFileIntoBuffer(argv[1], romFileSize);
+    if(!romBuffer)
+    {
+        fprintf(stderr, "ERROR: Couldn't read file %s\n", argv[1]);
+        return 1;
+    }
+    savBuffer = readFileIntoBuffer(argv[2], savFileSize);
+    if(!savBuffer)
+    {
+        fprintf(stderr, "ERROR: Couldn't read file %s\n", argv[2]);
+        free(romBuffer);
+        romBuffer = nullptr;
+        return 1;
+    }
+
+    GameboyCartridgeHeader cartridgeHeader;
+    BufferBasedRomReader romReader(romBuffer, romFileSize);
+    BufferBasedSaveManager saveManager(savBuffer, savFileSize);
+
+    readGameboyCartridgeHeader(romReader, cartridgeHeader);
+
+    // check if we're dealing with gen 1
+    const Gen1GameType gen1Type = gen1_determineGameType(cartridgeHeader);
+    const Gen2GameType gen2Type = gen2_determineGameType(cartridgeHeader);
+    if (gen1Type != Gen1GameType::INVALID)
+    {
+        Gen1GameReader gameReader(romReader, saveManager, gen1Type);
+        printf("%s", (gameReader.isMainChecksumValid()) ? "Game save valid!\n" : "Game save NOT valid!\n");
+    }
+    else if (gen2Type != Gen2GameType::INVALID)
+    {
+        Gen2GameReader gameReader(romReader, saveManager, gen2Type);
+        printf("%s", (gameReader.isMainChecksumValid()) ? "Main save valid!\n" : "Main save NOT valid!\n");
+        printf("%s", (gameReader.isBackupChecksumValid()) ? "Backup save valid!\n" : "Backup save NOT valid!\n");
+    }
+
+    free(romBuffer);
+    romBuffer = 0;
+
+    free(savBuffer);
+    savBuffer = 0;
+
+    return 0;
+}

src/gen2/Gen2GameReader.cpp

@@ -12,6 +12,9 @@
 #define EVENT_FLAGS_OFFSET_GOLDSILVER 0x261F
 #define EVENT_FLAGS_OFFSET_CRYSTAL 0x2600
 
+#define SAVE_CORRUPTION_CHECK1_EXPECTED_VALUE 99
+#define SAVE_CORRUPTION_CHECK2_EXPECTED_VALUE 127
+
 static const uint8_t crystalPicsBanks[] = {
     CRYSTAL_BANK_PICS_1 + 0,
     CRYSTAL_BANK_PICS_1 + 1,
@@ -203,6 +206,80 @@ static void updateMainChecksum(ISaveManager &saveManager, bool isCrystal)
     writeMainChecksum(saveManager, calculatedChecksum, isCrystal);
 }
 
+/**
+ * Gen 2 has 2 canary values for the main save to check whether SRAM actually contains a save
+ */
+static bool hasMainSave(ISaveManager& saveManager, bool isCrystal)
+{
+    // check first canary value
+    uint8_t saveCorruptionCheckValue1;
+    uint8_t saveCorruptionCheckValue2;
+
+    // this value is at the same offset for Gold/Silver/Crystal
+    saveManager.seekToBankOffset(1, 0x8);
+    saveManager.readByte(saveCorruptionCheckValue1);
+
+    // check second canary
+    // for this one the offset differs in crystal
+    if(isCrystal)
+    {
+        saveManager.seekToBankOffset(1, 0xD0F);
+    }
+    else
+    {
+        saveManager.seekToBankOffset(1, 0xD6B);
+    }
+    saveManager.readByte(saveCorruptionCheckValue2);
+
+    // for there to be an actual save, the canary values need to be set to these exact values.
+    // Taken from the PRET pokecrystal and pokegold projects
+    // https://github.com/pret/pokecrystal/blob/symbols/pokecrystal.sym
+    // https://github.com/pret/pokegold/blob/symbols/pokegold.sym
+    return (saveCorruptionCheckValue1 == SAVE_CORRUPTION_CHECK1_EXPECTED_VALUE) 
+        && (saveCorruptionCheckValue2 == SAVE_CORRUPTION_CHECK2_EXPECTED_VALUE);
+
+}
+
+/**
+ * Gen 2 has 2 canary values for the main save to check whether SRAM actually contains a save
+ */
+static bool hasBackupSave(ISaveManager& saveManager, bool isCrystal)
+{
+    // check first canary value
+    uint8_t saveCorruptionCheckValue1;
+    uint8_t saveCorruptionCheckValue2;
+
+    if(isCrystal)
+    {
+        saveManager.seekToBankOffset(0, 0x1208);
+    }
+    else
+    {
+        saveManager.seekToBankOffset(3, 0x1E38);
+    }
+    saveManager.readByte(saveCorruptionCheckValue1);
+
+    // check second canary
+    // for this one the offset differs in crystal
+    if(isCrystal)
+    {
+        saveManager.seekToBankOffset(0, 0x1F0F);
+    }
+    else
+    {
+        saveManager.seekToBankOffset(3, 0x1E6F);
+    }
+    saveManager.readByte(saveCorruptionCheckValue2);
+
+    // for there to be an actual save, the canary values need to be set to these exact values.
+    // Taken from the PRET pokecrystal and pokegold projects
+    // https://github.com/pret/pokecrystal/blob/symbols/pokecrystal.sym
+    // https://github.com/pret/pokegold/blob/symbols/pokegold.sym
+    return (saveCorruptionCheckValue1 == SAVE_CORRUPTION_CHECK1_EXPECTED_VALUE) 
+        && (saveCorruptionCheckValue2 == SAVE_CORRUPTION_CHECK2_EXPECTED_VALUE);
+
+}
+
 Gen2GameReader::Gen2GameReader(IRomReader &romReader, ISaveManager &saveManager, Gen2GameType gameType)
     : romReader_(romReader), saveManager_(saveManager), spriteDecoder_(romReader), gameType_(gameType)
 {
@@ -592,10 +669,17 @@ uint8_t Gen2GameReader::getPokedexCounter(PokedexFlag dexFlag)
     return result;
 }
 
+
+#include <cstdio>
 bool Gen2GameReader::isMainChecksumValid()
 {
     const bool isCrystal = isGameCrystal();
 
+    if(!hasMainSave(saveManager_, isCrystal))
+    {
+        return false;
+    }
+
     const uint16_t storedChecksum = readMainChecksum(saveManager_, isCrystal);
     const uint16_t calculatedChecksum = calculateMainRegionChecksum(saveManager_, isCrystal);
 
@@ -606,6 +690,11 @@ bool Gen2GameReader::isBackupChecksumValid()
 {
     const bool isCrystal = isGameCrystal();
 
+    if(!hasBackupSave(saveManager_, isCrystal))
+    {
+        return false;
+    }
+
     const uint16_t storedChecksum = readBackupChecksum(saveManager_, isCrystal);
     const uint16_t calculatedChecksum = calculateBackupRegionChecksum(saveManager_, isCrystal);
 
@@ -692,8 +781,6 @@ bool Gen2GameReader::getEventFlag(uint16_t flagNumber)
     return (byteVal & flag);
 }
 
-#include <cstdio>
-
 void Gen2GameReader::setEventFlag(uint16_t flagNumber, bool enabled)
 {
     const uint16_t saveOffset = (isGameCrystal()) ? EVENT_FLAGS_OFFSET_CRYSTAL : EVENT_FLAGS_OFFSET_GOLDSILVER;
@@ -713,7 +800,6 @@ void Gen2GameReader::setEventFlag(uint16_t flagNumber, bool enabled)
     {
         resultVal |= flag;
     }
-    printf("%s: flagNumber %hu, enabled: %d, orig byte: 0x%02x, new byte 0x%02x\n", __FUNCTION__, flagNumber, enabled, byteVal, resultVal);
 
     saveManager_.writeByte(resultVal);
 }