PatchSavefile:
	call CheckNotEmpty
	call InitializeRandomState
	seek sBuildNumber
	readhalfword #build
	ifne #build, 0xffff, .build_number_OK
	set #build, #zero
.build_number_OK
	call CheckSavefileIsValid
	call PrintSavefileMessage
	set #address, VersionInfo
	getbyteinc #result, #address
	getbyte #temp, #address
	mulacum #temp, #result, 0x100
	ifgt #build, 9999, .invalid_build
	ifgt #build, #temp, .future_build
	ifeq #build, #temp, .latest_build
	push #temp
	bufstring .savefile_build_string_1
	set #1, #build
	set #2, 4
	call PrintWithLeadingZeros
	bufstring .savefile_build_string_2
	printbuf
	menu #result, YesNoMenu
	jumpnz #result, .refused
	set #address, SavePatches
.loop
	gethalfwordinc #temp, #address
	jumpz #temp, .done
	getwordinc #patch, #address
	ifge #build, #temp, .loop
	push #address
	call #patch
	pop #address
	jump .loop
.done
	pop #build
	seek sBuildNumber
	writehalfword #build
	print .done_string
	exit #zero

.latest_build
	print .latest_build_string
	exit 1

.future_build
	bufstring .future_build_string_1
	set #1, #build
	set #2, 4
	call PrintWithLeadingZeros
	bufstring .future_build_string_2
	printbuf
.refused
	exit 1

.invalid_build
	print .invalid_build_string
	exit 1

.latest_build_string
	string "The savefile already corresponds to the current build of Pokémon Prism. No patching is necessary."
.future_build_string_1
	string "The savefile indicates it corresponds to build "
.future_build_string_2
	string ", which is higher than the current build of Pokémon Prism. The savefile cannot be patched." 
.invalid_build_string
	string "The savefile contains an invalid build number, and thus cannot be patched."
.savefile_build_string_1
	string "The savefile corresponds to build "
.savefile_build_string_2
	string ". Patch to the current build?"
.done_string
	string "The savefile was patched successfully."

CheckSavefileIsValid:
	; returns 0 if invalid, 1 if primary is valid, or 2 if backup is valid
	seek sValidCheck2
	readbyte #temp
	ifne #temp, 0x7f, .backup
	seek sValidCheck1
	readbyte #temp
	ifne #temp, 99, .backup
	; current file pointer points to sGameData now
	call CalculateSavefileChecksum
	seek sChecksum
	readhalfword #temp
	ifne #temp, #result, .backup
	iflt #build, 151, .main_OK
	seek sExtraData
	call CalculateExtraChecksum
	seek sExtraChecksum
	readhalfword #temp
	ifne #temp, #result, .backup
.main_OK
	set #savefile, 1
	return

.backup
	seek sBackupValidCheck2
	readbyte #temp
	ifne #temp, 0x7f, .invalid
	seek sBackupValidCheck1
	readbyte #temp
	ifne #temp, 99, .invalid
	; current file pointer points to sBackupGameData now
	call CalculateSavefileChecksum
	seek sBackupChecksum
	readhalfword #temp
	ifne #temp, #result, .invalid
	iflt #build, 151, .backup_OK
	seek sBackupExtraData
	call CalculateExtraChecksum
	seek sBackupExtraChecksum
	readhalfword #temp
	ifne #temp, #result, .invalid
.backup_OK
	set #savefile, 2
	return
	
.invalid
	set #savefile, #zero
	return

CalculateSavefileChecksum:
	; returns the checksum; calculates starting at the current address
	set #length, GAME_DATA_SIZE
DoSavefileChecksumCalculation:
	set #result, #zero
.loop
	readbyte #temp
	add #result, #temp
	decrement #length
	jumpnz #length, .loop
	and #result, 0xffff
	return

CalculateExtraChecksum:
	set #length, EXTRA_DATA_SIZE
	jump DoSavefileChecksumCalculation

PrintSavefileMessage:
	jumptable #savefile
	dw .invalid
	dw .ok
	dw .backup

.backup
	print .backup_string
.ok
	return

.invalid
	print .invalid_string
	menu #result, YesNoMenu
	set #savefile, 1
	retz #result
	exit #result

.backup_string
	string "The main save data in the savefile seems to be invalid, but the game can restore it from the backup. Unexpected errors may occur."
.invalid_string
	string "The savefile seems to be invalid or corrupted. Are you sure you want to continue?"

CheckNotEmpty:
	seek #zero
	getfilebyte #result
	set #length, SAVE_SIZE
.loop
	readbyte #temp
	subtract #temp, #result
	retnz #temp
	decrement #length
	jumpnz #length, .loop
	print .empty_savefile_string
	exit 1

.empty_savefile_string
	string "The savefile seems to be empty. Make sure to save the game from within the game before patching, not using emulator savestates. Also, remember that you only need to patch the savefile after updating to a new version; patching before starting a new game is not necessary."
