This the original design proposal for the generic import architecture.  The objective is to maximize code sharing between the QIF, HBCI and OFX modules.

The most important area of potential code sharing is the account and transaction matching code.  This code has 3 distinct roles:
-Finding the source account.
-Finding and eliminating transactions downloaded twice in the source account.
-Finding the destination account(s), and finding the matching transactions(s) if it/they exist(s).

The Online System specific module is responsible for any steps necessary for obtaining and processing the data.  During this process, it must use the generic-import module to:

-Identify and if necessary create the source account:  The account is identified using the account number for OFX and HBCI, and the account name or description for qif, if available.  The account number or identifier is stored in a kvp_string with key account_online_id.  The online system can use and format this frame however it sees fit, but it must be a kvp_string.  The account can be found using the function:
Account * gnc_import_select_account(char * account_online_id_value,
				    char * account_human_description,
				    gnc_commodity * new_account_default_commodity,
				    GNCAccountType new_account_default_type);
If no account is found with a matching online_id, the generic-module gives the user the option to select an existing account from a list, or create a new one.  The account_online_id is then stored in the selected or created account's kvp_frame.  The last 3 parameters of the function are defaults for new account creation and are optional.

-The Online System specific module is then responsible for creating a GnuCash transaction and adding the source split (associated with the source account, possibly created above), and filling it with as much information as it has at its disposal (much info is available for ofx, little for qif).  If a unique transaction id is available from the online system, it is stored in the split's kvp_frame, using key transaction_online_id.  No transaction matching or duplicate detection is done at this stage.

The generic module then receives the Transaction for the online system specific module using function:
void gnc_import_add_trans(Transaction *trans);
(We do not use GUID, because in all cases, the transaction was just created)
It uses the following enum:
enum gnc_match_probability {
	CERTAIN,
	PROBABLE,
	LIKELY,
	POSSIBLE,
	UNLIKELY,
	IMPOSSIBLE
	}

Here is the pseudocode of the gnc_import_add_trans function:
Variables:  matches (a list of possible match with likelihood)
	split_to_match = trans's first split.

In split_to_match's parent account; for each split where date >= split_to_match.date - 2 months:
	if transaction_online_id match
		add to matches using CERTAIN
		if preferences dictate: end search here
	if amount match
		if transaction_online_id exists but doesn't match
			add to matches using UNLIKELY (not IMPOSSIBLE, could be protocol change or prior error)
		else if memo match and date within 4 days
			add to matches using PROBABLE
		else if date within 24 hours
			add to matches using LIKELY
		else if date within 10 days
			add to matches using POSSIBLE
		else
			add to matches using UNLIKELY

Present the list of matches to the user in decreasing order of likelihood.  User has the option of selecting one of the match or creating a new transaction.
Add transaction_online_id to selected split
Erase from other CERTAIN splits
if transaction not balanced
	TODO:  gnc_balance_transaction(Transaction *trans) 
commit changes
return

gnc_balance_transaction((Transaction *trans) adds or matches other splits until the transaction is balanced, using whatever user interaction and heuristics are appropriate.  Since I haven't really used gnucash's current transaction matching before, I would like someone else to contribute the description of the process to match unbalanced transactions.

Remarks and things to remember:
-Credit card transactions can sometimes appear over a month after the purchase (clerk lost the paper, international transaction not always fast, etc.)
-void gnc_import_add_trans(Transaction *trans) should return as soon as possible (BEFORE user interaction) so that for systems that maintain a connection (such as HBCI) the user won't run into timeouts.  For example,  gnc_import_add_trans could check if its main dialog is open, and open it if it isn't, add to the list and return immediately.  The dialog is closed automatically once the list is empty.
-We may want to implement the function in such a way that it won't match any transaction that have been added as part of the current import process (flag them volatile or something).  This will solve the problems of multiple interac withdrawals in the same day for QIF, (possibly HBCI too?).
-The transaction passed to gnc_import_add_trans will have only one split for OFX and HBCI, but 1 or more for QIF.


