fix: importing of overspend handling now accounts for YNAB 4 oddities (#446)

In YNAB 4, when the overspendHandling is set to "Confined", it affects all months until
it is explicitly set back to "AffectsBuffer".

EZ on the other hand uses AFFECT_AVAILABLE as default (as does YNAB 4 with "AffectsBuffer")
but only changes to AFFECT_ENVELOPE (= "Confined" on YNAB 4) when explicitly configured for
that month.

This commit introduces parsing logic that accounts for this difference in behaviour when
importing.

Author: morremeyer

docs/import.md

@@ -12,13 +12,13 @@ The following is **not yet supported** in Envelope Zero and will therefore be ig
 
 - Recurring transactions. They will be implemented with [milestone #5](https://github.com/envelope-zero/backend/milestone/5) and import supported with https://github.com/envelope-zero/backend/issues/379.
 - Payee rename rules are not yet supported in Envelope Zero (https://github.com/envelope-zero/backend/issues/373)
-- Different handling of overspend. In Envelope Zero, overspend is always carried over to the next months budget for the category (https://github.com/envelope-zero/backend/issues/327)
 
 The following **work differently** on Envelope Zero:
 
 - Date formatting. While YNAB 4 does date formatting per Budget, in Envelope Zero, the formatting is decided by the browser (https://github.com/envelope-zero/frontend/issues/145) or by the users configuration (https://github.com/envelope-zero/backend/issues/33)
 - Transactions always need to have a source and destination. Transactions that do not have a Payee set in YNAB 4 will be imported with the opposing account as „YNAB 4 Import - No Payee“. If an account or Payee named „YNAB 4 Import - No Payee“ already exists in your budget, it will be used for those transactions.
 - Transactions can not have an amount of 0 - if no money was moved, no transaction is needed. Any transaction with an amount of 0 will be ignored during the import.
+- In YNAB 4, setting the overspend handling for an Envelope to “Subtract it from next month's category balance” affects all future months for that envelope until you set it back to ”Subtract it from next month's ’Available to Budget‘”. In Envelope Zero, setting overspend handling to `AFFECT_ENVELOPE` (where it is subtracted from the balance of the envelope in the next month) only affects the month it is set on. All months where no overspend handling is configured default to subtract it from the “Available to Budget” sum of the month after. If you set overspend handling to “Subtract it from next month's category balance” for a category in YNAB 4 and never changed it back on any of the following months, during the import, EZ will keep this behaviour for all months until the one you're executing the import in. For following months, you will have to set it manually.
 
 ### How to import
 

pkg/importer/parser/ynab4/parse.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"regexp"
+	"sort"
 	"strings"
 	"time"
 
@@ -67,6 +68,9 @@ func Parse(f io.Reader) (types.ParsedResources, error) {
 		return types.ParsedResources{}, fmt.Errorf("error parsing budget allocations: %w", err)
 	}
 
+	// Translate YNAB overspend handling behaviour to EZ overspending handling behaviour
+	fixOverspendHandling(&resources)
+
 	return resources, nil
 }
 
@@ -369,18 +373,15 @@ func parseMonthlyBudgets(resources *types.ParsedResources, monthlyBudgets []Mont
 					continue
 				}
 
-				// There's two modes in YNAB4: Confined, which equals AFFECT_ENVELOPE in EZ
-				// and "AffectsBuffer", which equals AFFECT_AVAILABLE in EZ.
-				// Since AFFECT_AVAILABLE is the default, we can skip everything that does not
-				// lead to AFFECT_ENVELOPE.
-				if subCategoryBudget.OverspendingHandling != "Confined" {
-					continue
+				var mode models.OverspendMode = "AFFECT_AVAILABLE"
+				if subCategoryBudget.OverspendingHandling == "Confined" {
+					mode = "AFFECT_ENVELOPE"
 				}
 
 				resources.MonthConfigs = append(resources.MonthConfigs, types.MonthConfig{
 					Model: models.MonthConfig{
 						MonthConfigCreate: models.MonthConfigCreate{
-							OverspendMode: "AFFECT_ENVELOPE",
+							OverspendMode: mode,
 						},
 						Month: month,
 					},
@@ -393,3 +394,92 @@ func parseMonthlyBudgets(resources *types.ParsedResources, monthlyBudgets []Mont
 
 	return nil
 }
+
+// fixOverspendHandling translates the overspend handling behaviour of YNAB 4 into
+// the overspend handling of EZ. In YNAB 4, when the overspendHandling is set to "Confined",
+// it affects all months until it is explicitly set back to "AffectsBuffer".
+//
+// EZ on the other hand uses AFFECT_AVAILABLE as default (as does YNAB 4 with "AffectsBuffer")
+// but only changes to AFFECT_ENVELOPE (= "Confined" on YNAB 4) when explicitly configured for
+// that month.
+func fixOverspendHandling(resources *types.ParsedResources) {
+	// sorter is a map of category names to a map of envelope names to the month configs
+	sorter := make(map[string]map[string][]types.MonthConfig, 0)
+
+	// Sort by envelope
+	for _, monthConfig := range resources.MonthConfigs {
+		_, ok := sorter[monthConfig.Category]
+		if !ok {
+			sorter[monthConfig.Category] = make(map[string][]types.MonthConfig, 0)
+		}
+
+		_, ok = sorter[monthConfig.Category][monthConfig.Envelope]
+		if !ok {
+			sorter[monthConfig.Category][monthConfig.Envelope] = make([]types.MonthConfig, 0)
+		}
+
+		sorter[monthConfig.Category][monthConfig.Envelope] = append(sorter[monthConfig.Category][monthConfig.Envelope], monthConfig)
+	}
+
+	// New slice for final MonthConfigs
+	var monthConfigs []types.MonthConfig
+
+	// Fix handling for all envelopes
+	for _, category := range sorter {
+		for _, envelope := range category {
+			// Sort by time so that earlier months are first
+			sort.Slice(envelope, func(i, j int) bool {
+				return envelope[i].Model.Month.Before(envelope[j].Model.Month)
+			})
+
+			for i, mConfig := range envelope {
+				// If we are switching back to "Available for budget", we don't need to do anything
+				if mConfig.Model.OverspendMode == "AFFECT_AVAILABLE" || mConfig.Model.OverspendMode == "" {
+					continue
+				}
+
+				monthConfigs = append(monthConfigs, mConfig)
+
+				// Start with the next month since we already appended the current one
+				checkMonth := mConfig.Model.Month.AddDate(0, 1, 0)
+
+				// If this is the last month, we set all months including the one of today to "AFFECT_ENVELOPE"
+				// to preserve the YNAB 4 behaviour up to the switch to EZ
+				if i+1 == len(envelope) {
+					for ok := true; ok; ok = !checkMonth.After(time.Now()) {
+						monthConfigs = append(monthConfigs, types.MonthConfig{
+							Model: models.MonthConfig{
+								Month: checkMonth,
+							},
+							Category: mConfig.Category,
+							Envelope: mConfig.Envelope,
+						})
+
+						checkMonth = checkMonth.AddDate(0, 1, 0)
+					}
+
+					continue
+				}
+
+				// Set all months up to the next one with a configuration to "AFFECT_ENVELOPE"
+				for ok := !checkMonth.Equal(envelope[i+1].Model.Month); ok; ok = !checkMonth.Equal(envelope[i+1].Model.Month) {
+					monthConfigs = append(monthConfigs, types.MonthConfig{
+						Model: models.MonthConfig{
+							Month: checkMonth,
+							MonthConfigCreate: models.MonthConfigCreate{
+								OverspendMode: "AFFECT_ENVELOPE",
+							},
+						},
+						Category: mConfig.Category,
+						Envelope: mConfig.Envelope,
+					})
+
+					checkMonth = checkMonth.AddDate(0, 1, 0)
+				}
+			}
+		}
+	}
+
+	// Overwrite the original MonthConfigs with the fixed ones
+	resources.MonthConfigs = monthConfigs
+}