1 files changed, 159 insertions, 0 deletions
diff --git a/backend/src/utils/calculate_budgets_accrual.rs b/backend/src/utils/calculate_budgets_accrual.rs
new file mode 100644
index 0000000..f14260d
--- /dev/null
+++ b/backend/src/utils/calculate_budgets_accrual.rs
@@ -0,0 +1,159 @@
+use crate::models::{budget_update::BudgetUpdate, category::Category, date_time_utc::DateTimeUtc};
+
+pub fn calculate_budgets_accrual(category: &Category, budget_updates: &[BudgetUpdate]) -> i64 {
+    let mut budget_updates = budget_updates
+        .iter()
+        .filter(|bu| bu.category_id == category.id)
+        .collect::<Vec<&BudgetUpdate>>();
+    budget_updates.sort_by_key(|bu| bu.date);
+    budget_updates.reverse();
+
+    let mut budgets_accrual = 0.0_f64;
+    let mut calculated_back_to_date = DateTimeUtc::now();
+
+    for budget_update in budget_updates {
+        let days_on_this_budget = calculated_back_to_date.day_diff(&budget_update.date);
+        if days_on_this_budget == 0_u32 {
+            continue;
+        }
+        let budget_per_day = budget_update.new_budget as f64 / budget_update.new_period as f64;
+        budgets_accrual += days_on_this_budget as f64 * budget_per_day;
+        calculated_back_to_date = budget_update.date;
+    }
+
+    budgets_accrual.floor() as i64
+}
+
+#[cfg(test)]
+mod test {
+    use chrono::{Days, Timelike, Utc};
+
+    use crate::models::{
+        budget_update::BudgetUpdate, category::Category, date_time_utc::DateTimeUtc,
+    };
+
+    use super::calculate_budgets_accrual;
+
+    #[test]
+    fn when_no_budget_updates_then_return_zero() {
+        let category = Category {
+            id: 0,
+            name: String::from("Groceries"),
+        };
+        let budget_updates = [];
+
+        let result = calculate_budgets_accrual(&category, &budget_updates);
+
+        assert_eq!(result, 0_i64);
+    }
+
+    #[test]
+    fn when_one_budget_update_for_another_category_then_return_zero() {
+        let category = Category {
+            id: 0,
+            name: String::from("Groceries"),
+        };
+        let budget_updates = [BudgetUpdate {
+            id: 0,
+            category_id: 1,
+            date: DateTimeUtc::now(),
+            new_budget: 100,
+            new_period: 1,
+        }];
+
+        let result = calculate_budgets_accrual(&category, &budget_updates);
+
+        assert_eq!(result, 0_i64);
+    }
+
+    #[test]
+    fn when_one_budget_update_then_return_quantity_times_days_in_past() {
+        let category = Category {
+            id: 0,
+            name: String::from("Groceries"),
+        };
+        let budget_updates = [BudgetUpdate {
+            id: 0,
+            category_id: 0,
+            date: DateTimeUtc::new(Utc::now().checked_sub_days(Days::new(120)).unwrap()),
+            new_budget: 100,
+            new_period: 1,
+        }];
+
+        let result = calculate_budgets_accrual(&category, &budget_updates);
+
+        assert_eq!(result, 12_000);
+    }
+
+    #[test]
+    fn when_two_budget_updates_then_calculates_as_expected() {
+        let category = Category {
+            id: 0,
+            name: String::from("Groceries"),
+        };
+        let budget_updates = [
+            BudgetUpdate {
+                id: 0,
+                category_id: 0,
+                date: DateTimeUtc::new(Utc::now().checked_sub_days(Days::new(60)).unwrap()),
+                new_budget: 200,
+                new_period: 1,
+            },
+            BudgetUpdate {
+                id: 0,
+                category_id: 0,
+                date: DateTimeUtc::new(Utc::now().checked_sub_days(Days::new(120)).unwrap()),
+                new_budget: 100,
+                new_period: 1,
+            },
+        ];
+
+        let result = calculate_budgets_accrual(&category, &budget_updates);
+
+        assert_eq!(result, 18_000);
+    }
+
+    #[test]
+    fn when_two_budget_updates_on_same_day_then_uses_latest_only() {
+        let category = Category {
+            id: 0,
+            name: String::from("Groceries"),
+        };
+        let budget_updates = [
+            BudgetUpdate {
+                id: 0,
+                category_id: 0,
+                date: DateTimeUtc::new(
+                    Utc::now()
+                        .checked_sub_days(Days::new(120))
+                        .unwrap()
+                        .with_hour(13)
+                        .unwrap(),
+                ),
+                new_budget: 200,
+                new_period: 1,
+            },
+            BudgetUpdate {
+                id: 0,
+                category_id: 0,
+                date: DateTimeUtc::new(
+                    Utc::now()
+                        .checked_sub_days(Days::new(120))
+                        .unwrap()
+                        .with_hour(12)
+                        .unwrap(),
+                ),
+                new_budget: 100,
+                new_period: 1,
+            },
+        ];
+        let mut budget_updates_reversed = budget_updates.clone();
+        budget_updates_reversed.reverse();
+
+        let result_leftways = calculate_budgets_accrual(&category, &budget_updates);
+        let result_rightways = calculate_budgets_accrual(&category, &budget_updates_reversed);
+
+        assert_eq!(result_leftways, 24_000);
+        assert_eq!(result_rightways, 24_000);
+    }
+}