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+# Database Utilities – Grade Snapshot System
+
+## Overview
+
+This module implements a **ClickHouse-backed grade snapshot and diffing system**.
+It ingests grade data from an external API, persists **immutable snapshots**, tracks **stable entities** (users, classes, assignments), and computes **changes over time** (new / updated / removed grades).
+
+The design emphasizes:
+
+* **Idempotent ingestion**
+* **Historical accuracy**
+* **Efficient change detection**
+* **Append-only semantics** (ClickHouse-friendly)
+
+All functionality lives in the `database_utils` namespace.
+
+---
+
+## Core Concepts
+
+### 1. Stable Entities vs Snapshots
+
+| Concept                 | Description                                                |
+| ----------------------- | ---------------------------------------------------------- |
+| **User**                | A logical account                                          |
+| **Class**               | A course belonging to a user (stable across time)          |
+| **Assignment**          | A specific graded item within a class (stable across time) |
+| **Snapshot (response)** | A point-in-time capture of all grades returned by the API  |
+| **Grade history**       | Per-assignment grades linked to a snapshot                 |
+| **Diffs**               | Computed changes between two snapshots                     |
+
+Stable entities are **created once and reused**.
+Snapshots are **immutable** and **time-ordered**.
+
+---
+
+## UUID Utilities
+
+### `parse_uuid(string) → UUID`
+
+Parses a standard UUID string (`xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx`) into ClickHouse’s `UUID { high, low }` format.
+
+* Validates format
+* Throws on malformed input
+* Used everywhere UUIDs enter the DB
+
+### `uuid_to_string(UUID) → string`
+
+Converts ClickHouse UUIDs back into standard string format.
+
+---
+
+## Date Handling
+
+### `parse_date_to_clickhouse(string) → uint16_t`
+
+Converts an API date string (`YYYY-MM-DD`) into ClickHouse `Date` format (days since Unix epoch).
+
+* Empty or invalid dates → epoch (`0`)
+* Logs parsing failures instead of throwing
+
+---
+
+## Database Handle
+
+```cpp
+using CHClient = std::shared_ptr<clickhouse::Client>;
+```
+
+All functions accept a shared ClickHouse client to allow:
+
+* Connection reuse
+* Thread-safe sharing
+* Easy dependency injection
+
+---
+
+## User Operations
+
+### `get_all_users()`
+
+Returns all users with:
+
+* `user_id`
+* `username`
+* `password`
+
+Primarily for administration/debugging.
+
+---
+
+### `register_user(username, password)`
+
+Inserts a new user row.
+
+⚠️ **Note:** Passwords are currently stored in plaintext.
+Hashing should be added before production use.
+
+---
+
+### `authenticate_user(username, password)`
+
+Returns `true` if a matching user exists.
+
+* Uses `count()` for minimal payload
+* Simple boolean authentication check
+
+---
+
+### `get_user_uuid(username)`
+
+Returns the user’s UUID if found.
+
+---
+
+## Snapshot Insertion Flow
+
+### `insert_grade_snapshot(user_id, api_response) → response_id`
+
+This is the **main ingestion pipeline**.
+
+#### Step-by-Step Flow
+
+1. **Insert `grade_responses`**
+
+   * One row per API fetch
+   * Contains metadata (`success`, `total_classes`, timestamp)
+
+2. **Fetch generated `response_id`**
+
+   * Most recent response for that user
+
+3. **Process each class**
+
+   * `get_or_create_class()`
+   * Ensures a stable `class_id`
+   * Links class to the response (`response_classes`)
+
+4. **Process assignments**
+
+   * `get_or_create_assignment()`
+   * Ensures stable `assignment_id`
+
+5. **Insert grade history**
+
+   * Batched inserts into `assignment_grade_history`
+   * Each row ties:
+
+     * response
+     * assignment
+     * score
+     * attempts
+
+Snapshots are **never updated**, only appended.
+
+---
+
+## Stable Entity Management
+
+### `get_or_create_class(user_id, class_data) → class_id`
+
+* Searches by `(user_id, class_name)`
+* If found:
+
+  * Updates metadata (teacher, period, category)
+* If not found:
+
+  * Inserts a new class record
+* Returns the stable `class_id`
+
+---
+
+### `get_or_create_assignment(user_id, class_id, assignment_data) → assignment_id`
+
+* Searches by `(user_id, class_id, assignment_name)`
+* Updates due date / major flag if it exists
+* Otherwise inserts a new assignment
+* Returns stable `assignment_id`
+
+---
+
+## Snapshot Loading
+
+### `load_latest_snapshot(user_id)`
+
+Loads the most recent snapshot for a user.
+
+Returns `std::nullopt` if none exists.
+
+---
+
+### `load_snapshot_by_id(response_id)`
+
+Loads a **fully hydrated snapshot**, including:
+
+* User
+* Classes
+* Assignments
+* Grades
+
+#### Result Structure
+
+```text
+GradeSnapshot
+├── response_id
+├── user_id
+├── classes[class_name] -> ClassRecord
+├── assignments[class::assignment] -> AssignmentRecord
+└── grades[assignment_id] -> GradeRecord
+```
+
+Used for:
+
+* Diffing
+* UI display
+* Historical comparisons
+
+---
+
+## Change Detection
+
+### `has_changes(user_id, new_api_response) → bool`
+
+Fast pre-check before inserting a new snapshot.
+
+Detects:
+
+* New assignments
+* Removed assignments
+* Score changes
+* Attempt changes
+
+If no prior snapshot exists → **changes detected**.
+
+---
+
+## Snapshot Diffing
+
+### `diff_snapshots(old, new) → vector<AssignmentDiff>`
+
+Computes **semantic differences** between two snapshots.
+
+#### Change Types
+
+| Type      | Meaning                         |
+| --------- | ------------------------------- |
+| `NEW`     | Assignment did not exist before |
+| `UPDATED` | Score or attempts changed       |
+| `REMOVED` | Assignment disappeared          |
+
+Each diff includes:
+
+* Assignment ID
+* Class name
+* Assignment name
+* Old grade (optional)
+* New grade
+
+---
+
+## Grade Update Logging
+
+### `insert_grade_updates(user_id, old_response_id, new_response_id, diffs)`
+
+Persists diffs into `grade_updates`.
+
+#### Features
+
+* Nullable old values for new assignments
+* Placeholder values for removed assignments
+* Compact enum encoding for change type
+* Batched insert for efficiency
+
+This table provides a **clear audit trail** of grade changes over time.
+
+---
+
+## Assignment Key Strategy
+
+```cpp
+"class_name::assignment_name"
+```
+
+Used as a human-stable lookup key when comparing snapshots.
+
+* Avoids relying on database IDs during diffing
+* Keeps logic resilient to ID reuse or refactors
+
+---
+
+## Design Guarantees
+
+* ✅ Snapshots are immutable
+* ✅ Stable IDs persist across time
+* ✅ Changes are explicitly logged
+* ✅ Efficient ClickHouse-friendly inserts
+* ⚠️ SQL string concatenation is used (should be parameterized later)
+* ⚠️ Password hashing not implemented
+
+# `database_utils` – Function Documentation
+
+---
+
+## **UUID Utilities**
+
+### `clickhouse::UUID parse_uuid(const std::string& str)`
+
+* **Purpose:** Converts a UUID string (`xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx`) into a ClickHouse `UUID` object.
+* **Input:** `str` – UUID string.
+* **Output:** `clickhouse::UUID` (high/low 64-bit parts).
+* **Behavior:**
+
+  * Throws `std::runtime_error` if string is not 36 characters or malformed.
+  * Removes hyphens and parses hex into two 64-bit integers.
+* **Usage:** Any place UUID strings need to be stored in ClickHouse.
+
+---
+
+### `std::string uuid_to_string(const clickhouse::UUID& u)`
+
+* **Purpose:** Converts a ClickHouse `UUID` back into a human-readable UUID string.
+* **Input:** `u` – ClickHouse UUID.
+* **Output:** Standard UUID string.
+* **Behavior:** Formats `high` and `low` 64-bit integers as a zero-padded 36-character string with hyphens.
+
+---
+
+## **Date Utilities**
+
+### `uint16_t parse_date_to_clickhouse(const std::string& date_str)`
+
+* **Purpose:** Converts a date string from the API into ClickHouse `Date` format.
+* **Input:** `date_str` – string in `YYYY-MM-DD` format.
+* **Output:** `uint16_t` representing days since 1970-01-01.
+* **Behavior:**
+
+  * Empty or invalid strings return `0` (epoch).
+  * Logs warnings for parse failures.
+* **Usage:** Assignments’ `due_date` conversion.
+
+---
+
+## **User Operations**
+
+### `std::vector<UserRecord> get_all_users(const CHClient& client)`
+
+* **Purpose:** Retrieves all users from the database.
+* **Input:** `client` – ClickHouse client.
+* **Output:** Vector of `UserRecord` (includes `user_id`, `login` info).
+* **Behavior:** Logs batch size and total retrieved.
+
+---
+
+### `bool register_user(const CHClient& client, const std::string& username, const std::string& password)`
+
+* **Purpose:** Inserts a new user into the database.
+* **Input:** `username`, `password`.
+* **Output:** `true` on success.
+* **Behavior:** Currently stores passwords in plaintext; logs success.
+
+---
+
+### `bool authenticate_user(const CHClient& client, const std::string& username, const std::string& password)`
+
+* **Purpose:** Checks if a user exists with given credentials.
+* **Input:** `username`, `password`.
+* **Output:** `true` if valid, `false` otherwise.
+* **Behavior:** Uses `SELECT count()` for boolean check. Logs results.
+
+---
+
+### `std::optional<clickhouse::UUID> get_user_uuid(const CHClient& client, const std::string& username)`
+
+* **Purpose:** Retrieves a user’s UUID based on their username.
+* **Input:** `username`.
+* **Output:** `optional<UUID>`; empty if user not found.
+* **Behavior:** Uses `LIMIT 1` for efficiency.
+
+---
+
+## **Class & Assignment Management**
+
+### `std::string get_or_create_class(const CHClient& client, const std::string& user_id, const api_utils::ClassGrades& class_data)`
+
+* **Purpose:** Ensures a stable `class_id` exists for a user.
+* **Input:** `user_id`, `class_data` (name, teacher, period, category).
+* **Output:** `class_id` as string.
+* **Behavior:**
+
+  * Searches for existing class.
+  * Updates metadata if found.
+  * Inserts new record if not found.
+  * Links class to the user in `user_classes`.
+
+---
+
+### `std::string get_or_create_assignment(const CHClient& client, const std::string& user_id, const std::string& class_id, const api_utils::AssignmentGrade& assignment_data)`
+
+* **Purpose:** Ensures a stable `assignment_id` exists within a class.
+* **Input:** `user_id`, `class_id`, `assignment_data` (name, dueDate, isMajorGrade).
+* **Output:** `assignment_id` as string.
+* **Behavior:**
+
+  * Updates existing assignment if found.
+  * Inserts a new assignment if not found.
+  * Uses `parse_date_to_clickhouse()` for due date.
+
+---
+
+## **Snapshot Insertion**
+
+### `std::string insert_grade_snapshot(const CHClient& client, const std::string& user_id, const api_utils::GradesResponse& api_response)`
+
+* **Purpose:** Inserts a complete snapshot of grades from the API.
+* **Input:** `user_id`, `api_response` (success flag, total classes, grades per class/assignment).
+* **Output:** `response_id` of the inserted snapshot; empty string on failure.
+* **Behavior:**
+
+  1. Inserts metadata into `grade_responses`.
+  2. Retrieves `response_id`.
+  3. Processes each class:
+
+     * Calls `get_or_create_class()`
+     * Links to response in `response_classes`
+  4. Processes each assignment:
+
+     * Calls `get_or_create_assignment()`
+     * Inserts grades into `assignment_grade_history`.
+* **Notes:** Immutable snapshots; append-only.
+
+---
+
+## **Snapshot Loading**
+
+### `std::optional<GradeSnapshot> load_latest_snapshot(const CHClient& client, const std::string& user_id)`
+
+* **Purpose:** Loads the most recent snapshot for a user.
+* **Output:** Fully populated `GradeSnapshot` or `nullopt` if none exists.
+* **Behavior:** Uses `fetched_at DESC LIMIT 1`.
+
+---
+
+### `std::optional<GradeSnapshot> load_snapshot_by_id(const CHClient& client, const std::string& response_id)`
+
+* **Purpose:** Loads a snapshot by `response_id`.
+* **Output:** `GradeSnapshot` including:
+
+  * Classes
+  * Assignments
+  * Grades
+* **Behavior:** Joins `user_classes`, `user_assignments`, `assignment_grade_history`, `response_classes`.
+
+---
+
+## **Diffing & Change Detection**
+
+### `bool has_changes(const CHClient& client, const std::string& user_id, const api_utils::GradesResponse& new_api_response)`
+
+* **Purpose:** Detects if new API response differs from the latest snapshot.
+* **Output:** `true` if changes exist, `false` otherwise.
+* **Checks for:**
+
+  * New assignments
+  * Removed assignments
+  * Score/attempt changes
+
+---
+
+### `std::vector<AssignmentDiff> diff_snapshots(const GradeSnapshot& old_snapshot, const GradeSnapshot& new_snapshot)`
+
+* **Purpose:** Returns detailed differences between two snapshots.
+* **Output:** Vector of `AssignmentDiff`.
+* **Change Types:** `NEW`, `UPDATED`, `REMOVED`.
+* **Behavior:** Compares old and new grades per assignment key.
+
+---
+
+### `void insert_grade_updates(const CHClient& client, const std::string& user_id, const std::string& old_response_id, const std::string& new_response_id, const std::vector<AssignmentDiff>& diffs)`
+
+* **Purpose:** Inserts diffs into `grade_updates` table.
+* **Behavior:**
+
+  * Maps `AssignmentDiff` to ClickHouse columns.
+  * Handles nullable old values for new assignments.
+  * Uses placeholder values for removed assignments.
+  * Logs number of inserted updates.
+