created basic project structure

backend/database.js

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+const sqlite3 = require('sqlite3').verbose();
+const path = require('path');
+
+// Database file path
+const DB_PATH = path.join(__dirname, '../data/gym-tracker.db');
+
+// Initialize database connection
+const db = new sqlite3.Database(DB_PATH, (err) => {
+  if (err) {
+    console.error('Error opening database:', err.message);
+  } else {
+    console.log('Connected to SQLite database at', DB_PATH);
+    initializeDatabase();
+  }
+});
+
+// Create tables if they don't exist
+function initializeDatabase() {
+  const createTableSQL = `
+    CREATE TABLE IF NOT EXISTS sessions (
+      id TEXT PRIMARY KEY,
+      date TEXT NOT NULL,
+      muscle_groups TEXT NOT NULL
+    )
+  `;
+
+  db.run(createTableSQL, (err) => {
+    if (err) {
+      console.error('Error creating sessions table:', err.message);
+    } else {
+      console.log('Sessions table ready');
+    }
+  });
+}
+
+// Get all sessions
+function getAllSessions(callback) {
+  const sql = 'SELECT * FROM sessions ORDER BY date DESC';
+  db.all(sql, [], (err, rows) => {
+    if (err) {
+      callback(err, null);
+    } else {
+      // Parse muscle_groups JSON string back to array
+      const sessions = rows.map(row => ({
+        id: row.id,
+        date: row.date,
+        muscle_groups: JSON.parse(row.muscle_groups)
+      }));
+      callback(null, sessions);
+    }
+  });
+}
+
+// Get session by ID
+function getSessionById(id, callback) {
+  const sql = 'SELECT * FROM sessions WHERE id = ?';
+  db.get(sql, [id], (err, row) => {
+    if (err) {
+      callback(err, null);
+    } else if (!row) {
+      callback(null, null);
+    } else {
+      const session = {
+        id: row.id,
+        date: row.date,
+        muscle_groups: JSON.parse(row.muscle_groups)
+      };
+      callback(null, session);
+    }
+  });
+}
+
+// Create new session
+function createSession(id, date, muscleGroups, callback) {
+  const sql = 'INSERT INTO sessions (id, date, muscle_groups) VALUES (?, ?, ?)';
+  const muscleGroupsJSON = JSON.stringify(muscleGroups);
+
+  db.run(sql, [id, date, muscleGroupsJSON], function(err) {
+    if (err) {
+      callback(err);
+    } else {
+      callback(null, { id, date, muscle_groups: muscleGroups });
+    }
+  });
+}
+
+// Update existing session
+function updateSession(id, date, muscleGroups, callback) {
+  const sql = 'UPDATE sessions SET date = ?, muscle_groups = ? WHERE id = ?';
+  const muscleGroupsJSON = JSON.stringify(muscleGroups);
+
+  db.run(sql, [date, muscleGroupsJSON, id], function(err) {
+    if (err) {
+      callback(err);
+    } else if (this.changes === 0) {
+      callback(new Error('Session not found'));
+    } else {
+      callback(null, { id, date, muscle_groups: muscleGroups });
+    }
+  });
+}
+
+// Delete session
+function deleteSession(id, callback) {
+  const sql = 'DELETE FROM sessions WHERE id = ?';
+
+  db.run(sql, [id], function(err) {
+    if (err) {
+      callback(err);
+    } else if (this.changes === 0) {
+      callback(new Error('Session not found'));
+    } else {
+      callback(null);
+    }
+  });
+}
+
+// Close database connection
+function closeDatabase() {
+  db.close((err) => {
+    if (err) {
+      console.error('Error closing database:', err.message);
+    } else {
+      console.log('Database connection closed');
+    }
+  });
+}
+
+module.exports = {
+  getAllSessions,
+  getSessionById,
+  createSession,
+  updateSession,
+  deleteSession,
+  closeDatabase
+};