import csv
from io import StringIO
from flask import Blueprint, render_template, redirect, request, url_for, flash, stream_with_context
from app.models import Reading, db
from flask_login import login_required, current_user
from datetime import datetime
from app.forms import DeleteForm
from app.insights import parse_range, readings_query, user_timezone

data = Blueprint('data', __name__)

@data.route('/', methods=['GET', 'POST'])
@login_required
def manage_data():
    form = DeleteForm()
    if request.method == 'POST':
        if not form.validate_on_submit():
            flash('Your form expired. Please try again.', 'danger')
            return redirect(url_for('data.manage_data'))
        # Handle CSV file upload
        file = request.files.get('file')
        if file and file.filename.endswith('.csv'):
            try:
                payload = file.read(2_000_001)
                if len(payload) > 2_000_000:
                    raise ValueError('CSV must be 2 MB or smaller.')
                csv_data = csv.reader(StringIO(payload.decode('utf-8-sig')))
                if next(csv_data, None) != ['Timestamp', 'Systolic', 'Diastolic', 'Heart Rate']:
                    raise ValueError('Use the column headings from a BP Tracker export.')
                readings_to_add = []
                for line, row in enumerate(csv_data, 2):
                    timestamp, systolic, diastolic, heart_rate = row
                    if not (50 <= int(systolic) <= 250 and 30 <= int(diastolic) <= 150 and 30 <= int(heart_rate) <= 200):
                        raise ValueError(f'Check the measurement values on row {line}.')
                    readings_to_add.append(Reading(
                        user_id=current_user.id,
                        timestamp=datetime.strptime(timestamp, '%Y-%m-%d %H:%M:%S'),
                        systolic=int(systolic),
                        diastolic=int(diastolic),
                        heart_rate=int(heart_rate),
                    ))
                db.session.bulk_save_objects(readings_to_add)
                db.session.commit()
                flash('Data imported successfully!', 'success')
            except Exception as e:
                db.session.rollback()
                flash(f'Error importing data: {str(e)}', 'danger')
        else:
            flash('Please upload a valid CSV file.', 'danger')
        return redirect(url_for('data.manage_data'))

    return render_template('data.html', form=form)

@data.route('/export', methods=['GET'])
@login_required
def export_data():
    import io
    from flask import Response
    user_id = current_user.id
    selected = None
    if any(request.args.get(key) for key in ('start_date', 'end_date', 'days')):
        try:
            selected = parse_range(request.args, user_timezone(), 30)
        except ValueError as error:
            return render_template('error.html', message=str(error)), 400

    def generate_csv():
        """Stream CSV rows to avoid loading all readings into memory."""
        # Write header
        output = io.StringIO()
        writer = csv.writer(output)
        writer.writerow(['Timestamp', 'Systolic', 'Diastolic', 'Heart Rate'])
        yield output.getvalue()
        output.seek(0)
        output.truncate(0)

        # Stream readings in chunks using yield_per
        readings = readings_query(user_id, selected).order_by(
            Reading.timestamp, Reading.id
        ).yield_per(500)

        for reading in readings:
            writer.writerow([
                reading.timestamp.strftime('%Y-%m-%d %H:%M:%S'),
                reading.systolic,
                reading.diastolic,
                reading.heart_rate,
            ])
            yield output.getvalue()
            output.seek(0)
            output.truncate(0)

    return Response(
        stream_with_context(generate_csv()),
        mimetype='text/csv',
        headers={'Content-Disposition': 'attachment; filename=readings.csv'}
    )