def select_participant(wheel, participant):
"""
Register the selection of a participant by updating the weights of all participants for a given wheel
:param wheel: Wheel dictionary:
{
"id": string ID of the wheel (DDB Hash Key),
"name": string name of the wheel,
"participant_count": number of participants in the wheel,
}
:param participant: Participant dictionary:
{
"id": string ID of the participant (DDB Hash Key),
"name": string name of the participant,
"url": Participant's URL,
"wheel_id": string ID of the wheel the participant belongs to,
"weight": participant's weight in the selection algorithm
}
:return: None
"""
participant_count = wheel['participant_count']
# All other participants get a slice of that participant's weight while the participant is weighted to 0
if participant_count > 1:
weight_share = participant['weight'] / (participant_count - 1)
with WheelParticipant.batch_writer() as batch:
for p in WheelParticipant.iter_query(
KeyConditionExpression=Key('wheel_id').eq(wheel['id'])):
if p['id'] == participant['id']:
p['weight'] = 0
else:
p['weight'] += Decimal(weight_share)
batch.put_item(Item=p)
def delete_wheel(event):
"""
Deletes the wheel and all of its participants
:param event: Lambda event containing the API Gateway request path parameter wheel_id
{
"pathParameters":
{
"wheel_id": string ID of the wheel (DDB Hash Key)
}
}
:return: response dictionary
"""
wheel_id = event['pathParameters']['wheel_id']
# DynamoDB always succeeds for delete_item,
Wheel.delete_item(Key={'id': wheel_id})
# Clear out all participants of the wheel. Query will be empty if it was already deleted
with WheelParticipant.batch_writer() as batch:
query_params = {
'KeyConditionExpression': Key('wheel_id').eq(wheel_id),
'ProjectionExpression': 'id'
}
# We don't use the default generator here because we don't want the deletes to change the query results
for p in list(WheelParticipant.iter_query(**query_params)):
batch.delete_item(Key={'id': p['id'], 'wheel_id': wheel_id})
def suggest_participant(wheel):
"""
Suggest a participant given weights of all participants with randomization.
This is weighted selection where all participants start with a weight of 1,
so the sum of the weights will always equal the number of participants
:param wheel: Wheel dictionary:
{
"id": string ID of the wheel (DDB Hash Key),
"name": string name of the wheel,
"participant_count": number of participants in the wheel,
}
:return: ID of the suggested participant
"""
if wheel['participant_count'] == 0:
raise BadRequestError("Cannot suggest a participant when the wheel doesn't have any!")
query_params = {'KeyConditionExpression': Key('wheel_id').eq(wheel['id'])}
participants = WheelParticipant.iter_query(**query_params)
selected_total_weight = random.random() * float(sum([participant['weight'] for participant in participants]))
# We do potentially want to return the last participant just as a safeguard for rounding errors
participant = None
for participant in WheelParticipant.iter_query(**query_params):
selected_total_weight -= float(participant['weight'])
if selected_total_weight <= 0:
return participant['id']
return participant['id']
def update_participant(event):
"""
Update a participant's name and/or url
:param event: Lambda event containing the API Gateway request body including updated name or url and the
path parameters wheel_id and participant_id
{
"pathParameters":
{
"wheel_id": string ID of the wheel (DDB Hash Key)
"participant_id": string ID of the participant (DDB Hash Key)
},
"body":
{
"id": string ID of the participant (DDB Hash Key),
"name": string name of the wheel (optional),
"url: Valid URL for the participant (optional),
}
}
:return: response dictionary containing the updated participant object if successful
{
"body":
{
"id": string ID of the participant (DDB Hash Key),
"wheel_id": string ID of the wheel (DDB Hash Key),
"name": string name of the wheel,
"url: URL for the participant,
"created_at": creation timestamp,
"updated_at": updated timestamp,
}
}
"""
wheel_id = event['pathParameters']['wheel_id']
participant_id = event['pathParameters']['participant_id']
# Check that the participant exists
participant = WheelParticipant.get_existing_item(Key={
'id': participant_id,
'wheel_id': wheel_id
})
body = event['body']
params = {'updated_at': get_utc_timestamp()}
if not check_string(body.get('name', 'Not Specified')) or not check_string(
body.get('url', 'Not Specified')):
raise base.BadRequestError(
"Participants names and urls must be at least 1 character in length"
)
if 'name' in body:
params['name'] = body['name']
if 'url' in body:
params['url'] = body['url']
WheelParticipant.update_item(Key={
'id': participant_id,
'wheel_id': wheel_id
},
**to_update_kwargs(params))
participant.update(params)
return participant
def test_selection_cycle(mock_dynamodb, setup_data, mock_participant_table):
def get_participant_with_id(participants, target_id):
for p in participants:
if p['id'] == target_id:
return p
return None
rngstate = random.getstate()
random.seed(0) # Make the (otherwise pseudorandom) test repeatable.
participants = WheelParticipant.scan({})['Items']
wheel = setup_data['wheel']
total_weight_of_chosens = 0
num_iterations = 200
distro = {}
for participant in participants:
distro[participant['name']] = 0
for _ in range(0, num_iterations):
chosen_id = choice_algorithm.suggest_participant(wheel)
chosen_was = get_participant_with_id(participants, chosen_id)
chosen_was_weight = chosen_was['weight']
distro[chosen_was['name']] = distro[chosen_was['name']] + 1
choice_algorithm.select_participant(wheel, chosen_was)
participants = WheelParticipant.scan({})['Items']
chosen_now = get_participant_with_id(participants, chosen_id)
chosen_now_weight = chosen_now['weight']
assert chosen_was_weight > 0.0
assert chosen_now_weight == 0
total_weight_of_chosens += chosen_was_weight
total_weight = sum(
[participant['weight'] for participant in participants])
assert abs(total_weight - len(participants)) < epsilon
# Must match human-inspected reasonable values for the RNG seed defined
# above for number of times each participant was chosen, and the total
# weight of participants selected. These are a rough equivalent to
# ensuring that the sequence of chosen participants matches the observed
# test run.
dv = list(distro.values())
list.sort(dv)
human_observed_selection_counts = [26, 27, 27, 29, 29, 31, 31]
human_observed_total_weight = 317.8786415239279
assert dv == human_observed_selection_counts
assert abs(float(total_weight_of_chosens) -
human_observed_total_weight) < epsilon
# Put things back the way they were.
random.setstate(rngstate)
def wrap_participant_creation(wheel, participant):
participant['weight'] = 1
yield
count = 0
with WheelParticipant.batch_writer() as batch:
for p in WheelParticipant.iter_query(
KeyConditionExpression=Key('wheel_id').eq(wheel['id'])):
count += 1
Wheel.update_item(Key={'id': wheel['id']},
**to_update_kwargs({'participant_count': count}))
def create_participant(event):
"""
Create a participant
:param event: Lambda event containing the API Gateway request body including a name and a url and the
path parameter wheel_id
{
"pathParameters":
{
"wheel_id": string ID of the wheel (DDB Hash Key)
},
"body":
{
"name": participant name string,
"url: Valid URL for the participant,
}
}
:return: response dictionary containing new participant object if successful
{
"body":
{
"id": string ID of the participant (DDB Hash Key),
"wheel_id": string ID of the wheel (DDB Hash Key),
"name": string name of the wheel,
"url: URL for the participant,
"created_at": creation timestamp,
"updated_at": updated timestamp,
}
}
"""
wheel_id = event['pathParameters']['wheel_id']
body = event['body']
if not check_string(body.get('name', None)) or not check_string(
body.get('url', None)):
raise base.BadRequestError(
"Participants require a name and url which must be at least 1 character in length"
)
wheel = Wheel.get_existing_item(Key={'id': wheel_id})
create_timestamp = get_utc_timestamp()
participant = {
'wheel_id': wheel_id,
'id': get_uuid(),
'name': body['name'],
'url': body['url'],
'created_at': create_timestamp,
'updated_at': create_timestamp,
}
with choice_algorithm.wrap_participant_creation(wheel, participant):
WheelParticipant.put_item(Item=participant)
return participant
def delete_participant(event):
"""
Deletes the participant from the wheel and redistributes wheel weights
:param event: Lambda event containing the API Gateway request path parameters wheel_id and participant_id
{
"pathParameters":
{
"wheel_id": string ID of the wheel (DDB Hash Key)
"participant_id": string ID of the participant (DDB Hash Key)
},
}
:return: response dictionary
"""
wheel_id = event['pathParameters']['wheel_id']
participant_id = event['pathParameters']['participant_id']
# Make sure the wheel exists
wheel = Wheel.get_existing_item(Key={'id': wheel_id})
# REST-ful Deletes are idempotent and should not error if it's already been deleted
response = WheelParticipant.delete_item(Key={
'wheel_id': wheel_id,
'id': participant_id
},
ReturnValues='ALL_OLD')
if 'Attributes' in response:
choice_algorithm.on_participant_deletion(wheel, response['Attributes'])
def suggest_participant(event):
"""
Returns a suggested participant to be selected by the next wheel spin
:param event: Lambda event containing the API Gateway request path parameter wheel_id
{
"pathParameters":
{
"wheel_id": string ID of the wheel (DDB Hash Key)
},
}
:return: response dictionary containing a selected participant_id
{
"body":
{
"participant_id": string ID of the suggested participant (DDB Hash Key),
"rigged": True (if rigged, otherwise this key is not present)
}
}
"""
wheel_id = event['pathParameters']['wheel_id']
wheel = Wheel.get_existing_item(Key={'id': wheel_id})
if 'rigging' in wheel:
participant_id = wheel['rigging']['participant_id']
# Use rigging only if the rigged participant is still available
if 'Item' in WheelParticipant.get_item(Key={
'wheel_id': wheel_id,
'id': participant_id
}):
return_value = {'participant_id': participant_id}
# Only return rigged: True if we're not using hidden rigging
if not wheel['rigging'].get('hidden', False):
return_value['rigged'] = True
return return_value
return {'participant_id': choice_algorithm.suggest_participant(wheel)}
def select_participant(event):
"""
Indicates selection of a participant by the wheel. This will cause updates to the weights for all participants
or removal of rigging if the wheel is rigged.
:param event: Lambda event containing the API Gateway request path parameters wheel_id and participant_id
{
"pathParameters":
{
"wheel_id": string ID of the wheel to rig (DDB Hash Key)
"participant_id": string ID of the participant to rig (DDB Hash Key)
},
}
:return: response dictionary
"""
wheel_id = event['pathParameters']['wheel_id']
participant_id = event['pathParameters']['participant_id']
wheel = Wheel.get_existing_item(Key={'id': wheel_id})
participant = WheelParticipant.get_existing_item(Key={
'id': participant_id,
'wheel_id': wheel_id
})
choice_algorithm.select_participant(wheel, participant)
# Undo any rigging that has been set up
Wheel.update_item(Key={'id': wheel['id']},
UpdateExpression='remove rigging')
def list_participants(event):
"""
Gets the participants for the specified wheel_id
:param event: Lambda event containing the API Gateway request path parameter wheel_id
{
"pathParameters":
{
"wheel_id": string ID of the wheel (DDB Hash Key)
},
}
:return: response dictionary containing a list of participants
{
"body":
[
participant1,
participant2,
...
participantn,
]
}
"""
wheel_id = event['pathParameters']['wheel_id']
# Make sure the wheel exists
Wheel.get_existing_item(Key={'id': wheel_id})
return list(
WheelParticipant.iter_query(
KeyConditionExpression=Key('wheel_id').eq(wheel_id)))
def on_participant_deletion(wheel, participant):
"""
Normalize the remaining participant weights to account for participant removal.
The ratio is based on the following:
1) The participant should be at weight=1 when it leaves the system (which is the same as it arrived)
2) That difference should be split by the remaining participants proportional by weight
This ensures that 'weight=0' participants are still at weight=0 and that the sum of all
weights is equal to the number of participants, so new additions are treated fairly
:param wheel: Wheel dictionary:
{
"id": string ID of the wheel (DDB Hash Key),
"name": string name of the wheel,
"participant_count": number of participants in the wheel,
}
:param participant: Participant dictionary:
{
"id": string ID of the wheel (DDB Hash Key),
"name": string name of the wheel,
"url": Participant's URL,
"wheel_id": string ID of the wheel the participant belongs to,
}
:return: None
"""
total_weight = participant['weight']
for p in WheelParticipant.iter_query(
KeyConditionExpression=Key('wheel_id').eq(wheel['id'])):
total_weight += p['weight']
weight = participant['weight']
remaining_weight = total_weight - weight # <-- no longer presumes existing weight balance via 'int(participant_count)'
ratio = (1 + ((weight - 1) / remaining_weight)) if (
remaining_weight != 0) else 1
num_participants = Decimal(0)
with WheelParticipant.batch_writer() as batch:
for p in WheelParticipant.iter_query(
KeyConditionExpression=Key('wheel_id').eq(wheel['id'])):
if p['id'] != participant['id']:
# This is cast to a string before turning into a decimal because of rounding/inexact guards in boto3
p['weight'] = Decimal(
str(float(p['weight']) *
float(ratio))) if (remaining_weight != 0) else 1
batch.put_item(Item=p)
num_participants = num_participants + 1
Wheel.update_item(Key={'id': wheel['id']},
**to_update_kwargs(
{'participant_count': num_participants}))
def select_participant(wheel, participant):
"""
Register the selection of a participant by updating the weights of all participants for a given wheel
:param wheel: Wheel dictionary:
{
"id": string ID of the wheel (DDB Hash Key),
"name": string name of the wheel,
"participant_count": number of participants in the wheel,
}
:param participant: Participant dictionary:
{
"id": string ID of the participant (DDB Hash Key),
"name": string name of the participant,
"url": Participant's URL,
"wheel_id": string ID of the wheel the participant belongs to,
"weight": participant's weight in the selection algorithm
}
:return: None
"""
num_participants = 0
total_weight = Decimal(0)
for p in WheelParticipant.iter_query(
KeyConditionExpression=Key('wheel_id').eq(wheel['id'])):
num_participants = num_participants + 1
total_weight += p['weight']
# Factor is the number by which all weights must be multiplied
# so total weight will be equal to the number of participants.
factor = Decimal(num_participants) / total_weight
if num_participants > 1:
weight_share = participant['weight'] / Decimal(num_participants - 1)
with WheelParticipant.batch_writer() as batch:
# Redistribute and normalize the weights.
for p in WheelParticipant.iter_query(
KeyConditionExpression=Key('wheel_id').eq(wheel['id'])):
if p['id'] == participant['id']:
p['weight'] = 0
else:
p['weight'] += Decimal(weight_share)
p['weight'] *= factor
batch.put_item(Item=p)
Wheel.update_item(Key={'id': wheel['id']},
**to_update_kwargs(
{'participant_count': num_participants}))
def reset_wheel(wheel):
"""
Resets the weights of all participants in the wheel and updates the wheel's participant count
:param wheel: Wheel dictionary:
{
"id": string ID of the wheel (DDB Hash Key),
"name": string name of the wheel,
"participant_count": number of participants in the wheel,
}
:return: None
"""
count = 0
with WheelParticipant.batch_writer() as batch:
for p in WheelParticipant.iter_query(KeyConditionExpression=Key('wheel_id').eq(wheel['id'])):
p['weight'] = get_sub_wheel_size(p['name'])
batch.put_item(Item=p)
count += 1
Wheel.update_item(Key={'id': wheel['id']}, **to_update_kwargs({'participant_count': count}))
def set_up_test(setup_data, mock_participant_table):
# Select a participant to take everyone off their 1.0 scores.
choice_algorithm.select_participant(setup_data['wheel'],
setup_data['participants'][0])
# Adjust participants to different weights to take the wheel out of balance.
participants = mock_participant_table.query(KeyConditionExpression=Key(
'wheel_id').eq(setup_data['wheel']['id']))['Items']
with WheelParticipant.batch_writer() as batch:
for p in participants:
p['weight'] += Decimal(.15)
batch.put_item(Item=p)
# Confirm that the wheel is out of balance.
participants = mock_participant_table.query(KeyConditionExpression=Key(
'wheel_id').eq(setup_data['wheel']['id']))['Items']
participant_weights = [
participant['weight'] for participant in participants
]
total_weight = Decimal(0)
for weight in participant_weights:
total_weight += weight
assert abs(total_weight - Decimal(8.05)) < epsilon
