fee-delta-and-impact-simulation

name	Fee_Delta_and_Impact_Simulation
description	Solve dabstep Fee_Delta_and_Impact_Simulation questions: computing fee deltas when a fee's rate changes, identifying which merchants are affected by fee rule changes, and computing fee deltas when a merchant's MCC changes. Use when asked about fee impact, delta payments, rate changes, MCC code changes, or which merchants would be affected by modifying a fee rule.

Fee Delta and Impact Simulation

This skill covers three question types:

1. Rate-change delta: How much more/less would a merchant pay if a specific fee's rate changed?
2. Affected-merchant identification: Which merchants would be impacted if a fee rule's applicability changed?
3. MCC-change delta: How much more/less would a merchant pay if its MCC code changed?

CRITICAL: Submit Immediately After Computing

Do NOT spend extra turns verifying individual transactions or investigating unmatched transactions. Once you have the computed delta, print it and submit. It is completely normal for 40–70% of transactions to have no matching fee rule — this is expected behavior, not a bug.

Data Files

File	Purpose
fees.json	1000 fee rules with matching criteria and rates
payments.csv	138,236 transactions (all year 2023)
merchant_data.json	30 merchants with account_type, MCC, capture_delay, acquirers
acquirer_countries.csv	Maps acquirer names → country codes (rarely needed)
manual.md	Domain definitions (read first)

Key Domain Rules

Fee Formula

fee = fixed_amount + rate * transaction_value / 10000

"Relative fee" = the rate field. When a question says "relative fee changed to X", it means the rate changes from its current value to X.

Fee Rule Matching

A transaction/merchant matches a fee rule when ALL conditions hold:

Fee field	Condition
card_scheme	Exact match with transaction's card_scheme
account_type	[] (empty list) = applies to all; else merchant's account_type must be in the list
merchant_category_code	[] (empty list) = applies to all; else merchant's MCC must be in the list
capture_delay	null = applies to all; else see mapping below
monthly_fraud_level	null = applies to all; else see calculation below
monthly_volume	null = applies to all; else see calculation below
is_credit	null = applies to all; else must equal transaction's is_credit
aci	[] (empty list) or null = applies to all; else transaction's aci must be in the list
intracountry	null = applies to all; 1.0 = issuing_country == acquirer_country; 0.0 = issuing_country != acquirer_country

Critical: Empty list [] for account_type, merchant_category_code, and aci means "applies to all values".

When multiple fees match a transaction: Use the first matching fee — iterate fees.json in order (fees are sorted by ascending ID) and return the FIRST fee that satisfies ALL criteria. Do NOT pick the most-specific one or sum them all.

Capture Delay Mapping

Merchant capture_delay values must be mapped to fee rule categories:

def capture_delay_matches(merchant_val, fee_val):
    if fee_val is None: return True
    if merchant_val in ('immediate', 'manual'): return merchant_val == fee_val
    n = int(merchant_val)
    if fee_val == '<3': return n < 3
    if fee_val == '3-5': return 3 <= n <= 5
    if fee_val == '>5': return n > 5
    return False

Monthly Fraud Level Calculation

Compute per merchant per natural calendar month (using day_of_year):

fraud_volume = sum(eur_amount where has_fraudulent_dispute == True)
total_volume = sum(eur_amount)
fraud_ratio = fraud_volume / total_volume * 100  # as percentage

# Map to fee rule category:
# '<7.2%'    : ratio < 7.2
# '7.2%-7.7%': 7.2 <= ratio <= 7.7
# '7.7%-8.3%': 7.7 <= ratio <= 8.3
# '>8.3%'    : ratio > 8.3

Monthly Volume Calculation

Compute per merchant per natural calendar month:

monthly_total = sum(eur_amount)  # in euros

# Map to fee rule category:
# '<100k'  : total < 100,000
# '100k-1m': 100,000 <= total <= 1,000,000
# '1m-5m'  : 1,000,000 < total <= 5,000,000
# '>5m'    : total > 5,000,000

Month → day_of_year (2023, non-leap year)

Month	day_of_year range
January	1–31
February	32–59
March	60–90
April	91–120
May	121–151
June	152–181
July	182–212
August	213–243
September	244–273
October	274–304
November	305–334
December	335–365

Intracountry

Use acquirer_country directly from payments.csv (already resolved). Compare with issuing_country:

is_intracountry = (row['issuing_country'] == row['acquirer_country'])

---

Question Type 1: Rate-Change Delta

Question pattern: "In [period] what delta would [merchant] pay if the relative fee of the fee with ID=[X] changed to [Y]?"

CRITICAL: Do NOT Check First-Match for Rate-Change

For Q1, do NOT verify whether fee X is the first matching fee for each transaction. The question asks for the direct impact of changing fee X's rate. Simply filter all transactions that match fee X's criteria (card_scheme, account_type, MCC, capture_delay, is_credit, aci, intracountry) and compute the delta. This is intentional — you are computing the delta on all transactions to which fee X's criteria apply.

Step-by-step Algorithm

import json, pandas as pd

# 1. Load fee rule
with open('fees.json') as f:
    fees = json.load(f)
fee = next(f for f in fees if f['ID'] == X)
old_rate = fee['rate']
new_rate = Y  # from question

# 2. Load merchant properties
with open('merchant_data.json') as f:
    merchants = json.load(f)
merchant = next(m for m in merchants if m['merchant'] == MERCHANT_NAME)

# 3. Load payments and filter by merchant + time period
payments = pd.read_csv('payments.csv')
mask = (payments['merchant'] == MERCHANT_NAME)
# For a specific month: also filter day_of_year range
# For a full year: no day_of_year filter needed

# 4. Check merchant-level criteria (checked once, not per transaction)
if fee['account_type'] and merchant['account_type'] not in fee['account_type']:
    print("Delta = 0 (merchant does not match fee's account_type)")
elif fee['merchant_category_code'] and merchant['merchant_category_code'] not in fee['merchant_category_code']:
    print()
 fee[]      capture_delay_matches(merchant[], fee[]):
    ()
:
    
    mask &= (payments[] == fee[])
     fee[]   :
        mask &= (payments[] == fee[])
     fee[]:  
        mask &= payments[].isin(fee[])
     fee[]   :
         fee[] == :
            mask &= (payments[] == payments[])
        :
            mask &= (payments[] != payments[])

    
    
    

    matching = payments[mask]
    total_amount = matching[].()
    delta = (new_rate - old_rate) * total_amount / 
    ()

Important: Monthly Conditions for Rate-Change

When the fee has monthly_fraud_level or monthly_volume != null, these are merchant-level, month-level conditions. For a single-month query, calculate for that month. For a full-year query, the fee may apply in some months but not others — filter transactions month-by-month.

---

Question Type 2: Affected Merchants (Account Type Change)

Question pattern: "During 2023, imagine if the Fee with ID [X] was only applied to account type [T], which merchants would have been affected by this change?"

Interpretation

"Affected" = merchants whose fee X application STATUS CHANGES:

• Currently: if account_type = [] (applies to all), ALL matching merchants use fee X
• After change: only account_type T merchants use fee X
• Affected = merchants whose current status ≠ new status AND who have transactions matched by other fee criteria

Step-by-step Algorithm

# 1. Load fee rule
fee = next(f for f in fees if f['ID'] == X)

# 2. For each merchant, check if fee status changes
affected = []
for m in merchants:
    # Check merchant-level criteria (MCC, capture_delay) - excluding account_type
    if fee['merchant_category_code'] and m['merchant_category_code'] not in fee['merchant_category_code']:
        continue
    if fee['capture_delay'] is not None and not capture_delay_matches(m['capture_delay'], fee['capture_delay']):
        continue

    # Check if merchant has any matching transactions (transaction-level filters)
    m_payments = payments[payments['merchant'] == m['merchant']]
    mask = (m_payments['card_scheme'] == fee['card_scheme'])
    if fee['is_credit'] is not None:
        mask &= (m_payments['is_credit'] == fee['is_credit'])
    if fee['aci']:
        mask &= m_payments['aci'].isin(fee['aci'])
    if fee['intracountry'] is not :
         fee[] == :
            mask &= (m_payments[] == m_payments[])
        :
            mask &= (m_payments[] != m_payments[])

     mask.() == :
          

    
    currently_applies =  fee[]  m[]  fee[]
    would_apply = (m[] == T)  

     currently_applies != would_apply:
        affected.append(m[])

(.join((affected)))

---

Question Type 3: MCC-Change Delta

Question pattern: "Imagine the merchant [M] had changed its MCC code to [NEW_MCC] before 2023 started, what amount delta will it have to pay in fees for the year 2023?"

Algorithm

For each transaction, find the first matching fee under the OLD MCC and NEW MCC separately. Delta = total_fees(new_mcc) - total_fees(old_mcc).

Write this as a single complete script — do not split into multiple exploration steps to avoid hitting turn limits.

import json
import pandas as pd

# Load data
with open('fees.json') as f:
    fees = json.load(f)
with open('merchant_data.json') as f:
    merchants = json.load(f)
payments = pd.read_csv('payments.csv')

# Merchant info
merchant = next(m for m in merchants if m['merchant'] == MERCHANT_NAME)
old_mcc = merchant['merchant_category_code']
new_mcc = NEW_MCC  # from question
acct_type = merchant['account_type']
cap_delay = str(merchant['capture_delay'])

# Filter transactions for this merchant
df = payments[payments['merchant'] == MERCHANT_NAME].copy()

MONTH_RANGES = {
    1:(1,31), 2:(32,59), 3:(60,90), 4:(91,120),
    5:(121,151), 6:(152,181), 7:(182,212), 8:(213,243),
    9:(244,), :(,), :(,), :(,)
}

 ():
     m, (s, e)  MONTH_RANGES.items():
         s <= day <= e:  m
     

df[] = df[].apply(day_to_month)


 ():
     ratio < :  
     ratio <= :  
     ratio <= :  
     

 ():
     vol < :  
     vol <= :  
     vol <= :  
     

monthly_cats = {}
 month  (, ):
    md = df[df[] == month]
    fv = md[md[] == ][].()
    tv = md[].()
    ratio = (fv / tv * )  tv >   
    monthly_cats[month] = (get_fraud_cat(ratio), get_vol_cat(tv))

 ():
     fee_val  :  
     merchant_val  (, ):  merchant_val == fee_val
    n = (merchant_val)
     fee_val == :  n < 
     fee_val == :   <= n <= 
     fee_val == :  n > 
     

 ():
    
    cs = tx[]
    ic = tx[]
    aci = tx[]
    intra = (tx[] == tx[])
     fee  fees:  
         fee[] != cs: 
         fee[]  acct_type   fee[]: 
          capture_delay_matches(cap_delay, fee[]): 
         fee[]     fee[] != fraud_cat: 
         fee[]     fee[] != vol_cat: 
         fee[]  mcc   fee[]: 
         fee[]     fee[] != ic: 
         fee[]  aci   fee[]: 
         fee[]   :
             fee[] ==    intra: 
             fee[] ==   intra: 
         fee
     


total_old = 
total_new = 

 _, tx  df.iterrows():
    month = (tx[])
    fraud_cat, vol_cat = monthly_cats[month]
    amount = tx[]

    fee_old = find_first_matching_fee(tx, old_mcc, fraud_cat, vol_cat)
     fee_old:
        total_old += fee_old[] + fee_old[] * amount / 

    fee_new = find_first_matching_fee(tx, new_mcc, fraud_cat, vol_cat)
     fee_new:
        total_new += fee_new[] + fee_new[] * amount / 

delta = total_new - total_old
()

---

Common Pitfalls

1. [Q1] Adding first-match verification: For Rate-Change Delta, do NOT check whether fee X is the first matching fee for transactions. Simply filter by fee X's criteria. The question asks for the direct impact on all transactions matching that fee's rules.

2. [Q3] Using "most specific" fee: Always use the FIRST matching fee (by ascending ID order). Do NOT rank by specificity. The first fee in fees.json that matches all criteria is the applicable one.

3. [Q3] Splitting computation into multiple steps: Write the entire MCC-change calculation as ONE script. Iterating through 20,000–30,000 transactions is fast in Python. Debugging step-by-step exhausts turn limits.

4. Empty list ≠ null for account_type/MCC/aci: Both mean "applies to all", but the data uses [] not null for these list fields.

5. Intracountry uses payments.csv acquirer_country directly — do NOT look up acquirer_countries.csv per transaction.

6. Capture delay numeric mapping: merchant value "1" or "2" maps to '<3', "7" maps to '>5'. Always convert the numeric string to int before comparing.

7. Delta sign: delta = (new_fees - old_fees). Positive = merchant pays more; negative = merchant pays less.

8. Month filtering: January = day_of_year 1–31, December = 335–365. Use day_of_year column directly.

9. Monthly conditions are merchant+month scoped: Calculate fraud level / volume from ALL of that merchant's transactions in that natural month (not just the filtered subset).

10. Rate-change output format: Round to 14 decimal places. Example: f"{delta:.14f}".

11. MCC-change output format: Round to 6 decimal places. Example: f"{delta:.6f}".

12. Unmatched transactions are normal: 40–70% of transactions having no matching fee is expected. Do not investigate or try to fix this.