"""BYL illustrative consolidation decisions v1.0.0. Standard library only.
Run: python model.py --output ./results
Not market observations, quotes, credit scoring or an underwriting model.
"""
import argparse, csv, json, math
from pathlib import Path

def payment(principal, annual_note_rate, months):
 r=annual_note_rate/1200
 return principal/months if r==0 else principal*r/-math.expm1(-months*math.log1p(r))

def fixed_plan(balance, annual_rate, monthly_payment):
 r=annual_rate/1200
 if balance>0 and monthly_payment<=balance*r:raise ValueError('Payment does not amortize balance')
 rows=[]
 while balance>1e-8:
  opening=balance;interest=opening*r;paid=min(monthly_payment,opening+interest);balance=max(0,opening+interest-paid)
  rows.append({'month':len(rows)+1,'opening':opening,'interest':interest,'payment':paid,'closing':balance})
  if len(rows)>1200:raise ValueError('Schedule exceeds model bound')
 return {'months':len(rows),'repayment':sum(x['payment'] for x in rows),'interest':sum(x['interest'] for x in rows),'schedule':rows}

def annualized_cashflow_rate(net_cash, monthly_payment, months):
 lo,hi=0.,1200.
 for _ in range(100):
  mid=(lo+hi)/2
  if payment(net_cash,mid,months)<monthly_payment:lo=mid
  else:hi=mid
 return (lo+hi)/2

def cards(scale=1.):
 return [{'name':'A','balance':3000*scale,'annual_rate':29.,'payment':120*scale,'limit':5000*scale},
         {'name':'B','balance':4000*scale,'annual_rate':24.,'payment':160*scale,'limit':8000*scale},
         {'name':'C','balance':3000*scale,'annual_rate':18.,'payment':120*scale,'limit':7000*scale}]

def baseline(accounts):
 plans=[fixed_plan(x['balance'],x['annual_rate'],x['payment']) for x in accounts]
 return {'balance':sum(x['balance'] for x in accounts),'monthly_payment_at_start':sum(x['payment'] for x in accounts),'repayment':sum(x['repayment'] for x in plans),'interest':sum(x['interest'] for x in plans),'months':max(x['months'] for x in plans),'plans':plans}

def break_even_note(net,fee,months,current_repayment):
 gross=net/(1-fee)
 if gross>current_repayment:return None
 lo,hi=0.,1200.
 for _ in range(100):
  mid=(lo+hi)/2
  if payment(gross,mid,months)*months<current_repayment:lo=mid
  else:hi=mid
 return (lo+hi)/2

def build():
 full=[]
 for amount in [5000,10000,20000]:
  accounts=cards(amount/10000);old=baseline(accounts)
  for rate in [15,22]:
   for n in [36,60]:
    for fee in [0,.03,.06]:
     gross=amount/(1-fee);m=payment(gross,rate,n);total=m*n
     full.append({'kind':'full','net_proceeds':amount,'annual_note_rate':rate,'months':n,'fee_fraction':fee,'principal':gross,'fee':gross-amount,'payment':m,'interest':total-gross,'repayment':total,'model_apr':annualized_cashflow_rate(amount,m,n),'current_repayment':old['repayment'],'current_months':old['months'],'current_payment_at_start':old['monthly_payment_at_start'],'total_future_repayment':total,'difference_vs_current':total-old['repayment']})
 partial=[];accounts=cards();old=baseline(accounts)
 for policy,allocation in [('Highest rate first',[3000,2000,0]),('Clear largest fixed payment first',[1000,4000,0])]:
  residual=[dict(a,balance=a['balance']-x) for a,x in zip(accounts,allocation)]
  residual_total=sum(fixed_plan(x['balance'],x['annual_rate'],x['payment'])['repayment'] for x in residual)
  residual_payment=sum(x['payment'] for x in residual if x['balance']>0)
  for rate in [15,22]:
   for n in [36,60]:
    net=5000;fee=.05;gross=net/(1-fee);m=payment(gross,rate,n);total=m*n+residual_total
    partial.append({'kind':'partial','policy':policy,'allocation':allocation,'net_proceeds':net,'annual_note_rate':rate,'months':n,'fee_fraction':fee,'principal':gross,'fee':gross-net,'payment':m,'interest':m*n-gross,'repayment':m*n,'model_apr':annualized_cashflow_rate(net,m,n),'residual_balances':[x['balance'] for x in residual],'residual_repayment':residual_total,'residual_initial_payment':residual_payment,'combined_initial_payment':m+residual_payment,'total_future_repayment':total,'current_repayment':old['repayment'],'difference_vs_current':total-old['repayment']})
 return {'version':'1.0.0','scope':'Hypothetical equal-month fixed-dollar card plans; no new purchases, promotional rate changes, minimum-payment rules, rollover, other charges, daily interest or discounting. Withheld fee financed once. Full precision; display rounding only. APR column is annualized monthly cash-flow IRR (12 times monthly rate), not certification of a legal APR disclosure.','cards':accounts,'baseline':old,'full':full,'partial':partial,'thresholds':[{'months':n,'fee_fraction':.03,'break_even_annual_note_rate':break_even_note(10000,.03,n,old['repayment']),'max_fee_fraction_at_15_percent_note':1-payment(10000,15,n)*n/old['repayment']} for n in [36,60]]}

def export(output):
 output.mkdir(parents=True,exist_ok=True);data=build()
 (output/'scenarios.json').write_text(json.dumps(data,indent=2)+'\n',encoding='utf-8')
 rows=data['full']+data['partial'];fields=list(dict.fromkeys(k for row in rows for k in row))
 with (output/'scenarios.csv').open('w',encoding='utf-8',newline='') as f:
  w=csv.DictWriter(f,fieldnames=fields);w.writeheader();w.writerows([{k:json.dumps(v) if isinstance(v,list) else v for k,v in row.items()} for row in rows])
 return data

if __name__=='__main__':
 p=argparse.ArgumentParser();p.add_argument('--output',type=Path,default=Path('results'));args=p.parse_args();d=export(args.output);print('Wrote36full and8partial illustrative scenarios to',args.output)