Chat Billing Calculation

Chat Billing Calculation (block-based per session, per plan) You are implementing a monthly billing component for a chat-based AI platform. Each chat session record contains: user_id (string) input_tokens (non-negative integer) output_tokens (non-negative integer) plan in { "payg", "fixed" } Given all session records for a single month (a user may appear multiple times), implement calculate_monthly_billing(sessions) and return a list of strings representing each user’s total spend in the format: "user_id: $xx.xx" Requirements: Output must be sorted alphabetically by user_id. Amounts are formatted to 2 decimal places (rounded to cents). Users with $0.00 spend must still appear in the output. Pricing rules Shared rule: block billing per session (important) Tokens are billed in blocks of 100 tokens. Any partial block is free. blocks = floor(tokens / 100). Block rounding is done per session; remainders from different sessions cannot be combined. Rates (used for payg and fixed overage) Input tokens: $0.03 per 100 tokens Output tokens: $0.04 per 100 tokens For one session: cost_in = floor(input_tokens/100) * 0.03 cost_out = floor(output_tokens/100) * 0.04 Requirement 1: Pay-as-you-go (plan = payg) For all payg sessions of a user, compute block-based costs per session and sum them. Requirement 2: Fixed plan without switching (plan = fixed only) If a user is on the fixed plan for the month: Flat monthly fee: $15.00 Included allowance: 40,000 input tokens 20,000 output tokens Usage above the included allowance (“overage”) is charged at the payg rates, still respecting per-session block rounding. A convenient interpretation: Add $15.00. For the user’s fixed sessions, compute billable token usage after per-session block rounding: billable_input_tokens = sum(floor(input_tokens/100)*100) billable_output_tokens = sum(floor(output_tokens/100)*100) Overage: over_input = max(0, billable_input_tokens - 40000) over_output = max(0, billable_output_tokens - 20000) over_cost = (over_input/100)*0.03 + (over_output/100)*0.04 Requirement 3: Plan switching (both payg and fixed sessions exist) Users may switch plans within the billing cycle. Let: N = total number of sessions in the month (payg + fixed) F = number of fixed sessions r = F / N Then prorate the fixed plan fee and allowance by r: Fixed fee: 15.00 * r Included allowance: input: 40000 * r output: 20000 * r Compute: payg_cost: sum payg session costs (per-session blocks) fixed_usage: only from fixed sessions, after per-session block rounding, converted back to billable tokens (floor/100*100) fixed_overage_cost: charge only the portion exceeding the prorated allowance at payg rates Total per user: payg_cost + prorated_fixed_fee + fixed_overage_cost Input / Output (OA-friendly) Input sessions: array of strings formatted as: "user_id,input_tokens,output_tokens,plan" Output Array of strings: ["user_id: $xx.xx", ...] sorted by user_id. Example 1 (Req 1) Input: [ "userA,100,120,payg", "userB,150,100,payg", "userB,100,130,payg" ] Output: [ "userA: $0.07", "userB: $0.14" ] Example 2 (Req 2) Input: [ "userA,100,100,payg", "userB,20000,10000,fixed", "userB,25000,12000,fixed" ] Output: [ "userA: $0.07", "userB: $17.30" ] Example 3 (Req 3) Input: [ "userA,100,100,payg", "userA,100,100,payg", "userA,20000,10000,fixed", "userA,100,100,fixed", "userB,100,100,payg" ] Output: [ "userA: $7.71", "userB: $0.07" ] Example Input userA,100,120,payg userB,150,100,payg userB,100,130,payg Output userA: $0.07 userB: $0.14