import { useMemo } from 'react';

export interface UseTokenCountOptions {
  /**
   * Per-1k-token USD cost (input price). Default 0 (no cost computed).
   * Override per-model, e.g. `0.00015` for gpt-4o-mini input.
   */
  costPer1kUsd?: number;
  /**
   * Chars-per-token approximation. Default 4 — the OpenAI rule-of-thumb
   * for English. Pass 3.5 for code-heavy contexts, ~6 for tightly
   * tokenised Indic / CJK scripts.
   */
  charsPerToken?: number;
}

export interface UseTokenCountValue {
  /** Approximate token count (rounded up). */
  tokens: number;
  /** Character count (raw length of `text`). */
  chars: number;
  /** Approximate USD cost. 0 if `costPer1kUsd` is 0 / undefined. */
  costUsd: number;
}

/**
 * `useTokenCount` — cheap, dependency-free token estimator for live
 * input previews.
 *
 * Wave 9.E.5. Not for billing. Hosts that need accurate counts should
 * call the model provider's tokenizer; this hook is for UI feedback in
 * composer surfaces ("you're typing ~412 tokens, ≈ \$0.0001").
 *
 * Pure function under the hood — wrapped in `useMemo` so the result
 * is stable across renders when `text` is unchanged.
 */
export function useTokenCount(
  text: string,
  options: UseTokenCountOptions = {},
): UseTokenCountValue {
  const { costPer1kUsd = 0, charsPerToken = 4 } = options;
  return useMemo(() => {
    const chars = text.length;
    const cpt = charsPerToken > 0 ? charsPerToken : 4;
    const tokens = chars === 0 ? 0 : Math.ceil(chars / cpt);
    const costUsd = costPer1kUsd > 0 ? (tokens / 1000) * costPer1kUsd : 0;
    return { tokens, chars, costUsd };
  }, [text, costPer1kUsd, charsPerToken]);
}