export interface CompressionResult {
  originalInput: string;
  input: string;
  applied: boolean;
  method: string;
  tokensBefore: number;
  tokensAfter: number;
  tokensSaved: number;
  ratio: number;
  strategy: string;
  tags: string[];
  notes: string[];
}

interface CompressionOptions {
  enabled?: boolean;
  mode?: string;
  targetTokens?: number;
}

function estimateTokens(text: string): number {
  return Math.max(1, Math.ceil(text.length / 4));
}

function noCompression(input: string, method = 'none:none'): CompressionResult {
  const tokens = estimateTokens(input);
  return {
    originalInput: input,
    input,
    applied: false,
    method,
    tokensBefore: tokens,
    tokensAfter: tokens,
    tokensSaved: 0,
    ratio: 0,
    strategy: 'none',
    tags: [],
    notes: ['compression not applied'],
  };
}

export function compressContext(input: string, options: CompressionOptions = {}): CompressionResult {
  if (options.enabled === false || input.trim().length === 0) return noCompression(input);

  const tokensBefore = estimateTokens(input);
  const targetTokens = options.targetTokens ?? (tokensBefore > 12_000 ? 6_000 : 0);
  if (!targetTokens || tokensBefore <= targetTokens) return noCompression(input);

  const targetChars = Math.max(2_000, targetTokens * 4);
  const headChars = Math.floor(targetChars * 0.62);
  const tailChars = Math.max(800, targetChars - headChars);
  const omittedChars = Math.max(0, input.length - headChars - tailChars);
  const compacted = [
    input.slice(0, headChars).trimEnd(),
    '',
    `[ctxlean omitted ${omittedChars} chars from the middle; preserve exact visible context]`,
    '',
    input.slice(-tailChars).trimStart(),
  ].join('\n');

  const tokensAfter = estimateTokens(compacted);
  return {
    originalInput: input,
    input: compacted,
    applied: true,
    method: `ctxlean:${options.mode ?? 'auto'}`,
    tokensBefore,
    tokensAfter,
    tokensSaved: Math.max(0, tokensBefore - tokensAfter),
    ratio: tokensBefore > 0 ? Math.max(0, 1 - tokensAfter / tokensBefore) : 0,
    strategy: 'head-tail-excerpt',
    tags: ['subscription-bridge-safe', 'lossy-excerpt'],
    notes: ['long context compacted before model routing'],
  };
}