/**
 * Context compaction: heuristic fallback + optional one-shot LLM summary.
 */

const truncate = require('./truncate.js');

const CHAR_PER_TOKEN = 3;
const THRESHOLD = 0.68;
const MIN_SUMMARY = 80;
const COMPACT_PROMPT =
  'Summarize this coding-agent conversation. Use exactly these sections:\n' +
  '1. Goal\n' +
  '2. Done\n' +
  '3. Files / decisions\n' +
  '4. Open work\n' +
  '5. Next action\n' +
  'Be specific (paths, names, errors). Do not say the conversation was compacted.';

function contentChars(content) {
  if (content == null) return 0;
  if (typeof content === 'string') return content.length;
  if (Array.isArray(content)) {
    let n = 0;
    for (let i = 0; i < content.length; i++) {
      const p = content[i];
      if (p == null) continue;
      if (typeof p === 'string') n += p.length;
      else if (p.text) n += String(p.text).length;
      else if (p.type === 'image_url' || p.image_url) n += 1600;
      else n += JSON.stringify(p).length;
    }
    return n;
  }
  return JSON.stringify(content).length;
}

function messageChars(m) {
  if (!m) return 0;
  let n = contentChars(m.content);
  if (m.tool_calls) n += JSON.stringify(m.tool_calls).length;
  if (m.name) n += String(m.name).length;
  return n + 8;
}

function estimateTokens(messages, tools) {
  let n = 0;
  for (const m of messages || []) n += messageChars(m);
  n += JSON.stringify(tools || []).length;
  return Math.ceil(n / CHAR_PER_TOKEN);
}

function historyBudget(ctxSize, tools, attempt) {
  const cap = ctxSize > 0 ? Number(ctxSize) : 8192;
  const toolTok = Math.ceil(JSON.stringify(tools || []).length / CHAR_PER_TOKEN);
  const reserve = Math.max(384, Math.floor(cap * (0.18 + (Number(attempt) || 0) * 0.08)));
  return Math.max(240, Math.floor(cap * 0.72) - toolTok - reserve);
}

function shouldCompact(messages, tools, ctxSize) {
  const cap = ctxSize > 0 ? ctxSize : 8192;
  return estimateTokens(messages, tools) > Math.floor(cap * THRESHOLD);
}

function isOverflowError(err) {
  const s = String((err && err.message) || err || '');
  return /context window|context.?length|too many tokens|maximum context|prompt too (?:long|large)|prompt exceeds|ContextOverflow|reduce the prompt size/i.test(
    s
  );
}

function usage(messages, tools, ctxSize) {
  return snapshot(estimateTokens(messages, tools), ctxSize);
}

function snapshot(used, ctxSize) {
  const limit = ctxSize > 0 ? Number(ctxSize) : 8192;
  const n = Math.max(0, Math.round(Number(used) || 0));
  const pct = limit ? Math.min(100, Math.round((n / limit) * 1000) / 10) : 0;
  return {
    used: n,
    limit,
    pct,
    threshold: Math.round(THRESHOLD * 100),
    over: n > Math.floor(limit * THRESHOLD),
  };
}

function isDegenerate(summary) {
  const s = String(summary || '').trim();
  if (s.length < MIN_SUMMARY) return true;
  if (/conversation compacted/i.test(s)) return true;
  if (/^\s*\[earlier conversation compacted\]\s*$/i.test(s)) return true;
  return false;
}

function isRealUser(m) {
  if (!m || m.role !== 'user') return false;
  const c = String(m.content || '');
  if (c.indexOf('<system-reminder>') >= 0) return false;
  if (c.indexOf('[conversation summary]') >= 0) return false;
  if (c.indexOf('[memory]') === 0) return false;
  if (c.indexOf('[git status]') === 0) return false;
  return true;
}

function rebuildHistory(messages, summary) {
  const list = Array.isArray(messages) ? messages : [];
  const sys =
    list[0] && list[0].role === 'system'
      ? { role: 'system', content: list[0].content }
      : { role: 'system', content: '' };
  let lastUser = null;
  for (let i = list.length - 1; i >= 0; i--) {
    if (isRealUser(list[i])) {
      lastUser = { role: 'user', content: list[i].content };
      break;
    }
  }
  const tail = [];
  for (let i = list.length - 1; i >= 1 && tail.length < 6; i--) {
    const m = list[i];
    if (!m || m.role === 'system') continue;
    if (lastUser && m.role === 'user' && m.content === lastUser.content) continue;
    tail.unshift({ role: m.role, content: m.content, name: m.name, tool_call_id: m.tool_call_id });
  }
  const out = [sys, { role: 'user', content: '[conversation summary]\n' + String(summary || '').trim() }];
  if (lastUser) out.push(lastUser);
  return out.concat(tail);
}

function truncateMsg(m, maxChars) {
  if (!m) return m;
  const copy = Object.assign({}, m);
  const c = copy.content;
  if (typeof c === 'string' && c.length > maxChars) {
    copy.content = truncate.truncateWithMarker(c, maxChars);
  } else if (Array.isArray(c)) {
    copy.content = c.map((part) => {
      if (!part || typeof part !== 'object') return part;
      if (typeof part.text === 'string' && part.text.length > maxChars) {
        return Object.assign({}, part, { text: truncate.truncateWithMarker(part.text, maxChars) });
      }
      return part;
    });
  }
  return copy;
}

function lastRealUserIndex(list) {
  for (let i = list.length - 1; i >= 0; i--) {
    if (isRealUser(list[i])) return i;
  }
  return -1;
}

function heuristicCompact(messages, opts) {
  opts = opts || {};
  const budget = opts.budgetTokens || 6000;
  const aggressive = !!opts.aggressive;
  let keep = messages.slice();
  while (estimateTokens(keep, opts.tools) > budget && keep.length > 4) {
    let idx = keep.findIndex((m, i) => i > 0 && m.role === 'tool');
    if (idx < 0) idx = keep.findIndex((m, i) => i > 1 && m.role === 'assistant');
    if (idx < 0) break;
    keep.splice(idx, 1);
  }
  const maxMsg = aggressive ? 1200 : 3200;
  if (estimateTokens(keep, opts.tools) > budget) {
    const lastUser = lastRealUserIndex(keep);
    keep = keep.map((m, i) => (i === 0 || i === lastUser ? m : truncateMsg(m, maxMsg)));
  }
  if (estimateTokens(keep, opts.tools) > budget && keep.length > 3) {
    const head = keep[0];
    const lastUserIdx = lastRealUserIndex(keep);
    const lastUser = lastUserIdx >= 0 ? keep[lastUserIdx] : null;
    const tail = [];
    for (let i = keep.length - 1; i >= 1 && tail.length < (aggressive ? 2 : 4); i--) {
      if (i === lastUserIdx) continue;
      tail.unshift(keep[i]);
    }
    keep = [head, { role: 'user', content: '[conversation summary]\nEarlier turns were compacted to fit the model window.' }];
    if (lastUser) keep.push(lastUser);
    keep = keep.concat(tail);
    keep = keep.map((m, i) => (i === 0 ? m : truncateMsg(m, aggressive ? 700 : 1800)));
  }
  while (estimateTokens(keep, opts.tools) > budget && keep.length > 3) {
    const dropAt = keep.findIndex((m, i) => i > 1 && !isRealUser(m));
    if (dropAt < 0) break;
    keep.splice(dropAt, 1);
  }
  return keep;
}

function compact(messages, opts) {
  return heuristicCompact(messages, opts);
}

function transcript(messages) {
  return (messages || [])
    .map((m) => {
      const role = m && m.role ? m.role : 'unknown';
      const name = m && m.name ? ' ' + m.name : '';
      return role + name + ':\n' + String(m && m.content != null ? m.content : '');
    })
    .join('\n\n');
}

function autoContinue(messages) {
  const list = messages || [];
  const last = list[list.length - 1];
  if (!last) return null;
  if (last.role === 'tool' || last.role === 'assistant') {
    return {
      role: 'user',
      content:
        '<system-reminder>\nContinue the work from the summary. Do not wait for a new user request.\n</system-reminder>',
    };
  }
  return null;
}

function compactReminder() {
  return (
    '<system-reminder>\n' +
    'Context was compacted. Trust the summary and the last user request. Re-read files before further edits. Follow AGENTS.md if present.\n' +
    '</system-reminder>'
  );
}

async function compactWithLlm(messages, opts) {
  opts = opts || {};
  const fallback = () => heuristicCompact(messages, opts);
  const complete = opts.complete;
  if (typeof complete !== 'function') return fallback();
  const cap =
    opts.maxTranscriptChars ||
    Math.max(1500, Math.min(24000, Math.floor((opts.budgetTokens || 4000) * CHAR_PER_TOKEN * 0.45)));
  const body = truncate.truncateWithMarker(transcript(messages), cap);
  try {
    const result = await complete({
      history: [
        { role: 'system', content: 'Reply with the five summary sections only. No tools.' },
        { role: 'user', content: COMPACT_PROMPT + '\n\n---\n\n' + body },
      ],
      tools: [],
    });
    const summary = result && result.text;
    if (isDegenerate(summary)) return fallback();
    return rebuildHistory(messages, summary);
  } catch (_) {
    return fallback();
  }
}

module.exports = {
  CHAR_PER_TOKEN,
  THRESHOLD,
  MIN_SUMMARY,
  COMPACT_PROMPT,
  estimateTokens,
  historyBudget,
  shouldCompact,
  isOverflowError,
  usage,
  snapshot,
  isDegenerate,
  rebuildHistory,
  heuristicCompact,
  compact,
  autoContinue,
  compactReminder,
  compactWithLlm,
};