/**
 * Session-scoped `codegraph_explore` call state (CG-17).
 *
 * What it holds: for ONE MCP session, per project it queried, what explore has
 * already returned — the files, the line ranges of source inside them, the bytes
 * they cost, and where in the session each call fell. Nothing else in the server
 * knows this today: every explore call is answered as if it were the first one,
 * which is why a 4th call happily re-serves the same spine it already sent
 * (#1500) and why the tier's call budget can only be *asked* for rather than
 * enforced. This module is the record those two behaviours are built on
 * (CG-18 cross-call dedup, CG-19 budget decay). It changes no response itself.
 *
 * Four constraints shape the design, all of them from how the daemon actually
 * runs:
 *
 *   1. **Per session, never persisted.** One instance is owned by an
 *      {@link ../mcp/session.MCPSession} and dies with the socket. A new agent
 *      session starts clean — dedup across sessions would suppress source the
 *      new agent has never seen.
 *   2. **Per project inside the session.** A session can query several projects
 *      by `projectPath`, so state is keyed by the RESOLVED project root
 *      (`cg.getProjectRoot()`), not by whatever path the agent typed.
 *   3. **Bounded.** A long-lived session must not grow without limit, so
 *      everything is capped — see {@link EXPLORE_SESSION_LIMITS}. Eviction drops
 *      DETAIL only: `callCount` and `responseBytes` keep counting past it, since
 *      decay (CG-19) reads the count and must not be reset by its own bound.
 *   4. **Daemon-safe.** The daemon shares ONE {@link ../mcp/tools.ToolHandler}
 *      (and a pool of worker threads) across every connected session, so this
 *      state can live neither on the handler nor in a worker. It lives on the
 *      session; the handler is handed it per call, and the record of what a call
 *      emitted travels back on the {@link ToolResult} so it can be recorded on
 *      the main thread whether dispatch ran in-process or on a worker.
 *
 * Over- vs under-reporting: where a bound forces a choice, this module keeps
 * FEWER ranges than were emitted, never more. A consumer that under-knows
 * re-serves something the agent already has (wasteful); one that over-knows
 * withholds source the agent never saw (a Read — the failure this whole area
 * exists to prevent).
 */

import * as path from 'path';

/**
 * Property on a {@link ../mcp/tools.ToolResult} carrying what an explore call
 * emitted. INTERNAL: `ToolHandler.execute` records it and deletes it before the
 * result reaches the wire, so the agent-facing response is unchanged. It is a
 * plain-object property (not a Symbol) on purpose — it has to survive the
 * structured clone back from a query-pool worker.
 */
export const EXPLORE_EMISSION_KEY = '_cgExploreEmission';

/**
 * Argument key carrying this session's prior-call view INTO a tool call. Same
 * reasoning as {@link EXPLORE_EMISSION_KEY}: it crosses the worker boundary, so
 * it must be a serializable property on the args object.
 */
export const EXPLORE_SESSION_VIEW_ARG = '_cgExploreSession';

/** An inclusive 1-based line span of a file that was emitted. */
export interface ExploreLineRange {
  start: number;
  end: number;
}

/** What one call emitted for one file. */
export interface ExploreFileEmission {
  /** Project-relative path, exactly as the response's file header spells it. */
  path: string;
  /** Coalesced line spans whose source was in the response. */
  ranges: ExploreLineRange[];
  /** Source chars emitted for this file (excludes headers / fences). */
  bytes: number;
  /**
   * Identity of the bytes those ranges were sliced from (CG-18). Cross-call
   * dedup withholds a span only when the file still hashes to this, so an edit
   * between two calls re-serves instead of pointing at source the agent holds a
   * now-wrong copy of. Absent = unprovable, which dedup treats as "re-serve".
   */
  fingerprint?: string;
  /** Set when ranges were dropped to stay under the per-file bound. */
  rangesTruncated?: boolean;
}

/** What one explore call emitted, as reported by the handler. */
export interface ExploreEmission {
  /** Resolved project root — the key state is filed under. */
  projectRoot: string;
  /** Normalized query text (post `normalizeQuerySpelling`). */
  query: string;
  files: ExploreFileEmission[];
  /** Source chars across all files. */
  sourceBytes: number;
  /** Total chars of the response the agent received. */
  responseBytes: number;
}

/** A recorded call: an emission plus where it fell in the session. */
export interface ExploreCallRecord extends ExploreEmission {
  /** 1-based call index within this session FOR THIS PROJECT. Survives eviction. */
  index: number;
}

/** Everything the session knows about one project. */
export interface ExploreProjectState {
  projectRoot: string;
  /** Explore calls made this session against this project, including evicted ones. */
  callCount: number;
  /** Response chars across every call, including evicted ones. */
  responseBytes: number;
  /** Retained call records, oldest first. Bounded — may omit early calls. */
  calls: ExploreCallRecord[];
}

/**
 * The bounded, serializable read-view handed to a tool call. Deliberately
 * smaller than the full state: only the most recent calls carry their ranges,
 * because that is what a dedup/decay decision reads and the whole thing is
 * structured-cloned to a worker on every call.
 */
export interface ExploreSessionView {
  projects: ExploreProjectState[];
}

/**
 * Memory bounds. Every one of them caps DETAIL; none caps the counters that
 * CG-19's decay reads.
 *
 * Sized against how sessions actually behave: an agent explores one project
 * (occasionally a second in a monorepo) and the tier call budget is 1–5, so the
 * retained window covers a whole realistic session and the caps only bite on
 * pathological ones.
 */
export const EXPLORE_SESSION_LIMITS = {
  /** Distinct projects kept per session; least-recently-used evicted first. */
  MAX_PROJECTS: 4,
  /** Call records kept per project (oldest dropped; `callCount` keeps counting). */
  MAX_CALLS_RETAINED: 8,
  /** Files kept per call — the ones that got the most source. */
  MAX_FILES_PER_CALL: 24,
  /** Line ranges kept per file after coalescing — the largest spans. */
  MAX_RANGES_PER_FILE: 24,
  /** Most-recent calls per project included in {@link ExploreSessionView}. */
  MAX_VIEW_CALLS: 4,
} as const;

/**
 * Key a project root is filed under. Resolved so `/repo` and `/repo/` agree;
 * case-folded on the two platforms whose filesystems are case-insensitive, so a
 * drive-letter or capitalization difference doesn't split one project in two.
 */
export function exploreProjectKey(projectRoot: string): string {
  const resolved = path.resolve(projectRoot);
  return process.platform === 'win32' || process.platform === 'darwin'
    ? resolved.toLowerCase()
    : resolved;
}

/**
 * Merge overlapping / adjacent spans into the smallest equivalent set, then cap
 * it. Adjacency (`next.start <= cur.end + 1`) counts as overlap: two ranges that
 * touch describe one contiguous block of emitted source.
 *
 * When the cap bites, the LARGEST spans are kept and the result is re-sorted by
 * line so the set still reads top-to-bottom — dropping small fragments loses the
 * least information, and under-reporting is the safe direction (see the module
 * header).
 */
export function coalesceRanges(
  ranges: ReadonlyArray<ExploreLineRange>,
  max: number = EXPLORE_SESSION_LIMITS.MAX_RANGES_PER_FILE,
): { ranges: ExploreLineRange[]; truncated: boolean } {
  const valid = ranges
    .filter((r) => Number.isFinite(r.start) && Number.isFinite(r.end) && r.end >= r.start && r.start >= 1)
    .map((r) => ({ start: Math.floor(r.start), end: Math.floor(r.end) }))
    .sort((a, b) => a.start - b.start || a.end - b.end);

  const merged: ExploreLineRange[] = [];
  for (const r of valid) {
    const last = merged[merged.length - 1];
    if (last && r.start <= last.end + 1) last.end = Math.max(last.end, r.end);
    else merged.push({ ...r });
  }
  if (merged.length <= max) return { ranges: merged, truncated: false };

  const kept = [...merged]
    .sort((a, b) => (b.end - b.start) - (a.end - a.start) || a.start - b.start)
    .slice(0, max)
    .sort((a, b) => a.start - b.start);
  return { ranges: kept, truncated: true };
}

/** Whether a line falls inside any of the (sorted, coalesced) ranges. */
export function rangesCover(ranges: ReadonlyArray<ExploreLineRange>, line: number): boolean {
  return ranges.some((r) => line >= r.start && line <= r.end);
}

interface MutableProjectState {
  projectRoot: string;
  callCount: number;
  responseBytes: number;
  calls: ExploreCallRecord[];
}

/**
 * One MCP session's explore history. Created per session, thrown away with it.
 *
 * Not thread-shared and not a singleton: two sessions on the same daemon own two
 * instances and can never observe each other's calls. Every method is total —
 * malformed input is normalized away rather than thrown, because this sits on
 * the tool-call path and a bookkeeping bug must never fail an explore.
 */
export class ExploreSessionState {
  /** Insertion-ordered; a touched project is re-inserted, so the head is the LRU. */
  private readonly projects = new Map<string, MutableProjectState>();

  /**
   * File an emission. Returns the record as stored (with its session call
   * index), or `null` if the emission was unusable.
   */
  record(emission: ExploreEmission): ExploreCallRecord | null {
    if (!emission || typeof emission.projectRoot !== 'string' || !emission.projectRoot) return null;
    const key = exploreProjectKey(emission.projectRoot);
    const state = this.touch(key, emission.projectRoot);

    state.callCount += 1;
    state.responseBytes += Math.max(0, emission.responseBytes || 0);

    const record: ExploreCallRecord = {
      index: state.callCount,
      projectRoot: emission.projectRoot,
      query: typeof emission.query === 'string' ? emission.query : '',
      files: this.boundFiles(emission.files),
      sourceBytes: Math.max(0, emission.sourceBytes || 0),
      responseBytes: Math.max(0, emission.responseBytes || 0),
    };
    state.calls.push(record);
    if (state.calls.length > EXPLORE_SESSION_LIMITS.MAX_CALLS_RETAINED) {
      state.calls.splice(0, state.calls.length - EXPLORE_SESSION_LIMITS.MAX_CALLS_RETAINED);
    }
    return record;
  }

  /** Full state for one project, or `null` if it was never queried this session. */
  forProject(projectRoot: string): ExploreProjectState | null {
    const state = this.projects.get(exploreProjectKey(projectRoot));
    return state ? cloneProject(state) : null;
  }

  /** Explore calls made this session against a project (including evicted ones). */
  callCount(projectRoot: string): number {
    return this.projects.get(exploreProjectKey(projectRoot))?.callCount ?? 0;
  }

  /** Every project this session has queried, least-recently-used first. */
  snapshot(): ExploreProjectState[] {
    return [...this.projects.values()].map(cloneProject);
  }

  /**
   * The bounded view passed INTO a tool call. Trimmed to the most recent
   * {@link EXPLORE_SESSION_LIMITS.MAX_VIEW_CALLS} calls per project: it crosses a
   * worker boundary on every explore, so it carries what a dedup/decay decision
   * needs and not the whole history.
   */
  view(): ExploreSessionView {
    return {
      projects: [...this.projects.values()].map((state) => ({
        projectRoot: state.projectRoot,
        callCount: state.callCount,
        responseBytes: state.responseBytes,
        calls: state.calls
          .slice(-EXPLORE_SESSION_LIMITS.MAX_VIEW_CALLS)
          .map((c) => ({ ...c, files: c.files.map((f) => ({ ...f, ranges: [...f.ranges] })) })),
      })),
    };
  }

  /** Drop everything. Used by tests; a real session just goes away instead. */
  clear(): void {
    this.projects.clear();
  }

  /**
   * Fetch a project's state, creating it if new, and mark it most-recently-used.
   * Evicts the LRU project past the bound — dropping a project entirely (rather
   * than its detail) is right here: a session that has moved on to four other
   * repos is not about to re-ask the first one.
   */
  private touch(key: string, projectRoot: string): MutableProjectState {
    const existing = this.projects.get(key);
    if (existing) {
      this.projects.delete(key);
      this.projects.set(key, existing);
      return existing;
    }
    const created: MutableProjectState = { projectRoot, callCount: 0, responseBytes: 0, calls: [] };
    this.projects.set(key, created);
    while (this.projects.size > EXPLORE_SESSION_LIMITS.MAX_PROJECTS) {
      const lru = this.projects.keys().next().value as string | undefined;
      if (lru === undefined) break;
      this.projects.delete(lru);
    }
    return created;
  }

  /**
   * Normalize + bound one call's files: coalesce each file's ranges, then keep
   * the files that got the most source. A call that renders more files than the
   * bound has already spread its envelope thin, so the tail files carry the
   * least — and losing them costs the least.
   */
  private boundFiles(files: ReadonlyArray<ExploreFileEmission> | undefined): ExploreFileEmission[] {
    if (!Array.isArray(files) || files.length === 0) return [];
    const normalized = files
      .filter((f) => f && typeof f.path === 'string' && f.path.length > 0)
      .map((f) => {
        const { ranges, truncated } = coalesceRanges(f.ranges ?? []);
        const out: ExploreFileEmission = { path: f.path, ranges, bytes: Math.max(0, f.bytes || 0) };
        if (typeof f.fingerprint === 'string' && f.fingerprint) out.fingerprint = f.fingerprint;
        if (truncated) out.rangesTruncated = true;
        return out;
      });
    if (normalized.length <= EXPLORE_SESSION_LIMITS.MAX_FILES_PER_CALL) return normalized;
    return [...normalized]
      .sort((a, b) => b.bytes - a.bytes)
      .slice(0, EXPLORE_SESSION_LIMITS.MAX_FILES_PER_CALL);
  }
}

function cloneProject(state: MutableProjectState): ExploreProjectState {
  return {
    projectRoot: state.projectRoot,
    callCount: state.callCount,
    responseBytes: state.responseBytes,
    calls: state.calls.map((c) => ({ ...c, files: c.files.map((f) => ({ ...f, ranges: [...f.ranges] })) })),
  };
}

/**
 * Read the session view a caller injected into tool args, if any. Defensive:
 * the key is internal, but the args object comes off the wire, so a client that
 * spells it itself gets ignored rather than trusted into a crash.
 */
export function readExploreSessionView(args: Record<string, unknown>): ExploreSessionView | null {
  const raw = args?.[EXPLORE_SESSION_VIEW_ARG];
  if (!raw || typeof raw !== 'object') return null;
  const projects = (raw as ExploreSessionView).projects;
  if (!Array.isArray(projects)) return null;
  return { projects: projects.filter((p) => p && typeof p.projectRoot === 'string') };
}

/**
 * This session's prior state for one project, from an injected view.
 *
 * `null` means NOBODY IS TRACKING (no view was injected — the CLI, a bare
 * handler). A view that simply hasn't seen this project yet returns an EMPTY
 * state, not null: the distinction matters to consumers, since "first call of a
 * tracked session" and "untracked" are different situations.
 */
export function viewForProject(
  view: ExploreSessionView | null,
  projectRoot: string,
): ExploreProjectState | null {
  if (!view) return null;
  const key = exploreProjectKey(projectRoot);
  return view.projects.find((p) => exploreProjectKey(p.projectRoot) === key)
    ?? { projectRoot, callCount: 0, responseBytes: 0, calls: [] };
}