/**
 * window/strategy — PUBLIC. How a window strategy is shaped.
 *
 * Pattern: Strategy (GoF), with the dangerous half of the decision handed in
 *          pre-bound rather than left to the implementer to re-derive.
 * Role:    core/ layer. The window stage does the wiring — read the meter,
 *          write the window, emit, record, cost — and delegates the one
 *          interesting question to a strategy:
 *
 *              given the segmented turns and what the provider actually
 *              counted → what should the window become, and what does the
 *              ledger need to be told about it?
 *
 * Emits:   N/A. A strategy is pure decision + (optionally) its own LLM call;
 *          it never touches scope, never emits, and never writes. That is
 *          what makes it testable without a chart, and what keeps the
 *          "record everything you did" duty in ONE place (the stage) rather
 *          than duplicated per strategy.
 *
 * Three strategies ship — `summarizeOldest` (what `.compaction()`
 * configures), `slidingWindow`, `tokenBudget` — and `.window(...)` takes any
 * object that satisfies `WindowStrategy`.
 *
 * Two things are deliberately NOT left to the implementer:
 *
 *   1. **The refusal rules.** `planRemoval` arrives already bound to this
 *      iteration's turns and guards. A strategy cannot forget that an
 *      unanswered tool call must not leave the window — or that the CURRENT
 *      REQUEST never does (9.55.0) — because it never gets the chance to
 *      decide that for itself: it asks, and it is told, with every refusal
 *      named. That is safety by construction, not by docs. It is also why a
 *      rule added there arrives in every strategy at once, including the ones
 *      consumers wrote before the rule existed.
 *   2. **Provenance.** `removalFacts` turns "these indices left" into the
 *      stage ids that wrote them and how long each lived. A strategy cannot
 *      file a removal it cannot name.
 *
 * The TRIGGER, by contrast, is entirely the strategy's own: `plan` is called
 * at every ReAct iteration boundary and answers `undefined` when it did not
 * engage. That is why `slidingWindow` can run on a provider that reports no
 * usage at all while `summarizeOldest` and `tokenBudget` refuse by name.
 */
import type { LLMMessage, LLMProvider } from '../../../adapters/types.js';
import type { Turn, RemovalPlan } from './turns.js';
import type { FoldedSpan, WindowRecord } from './types.js';
/** One message leaving the window, with the facts an eviction event needs. */
export interface WindowEviction {
    /** Index in the PRE-change window — the index the content hash was built on. */
    readonly index: number;
    /** How long it lived in the window. Exact; 0 when its birth is unknown. */
    readonly survivalMs: number;
}
/** The provenance of a set of removed messages, as the ledger needs it. */
export interface RemovalFacts {
    /** `runtimeStageId`s of the stages that appended those messages, in order. */
    readonly removedStageIds: readonly string[];
    /** One eviction per message, with its measured lifetime. */
    readonly evictions: readonly WindowEviction[];
}
/** Everything a strategy is allowed to look at. */
export interface WindowStrategyInput {
    /** The window as it stands, detached. */
    readonly history: readonly LLMMessage[];
    /** The same window, segmented into turns. */
    readonly turns: readonly Turn[];
    /**
     * What the provider REPORTED for the last completed call. Counted, never
     * guessed. `undefined` before the first call of the run — a strategy that
     * acted on that would be guessing, which is the one thing this family
     * refuses to do.
     *
     * `{ input: 0, output: 0 }` is a provider that reported NOTHING, not a call
     * that cost nothing. A token-triggered strategy should throw
     * `CompactionUnmeasurableError` there rather than invent a size.
     */
    readonly measured: {
        readonly input: number;
        readonly output: number;
    } | undefined;
    /** The ReAct iteration this decision belongs to. */
    readonly iteration: number;
    /**
     * The run this decision belongs to.
     *
     * A strategy that retains what it removed has to name the run whose commit
     * log held it — that is the honest answer to "where else could I have found
     * this?", and the answer is "nowhere, once that process ended", which is the
     * whole reason retention exists. `'unknown'` when the runtime could not name
     * the run, never a fabricated id.
     */
    readonly runId: string;
    /** The agent's own model — the sensible default for a strategy that bills. */
    readonly agentModel: string;
    /** `provider.name` of the MAIN provider, for a refusal that names it. */
    readonly providerName: string;
    /** The run's cancellation signal, when there is one. */
    readonly signal: AbortSignal | undefined;
    /** Wall clock, injectable so a caller can pin `survivalMs`. */
    readonly now: () => number;
    /**
     * THE shared refusal engine, bound to this iteration.
     *
     * Answers: which contiguous span of turns may leave, and every turn that
     * refused, named. Never removes the system envelope, the CURRENT REQUEST,
     * the last `keepRecentTurns` turns, an unanswered tool call, the paused
     * tool, or a pending check-in.
     *
     * @param keepRecentTurns   how many trailing turns are off-limits
     * @param isExistingSummary optional predicate marking a turn that is a
     *   summary a previous fold wrote; when the whole span is one of those, the
     *   plan refuses with `only-existing-summary`. Pass it only if your strategy
     *   spends an LLM call — a drop has nothing to protect against.
     */
    readonly planRemoval: (keepRecentTurns: number, isExistingSummary?: (turn: Turn) => boolean) => RemovalPlan;
    /**
     * Turn removed message indices into the facts the ledger needs: which
     * stages wrote them, and how long each lived in the window.
     *
     * @param indices indices in the PRE-change window that are leaving
     * @param atMs    the moment they leave (usually `input.now()`)
     */
    readonly removalFacts: (indices: readonly number[], atMs: number) => RemovalFacts;
}
/** What the stage should do next. */
export interface WindowStrategyResult {
    /** The new window. Absent = leave the window alone. */
    readonly window?: readonly LLMMessage[];
    /**
     * How the meter must re-align its provenance to the new window, which is
     * `[...head, (one new message)?, ...tail]`. Present exactly when `window`
     * is. `insertedAtMs` is the birth of the message the strategy put in the
     * span's place — omit it when the strategy removed messages and inserted
     * nothing.
     */
    readonly rebase?: {
        readonly headCount: number;
        readonly keptTailCount: number;
        readonly insertedAtMs?: number;
    };
    /** What the ledger is told. Always present — an engaged visit explains itself. */
    readonly record: WindowRecord;
    /** Messages that left the window, for `context.evicted`. */
    readonly evictions: readonly WindowEviction[];
    /**
     * Spans this visit removed, in the form that OUTLIVES the process: appended
     * to the conversation checkpoint, so a restart can still say what a summary
     * stands for — and, under `retain: 'conversation'`, produce it verbatim.
     *
     * OMIT IT unless your strategy replaced messages with something that stands
     * for them. `summarizeOldest` fills it because a summary is a claim that
     * needs its evidence; the drop strategies do not, because a drop replaces
     * nothing and its authored notice claims nothing.
     *
     * The stage writes these in the SAME commit as the window change, so there
     * is no state in which messages left the window and the record of what they
     * were did not follow them.
     */
    readonly folded?: readonly FoldedSpan[];
    /**
     * The budget reading to report on `agentfootprint.context.budget_pressure`.
     *
     * OMIT IT when the strategy has no token budget. `slidingWindow` does: it
     * triggers on turn count, and filling `capTokens` with a number nobody
     * configured would be the invented figure this family refuses. No budget,
     * no budget_pressure event.
     *
     * `unit` says what the two numbers count, because the context SLOTS emit
     * this same event name with the same `slot: 'messages'` and count in CHARS.
     * It defaults to `'tokens'` — every shipped strategy compares against a
     * `thresholdTokens`, so that is what all three already mean. Set it to
     * `'chars'` if yours measures characters, and the event will say so.
     */
    readonly budgetPressure?: {
        readonly capTokens: number;
        readonly projectedTokens: number;
        readonly planAction: 'evict' | 'summarize' | 'none';
        readonly unit?: 'chars' | 'tokens';
    };
    /** A billed call the strategy made, for the cost channel. */
    readonly spend?: {
        readonly model: string;
        readonly usage: {
            readonly input: number;
            readonly output: number;
        };
    };
    /** A one-per-run dev warning the stage should print. */
    readonly warning?: string;
}
/**
 * A window strategy: what the live window should become at this iteration
 * boundary, and what the record must say about the change.
 *
 * Pass one to `AgentBuilder.window(...)`. Exactly one per agent —
 * `.compaction(...)` is the same door with `summarizeOldest` already in it.
 */
export interface WindowStrategy {
    /**
     * Stable name — it is written onto every record this strategy files
     * (`WindowRecord.strategy`), so a reader can tell which policy produced a
     * window, and it names the strategy on the chart's `compact` stage.
     */
    readonly name: string;
    /**
     * What this strategy will BILL, when it bills anything (8.14.0).
     *
     * Omit it if your strategy makes no LLM call — the drop strategies do, and
     * a strategy that spends nothing has no billing to declare.
     *
     * It exists so the agent BUILDER can check a strategy's spending against
     * the agent's own provider and model before the first run. Without it,
     * `.compaction({...})` could be refused for a configuration that
     * `.window(summarizeOldest({...}))` — the same strategy through the other
     * door — accepted silently. Two doors onto one policy must refuse the same
     * things, or the refusal is advice rather than a rule.
     */
    readonly billing?: {
        readonly provider: LLMProvider;
        readonly model: string;
    };
    /**
     * Decide. Called at EVERY ReAct iteration boundary.
     *
     * Return `undefined` when this strategy did not engage — nothing was over
     * budget, nothing was old enough, nothing has been counted yet. The ledger
     * stays untouched and the run proceeds.
     *
     * Return a result for anything else, INCLUDING a visit that changed
     * nothing because every candidate refused. Those are the visits a person
     * debugging an oversized window actually needs.
     */
    plan(input: WindowStrategyInput): Promise<WindowStrategyResult | undefined>;
}