import { VAULT_SYNC_CODES } from "../errors/codes.js";
import { VaultSyncError } from "../errors/vault-sync-error.js";
import { getVaultSyncInstruments } from "../otel/instruments.js";
import type {
    MergeStrategy,
    VaultHistoryEntry,
    VaultItem,
    VectorClockState,
} from "../types/types.js";
import { compare, lwwTotalOrder, merge } from "../vector-clock/vector-clock.js";

/**
 * Permitted merge strategies. Exposed for runtime checks and exhaustive
 * `switch` statements in consumers.
 */
export const MERGE_STRATEGIES: readonly MergeStrategy[] = [
    "lww",
    "append",
    "atomic",
] as const;

function clockUnion(
    a: VectorClockState,
    b: VectorClockState
): VectorClockState {
    return merge(a, b);
}

function chooseWinner(local: VaultItem, remote: VaultItem): VaultItem {
    const order = lwwTotalOrder(local, remote);
    if (order === -1) return local;
    if (order === 1) return remote;
    // Identical clock+updatedAt+nodeId but possibly different content (a
    // pathological case from anomalous inputs). Pick deterministically by
    // canonical-string content so commutativity is preserved.
    const localCanonical = canonicalString(local);
    const remoteCanonical = canonicalString(remote);
    return localCanonical <= remoteCanonical ? local : remote;
}

function canonicalString(item: VaultItem): string {
    return [
        item.title,
        item.password,
        item.folderId ?? "",
        String(item.deleted ?? false),
    ].join(" ");
}

function lwwMerge(local: VaultItem, remote: VaultItem): VaultItem {
    const winner = chooseWinner(local, remote);
    const mergedClock = clockUnion(local.clock, remote.clock);
    const mergedHistory = appendHistory(local.history, remote.history);
    const result: VaultItem = {
        ...winner,
        clock: mergedClock,
        history: mergedHistory,
    };
    if (local.deleted === true || remote.deleted === true) {
        return { ...result, deleted: true };
    }
    return result;
}

function pickHistoryEntry(
    a: VaultHistoryEntry,
    b: VaultHistoryEntry
): VaultHistoryEntry {
    // Deterministic, commutative selection when two sides claim the same
    // historyId with different content: prefer the entry with the higher
    // timestamp; on timestamp ties, prefer the lexicographically smaller note.
    if (a.timestamp !== b.timestamp) {
        return a.timestamp > b.timestamp ? a : b;
    }
    if (a.note < b.note) return a;
    if (a.note > b.note) return b;
    return a;
}

function appendHistory(
    left: readonly VaultHistoryEntry[],
    right: readonly VaultHistoryEntry[]
): readonly VaultHistoryEntry[] {
    const seen = new Map<string, VaultHistoryEntry>();
    for (const entry of left) {
        const existing = seen.get(entry.historyId);
        seen.set(
            entry.historyId,
            existing === undefined ? entry : pickHistoryEntry(existing, entry)
        );
    }
    for (const entry of right) {
        const existing = seen.get(entry.historyId);
        seen.set(
            entry.historyId,
            existing === undefined ? entry : pickHistoryEntry(existing, entry)
        );
    }
    const out = Array.from(seen.values());
    out.sort((a, b) => {
        if (a.timestamp !== b.timestamp) return a.timestamp - b.timestamp;
        return a.historyId < b.historyId ? -1 : 1;
    });
    return out;
}

function appendMerge(local: VaultItem, remote: VaultItem): VaultItem {
    const winner = chooseWinner(local, remote);
    return {
        ...winner,
        clock: clockUnion(local.clock, remote.clock),
        history: appendHistory(local.history, remote.history),
        deleted:
            local.deleted === true || remote.deleted === true
                ? true
                : undefined,
        // Preserve declared shape (no extra fields)
    } as VaultItem;
}

function atomicMerge(local: VaultItem, remote: VaultItem): VaultItem {
    const relation = compare(local.clock, remote.clock);
    if (relation === "concurrent") {
        const conflictingFields: string[] = [];
        if (local.folderId !== remote.folderId)
            conflictingFields.push("folderId");
        if (local.title !== remote.title) conflictingFields.push("title");
        if (local.password !== remote.password)
            conflictingFields.push("password");
        if ((local.deleted ?? false) !== (remote.deleted ?? false))
            conflictingFields.push("deleted");
        if (conflictingFields.length > 0) {
            getVaultSyncInstruments().mergeOutcomeTotal.add(1, {
                status: "conflict",
            });
            throw new VaultSyncError(
                VAULT_SYNC_CODES["MERGE.CONFLICT_UNRESOLVED"],
                "error.vault-sync.merge.conflict-unresolved",
                { context: { fieldCount: conflictingFields.length } }
            );
        }
    }
    if (relation === "<") {
        return {
            ...remote,
            clock: clockUnion(local.clock, remote.clock),
            history: appendHistory(local.history, remote.history),
        };
    }
    if (relation === ">") {
        return {
            ...local,
            clock: clockUnion(local.clock, remote.clock),
            history: appendHistory(local.history, remote.history),
        };
    }
    // Identical clocks → identical state by causal definition.
    return {
        ...local,
        clock: clockUnion(local.clock, remote.clock),
        history: appendHistory(local.history, remote.history),
    };
}

/**
 * Merge two divergent copies of a vault item using the supplied strategy.
 *
 * The merge is commutative and idempotent for the `"lww"` and `"append"`
 * strategies. The `"atomic"` strategy throws
 * `VaultSyncError(MERGE.CONFLICT_UNRESOLVED)` when the two clocks are
 * concurrent and any atomic-protected field differs.
 *
 * @throws VaultSyncError(MERGE.STRATEGY_UNKNOWN) — strategy not in {lww, append, atomic}
 * @throws VaultSyncError(MERGE.CONFLICT_UNRESOLVED) — atomic strategy with concurrent edits
 */
export function mergeVaultItems(
    local: VaultItem,
    remote: VaultItem,
    strategy: MergeStrategy
): VaultItem {
    if (local.id !== remote.id) {
        throw new VaultSyncError(
            VAULT_SYNC_CODES["MERGE.CONFLICT_UNRESOLVED"],
            "error.vault-sync.merge.conflict-unresolved",
            { context: { reason: "id mismatch" } }
        );
    }
    const start = Date.now();
    try {
        let merged: VaultItem;
        switch (strategy) {
            case "lww":
                merged = lwwMerge(local, remote);
                break;
            case "append":
                merged = appendMerge(local, remote);
                break;
            case "atomic":
                merged = atomicMerge(local, remote);
                break;
            default: {
                const exhaustive: never = strategy;
                throw new VaultSyncError(
                    VAULT_SYNC_CODES["MERGE.STRATEGY_UNKNOWN"],
                    "error.vault-sync.merge.strategy-unknown",
                    { context: { strategy: String(exhaustive) } }
                );
            }
        }
        const i = getVaultSyncInstruments();
        i.mergeDurationMs.record(Date.now() - start);
        i.mergeOutcomeTotal.add(1, { status: "success" });
        return normalizeItem(merged);
    } catch (err) {
        if (
            err instanceof VaultSyncError &&
            err.code !== VAULT_SYNC_CODES["MERGE.CONFLICT_UNRESOLVED"]
        ) {
            getVaultSyncInstruments().mergeOutcomeTotal.add(1, {
                status: "rejected",
            });
        }
        throw err;
    }
}

/**
 * Validate that `strategy` is recognised. Returns the strategy if valid;
 * throws `VaultSyncError(MERGE.STRATEGY_UNKNOWN)` otherwise. Useful when a
 * caller has accepted user input that has not yet been Zod-validated.
 */
export function assertMergeStrategy(value: string): MergeStrategy {
    if ((MERGE_STRATEGIES as readonly string[]).includes(value)) {
        return value as MergeStrategy;
    }
    throw new VaultSyncError(
        VAULT_SYNC_CODES["MERGE.STRATEGY_UNKNOWN"],
        "error.vault-sync.merge.strategy-unknown",
        { context: { strategy: value } }
    );
}

function normalizeItem(item: VaultItem): VaultItem {
    if (item.deleted === undefined) {
        const { deleted: _omit, ...rest } = item;
        return rest as VaultItem;
    }
    return item;
}