const std = @import("std"); const Sha256 = std.crypto.hash.sha2.Sha256; pub const Digest = [Sha256.digest_length]u8; pub const CacheScope = enum {    private,    public,}; pub const Capability = enum {    tools_list,    resources_list,    resource_templates_list,    resource_read,    prompts_list,    prompt_get,    completion,}; pub const CacheKey = struct {    server_identity: Digest,    protocol_version: Digest,    capability: Capability,    normalized_arguments: Digest,    private_auth_identity: ?Digest,     pub fn eql(left: CacheKey, right: CacheKey) bool {        return std.mem.eql(u8, &left.server_identity, &right.server_identity) and            std.mem.eql(u8, &left.protocol_version, &right.protocol_version) and            left.capability == right.capability and            std.mem.eql(u8, &left.normalized_arguments, &right.normalized_arguments) and            optionalDigestEql(left.private_auth_identity, right.private_auth_identity);    }}; const CacheKeyInput = struct {    server_identity: []const u8,    protocol_version: []const u8,    capability: Capability,    normalized_arguments: []const u8,    scope: CacheScope,    auth_identity: Digest,}; pub fn buildCacheKey(input: CacheKeyInput) CacheKey {    return .{        .server_identity = digest(input.server_identity),        .protocol_version = digest(input.protocol_version),        .capability = input.capability,        .normalized_arguments = digest(input.normalized_arguments),        .private_auth_identity = if (input.scope == .private)            input.auth_identity        else            null,    };} pub const Header = struct {    name: []const u8,    value: []const u8,}; /// Produces an opaque partition identity without retaining header names or/// credential values. Header order is significant because it is part of the/// effective request identity.pub fn authIdentity(headers: []const Header) Digest {    var hasher = Sha256.init(.{});    for (headers) |header| {        hashLengthPrefixed(&hasher, header.name);        hashLengthPrefixed(&hasher, header.value);    }    var result: Digest = undefined;    hasher.final(&result);    return result;} pub const Freshness = enum {    fresh,    stale,    refreshing,    failed_refresh,}; pub const SubscriptionIdentity = struct {    request_id: u64,    generation: u64,}; pub const SnapshotMetadata = struct {    key: CacheKey,    connection_generation: u64,    catalog_generation: u64,    fetched_at_ms: u64,    expires_at_ms: u64,    scope: CacheScope,    freshness: Freshness = .fresh,    refresh_attempt: u8 = 0,    retry_at_ms: u64 = 0,    content_digest: Digest,    subscription: ?SubscriptionIdentity = null,}; fn expiryFromTtl(fetched_at_ms: u64, ttl_ms: u64) u64 {    return fetched_at_ms +| ttl_ms;} pub fn pageExpiry(    protocol: Protocol,    received_at_ms: u64,    ttl_present: bool,    ttl_ms: u64,) u64 {    return expiryFromTtl(        received_at_ms,        snapshotTtl(protocol, ttl_present, ttl_ms),    );} pub fn earliestExpiry(current: ?u64, page_expiry_ms: u64) u64 {    return if (current) |expiry_ms|        @min(expiry_ms, page_expiry_ms)    else        page_expiry_ms;} const RefreshDecision = struct {    action: enum {        hit,        refresh,        retry_later,        already_refreshing,    },    may_serve_snapshot: bool,}; pub fn decideRefresh(    metadata: SnapshotMetadata,    requested_key: CacheKey,    now_ms: u64,    invalidated: bool,) RefreshDecision {    const same_partition = metadata.key.eql(requested_key);    if (!same_partition) return .{        .action = .refresh,        .may_serve_snapshot = false,    };    if (metadata.freshness == .refreshing) return .{        .action = .already_refreshing,        .may_serve_snapshot = true,    };    if (metadata.freshness == .failed_refresh and now_ms < metadata.retry_at_ms) {        return .{            .action = .retry_later,            .may_serve_snapshot = true,        };    }    if (invalidated) return .{        .action = .refresh,        .may_serve_snapshot = true,    };    if (metadata.freshness == .fresh and now_ms < metadata.expires_at_ms) {        return .{            .action = .hit,            .may_serve_snapshot = true,        };    }    return .{        .action = .refresh,        .may_serve_snapshot = true,    };} pub fn effectiveFreshness(    metadata: SnapshotMetadata,    now_ms: u64,    invalidated: bool,) Freshness {    return switch (metadata.freshness) {        .refreshing, .failed_refresh => metadata.freshness,        .fresh, .stale => if (invalidated or now_ms >= metadata.expires_at_ms)            .stale        else            .fresh,    };} pub fn beginRefresh(metadata: SnapshotMetadata) SnapshotMetadata {    var next = metadata;    next.freshness = .refreshing;    return next;} const retry_initial_ms: u64 = 100;const retry_max_ms: u64 = 5_000;const retry_max_attempt: u8 = 8; pub fn retryDelayMs(attempt: u8) u64 {    var delay = retry_initial_ms;    var remaining = @min(attempt, retry_max_attempt);    while (remaining > 0) : (remaining -= 1) {        delay = @min(delay *| 2, retry_max_ms);    }    return delay;} pub fn failedRefresh(metadata: SnapshotMetadata, now_ms: u64) SnapshotMetadata {    var next = metadata;    next.freshness = .failed_refresh;    next.refresh_attempt = @min(metadata.refresh_attempt +| 1, retry_max_attempt);    next.retry_at_ms = now_ms +| retryDelayMs(metadata.refresh_attempt);    return next;} const ReplacementDecision = enum {    reject_stale_generation,    reject_cache_key,    metadata_only,    replace_snapshot,}; pub fn authorizeReplacement(    current: SnapshotMetadata,    source_connection_generation: u64,    source_catalog_generation: u64,    active_key: CacheKey,    replacement_key: CacheKey,    replacement_content_digest: Digest,) ReplacementDecision {    if (current.connection_generation != source_connection_generation or        current.catalog_generation != source_catalog_generation)    {        return .reject_stale_generation;    }    if (!active_key.eql(replacement_key)) return .reject_cache_key;    if (std.mem.eql(u8, &current.content_digest, &replacement_content_digest)) {        return .metadata_only;    }    return .replace_snapshot;} pub const Protocol = enum {    legacy,    modern,}; fn snapshotTtl(protocol: Protocol, ttl_present: bool, ttl_ms: u64) u64 {    if (ttl_present) return ttl_ms;    return switch (protocol) {        .modern => 0,        .legacy => std.math.maxInt(u64),    };} pub const NotificationKind = enum {    subscription_acknowledged,    subscription_filter_unsupported,    tools_list_changed,    resources_list_changed,    resource_updated,    prompts_list_changed,    subscription_cancelled,    other,}; pub const Notification = struct {    kind: NotificationKind,    subscription_id: ?u64 = null,    cancelled_request_id: ?u64 = null,    resource_uri: ?[]const u8 = null,}; const SubscriptionExpectation = struct {    protocol: Protocol,    capability_advertised: bool,    connection_generation: u64,    event_connection_generation: u64,    subscription: ?SubscriptionIdentity,    acknowledged: bool,}; const NotificationDecision = enum {    acknowledge,    invalidate,    close_subscription,    close_unsupported,    ignore_unadvertised,    ignore_malformed,    ignore_duplicate,    ignore_late_generation,    ignore_unrelated,}; pub fn classifyNotification(    expectation: SubscriptionExpectation,    notification: Notification,) NotificationDecision {    if (!expectation.capability_advertised) return .ignore_unadvertised;    if (expectation.connection_generation != expectation.event_connection_generation) {        return .ignore_late_generation;    }    if (expectation.protocol == .legacy) {        return switch (notification.kind) {            .tools_list_changed,            .resources_list_changed,            .resource_updated,            .prompts_list_changed,            => .invalidate,            else => .ignore_unrelated,        };    }     const subscription = expectation.subscription orelse return .ignore_late_generation;    return switch (notification.kind) {        .subscription_acknowledged => {            const subscription_id = notification.subscription_id orelse                return .ignore_malformed;            if (subscription_id != subscription.request_id) return .ignore_late_generation;            if (expectation.acknowledged) return .ignore_duplicate;            return .acknowledge;        },        .subscription_filter_unsupported => {            const subscription_id = notification.subscription_id orelse                return .ignore_malformed;            if (subscription_id != subscription.request_id) return .ignore_late_generation;            if (expectation.acknowledged) return .ignore_duplicate;            return .close_unsupported;        },        .tools_list_changed,        .resources_list_changed,        .resource_updated,        .prompts_list_changed,        => {            const subscription_id = notification.subscription_id orelse                return .ignore_malformed;            if (subscription_id != subscription.request_id) return .ignore_late_generation;            if (!expectation.acknowledged) return .ignore_late_generation;            return .invalidate;        },        .subscription_cancelled => {            const request_id = notification.cancelled_request_id orelse                return .ignore_malformed;            return if (request_id == subscription.request_id)                .close_subscription            else                .ignore_late_generation;        },        .other => .ignore_unrelated,    };} const CoalesceDecision = enum {    enqueue,    already_pending,    ignore,}; pub fn decideCoalesce(pending: bool, notification: NotificationDecision) CoalesceDecision {    if (notification != .invalidate) return .ignore;    return if (pending) .already_pending else .enqueue;} pub fn digest(bytes: []const u8) Digest {    var result: Digest = undefined;    Sha256.hash(bytes, &result, .{});    return result;} fn optionalDigestEql(left: ?Digest, right: ?Digest) bool {    if ((left == null) != (right == null)) return false;    if (left) |value| return std.mem.eql(u8, &value, &right.?);    return true;} fn hashLengthPrefixed(hasher: *Sha256, bytes: []const u8) void {    var length: [8]u8 = undefined;    std.mem.writeInt(u64, &length, bytes.len, .big);    hasher.update(&length);    hasher.update(bytes);} fn testMetadata(key: CacheKey, generation: u64, content: []const u8) SnapshotMetadata {    return .{        .key = key,        .connection_generation = 4,        .catalog_generation = generation,        .fetched_at_ms = 100,        .expires_at_ms = 200,        .scope = if (key.private_auth_identity == null) .public else .private,        .content_digest = digest(content),    };} test "duplicate notifications are idempotent" {    const subscription = SubscriptionIdentity{ .request_id = 17, .generation = 3 };    const notification = Notification{        .kind = .tools_list_changed,        .subscription_id = 17,    };    const expectation = SubscriptionExpectation{        .protocol = .modern,        .capability_advertised = true,        .connection_generation = 4,        .event_connection_generation = 4,        .subscription = subscription,        .acknowledged = true,    };    const classified = classifyNotification(expectation, notification);    try std.testing.expectEqual(NotificationDecision.invalidate, classified);    try std.testing.expectEqual(CoalesceDecision.enqueue, decideCoalesce(false, classified));    try std.testing.expectEqual(CoalesceDecision.already_pending, decideCoalesce(true, classified));} test "failed refresh preserves the last valid snapshot" {    const identity = authIdentity(&.{});    const key = buildCacheKey(.{        .server_identity = "server",        .protocol_version = "2026-07-28",        .capability = .tools_list,        .normalized_arguments = "{}",        .scope = .private,        .auth_identity = identity,    });    const current = testMetadata(key, 9, "valid snapshot");    const failed = failedRefresh(current, 250);    try std.testing.expectEqual(Freshness.failed_refresh, failed.freshness);    try std.testing.expectEqual(current.catalog_generation, failed.catalog_generation);    try std.testing.expectEqualSlices(u8, &current.content_digest, &failed.content_digest);    try std.testing.expect(failed.retry_at_ms > 250);} test "a stale generation cannot replace newer state" {    const key = buildCacheKey(.{        .server_identity = "server",        .protocol_version = "2026-07-28",        .capability = .tools_list,        .normalized_arguments = "{}",        .scope = .public,        .auth_identity = authIdentity(&.{}),    });    const current = testMetadata(key, 12, "newer");    try std.testing.expectEqual(        ReplacementDecision.reject_stale_generation,        authorizeReplacement(current, 4, 11, key, key, digest("older")),    );} test "private cache state cannot cross authentication identities" {    const first_identity = authIdentity(&.{.{ .name = "Authorization", .value = "first" }});    const second_identity = authIdentity(&.{.{ .name = "Authorization", .value = "second" }});    const first = buildCacheKey(.{        .server_identity = "server",        .protocol_version = "2026-07-28",        .capability = .tools_list,        .normalized_arguments = "{}",        .scope = .private,        .auth_identity = first_identity,    });    const second = buildCacheKey(.{        .server_identity = "server",        .protocol_version = "2026-07-28",        .capability = .tools_list,        .normalized_arguments = "{}",        .scope = .private,        .auth_identity = second_identity,    });    const decision = decideRefresh(testMetadata(first, 1, "private"), second, 101, false);    try std.testing.expectEqual(.refresh, decision.action);    try std.testing.expect(!decision.may_serve_snapshot);     const public_first = buildCacheKey(.{        .server_identity = "server",        .protocol_version = "2026-07-28",        .capability = .tools_list,        .normalized_arguments = "{}",        .scope = .public,        .auth_identity = first_identity,    });    const public_second = buildCacheKey(.{        .server_identity = "server",        .protocol_version = "2026-07-28",        .capability = .tools_list,        .normalized_arguments = "{}",        .scope = .public,        .auth_identity = second_identity,    });    try std.testing.expect(public_first.eql(public_second));    try std.testing.expectEqual(        ReplacementDecision.reject_cache_key,        authorizeReplacement(            testMetadata(first, 1, "private"),            4,            1,            second,            first,            digest("replacement"),        ),    );}

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