diff --git a/app/logical/apng_inspector.rb b/app/logical/apng_inspector.rb
index 68cc62c06..2ff1d54de 100644
--- a/app/logical/apng_inspector.rb
+++ b/app/logical/apng_inspector.rb
@@ -1,3 +1,4 @@
+# Parse an PNG and return whether or not it's animated.
 class APNGInspector
   attr_reader :frames
 
diff --git a/app/logical/application_responder.rb b/app/logical/application_responder.rb
index 8e29aa7b2..3ca29de86 100644
--- a/app/logical/application_responder.rb
+++ b/app/logical/application_responder.rb
@@ -1,5 +1,8 @@
-# https://github.com/plataformatec/responders
-# https://github.com/plataformatec/responders/blob/master/lib/action_controller/responder.rb
+# Hooks into `respond_with` to add some custom behavior, including support for
+# the `expires_in`, `expiry`, and `only` params.
+#
+# @see https://github.com/plataformatec/responders
+# @see https://github.com/plataformatec/responders/blob/master/lib/action_controller/responder.rb
 class ApplicationResponder < ActionController::Responder
   # this is called by respond_with for non-html, non-js responses.
   def to_format
diff --git a/app/logical/approver_pruner.rb b/app/logical/approver_pruner.rb
index 67e7933b5..c494aa688 100644
--- a/app/logical/approver_pruner.rb
+++ b/app/logical/approver_pruner.rb
@@ -1,9 +1,17 @@
+# Demote all approvers who haven't approved at least 30 posts in the last 45
+# days. Moderators and recently promoted approvers are exempt. Runs as a monthly
+# maintenance task. Approvers who are facing demotion are sent a weekly warning
+# dmail first.
+#
+# @see DanbooruMaintenance#monthly
 module ApproverPruner
   module_function
 
   APPROVAL_PERIOD = 45.days
   MINIMUM_APPROVALS = 30
 
+  # Get the list of inactive approvers.
+  # @return [Array<User>] the list of inactive approvers
   def inactive_approvers
     approvers = User.bit_prefs_match(:can_approve_posts, true)
     approvers = approvers.where("level < ?", User::Levels::MODERATOR)
@@ -16,9 +24,10 @@ module ApproverPruner
     end
   end
 
+  # Demote all inactive approvers
   def prune!
     inactive_approvers.each do |user|
-      CurrentUser.scoped(User.system, "127.0.0.1") do
+      CurrentUser.scoped(User.system) do
         user.update!(can_approve_posts: false)
         user.feedback.create(category: "neutral", body: "Lost approval privileges", creator: User.system)
 
@@ -31,6 +40,7 @@ module ApproverPruner
     end
   end
 
+  # Send a warning dmail to approvers who are pending demotion.
   def dmail_inactive_approvers!
     days_until_next_month = (Date.current.next_month.beginning_of_month - Date.current).to_i
     return unless days_until_next_month <= 21
diff --git a/app/logical/artist_finder.rb b/app/logical/artist_finder.rb
index c40b323e3..efa5cb719 100644
--- a/app/logical/artist_finder.rb
+++ b/app/logical/artist_finder.rb
@@ -1,3 +1,4 @@
+# Find the artist entry for a given artist profile URL.
 module ArtistFinder
   module_function
 
@@ -131,6 +132,14 @@ module ArtistFinder
     %r{\Ahttps?://(?:[a-zA-Z0-9_-]+\.)*#{domain}/\z}i
   end)
 
+  # Find the artist for a given artist profile URL. May return multiple Artists
+  # in the event of duplicate artist entries.
+  #
+  # Uses a path-stripping algorithm to find any artist URL that is a prefix
+  # of the given URL. A site blacklist is used to prevent false positives.
+  #
+  # @param url [String] the artist profile URL
+  # @return [Array<Artist>] the list of matching artists
   def find_artists(url)
     url = ArtistUrl.normalize(url)
     artists = []
diff --git a/app/logical/autocomplete_service.rb b/app/logical/autocomplete_service.rb
index da91fbfc3..733a67b1d 100644
--- a/app/logical/autocomplete_service.rb
+++ b/app/logical/autocomplete_service.rb
@@ -1,3 +1,10 @@
+# Autocomplete tags, usernames, pools, and more.
+#
+# @example
+#   AutocompleteService.new("touho", :tag).autocomplete_results
+#   #=> [{ type: :tag, label: "touhou", value: "touhou", category: 3, post_count: 42 }]
+#
+# @see AutocompleteController
 class AutocompleteService
   extend Memoist
 
@@ -18,6 +25,11 @@ class AutocompleteService
 
   attr_reader :query, :type, :limit, :current_user
 
+  # Perform completion for the given search type and query.
+  # @param query [String] the string being completed
+  # @param type [String] the type of completion being performed
+  # @param current_user [User] the user we're performing completion for
+  # @param limit [Integer] the max number of results to return
   def initialize(query, type, current_user: User.anonymous, limit: 10)
     @query = query.to_s
     @type = type.to_s.to_sym
@@ -25,6 +37,8 @@ class AutocompleteService
     @limit = limit
   end
 
+  # Return the results of the completion.
+  # @return [Array<Hash>] the autocomplete results
   def autocomplete_results
     case type
     when :tag_query
@@ -52,6 +66,9 @@ class AutocompleteService
     end
   end
 
+  # Complete a tag search (a regular tag or a metatag)
+  # @param string [String] the string to complete
+  # @return [Array<Hash>] the autocomplete results
   def autocomplete_tag_query(string)
     term = PostQueryBuilder.new(string).terms.first
     return [] if term.nil?
@@ -64,10 +81,20 @@ class AutocompleteService
     end
   end
 
+  # Find tags matching a search.
+  #
+  # If the string is non-English, translate it to a Danbooru tag.
+  # If the string is a slash abbreviation, expand the abbreviation.
+  # If the string has a wildcard, do a wildcard search.
+  # If the string doesn't match anything, perform autocorrect.
+  #
+  # @param string [String] the string to complete
+  # @return [Array<Hash>] the autocomplete results
   def autocomplete_tag(string)
     return [] if string.size > TagNameValidator::MAX_TAG_LENGTH
     return [] if string.start_with?("http://", "https://")
 
+    # XXX convert to NFKC? deaccent?
     if !string.ascii_only?
       results = tag_other_name_matches(string)
     elsif string.starts_with?("/")
@@ -90,6 +117,9 @@ class AutocompleteService
     results
   end
 
+  # Find tags or tag aliases matching a wildcard search.
+  # @param string [String] the string to complete
+  # @return [Array<Hash>] the autocomplete results
   def tag_matches(string)
     name_matches = Tag.nonempty.name_matches(string).order(post_count: :desc).limit(limit)
     alias_matches = Tag.nonempty.alias_matches(string).order(post_count: :desc).limit(limit)
@@ -103,6 +133,12 @@ class AutocompleteService
     end
   end
 
+  # Find tags matching a slash abbreviation.
+  # Example: /evth => eyebrows_visible_through_hair
+  #
+  # @param string [String] the string to complete
+  # @param max_length [Integer] the max abbreviation length
+  # @return [Array<Hash>] the autocomplete results
   def tag_abbreviation_matches(string, max_length: 10)
     return [] if string.size > max_length
 
@@ -113,6 +149,11 @@ class AutocompleteService
     end
   end
 
+  # Find tags matching a mispelled tag.
+  # Example: logn_hair => long_hair
+  #
+  # @param string [String] the string to complete
+  # @return [Array<Hash>] the autocomplete results
   def tag_autocorrect_matches(string)
     # autocorrect uses trigram indexing, which needs at least 3 alphanumeric characters to work.
     return [] if string.remove(/[^a-zA-Z0-9]/).size < 3
@@ -124,6 +165,12 @@ class AutocompleteService
     end
   end
 
+  # Find tags matching a non-English string. Does a `name*` search in wiki page
+  # and artist other names to translate the non-English tag to a Danbooru tag.
+  # Example: 東方 => touhou.
+  #
+  # @param string [String] the string to complete
+  # @return [Array<Hash>] the autocomplete results
   def tag_other_name_matches(string)
     artists = Artist.undeleted.where_any_in_array_starts_with(:other_names, string)
     wikis = WikiPage.undeleted.where_any_in_array_starts_with(:other_names, string)
@@ -137,6 +184,10 @@ class AutocompleteService
     end
   end
 
+  # Complete a metatag.
+  # @param metatag [String] the type of metatag to complete
+  # @param value [String] the value of the metatag
+  # @return [Array<Hash>] the autocomplete results
   def autocomplete_metatag(metatag, value)
     results = case metatag.to_sym
     when :user, :approver, :commenter, :comm, :noter, :noteupdater, :commentaryupdater,
@@ -159,6 +210,10 @@ class AutocompleteService
     end
   end
 
+  # Complete a static metatag: rating, filetype, etc.
+  # @param metatag [String] the type of metatag to complete
+  # @param value [String] the value of the metatag
+  # @return [Array<Hash>] the autocomplete results
   def autocomplete_static_metatag(metatag, value)
     values = STATIC_METATAGS[metatag.to_sym]
     results = values.select { |v| v.starts_with?(value.downcase) }.sort.take(limit)
@@ -168,6 +223,9 @@ class AutocompleteService
     end
   end
 
+  # Complete a pool name. Does a `*name*` search.
+  # @param string [String] the name of the pool
+  # @return [Array<Hash>] the autocomplete results
   def autocomplete_pool(string)
     string = "*" + string + "*" unless string.include?("*")
     pools = Pool.undeleted.name_matches(string).search(order: "post_count").limit(limit)
@@ -177,6 +235,9 @@ class AutocompleteService
     end
   end
 
+  # Complete a favorite group name. Does a `*name*` search.
+  # @param string [String] the name of the favgroup
+  # @return [Array<Hash>] the autocomplete results
   def autocomplete_favorite_group(string)
     string = "*" + string + "*" unless string.include?("*")
     favgroups = FavoriteGroup.visible(current_user).where(creator: current_user).name_matches(string).search(order: "post_count").limit(limit)
@@ -186,6 +247,9 @@ class AutocompleteService
     end
   end
 
+  # Complete a saved search label. Does a `*name*` search.
+  # @param string [String] the name of the label
+  # @return [Array<Hash>] the autocomplete results
   def autocomplete_saved_search_label(string)
     string = "*" + string + "*" unless string.include?("*")
     labels = current_user.saved_searches.labels_like(string).take(limit)
@@ -195,6 +259,9 @@ class AutocompleteService
     end
   end
 
+  # Complete an artist name. Does a `name*` search.
+  # @param string [String] the name of the artist
+  # @return [Array<Hash>] the autocomplete results
   def autocomplete_artist(string)
     string = string + "*" unless string.include?("*")
     artists = Artist.undeleted.name_matches(string).search(order: "post_count").limit(limit)
@@ -204,6 +271,9 @@ class AutocompleteService
     end
   end
 
+  # Complete a wiki name. Does a `name*` search.
+  # @param string [String] the name of the wiki
+  # @return [Array<Hash>] the autocomplete results
   def autocomplete_wiki_page(string)
     string = string + "*" unless string.include?("*")
     wiki_pages = WikiPage.undeleted.title_matches(string).search(order: "post_count").limit(limit)
@@ -213,6 +283,9 @@ class AutocompleteService
     end
   end
 
+  # Complete a user name. Does a `name*` search.
+  # @param string [String] the name of the user
+  # @return [Array<Hash>] the autocomplete results
   def autocomplete_user(string)
     string = string + "*" unless string.include?("*")
     users = User.search(name_matches: string, current_user_first: true, order: "post_upload_count").limit(limit)
@@ -222,17 +295,27 @@ class AutocompleteService
     end
   end
 
+  # Complete an @mention for a user name. Does a `name*` search.
+  # @param string [String] the name of the user
+  # @return [Array<Hash>] the autocomplete results
   def autocomplete_mention(string)
     autocomplete_user(string).map do |result|
       { **result, value: "@" + result[:value] }
     end
   end
 
+  # Complete a search typed in the browser address bar.
+  # @param string [String] the name of the tag
+  # @return [Array<(String, [Array<String>])>] the autocomplete results
+  # @see https://en.wikipedia.org/wiki/OpenSearch
+  # @see https://developer.mozilla.org/en-US/docs/Web/OpenSearch
   def autocomplete_opensearch(string)
     results = autocomplete_tag(string).map { |result| result[:value] }
     [query, results]
   end
 
+  # How long autocomplete results can be cached. Cache short result lists (<10
+  # results) for less time because they're more likely to change.
   def cache_duration
     if autocomplete_results.size == limit
       24.hours
@@ -241,6 +324,7 @@ class AutocompleteService
     end
   end
 
+  # Whether the results can be safely cached with `Cache-Control: public`.
   # Queries that don't depend on the current user are safe to cache publicly.
   def cache_publicly?
     if type == :tag_query && parsed_search&.type == :tag
diff --git a/app/logical/bigquery_export_service.rb b/app/logical/bigquery_export_service.rb
index 7320eb2c9..23cb56351 100644
--- a/app/logical/bigquery_export_service.rb
+++ b/app/logical/bigquery_export_service.rb
@@ -1,22 +1,40 @@
-# Export all public data in a model to BigQuery and to Google Cloud Storage.
-
+# Perform a daily database dump to BigQuery and to Google Cloud Storage. This
+# contains all data visible to anonymous users.
+#
+# The database dumps are publicly accessible. The BigQuery data is at
+# `danbooru1.danbooru_public.{table}`. The Google Cloud Storage data is at
+# `gs://danbooru_public/data/{table}.json`. The storage bucket contains the data
+# in newline-delimited JSON format.
+#
+# @see DanbooruMaintenance#daily
+# @see https://console.cloud.google.com/storage/browser/danbooru_public
+# @see https://console.cloud.google.com/bigquery?d=danbooru_public&p=danbooru1&t=posts&page=table
+# @see https://cloud.google.com/bigquery/docs
+# @see https://cloud.google.com/storage/docs
+# @see https://en.wikipedia.org/wiki/JSON_streaming#Line-delimited_JSON
 class BigqueryExportService
   extend Memoist
 
   attr_reader :model, :dataset_name, :credentials
 
+  # Prepare to dump a table. Call {#export!} to dump it.
+  # @param model [ApplicationRecord] the database table to dump
+  # @param dataset_name [String] the BigQuery dataset name
+  # @param credentials [String] the Google Cloud credentials (in JSON format)
   def initialize(model = nil, dataset_name: "danbooru_public", credentials: default_credentials)
     @model = model
     @dataset_name = dataset_name
     @credentials = credentials
   end
 
+  # Start a background job for each table to export it to BigQuery.
   def self.async_export_all!(**options)
     models.each do |model|
       BigqueryExportJob.perform_later(model: model, **options)
     end
   end
 
+  # The list of database tables to dump.
   def self.models
     Rails.application.eager_load!
 
@@ -29,6 +47,7 @@ class BigqueryExportService
     credentials.present?
   end
 
+  # Dump the table to Cloud Storage and BigQuery.
   def export!
     return unless enabled? && records.any?
 
@@ -36,7 +55,7 @@ class BigqueryExportService
     upload_to_bigquery!(file)
   end
 
-  # Dump the model records to a gzipped, newline-delimited JSON tempfile.
+  # Dump the table's records to a gzipped, newline-delimited JSON tempfile.
   def dump_records!
     file = Tempfile.new("danbooru-export-dump-", binmode: true)
     file = Zlib::GzipWriter.new(file)
@@ -51,8 +70,7 @@ class BigqueryExportService
     file
   end
 
-  # GCS: gs://danbooru_public/data/{model}.json
-  # BQ: danbooru1.danbooru_public.{model}
+  # Upload the JSON dump to Cloud Storage, then load it into BigQuery.
   def upload_to_bigquery!(file)
     table_name = model.model_name.collection
     gsfilename = "data/#{table_name}.json"
@@ -64,24 +82,27 @@ class BigqueryExportService
     job
   end
 
-  # private
-
+  # The list of records to dump.
   def records
     model.visible(User.anonymous)
   end
 
+  # Find or create the BigQuery dataset.
   def dataset
     bigquery.dataset(dataset_name) || bigquery.create_dataset(dataset_name)
   end
 
+  # Find or create the Google Storage bucket.
   def bucket
     storage.bucket(dataset_name) || storage.create_bucket(dataset_name, acl: "public", default_acl: "public", storage_class: "standard", location: "us-east1")
   end
 
+  # The BigQuery API client.
   def bigquery
     Google::Cloud::Bigquery.new(credentials: credentials)
   end
 
+  # The Cloud Storage API client.
   def storage
     Google::Cloud::Storage.new(credentials: credentials)
   end
diff --git a/app/logical/bulk_update_request_processor.rb b/app/logical/bulk_update_request_processor.rb
index ea77175fb..332e90f3f 100644
--- a/app/logical/bulk_update_request_processor.rb
+++ b/app/logical/bulk_update_request_processor.rb
@@ -1,3 +1,5 @@
+# Process a bulk update request. Parses the request and applies each line in
+# sequence.
 class BulkUpdateRequestProcessor
   # Maximum tag size allowed by the rename command before an alias must be used.
   MAXIMUM_RENAME_COUNT = 200
@@ -17,6 +19,7 @@ class BulkUpdateRequestProcessor
   validate :validate_script
   validate :validate_script_length
 
+  # @param bulk_update_request [String] the BUR
   def initialize(bulk_update_request)
     @bulk_update_request = bulk_update_request
   end
@@ -50,6 +53,8 @@ class BulkUpdateRequestProcessor
     end
   end
 
+  # Validate the bulk update request when it is created or approved.
+  #
   # validation_context will be either :request (when the BUR is first created
   # or edited) or :approval (when the BUR is approved). Certain validations
   # only run when the BUR is requested, not when it's approved.
@@ -117,12 +122,15 @@ class BulkUpdateRequestProcessor
     end
   end
 
+  # Validate that the script isn't too long.
   def validate_script_length
     if commands.size > MAXIMUM_SCRIPT_LENGTH
       errors.add(:base, "Bulk update request is too long (maximum size: #{MAXIMUM_SCRIPT_LENGTH} lines). Split your request into smaller chunks and try again.")
     end
   end
 
+  # Apply the script.
+  # @param approver [User] the approver of the request
   def process!(approver)
     ActiveRecord::Base.transaction do
       commands.map do |command, *args|
@@ -162,6 +170,8 @@ class BulkUpdateRequestProcessor
     end
   end
 
+  # The list of tags in the script. Used for search BURs by tag.
+  # @return [Tag] the list of tags
   def affected_tags
     commands.flat_map do |command, *args|
       case command
@@ -178,6 +188,7 @@ class BulkUpdateRequestProcessor
     end.sort.uniq
   end
 
+  # Returns true if a non-Admin is allowed to approve a rename or alias request.
   def is_tag_move_allowed?
     commands.all? do |command, *args|
       case command
@@ -194,6 +205,8 @@ class BulkUpdateRequestProcessor
     end
   end
 
+  # Convert the BUR to DText format.
+  # @return [String]
   def to_dtext
     commands.map do |command, *args|
       case command
diff --git a/app/logical/bulk_update_request_pruner.rb b/app/logical/bulk_update_request_pruner.rb
index 38ed0c5c0..88aa75937 100644
--- a/app/logical/bulk_update_request_pruner.rb
+++ b/app/logical/bulk_update_request_pruner.rb
@@ -1,6 +1,8 @@
+# Rejects bulk update requests that haven't been approved in 60 days.
 module BulkUpdateRequestPruner
   module_function
 
+  # Posts a warning when a bulk update request is pending automatic rejection in 5 days.
   def warn_old
     BulkUpdateRequest.old.pending.find_each do |bulk_update_request|
       if bulk_update_request.forum_topic
@@ -12,6 +14,7 @@ module BulkUpdateRequestPruner
     end
   end
 
+  # Rejects bulk update requests that haven't been approved in 60 days.
   def reject_expired
     BulkUpdateRequest.expired.pending.find_each do |bulk_update_request|
       ApplicationRecord.transaction do
diff --git a/app/logical/cache.rb b/app/logical/cache.rb
index c447ccd88..459d063da 100644
--- a/app/logical/cache.rb
+++ b/app/logical/cache.rb
@@ -1,4 +1,17 @@
+# A wrapper around `Rails.cache` that adds some extra helper methods. All
+# Danbooru code should use this class instead of using `Rails.cache` directly.
+#
+# In production, the cache is backed by Redis. In development, it's a temporary
+# in-memory cache.
+#
+# @see config/initializers/cache_store.rb
+# @see https://guides.rubyonrails.org/caching_with_rails.html#cache-stores
+# @see https://api.rubyonrails.org/classes/ActiveSupport/Cache/RedisCacheStore.html
 class Cache
+  # Get multiple values from the cache at once.
+  # @param keys [Array<String>] the list of keys to fetch
+  # @param prefix [String] a prefix for each cache key
+  # @return [Hash<String, Object>] a map from cache keys to cache values
   def self.get_multi(keys, prefix)
     sanitized_key_to_key_hash = keys.map do |key|
       ["#{prefix}:#{Cache.hash(key)}", key]
@@ -14,28 +27,41 @@ class Cache
     keys_to_values_hash
   end
 
+  # Get a value from the cache. If the value isn't in the cache, use the block
+  # to generate the value.
+  # @param key [String] the key to fetch
+  # @param expiry_in_seconds [Integer] the lifetime of the cached value
+  # @param options [Hash] options to pass to Rails.cache.fetch
+  # @return the cached value
   def self.get(key, expiry_in_seconds = nil, **options, &block)
     Rails.cache.fetch(key, expires_in: expiry_in_seconds, **options, &block)
   end
 
+  # Write a value to the cache.
+  # @param key [String] the key to cache
+  # @param value [Object] the value to cache
+  # @param expiry_in_seconds [Integer] the lifetime of the cached value
+  # @return the cached value
   def self.put(key, value, expiry_in_seconds = nil)
     Rails.cache.write(key, value, expires_in: expiry_in_seconds)
     value
   end
 
+  # Remove a value from the cache.
+  # @param key [String] the key to remove
   def self.delete(key)
     Rails.cache.delete(key)
     nil
   end
 
+  # Clear the entire cache.
   def self.clear
     Rails.cache.clear
   end
 
-  def self.sanitize(key)
-    key.gsub(/\W/) {|x| "%#{x.ord}"}.slice(0, 230)
-  end
-
+  # Hash a cache key.
+  # @param string [String] the cache key to hash
+  # @return [String] the hashed key
   def self.hash(string)
     Digest::SHA256.base64digest(string)
   end
diff --git a/app/logical/cloudflare_service.rb b/app/logical/cloudflare_service.rb
index 8cc1be195..585b43ac2 100644
--- a/app/logical/cloudflare_service.rb
+++ b/app/logical/cloudflare_service.rb
@@ -1,3 +1,5 @@
+# A simple Cloudflare API client for purging cached images after they're
+# regenerated or deleted.
 class CloudflareService
   attr_reader :api_token, :zone
 
@@ -9,6 +11,9 @@ class CloudflareService
     api_token.present? && zone.present?
   end
 
+  # Purge a list of URLs from Cloudflare's cache.
+  # @param urls [Array<String>] the list of URLs to purge
+  # @see https://api.cloudflare.com/#zone-purge-files-by-url
   def purge_cache(urls)
     return unless enabled?
 
diff --git a/app/logical/color_palette.rb b/app/logical/color_palette.rb
index baa106c54..abe75562c 100644
--- a/app/logical/color_palette.rb
+++ b/app/logical/color_palette.rb
@@ -1,3 +1,18 @@
+# Generates a set of CSS variables for site's color palette.
+#
+# The CSS variables are named like `var(--{hue}-{i})`, where `hue` is the color
+# name (see below) and `i` is the brightless level (0-9, 0 is white and 9 is
+# black).
+#
+# Based on the HSLuv color space. The color palette is designed to be
+# perceptually uniform, that is, to have equal brightness levels across each
+# color.
+#
+# @see https://www.hsluv.org/
+# @see https://en.wikipedia.org/wiki/HSLuv
+# @see https://github.com/hsluv/hsluv
+# @see https://danbooru.donmai.us/static/colors
+# @see app/javascript/src/styles/base/040_colors.css
 module ColorPalette
   module_function
 
diff --git a/app/logical/current_user.rb b/app/logical/current_user.rb
index f40c679e0..6846b3f52 100644
--- a/app/logical/current_user.rb
+++ b/app/logical/current_user.rb
@@ -1,3 +1,15 @@
+# A global variable containing the current user, the current IP address, the
+# current user's country code, and whether safe mode is enabled.
+#
+# The current user is set during a request by {ApplicationController#set_current_user},
+# which calls {SessionLoader#load}. The current user will not be set outside of
+# the request cycle, for example, during background jobs, cron jobs, or on the
+# console. For this reason, code outside of controllers and views, such as
+# models, shouldn't rely on or assume the current user exists.
+#
+# @see https://api.rubyonrails.org/classes/ActiveSupport/CurrentAttributes.html
+# @see ApplicationController#set_current_user
+# @see SessionLoader#load
 class CurrentUser < ActiveSupport::CurrentAttributes
   attribute :user, :ip_addr, :country, :safe_mode
 
@@ -5,6 +17,10 @@ class CurrentUser < ActiveSupport::CurrentAttributes
   delegate :id, to: :user, allow_nil: true
   delegate_missing_to :user
 
+  # Run a block of code as another user.
+  # @param user [User] the user to run as
+  # @param ip_addr [String] the IP address to run as
+  # @yield the block
   def self.scoped(user, ip_addr = "127.0.0.1", &block)
     set(user: user, ip_addr: ip_addr) do
       yield user
diff --git a/app/logical/d_text.rb b/app/logical/d_text.rb
index 61c34db6f..222d51795 100644
--- a/app/logical/d_text.rb
+++ b/app/logical/d_text.rb
@@ -1,20 +1,38 @@
 require 'cgi'
 require 'uri'
 
+# The DText class handles Danbooru's markup language, DText. Parsing DText is
+# handled by the DTextRagel class in the dtext_rb gem.
+#
+# @see https://github.com/evazion/dtext_rb
+# @see https://danbooru.donmai.us/wiki_pages/help:dtext
 class DText
   MENTION_REGEXP = /(?<=^| )@\S+/
 
-  def self.format_text(text, data: nil, **options)
+  # Convert a string of DText to HTML.
+  # @param text [String] The DText input
+  # @param inline [Boolean] if true, allow only inline constructs. Ignore
+  #   block-level constructs, such as paragraphs, quotes, lists, and tables.
+  # @param disable_mentions [Boolean] if true, don't generate @mentions.
+  # @param base_url [String, nil] if present, convert relative URLs to absolute URLs.
+  # @param data cached wiki/tag/artist data generated by {#preprocess}.
+  # @return [String, nil] The HTML output
+  def self.format_text(text, data: nil, inline: false, disable_mentions: false, base_url: nil)
     return nil if text.nil?
     data = preprocess([text]) if data.nil?
     text = parse_embedded_tag_request(text)
-    html = DTextRagel.parse(text, **options)
+    html = DTextRagel.parse(text, inline: inline, disable_mentions: disable_mentions, base_url: base_url)
     html = postprocess(html, *data)
     html
   rescue DTextRagel::Error
     ""
   end
 
+  # Preprocess a set of DText messages and collect all tag, artist, and wiki
+  # page references. Called before rendering a collection of DText messages
+  # (e.g. comments or forum posts) to do all database lookups in one batch.
+  # @param [Array<String>] a list of DText strings
+  # @return an array of wiki pages, tags, and artists
   def self.preprocess(dtext_messages)
     dtext_messages = dtext_messages.map { |message| parse_embedded_tag_request(message) }
     names = dtext_messages.map { |message| parse_wiki_titles(message) }.flatten.uniq
@@ -25,6 +43,11 @@ class DText
     [wiki_pages, tags, artists]
   end
 
+  # Rewrite the HTML produced by {#format_text} to colorize wiki links.
+  # @param wiki_pages [Array<WikiPage>]
+  # @param tags [Array<Tag>]
+  # @param artists [Array<Artist>]
+  # @return [String] the HTML output
   def self.postprocess(html, wiki_pages, tags, artists)
     fragment = Nokogiri::HTML.fragment(html)
 
@@ -69,11 +92,18 @@ class DText
     fragment.to_s
   end
 
+  # Wrap a DText message in a [quote] block.
+  # @param message [String] the DText to quote
+  # @param creator_name [String] the name of the user to quote.
+  # @return [String] the quoted DText
   def self.quote(message, creator_name)
     stripped_body = DText.strip_blocks(message, "quote")
     "[quote]\n#{creator_name} said:\n\n#{stripped_body}\n[/quote]\n\n"
   end
 
+  # Convert `[bur:<id>]`, `[ta:<id>]`, `[ti:<id>]` tags to DText.
+  # @param text [String] the DText input
+  # @return [String] the DText output
   def self.parse_embedded_tag_request(text)
     text = parse_embedded_tag_request_type(text, TagAlias, /\[ta:(?<id>\d+)\]/m)
     text = parse_embedded_tag_request_type(text, TagImplication, /\[ti:(?<id>\d+)\]/m)
@@ -81,6 +111,11 @@ class DText
     text
   end
 
+  # Convert a `[bur:<id>]`, `[ta:<id>]`, or `[ti:<id>]` tag to DText.
+  # @param text [String] the DText input
+  # @param tag_request [BulkUpdateRequest, TagAlias, TagImplication]
+  # @param pattern [Regexp]
+  # @return [String] the DText output
   def self.parse_embedded_tag_request_type(text, tag_request, pattern)
     text.gsub(pattern) do |match|
       obj = tag_request.find_by_id($~[:id])
@@ -88,6 +123,9 @@ class DText
     end
   end
 
+  # Convert a `[bur:<id>]`, `[ta:<id>]`, or `[ti:<id>]` tag to DText.
+  # @param obj [BulkUpdateRequest, TagAlias, TagImplication] the object to convert
+  # @return [String] the DText output
   def self.tag_request_message(obj)
     if obj.is_a?(TagRelationship)
       if obj.is_active?
@@ -118,6 +156,9 @@ class DText
     end
   end
 
+  # Return a list of user names mentioned in a string of DText. Ignore mentions in [quote] blocks.
+  # @param text [String] the string of DText
+  # @return [Array<String>] the list of user names
   def self.parse_mentions(text)
     text = strip_blocks(text.to_s, "quote")
 
@@ -128,6 +169,9 @@ class DText
     names.uniq
   end
 
+  # Return a list of wiki pages mentioned in a string of DText.
+  # @param text [String] the string of DText
+  # @return [Array<String>] the list of wiki page names
   def self.parse_wiki_titles(text)
     html = DTextRagel.parse(text)
     fragment = Nokogiri::HTML.fragment(html)
@@ -142,6 +186,9 @@ class DText
     titles.uniq
   end
 
+  # Return a list of external links mentioned in a string of DText.
+  # @param text [String] the string of DText
+  # @return [Array<String>] the list of external URLs
   def self.parse_external_links(text)
     html = DTextRagel.parse(text)
     fragment = Nokogiri::HTML.fragment(html)
@@ -150,6 +197,10 @@ class DText
     links.uniq
   end
 
+  # Return whether the two strings of DText have the same set of links.
+  # @param a [String] a string of DText
+  # @param b [String] a string of DText
+  # @return [Boolean]
   def self.dtext_links_differ?(a, b)
     Set.new(parse_wiki_titles(a)) != Set.new(parse_wiki_titles(b)) ||
       Set.new(parse_external_links(a)) != Set.new(parse_external_links(b))
@@ -159,6 +210,10 @@ class DText
   # the capitalization of the old tag when rewriting it to the new tag, but if
   # we can't determine how the new tag should be capitalized based on some
   # simple heuristics, then we skip rewriting the tag.
+  # @param dtext [String] the DText input
+  # @param old_name [String] the old wiki name
+  # @param new_name [String] the new wiki name
+  # @return [String] the DText output
   def self.rewrite_wiki_links(dtext, old_name, new_name)
     old_name = old_name.downcase.squeeze("_").tr("_", " ").strip
     new_name = new_name.downcase.squeeze("_").tr("_", " ").strip
@@ -201,6 +256,10 @@ class DText
     end
   end
 
+  # Remove all [<tag>] blocks from the DText.
+  # @param string [String] the DText input
+  # @param tag [String] the type of block to remove
+  # @return [String] the DText output
   def self.strip_blocks(string, tag)
     n = 0
     stripped = ""
@@ -229,12 +288,18 @@ class DText
     stripped.strip
   end
 
+  # Remove all DText formatting from a string of DText, converting it to plain text.
+  # @param dtext [String] the DText input
+  # @return [String] the plain text output
   def self.strip_dtext(dtext)
     html = DTextRagel.parse(dtext)
     text = to_plaintext(html)
     text
   end
 
+  # Remove all formatting from a string of HTML, converting it to plain text.
+  # @param html [String] the HTML input
+  # @return [String] the plain text output
   def self.to_plaintext(html)
     text = from_html(html) do |node|
       case node.name
@@ -250,10 +315,16 @@ class DText
     text.gsub(/\A[[:space:]]+|[[:space:]]+\z/, "")
   end
 
+  # Convert DText formatting to Markdown.
+  # @param dtext [String] the DText input
+  # @return [String] the Markdown output
   def self.to_markdown(dtext)
     html_to_markdown(format_text(dtext))
   end
 
+  # Convert HTML to Markdown.
+  # @param html [String] the HTML input
+  # @return [String] the Markdown output
   def self.html_to_markdown(html)
     html = Nokogiri::HTML.fragment(html)
 
@@ -273,6 +344,10 @@ class DText
     end.join
   end
 
+  # Convert HTML to DText.
+  # @param html [String] the HTML input
+  # @param inline [Boolean] if true, convert <img> tags to plaintext
+  # @return [String] the DText output
   def self.from_html(text, inline: false, &block)
     html = Nokogiri::HTML.fragment(text)
 
@@ -336,13 +411,19 @@ class DText
     dtext
   end
 
-  # extract the first paragraph `needle` occurs in.
+  # Return the first paragraph the search string `needle` occurs in.
+  # @param needle [String] the string to search for
+  # @param dtext [String] the DText input
+  # @return [String] the first paragraph mentioning the search string
   def self.extract_mention(dtext, needle)
     dtext = dtext.gsub(/\r\n|\r|\n/, "\n")
     excerpt = ActionController::Base.helpers.excerpt(dtext, needle, separator: "\n\n", radius: 1, omission: "")
     excerpt
   end
 
+  # Generate a short plain text excerpt from a DText string.
+  # @param length [Integer] the max length of the output
+  # @return [String] a plain text string
   def self.excerpt(text, length: 160)
     strip_dtext(text).split(/\r\n|\r|\n/).first.to_s.truncate(length)
   end
diff --git a/app/logical/danbooru_logger.rb b/app/logical/danbooru_logger.rb
index 30989d162..e20b6f58c 100644
--- a/app/logical/danbooru_logger.rb
+++ b/app/logical/danbooru_logger.rb
@@ -1,6 +1,13 @@
+# The DanbooruLogger class handles logging messages to the Rails log and to NewRelic.
+#
+# @see https://guides.rubyonrails.org/debugging_rails_applications.html#the-logger
+# @see https://docs.newrelic.com
 class DanbooruLogger
   HEADERS = %w[referer sec-fetch-dest sec-fetch-mode sec-fetch-site sec-fetch-user]
 
+  # Log a message to the Rails log and to NewRelic.
+  # @param message [String] the message to log
+  # @param params [Hash] optional key-value data to log with the message
   def self.info(message, params = {})
     Rails.logger.info(message)
 
@@ -10,6 +17,13 @@ class DanbooruLogger
     end
   end
 
+  # Log an exception to the Rails log and to NewRelic. The `expected` flag is
+  # used to separate expected exceptions, like search timeouts or auth failures,
+  # from unexpected exceptions, like runtime errors, in the NewRelic error log.
+  #
+  # @param message [Exception] the exception to log
+  # @param expected [Boolean] whether the exception was expected
+  # @param params [Hash] optional key-value data to log with the exception
   def self.log(exception, expected: false, **params)
     if expected
       Rails.logger.info("#{exception.class}: #{exception.message}")
@@ -23,6 +37,12 @@ class DanbooruLogger
     end
   end
 
+  # Log extra HTTP request data to NewRelic. Logs the user's IP, user agent,
+  # request params, and session cookies.
+  #
+  # @param request the HTTP request
+  # @param session the Rails session
+  # @param user [User] the current user
   def self.add_session_attributes(request, session, user)
     add_attributes("request", { path: request.path })
     add_attributes("request.headers", header_params(request))
@@ -31,6 +51,7 @@ class DanbooruLogger
     add_attributes("user", user_params(request, user))
   end
 
+  # Get logged HTTP headers from request.
   def self.header_params(request)
     headers = request.headers.to_h.select { |header, value| header.match?(/\AHTTP_/) }
     headers = headers.transform_keys { |header| header.delete_prefix("HTTP_").downcase }
@@ -65,6 +86,7 @@ class DanbooruLogger
 
   private_class_method
 
+  # @see https://docs.newrelic.com/docs/using-new-relic/data/customize-data/collect-custom-attributes/#ruby-att
   def self.add_custom_attributes(attributes)
     return unless defined?(::NewRelic)
     ::NewRelic::Agent.add_custom_attributes(attributes)
diff --git a/app/logical/deviant_art_api_client.rb b/app/logical/deviant_art_api_client.rb
index 53abf1c8a..4b5fd3139 100644
--- a/app/logical/deviant_art_api_client.rb
+++ b/app/logical/deviant_art_api_client.rb
@@ -1,3 +1,5 @@
+# A DeviantArt API client.
+#
 # Authentication is via OAuth2 with the client credentials grant. Register a
 # new app at https://www.deviantart.com/developers/ to obtain a client_id and
 # client_secret. The app doesn't need to be published.
diff --git a/app/logical/diff_builder.rb b/app/logical/diff_builder.rb
index 1d623261f..073f5a1a9 100644
--- a/app/logical/diff_builder.rb
+++ b/app/logical/diff_builder.rb
@@ -1,3 +1,4 @@
+# Builds an HTML diff between two pieces of text.
 class DiffBuilder
   attr_reader :this_text, :that_text, :pattern
 
diff --git a/app/logical/discord_api_client.rb b/app/logical/discord_api_client.rb
index 3ecc59f26..c2a2aab9f 100644
--- a/app/logical/discord_api_client.rb
+++ b/app/logical/discord_api_client.rb
@@ -1,3 +1,6 @@
+# A Discord API Client.
+#
+# @see https://discord.com/developers/docs/intro
 class DiscordApiClient
   extend Memoist
 
diff --git a/app/logical/discord_slash_command.rb b/app/logical/discord_slash_command.rb
index e8e35bffb..78ffbd11e 100644
--- a/app/logical/discord_slash_command.rb
+++ b/app/logical/discord_slash_command.rb
@@ -1,3 +1,21 @@
+# The parent class for a Discord slash command. Each slash command handled by
+# Danbooru is a subclass of this class. Subclasses should set the {#name},
+# {#description}, and {#options} class attributes defining the slash command's
+# parameters, then implement the {#call} method to return a response to the
+# command.
+#
+# The lifecycle of a Discord slash command is this:
+#
+# * First we register the command with Discord with an API call ({DiscordApiClient#register_slash_command}).
+# * Then whenever someone uses the command, Discord sends us a HTTP request to
+#   https://danbooru.donmai.us/webhooks/receive.
+# * We validate the request really came from Discord, then return the
+#   command's response.
+#
+# @abstract
+# @see DiscordApiClient
+# @see WebhooksController#receive
+# @see https://discord.com/developers/docs/interactions/slash-commands
 class DiscordSlashCommand
   class WebhookVerificationError < StandardError; end
 
@@ -47,7 +65,7 @@ class DiscordSlashCommand
   end
 
   concerning :HelperMethods do
-    # The parameters passed to the command. A hash.
+    # @return [Hash] The parameters passed to the slash command by the Discord user.
     def params
       @params ||= data.dig(:data, :options).to_a.map do |opt|
         [opt[:name], opt[:value]]
diff --git a/app/logical/email_validator.rb b/app/logical/email_validator.rb
index 6e4b60194..780ebe63c 100644
--- a/app/logical/email_validator.rb
+++ b/app/logical/email_validator.rb
@@ -1,14 +1,25 @@
 require 'resolv'
 
+# Validates that an email address is well-formed, is deliverable, and is not a
+# disposable or throwaway email address. Also normalizes equivalent addresses to
+# a single canonical form, so that users can't use different forms of the same
+# address to register multiple accounts.
 module EmailValidator
   module_function
 
   # https://www.regular-expressions.info/email.html
   EMAIL_REGEX = /\A[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}\z/
 
+  # Sites that ignore dots in email addresses, e.g. where `te.st@gmail.com` is
+  # the same as `test@gmail.com`.
   IGNORE_DOTS = %w[gmail.com]
+
+  # Sites that allow plus addressing, e.g. `test+nospam@gmail.com`.
+  # @see https://en.wikipedia.org/wiki/Email_address#Subaddressing
   IGNORE_PLUS_ADDRESSING = %w[gmail.com hotmail.com outlook.com live.com]
   IGNORE_MINUS_ADDRESSING = %w[yahoo.com]
+
+  # Sites that have multiple domains mapping to the same logical email address.
   CANONICAL_DOMAINS = {
     "googlemail.com" => "gmail.com",
     "hotmail.co.uk" => "outlook.com",
@@ -66,10 +77,16 @@ module EmailValidator
     "gmx.us" => "gmx.net",
   }
 
+  # Returns true if it's okay to connect to port 25. Disabled outside of
+  # production because many home ISPs blackhole port 25.
   def smtp_enabled?
     Rails.env.production?
   end
 
+  # Normalize an email address by stripping out plus addressing and dots, if
+  # applicable, and rewriting the domain to a canonical domain.
+  # @param address [String] the email address to normalize
+  # @return [String] the normalized address
   def normalize(address)
     return nil unless address.count("@") == 1
 
@@ -83,10 +100,16 @@ module EmailValidator
     "#{name}@#{domain}"
   end
 
+  # Returns true if the email address is correctly formatted.
+  # @param [String] the email address
+  # @return [Boolean]
   def is_valid?(address)
     address.match?(EMAIL_REGEX)
   end
 
+  # Returns true if the email is a throwaway or disposable email address.
+  # @param [String] the email address
+  # @return [Boolean]
   def is_restricted?(address)
     return false if Danbooru.config.email_domain_verification_list.blank?
 
@@ -96,6 +119,11 @@ module EmailValidator
     true
   end
 
+  # Returns true if the email can't be delivered. Checks if the domain has an MX
+  # record and responds to the RCPT TO command.
+  # @param to_address [String] the email address to check
+  # @param from_address [String] the email address to check from
+  # @return [Boolean]
   def undeliverable?(to_address, from_address: Danbooru.config.contact_email, timeout: 3)
     mail_server = mx_domain(to_address, timeout: timeout)
     mail_server.nil? || rcpt_to_failed?(to_address, from_address, mail_server, timeout: timeout)
@@ -103,6 +131,13 @@ module EmailValidator
     false
   end
 
+  # Returns true if the email can't be delivered. Sends a RCPT TO command over
+  # port 25 to check if the mailbox exists.
+  # @param to_address [String] the email address to check
+  # @param from_address [String] the email address to check from
+  # @param mail_server [String] the DNS name of the SMTP server
+  # @param timeout [Integer] the network timeout
+  # @return [Boolean]
   def rcpt_to_failed?(to_address, from_address, mail_server, timeout: nil)
     return false unless smtp_enabled?
 
@@ -121,6 +156,11 @@ module EmailValidator
     end
   end
 
+  # Does a DNS MX record lookup of the domain in the email address and returns the
+  # name of the mail server, if it exists.
+  # @param to_address [String] the email address to check
+  # @param timeout [Integer] the network timeout
+  # @return [String] the DNS name of the mail server
   def mx_domain(to_address, timeout: nil)
     domain = Mail::Address.new(to_address).domain
 
diff --git a/app/logical/explain_parser.rb b/app/logical/explain_parser.rb
index 2afc6f57f..65b7f3564 100644
--- a/app/logical/explain_parser.rb
+++ b/app/logical/explain_parser.rb
@@ -1,21 +1,48 @@
+# Parse the output of the Postgres EXPLAIN command to get how many rows Postgres
+# *thinks* a SQL query will return. This is an estimate, and only accurate for
+# queries that have a single condition. If the query has multiple conditions,
+# then Postgres will assume they're independent (have no overlap), which is not
+# always the case.
+#
+# Used by {PostQueryBuilder#fast_count} to get a fast post count estimate for
+# certain single metatag searches.
+#
+# ExplainParser.new(Post.all).query_plan
+# => EXPLAIN (FORMAT JSON) SELECT "posts".* FROM "posts"
+# => {
+#    "Node Type"=>"Seq Scan",
+#    "Parallel Aware"=>false,
+#    "Relation Name"=>"posts",
+#    "Alias"=>"posts",
+#    "Startup Cost"=>0.0,
+#    "Total Cost"=>780900.02,
+#    "Plan Rows"=>4413102,
+#    "Plan Width"=>1268
+#  }
+#
+# @see https://www.postgresql.org/docs/current/sql-explain.html
 class ExplainParser
   extend Memoist
   attr_reader :relation
 
+  # @param the ActiveRecord relation
   def initialize(relation)
     @relation = relation
   end
 
+  # @return [Hash] the Postgres query plan
   def query_plan
     result = ApplicationRecord.connection.select_one("EXPLAIN (FORMAT JSON) #{sql}")
     json = JSON.parse(result["QUERY PLAN"])
     json.first["Plan"]
   end
 
+  # @return [Integer] the number of rows Postgres *thinks* the query will return
   def row_count
     query_plan["Plan Rows"]
   end
 
+  # @return [String] the query's SQL
   def sql
     relation.reorder(nil).to_sql
   end
diff --git a/app/logical/forum_updater.rb b/app/logical/forum_updater.rb
index 423b3a62b..7d2f66f85 100644
--- a/app/logical/forum_updater.rb
+++ b/app/logical/forum_updater.rb
@@ -1,3 +1,4 @@
+# Add a post to a forum topic.
 class ForumUpdater
   attr_reader :forum_topic
 
diff --git a/app/logical/ip_address_type.rb b/app/logical/ip_address_type.rb
index 5cce74f92..f3921e727 100644
--- a/app/logical/ip_address_type.rb
+++ b/app/logical/ip_address_type.rb
@@ -1,13 +1,20 @@
-# See also config/initializers/types.rb
-
+# Ensure ActiveRecord::ConnectionAdapters::PostgreSQL::OID::Inet is loaded in
+# the dev environment when eager loading is disabled.
 require "active_record/connection_adapters/postgresql_adapter"
 
+# Define a custom IP address type for IP columns in the database. IP columns
+# will be serialized and deserialized as a {Danbooru::IpAddress} object.
+#
+# @see config/initializers/types.rb
+# @see https://www.bigbinary.com/blog/rails-5-attributes-api
 class IpAddressType < ActiveRecord::ConnectionAdapters::PostgreSQL::OID::Inet
+  # @param value [String] the IP address from the database
   def cast(value)
     return nil if value.blank?
     super(Danbooru::IpAddress.new(value))
   end
 
+  # @param [Danbooru::IpAddress] the IP address object
   def serialize(value)
     value&.to_string
   end
diff --git a/app/logical/ip_lookup.rb b/app/logical/ip_lookup.rb
index 54aa98c1b..13085b009 100644
--- a/app/logical/ip_lookup.rb
+++ b/app/logical/ip_lookup.rb
@@ -1,5 +1,7 @@
-# API client for https://ipregistry.co/
-
+# An API client for https://ipregistry.co. Looks up IP address information,
+# including the geolocation and whether the IP is a proxy.
+#
+# @see https://ipregistry.co/docs
 class IpLookup
   extend Memoist
 
diff --git a/app/logical/iqdb_client.rb b/app/logical/iqdb_client.rb
index e8ad04861..3906553d6 100644
--- a/app/logical/iqdb_client.rb
+++ b/app/logical/iqdb_client.rb
@@ -1,13 +1,21 @@
+# An API client for Danbooru's internal IQDB instance. Can add images, remove
+# images, and search for images in IQDB.
+#
+# @see https://github.com/danbooru/iqdb
 class IqdbClient
   class Error < StandardError; end
   attr_reader :iqdb_url, :http
 
+  # Create a new IQDB API client.
+  # @param iqdb_url [String] the base URL of the IQDB server
+  # @param http [Danbooru::Http] the HTTP client to use
   def initialize(iqdb_url: Danbooru.config.iqdb_url.to_s, http: Danbooru::Http.new)
     @iqdb_url = iqdb_url.chomp("/")
     @http = http
   end
 
   concerning :QueryMethods do
+    # Search for an image by file, URL, or post ID.
     def search(post_id: nil, file: nil, url: nil, image_url: nil, file_url: nil, similarity: 0.0, high_similarity: 65.0, limit: 20)
       limit = limit.to_i.clamp(1, 1000)
       similarity = similarity.to_f.clamp(0.0, 100.0)
@@ -37,6 +45,10 @@ class IqdbClient
       file.try(:close)
     end
 
+    # Download an URL to a file.
+    # @param url [String] the URL to download
+    # @param type [Symbol] the type of URL to download (:preview_url or full :image_url)
+    # @return [MediaFile] the downloaded file
     def download(url, type)
       strategy = Sources::Strategies.find(url)
       download_url = strategy.send(type)
@@ -44,8 +56,12 @@ class IqdbClient
       file
     end
 
-    def decorate_posts(json)
-      post_ids = json.map { |match| match["post_id"] }
+    # Transform the JSON returned by IQDB to add the full post data for each
+    # match.
+    # @param matches [Array<Hash>] the array of IQDB matches
+    # @return [Array<Hash>] the array of IQDB matches, with post data
+    def decorate_posts(matches)
+      post_ids = matches.map { |match| match["post_id"] }
       posts = Post.where(id: post_ids).group_by(&:id).transform_values(&:first)
 
       json.map do |match|
@@ -55,6 +71,8 @@ class IqdbClient
     end
   end
 
+  # Add a post to IQDB.
+  # @param post [Post] the post to add
   def add_post(post)
     return unless post.has_preview?
     preview_file = post.file(:preview)
@@ -62,20 +80,31 @@ class IqdbClient
   end
 
   concerning :HttpMethods do
+    # Search for an image in IQDB.
+    # @param file [File] the image to search
     def query(file, limit: 20)
       file = HTTP::FormData::File.new(file)
       request(:post, "query", form: { file: file }, params: { limit: limit })
     end
 
+    # Add a post to IQDB.
+    # @param post_id [Integer] the post to add
+    # @param file [File] the image to add
     def add(post_id, file)
       file = HTTP::FormData::File.new(file)
       request(:post, "images/#{post_id}", form: { file: file })
     end
 
+    # Remove an image from IQDB.
+    # @param post_id [Integer] the post to remove
     def remove(post_id)
       request(:delete, "images/#{post_id}")
     end
 
+    # Send a request to IQDB.
+    # @param method [String] the HTTP method
+    # @param url [String] the IQDB url
+    # @param options [Hash] the URL params to send
     def request(method, url, **options)
       return [] if iqdb_url.blank? # do nothing if iqdb isn't configured
       response = http.timeout(30).send(method, "#{iqdb_url}/#{url}", **options)
diff --git a/app/logical/mastodon_api_client.rb b/app/logical/mastodon_api_client.rb
index d6425d999..a8a40c36c 100644
--- a/app/logical/mastodon_api_client.rb
+++ b/app/logical/mastodon_api_client.rb
@@ -1,3 +1,6 @@
+# An API client for Mastodon.
+#
+# @see https://docs.joinmastodon.org/api
 class MastodonApiClient
   extend Memoist
   attr_reader :json
diff --git a/app/logical/nico_seiga_api_client.rb b/app/logical/nico_seiga_api_client.rb
index d307809f4..5979c85fc 100644
--- a/app/logical/nico_seiga_api_client.rb
+++ b/app/logical/nico_seiga_api_client.rb
@@ -1,3 +1,4 @@
+# An API client for the NicoSeiga XML API.
 class NicoSeigaApiClient
   extend Memoist
   XML_API = "https://seiga.nicovideo.jp/api"
diff --git a/app/logical/note_sanitizer.rb b/app/logical/note_sanitizer.rb
index 6405f8283..82ee9e12b 100644
--- a/app/logical/note_sanitizer.rb
+++ b/app/logical/note_sanitizer.rb
@@ -1,3 +1,5 @@
+# Sanitizes the HTML used in notes. Only safe HTML tags, HTML attributes, and
+# CSS properties are allowed.
 module NoteSanitizer
   ALLOWED_ELEMENTS = %w[
     code center tn h1 h2 h3 h4 h5 h6 a span div blockquote br p ul li ol em
@@ -50,6 +52,9 @@ module NoteSanitizer
     vertical-align
   ]
 
+  # Sanitize a string of HTML.
+  # @param text [String] the HTML to sanitize
+  # @return [String] the sanitized HTML
   def self.sanitize(text)
     text.gsub!(/<( |-|3|:|>|\Z)/, "&lt;\\1")
 
@@ -76,6 +81,8 @@ module NoteSanitizer
     )
   end
 
+  # Convert absolute Danbooru links inside notes to relative links.
+  # https://danbooru.donmai.us/posts/1 is converted to /posts/1.
   def self.relativize_links(node:, **env)
     return unless node.name == "a" && node["href"].present?
 
diff --git a/app/logical/pixiv_ajax_client.rb b/app/logical/pixiv_ajax_client.rb
index 9b79525c9..1dbf5c0af 100644
--- a/app/logical/pixiv_ajax_client.rb
+++ b/app/logical/pixiv_ajax_client.rb
@@ -1,4 +1,6 @@
-# Notes on Pixiv's AJAX API:
+# A client for Pixiv's AJAX API.
+#
+# Notes on the API:
 #
 # * The user agent must be spoofed as a browser user agent, otherwise the API
 #   will return an error.
@@ -359,23 +361,40 @@ class PixivAjaxClient
 
   attr_reader :phpsessid, :http
 
+  # @param phpsessid [String] the Pixiv login cookie
+  # @param http [Danbooru::Http] the HTTP client to use for Pixiv
   def initialize(phpsessid, http: Danbooru::Http.new)
     @phpsessid = phpsessid
     @http = http
   end
 
+  # Return the illust data for a Pixiv illustration.
+  # @see https://www.pixiv.net/ajax/illust/87598468
+  # @param illust_id [Integer] the Pixiv illustration id
+  # @return [Hash] the illustration data
   def illust(illust_id)
     get("https://www.pixiv.net/ajax/illust/#{illust_id}").with_indifferent_access
   end
 
+  # Return the data for a Pixiv manga illust with multiple pages.
+  # @see https://www.pixiv.net/ajax/illust/87598468/pages
+  # @param illust_id [Integer] the Pixiv illustration id
+  # @return [Hash] the illustration data
   def pages(illust_id)
     get("https://www.pixiv.net/ajax/illust/#{illust_id}/pages")
   end
 
+  # Return the data for a Pixiv ugoira.
+  # @see https://www.pixiv.net/ajax/illust/74932152/ugoira_meta
+  # @param illust_id [Integer] the Pixiv illustration id
+  # @return [Hash] the ugoira data
   def ugoira_meta(illust_id)
     get("https://www.pixiv.net/ajax/illust/#{illust_id}/ugoira_meta").with_indifferent_access
   end
 
+  # Perform a GET request for a Pixiv URL.
+  # @param url [String] the Pixiv URL
+  # @return [Hash] the parsed response, or blank on error
   def get(url)
     response = client.cache(1.minute).get(url)
 
@@ -387,6 +406,7 @@ class PixivAjaxClient
     end
   end
 
+  # @return [Danbooru::Http] the HTTP client used for Pixiv
   def client
     @client ||= http.headers("User-Agent": USER_AGENT).cookies(PHPSESSID: phpsessid)
   end
diff --git a/app/logical/post_appeal_forum_updater.rb b/app/logical/post_appeal_forum_updater.rb
index cbb3881c5..0e9365307 100644
--- a/app/logical/post_appeal_forum_updater.rb
+++ b/app/logical/post_appeal_forum_updater.rb
@@ -1,3 +1,6 @@
+# Posts new appeals to the Deletion appeal thread once per hour.
+#
+# @see DanbooruMaintenance#hourly
 module PostAppealForumUpdater
   APPEAL_TOPIC_TITLE = "Deletion appeal thread"
 
diff --git a/app/logical/post_pruner.rb b/app/logical/post_pruner.rb
index 924fc7152..cb1707d2c 100644
--- a/app/logical/post_pruner.rb
+++ b/app/logical/post_pruner.rb
@@ -1,3 +1,4 @@
+# Delete posts that were unapproved after three days.
 module PostPruner
   module_function
 
diff --git a/app/logical/tag_name_validator.rb b/app/logical/tag_name_validator.rb
index c71d32852..de50d0bdd 100644
--- a/app/logical/tag_name_validator.rb
+++ b/app/logical/tag_name_validator.rb
@@ -1,3 +1,11 @@
+# Define a custom validator for tag names. Tags must be plain ASCII, no spaces,
+# no redundant underscores, no conflicts with metatags, and can't begin with
+# certain special characters.
+#
+# @example
+#   validates :name, tag_name: true
+#
+# @see https://guides.rubyonrails.org/active_record_validations.html#custom-validators
 class TagNameValidator < ActiveModel::EachValidator
   MAX_TAG_LENGTH = 170
 
diff --git a/app/logical/tag_relationship_retirement_service.rb b/app/logical/tag_relationship_retirement_service.rb
index b35403ec7..4d7917a5d 100644
--- a/app/logical/tag_relationship_retirement_service.rb
+++ b/app/logical/tag_relationship_retirement_service.rb
@@ -1,3 +1,8 @@
+# Removes tag aliases and implications if they haven't had any new uploads in
+# the last two years. Runs weekly. Posts a message to the forum when aliases or
+# implications are retired.
+#
+# @see DanbooruMaintenance#weekly
 module TagRelationshipRetirementService
   module_function
 
diff --git a/app/logical/user_name_validator.rb b/app/logical/user_name_validator.rb
index 2df2decf4..06e052d88 100644
--- a/app/logical/user_name_validator.rb
+++ b/app/logical/user_name_validator.rb
@@ -1,3 +1,9 @@
+# Define a custom validator for user names.
+#
+# @example
+#   validates :name, user_name: true
+#
+# @see https://guides.rubyonrails.org/active_record_validations.html#custom-validators
 class UserNameValidator < ActiveModel::EachValidator
   def validate_each(rec, attr, value)
     name = value