Migration.hs

module Seihou.Core.Migration
  ( -- * Author-declared migrations
    Migration (..),
    MigrationOp (..),
    BlueprintMigration (..),

    -- * Migration planning
    MigrationPlan (..),
    BlueprintMigrationPlan (..),
    MigrationPlanError (..),
    planMigrationChain,
    planBlueprintMigrationChain,
  )
where

import Data.List (sortOn)
import GHC.Generics (Generic)
import Seihou.Core.Version (Version, parseVersion)
import Seihou.Prelude

-- | A single filesystem operation declared by a migration.
--
-- The variants mirror the Dhall union @schema/MigrationOp.dhall@ exactly:
--
--   * 'MoveFile'   — rename a tracked file. The migration engine rewrites
--     the manifest's @files@ map key from @src@ to @dest@.
--   * 'MoveDir'    — rename a directory. Every manifest @files@ entry whose
--     path starts with @src/@ has its key rewritten with the @dest/@ prefix.
--   * 'DeleteFile' — remove a tracked file from disk and drop it from the
--     manifest.
--   * 'DeleteDir'  — remove a directory recursively and drop every manifest
--     entry under that prefix.
--   * 'RunCommand' — execute a shell command. The manifest is not rewritten
--     by this op; if the command moves files, the migration author is
--     responsible for following it with explicit move/delete ops.
data MigrationOp
  = MoveFile {src :: FilePath, dest :: FilePath}
  | MoveDir {src :: FilePath, dest :: FilePath}
  | DeleteFile {path :: FilePath}
  | DeleteDir {path :: FilePath}
  | RunCommand {run :: Text, workDir :: Maybe FilePath}
  deriving stock (Eq, Show, Generic)

-- | A migration that moves a project from module version @from@ to module
-- version @to@. The 'ops' list is applied in declaration order.
data Migration = Migration
  { from :: Text,
    to :: Text,
    ops :: [MigrationOp]
  }
  deriving stock (Eq, Show, Generic)

-- | One agent-guided source migration declared by a blueprint. The
-- version strings use the same dotted-numeric format as module migrations,
-- while 'prompt' describes only the changes needed for this edge.
data BlueprintMigration = BlueprintMigration
  { from :: Text,
    to :: Text,
    prompt :: Text
  }
  deriving stock (Eq, Show, Generic)

-- ----------------------------------------------------------------------------
-- Pure planner — gap-tolerant version-window walker
--
-- The planner is the bridge between the unsorted list of author-declared
-- migrations on a 'Module' and the ordered sequence that the migration
-- engine actually executes. It is a pure function: no IO, no filesystem,
-- no manifest. The @moduleName@ parameter is the rendered module name
-- (a 'Text', not 'Seihou.Core.Types.ModuleName') so this module can stay
-- self-contained and avoid a circular dependency with @Types@ (which
-- imports 'Migration' for the @migrations@ field on @Module@). The CLI
-- handler unwraps the 'ModuleName' newtype before calling.
--
-- The planner's contract is simple: given an installed manifest version
-- and a target version, apply every declared migration @m@ such that
-- @installed ≤ m.from@ and @m.to ≤ target@, in ascending @from@ order,
-- advancing a cursor as you go (skipping any edge whose @from@ has
-- fallen behind the cursor). After all applicable edges have been
-- collected, the manifest's recorded version always advances to
-- @target@, even when no migration applies (a "pure version bump").
-- ----------------------------------------------------------------------------

-- | The planner result for a non-trivial @installed → target@ request.
--
-- The plan carries the module name for rendering, the start and end
-- versions of the user-visible "X → Y" header, and the ordered list of
-- migrations that will run. A plan with @planSteps == []@ means the
-- manifest will advance from @planFrom@ to @planTo@ without running
-- any migration ops (a pure version bump).
data MigrationPlan = MigrationPlan
  { planModule :: Text,
    -- | Installed (manifest) version at the start.
    planFrom :: Version,
    -- | Target version. The manifest will land here after the plan
    -- runs, regardless of whether any of the declared migrations
    -- bridge every gap inside @[planFrom, planTo]@.
    planTo :: Version,
    -- | The migrations that actually apply, in ascending @from@
    -- order. May be empty.
    planSteps :: [Migration]
  }
  deriving stock (Eq, Show, Generic)

-- | The ordered blueprint migrations selected for a requested version
-- window. A non-trivial window may have no selected steps when the author
-- declared no agent intervention for that range.
data BlueprintMigrationPlan = BlueprintMigrationPlan
  { blueprintPlanName :: Text,
    blueprintPlanFrom :: Version,
    blueprintPlanTo :: Version,
    blueprintPlanSteps :: [BlueprintMigration]
  }
  deriving stock (Eq, Show, Generic)

-- | All the ways planning can fail. Each carries enough information to
-- write a useful error message at the CLI layer.
data MigrationPlanError
  = -- | A 'Migration' had a 'from' or 'to' string that didn't parse.
    -- Carries the offending string verbatim.
    MigrationVersionUnparseable Text
  | -- | Refusing to plan a downgrade. @installed → target@ where @target@
    -- compares strictly less than @installed@.
    MigrationDowngradeNotSupported Version Version
  | -- | Two migrations declare the same 'from' version, so the planner
    -- can't unambiguously pick a successor. Args: the duplicated 'from'
    -- and one of the conflicting 'to' versions.
    MigrationDuplicateEdge Version Version
  deriving stock (Eq, Show, Generic)

-- | Compute the migration plan that spans installed → target.
--
-- Returns:
--
--   * @Right Nothing@ — installed and target are equal; no work to do.
--   * @Right (Just plan)@ — the plan carries every migration whose
--     version range falls inside @[installed, target]@, plus the
--     installed and target versions for downstream rendering and
--     manifest-advance logic. The list may be empty (a pure version
--     bump where no migration applies); the manifest still advances
--     to @target@ in that case.
--   * @Left e@ — planning failed; the error variant explains why.
--     Author-side mistakes (duplicate edge, unparseable version) and
--     downgrades are still hard errors. Partial coverage is not, and
--     overshoots are silently skipped.
--
-- Algorithm: parse every declared migration's @from@/@to@ into
-- 'Version' values, reject duplicate @from@s, sort the remaining edges
-- by @from@ ascending, then walk the sorted list with a cursor. Each
-- edge is either picked (and the cursor advances to its @to@), skipped
-- because its @from@ has fallen behind the cursor (already covered by
-- an earlier picked edge), skipped because its @to@ overshoots the
-- target (the user hasn't asked to go that far), or terminates the
-- walk because its @from@ has reached or exceeded the target (no
-- subsequent edge in the sorted list can contribute either).
planMigrationChain ::
  -- | Module name (already rendered to text)
  Text ->
  -- | All declared migrations on the module
  [Migration] ->
  -- | Installed version
  Version ->
  -- | Target version
  Version ->
  Either MigrationPlanError (Maybe MigrationPlan)
planMigrationChain modName migrations installed target =
  fmap
    ( fmap
        ( \steps ->
            MigrationPlan
              { planModule = modName,
                planFrom = installed,
                planTo = target,
                planSteps = steps
              }
        )
    )
    (planMigrationWindow (.from) (.to) migrations installed target)

-- | Compute the ordered agent-guided migrations for a blueprint and version
-- window. Selection and errors deliberately match 'planMigrationChain'.
planBlueprintMigrationChain ::
  Text ->
  [BlueprintMigration] ->
  Version ->
  Version ->
  Either MigrationPlanError (Maybe BlueprintMigrationPlan)
planBlueprintMigrationChain blueprintName migrations current target =
  fmap
    ( fmap
        ( \steps ->
            BlueprintMigrationPlan
              { blueprintPlanName = blueprintName,
                blueprintPlanFrom = current,
                blueprintPlanTo = target,
                blueprintPlanSteps = steps
              }
        )
    )
    (planMigrationWindow (.from) (.to) migrations current target)

-- | Shared gap-tolerant version-window planner. Keeping parsing, duplicate
-- detection, ordering, overlap handling, and overshoot handling here prevents
-- module and blueprint migrations from developing subtly different rules.
planMigrationWindow ::
  (a -> Text) ->
  (a -> Text) ->
  [a] ->
  Version ->
  Version ->
  Either MigrationPlanError (Maybe [a])
planMigrationWindow getFrom getTo migrations current target
  | current == target = Right Nothing
  | target < current = Left (MigrationDowngradeNotSupported current target)
  | otherwise = do
      parsed <- traverse parseEdge migrations
      checkDuplicates parsed
      let sorted = sortOn (\(_, f, _) -> f) parsed
      Right (Just (pickInWindow current target sorted))
  where
    parseEdge migration = do
      fromVersion <- parseVersionE (getFrom migration)
      toVersion <- parseVersionE (getTo migration)
      Right (migration, fromVersion, toVersion)

    parseVersionE versionText =
      case parseVersion versionText of
        Just version -> Right version
        Nothing -> Left (MigrationVersionUnparseable versionText)

    checkDuplicates [] = Right ()
    checkDuplicates ((_, fromVersion, _) : rest) =
      case [toVersion | (_, duplicateFrom, toVersion) <- rest, duplicateFrom == fromVersion] of
        (duplicateTo : _) -> Left (MigrationDuplicateEdge fromVersion duplicateTo)
        [] -> checkDuplicates rest

    pickInWindow _cursor _end [] = []
    pickInWindow cursor end ((migration, fromVersion, toVersion) : rest)
      | fromVersion < cursor = pickInWindow cursor end rest
      | fromVersion >= end = []
      | toVersion > end = pickInWindow cursor end rest
      | otherwise = migration : pickInWindow toVersion end rest