package persistence

import (
	"database/sql"
	"errors"
	"fmt"
	"strings"
	"time"

	sqlite "modernc.org/sqlite"
)

// Sidecar schema migrations.
//
// The sidecar DB has no migration framework historically: sidecarSchema was
// run with CREATE TABLE / CREATE INDEX IF NOT EXISTS, and columns added after
// a table's original shape were retrofitted with best-effort ALTERs. That
// breaks when an index (or any statement) inside sidecarSchema references a
// column the IF-NOT-EXISTS create never adds to a pre-existing table — the
// whole schema batch aborts with "no such column" and the install can no
// longer open its ledger.
//
// runMigrations replaces that with a forward-only, version-stamped sequence
// keyed on SQLite's built-in PRAGMA user_version. sidecarSchema keeps only the
// idempotent base shape (tables + original-shape indexes); every later column
// and every column-dependent index lives in a migration that runs after the
// column's ALTER. Existing installs upgrade in place on the next OpenSidecar —
// no user action, no data loss (migrations are additive only).
//
// Concurrency: applyOne relies on the sidecar DSN's _txlock=immediate so
// db.Begin() takes the write lock at BEGIN, making the in-transaction
// user_version check authoritative. Two processes opening a stale DB at once
// serialise on busy_timeout; the loser re-reads the bumped version and skips.

// migration is one forward step. Steps are append-only and ascending: never
// edit or renumber a migration that has shipped — add a new higher version.
type migration struct {
	version int
	name    string
	fn      func(tx *sql.Tx) error
}

// currentSidecarVersion is the schema version a fully-migrated DB reports via
// PRAGMA user_version. It must equal the highest version in sidecarMigrations.
const currentSidecarVersion = 1

// sidecarMigrations is the ordered, forward-only migration list.
var sidecarMigrations = []migration{
	{version: 1, name: "baseline-reconcile", fn: migrateV1Baseline},
}

// runMigrations applies every pending migration in order. Each runs in its own
// IMMEDIATE-locked transaction and bumps user_version on success, so a failure
// at version N leaves versions < N committed and stamped and is safe to retry
// on the next open.
func runMigrations(db *sql.DB) error {
	for _, m := range sidecarMigrations {
		if err := applyOne(db, m); err != nil {
			return fmt.Errorf("persistence: sidecar migration v%d (%s): %w", m.version, m.name, err)
		}
	}
	return nil
}

// isSidecarBusyErr reports whether err is a SQLite BUSY/LOCKED result code
// from the modernc driver (matching extended codes by their base value).
func isSidecarBusyErr(err error) bool {
	var se *sqlite.Error
	if !errors.As(err, &se) {
		return false
	}
	// Mask the high byte so extended codes (SQLITE_BUSY_SNAPSHOT, ...) match
	// their base SQLITE_BUSY (5) / SQLITE_LOCKED (6).
	switch se.Code() & 0xff {
	case 5, 6:
		return true
	}
	return false
}

// withSidecarBusyRetry runs fn, retrying on a SQLite BUSY/LOCKED error with
// bounded exponential backoff.
//
// busy_timeout (set in the sidecar DSN) covers ordinary write-lock contention
// once the file is in WAL mode, but it does NOT cover the rollback-journal ->
// WAL conversion the very first opener performs: that conversion takes a brief
// EXCLUSIVE lock whose acquisition SQLite answers with an immediate, un-retried
// SQLITE_BUSY (the busy handler is not consulted for a journal-mode change)
// when another process has the file open. The daemon, every per-repo
// `gortex mcp` subprocess, and the CLI can all open a fresh/stale sidecar at
// the same instant, so a bounded application-level retry is required on top of
// busy_timeout. fn must be idempotent — runBaseSchema (CREATE ... IF NOT
// EXISTS) and runMigrations (user_version-gated) both are.
func withSidecarBusyRetry(fn func() error) error {
	const (
		maxAttempts = 40
		baseDelay   = 5 * time.Millisecond
		maxDelay    = 250 * time.Millisecond
	)
	delay := baseDelay
	var err error
	for attempt := 0; attempt < maxAttempts; attempt++ {
		if err = fn(); err == nil || !isSidecarBusyErr(err) {
			return err
		}
		time.Sleep(delay)
		if delay *= 2; delay > maxDelay {
			delay = maxDelay
		}
	}
	return err
}

// runBaseSchema applies the idempotent base shape (sidecarSchema) inside one
// IMMEDIATE-locked transaction. The sidecar DSN's _txlock=immediate makes
// db.Begin() emit BEGIN IMMEDIATE, so the reserved write lock is held from the
// start of the batch.
//
// This matters for the same reason applyOne wraps its work: the base schema's
// CREATE TABLE / CREATE INDEX IF NOT EXISTS statements write, and run in
// autocommit each one first takes a read lock and then tries to promote to a
// write lock. SQLite answers that promotion with an un-retryable SQLITE_BUSY
// (busy_timeout is not consulted on a lock upgrade) when another process is
// concurrently creating the same objects — so several gortex processes opening
// a fresh or stale DB at once would race here, before runMigrations' own
// IMMEDIATE transactions ever run, and the losers would fail the open. Taking
// the write lock at BEGIN instead makes a concurrent opener block on
// busy_timeout and then re-run the IF NOT EXISTS statements as clean no-ops, so
// every opener succeeds.
func runBaseSchema(db *sql.DB) error {
	tx, err := db.Begin()
	if err != nil {
		return fmt.Errorf("begin: %w", err)
	}
	defer func() { _ = tx.Rollback() }() // no-op once Commit succeeds
	if _, err := tx.Exec(sidecarSchema); err != nil {
		return err
	}
	return tx.Commit()
}

// applyOne applies a single migration if the DB's user_version is below it.
//
// The sidecar DSN sets _txlock=immediate, so db.Begin() emits BEGIN IMMEDIATE
// and the reserved write lock is held before the user_version read below —
// the gate is therefore authoritative across processes. A concurrent opener
// blocks on busy_timeout at BEGIN, then (this winner having committed) reads
// the bumped version and returns without writing. PRAGMA user_version is
// transactional, so on any failure the deferred Rollback reverts both the
// partial DDL and the version bump.
func applyOne(db *sql.DB, m migration) error {
	tx, err := db.Begin()
	if err != nil {
		return fmt.Errorf("begin: %w", err)
	}
	defer func() { _ = tx.Rollback() }() // no-op once Commit succeeds

	var cur int
	if err := tx.QueryRow("PRAGMA user_version").Scan(&cur); err != nil {
		return fmt.Errorf("read user_version: %w", err)
	}
	if cur >= m.version {
		return nil // already applied (this or another process)
	}

	if err := m.fn(tx); err != nil {
		return err
	}

	// PRAGMA takes no bound parameters; m.version is an int constant we own.
	// Bumped last so it rolls back with the transaction on any earlier error.
	if _, err := tx.Exec(fmt.Sprintf("PRAGMA user_version = %d", m.version)); err != nil {
		return fmt.Errorf("bump user_version: %w", err)
	}
	return tx.Commit()
}

// migrateV1Baseline reconciles any historical sidecar shape (which all report
// user_version 0) to the current schema. It is purely additive and idempotent:
// CREATE TABLE IF NOT EXISTS for every table runs earlier in sidecarSchema, so
// this only adds the columns and column-dependent indexes that a pre-existing
// table cannot gain from CREATE TABLE.
//
// savings_events.model / .client are the only column-level drift in the whole
// sidecar history; every other shape difference is a whole-table addition
// already covered by the base schema.
func migrateV1Baseline(tx *sql.Tx) error {
	if err := addColumnIfMissingTx(tx, "savings_events", "model", "TEXT NOT NULL DEFAULT ''"); err != nil {
		return err
	}
	if err := addColumnIfMissingTx(tx, "savings_events", "client", "TEXT NOT NULL DEFAULT ''"); err != nil {
		return err
	}
	// Column-dependent indexes LAST — only after the columns are guaranteed to
	// exist. These are exactly the statements that aborted sidecarSchema on a
	// database created before model/client existed.
	if _, err := tx.Exec(`CREATE INDEX IF NOT EXISTS idx_savings_events_model ON savings_events (model, ts)`); err != nil {
		return fmt.Errorf("index idx_savings_events_model: %w", err)
	}
	if _, err := tx.Exec(`CREATE INDEX IF NOT EXISTS idx_savings_events_client ON savings_events (client, ts)`); err != nil {
		return fmt.Errorf("index idx_savings_events_client: %w", err)
	}
	return nil
}

// addColumnIfMissingTx runs ALTER TABLE ... ADD COLUMN inside a transaction,
// swallowing ONLY the "duplicate column name" error that a table which already
// has the column returns (idempotency for fresh / already-upgraded databases).
// Every other error propagates — surfacing a genuine failure with its real
// cause instead of letting a later column-dependent statement fail with a
// misleading "no such column".
func addColumnIfMissingTx(tx *sql.Tx, table, column, decl string) error {
	if _, err := tx.Exec("ALTER TABLE " + table + " ADD COLUMN " + column + " " + decl); err != nil {
		if strings.Contains(err.Error(), "duplicate column name") {
			return nil
		}
		return fmt.Errorf("add column %s.%s: %w", table, column, err)
	}
	return nil
}