// SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
// SPDX-License-Identifier: Apache-2.0

package oblast

import (
	"database/sql"
	"fmt"
	"reflect"
)

// PrepareThreshold is a tuning parameter for the strategy used by all methods of [Store] operating on batches of records provided by the caller
// (specifically, [Store.Insert], [Store.Update] and [Store.Delete]).
//
// For large amounts of records, it is obviously advantageous to build a prepared statement for the one query that will be used repeatedly on all of them.
// However, building a prepared statement is associated with some amount of bookkeeping on the level of the database/sql library.
// When operating on individual records or small amounts of records at a time (that is, in OLTP rather than OLAP workloads), this overhead becomes a measurable performance burden.
//
// This tuning parameter defines the minimum number of records that will justify maintaining a prepared statement.
// Our benchmarking with the mattn/go-sqlite3 driver (and last checked with Go 1.26.2 on x86_64) indicates that this becomes a worthwhile investment at 8 or more records, so this is our default.
// If your benchmarking indicates a different tradeoff depending on your choice of Go version or SQL driver, you may adjust this variable accordingly.
var PrepareThreshold int = 8

// preparedStatement behaves like sql.Stmt, but only uses *sql.Stmt when it is useful (see explanation above).
type preparedStatement struct {
	db    Handle
	query string
	stmt  *sql.Stmt // nil for input sizes below PrepareThreshold
}

// prepare behaves like [Handle.Prepare].
func prepare(db Handle, query, operation string, inputSize int) (preparedStatement, error) {
	if query == "" {
		return preparedStatement{}, fmt.Errorf("cannot execute %s() because query could not be autogenerated", operation)
	}

	if inputSize < PrepareThreshold {
		return preparedStatement{db, query, nil}, nil
	}
	stmt, err := db.Prepare(query)
	if err != nil {
		return preparedStatement{}, fmt.Errorf("during Prepare(): %w", err)
	}
	return preparedStatement{db, query, stmt}, nil
}

// Close behaves like [sql.Stmt.Close].
func (s preparedStatement) Close() error {
	if s.stmt == nil {
		return nil
	}
	return s.stmt.Close()
}

// Exec behaves like [sql.Stmt.Exec].
func (s preparedStatement) Exec(args ...any) (sql.Result, error) {
	if s.stmt == nil {
		return s.db.Exec(s.query, args...)
	}
	return s.stmt.Exec(args...)
}

// QueryRow behaves like [sql.Stmt.QueryRow].
func (s preparedStatement) QueryRow(args ...any) *sql.Row {
	if s.stmt == nil {
		return s.db.QueryRow(s.query, args...)
	}
	return s.stmt.QueryRow(args...)
}

// Insert executes an SQL INSERT statement for each of the provided records.
//
// Fields that are declared with the "auto" tag will not be written into the DB,
// and instead their value (as auto-generated by the DB on insert) will be placed in the record.
//
// Returns an error if [NewStore] was called without the [TableNameIs] option, which is required to generate a query for this method.
//
// Returns an error if any of the `records` has a non-zero value in any column marked as `db:",auto"`.
// Records that already exist in the database should be handled with [Store.Update] instead.
func (s Store[R]) Insert(db Handle, records ...*R) error {
	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
	//       Any expression that does not depend on type R should be factored out into a reusable function.

	stmt, err := prepare(db, s.plan.Insert.Query, "Insert", len(records))
	if err != nil {
		return err
	}

	var (
		argumentIndexes = s.plan.Insert.ArgumentIndexes
		argumentSlots   = make([]any, len(argumentIndexes))
		scanIndexes     = s.plan.Insert.ScanIndexes
		scanSlots       = make([]any, len(scanIndexes))
	)

	for idx := range records {
		v := reflect.ValueOf(records[idx]).Elem()
		err := insertRecord(v, idx, stmt, argumentIndexes, argumentSlots, scanIndexes, scanSlots)
		if err != nil {
			return newIOError(err, "Stmt.Close", stmt.Close())
		}
	}

	return newIOError(nil, "Stmt.Close", stmt.Close())
}

func insertRecord(v reflect.Value, recordIndex int, stmt preparedStatement, argumentIndexes [][]int, argumentSlots []any, scanIndexes [][]int, scanSlots []any) error {
	for idx, index := range argumentIndexes {
		argumentSlots[idx] = v.FieldByIndex(index).Interface()
	}
	for idx, index := range scanIndexes {
		f := v.FieldByIndex(index)
		if !f.IsZero() {
			return fmt.Errorf(`refusing to INSERT record with idx = %d that already has non-zero values in its "auto" columns`, recordIndex)
		}
		scanSlots[idx] = f.Addr().Interface()
	}
	var err error
	if len(scanSlots) == 0 {
		_, err = stmt.Exec(argumentSlots...)
	} else {
		err = stmt.QueryRow(argumentSlots...).Scan(scanSlots...)
	}
	if err != nil {
		return fmt.Errorf("while inserting record with idx = %d: %w", recordIndex, err)
	}
	return nil
}

// Update executes an SQL UPDATE statement for each of the provided records, updating all non-primary-key columns with the values in the records.
// Returns [MissingRecordError] if any of the records does not exist in the database, that is, if for any of the records, the database contains no row with the same primary key values.
//
// Returns an error if [NewStore] was called without the [TableNameIs] or [PrimaryKeyIs] options, which are both required to generate a query for this method.
func (s Store[R]) Update(db Handle, records ...R) error {
	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
	//       Any expression that does not depend on type R should be factored out into a reusable function.

	stmt, err := prepare(db, s.plan.Update.Query, "Update", len(records))
	if err != nil {
		return err
	}

	var (
		argumentIndexes = s.plan.Update.ArgumentIndexes
		argumentSlots   = make([]any, len(argumentIndexes))
	)

	for idx, r := range records {
		v := reflect.ValueOf(&r).Elem()
		rowsAffected, err := updateRecord(v, idx, stmt, argumentIndexes, argumentSlots)
		if err == nil && rowsAffected == 0 {
			err = MissingRecordError[R]{r, s.plan}
		}
		if err != nil {
			return newIOError(err, "Stmt.Close", stmt.Close())
		}
	}
	return newIOError(nil, "Stmt.Close", stmt.Close())
}

func updateRecord(v reflect.Value, recordIndex int, stmt preparedStatement, argumentIndexes [][]int, argumentSlots []any) (int64, error) {
	for idx, index := range argumentIndexes {
		argumentSlots[idx] = v.FieldByIndex(index).Interface()
	}
	result, err := stmt.Exec(argumentSlots...)
	if err != nil {
		return 0, fmt.Errorf("while updating record with idx = %d: %w", recordIndex, err)
	}
	rowsAffected, err := result.RowsAffected()
	if err != nil {
		return 0, fmt.Errorf("during RowsAffected() for record with idx = %d: %w", recordIndex, err)
	}
	return rowsAffected, nil
}

// Delete executes an SQL DELETE statement for each of the provided records, using their primary keys to locate the respective table rows.
//
// Returns an error if [NewStore] was called without the [TableNameIs] or [PrimaryKeyIs] options, which are both required to generate a query for this method.
func (s Store[R]) Delete(db Handle, records ...R) error {
	// NOTE: This function body should be as short as possible to reduce the binary size after monomorphization.
	//       Any expression that does not depend on type R should be factored out into a reusable function.

	stmt, err := prepare(db, s.plan.Delete.Query, "Delete", len(records))
	if err != nil {
		return err
	}

	var (
		argumentIndexes = s.plan.Delete.ArgumentIndexes
		argumentSlots   = make([]any, len(argumentIndexes))
	)

	for idx, r := range records {
		v := reflect.ValueOf(&r).Elem()
		err := deleteRecord(v, idx, stmt, argumentIndexes, argumentSlots)
		if err != nil {
			return newIOError(err, "Stmt.Close", stmt.Close())
		}
	}

	return newIOError(nil, "Stmt.Close", stmt.Close())
}

func deleteRecord(v reflect.Value, recordIndex int, stmt preparedStatement, argumentIndexes [][]int, argumentSlots []any) error {
	for idx, index := range argumentIndexes {
		argumentSlots[idx] = v.FieldByIndex(index).Interface()
	}
	_, err := stmt.Exec(argumentSlots...)
	if err != nil {
		return fmt.Errorf("while deleting record with idx = %d: %w", recordIndex, err)
	}
	return nil
}