package statement

import (
	"errors"

	"github.com/genjidb/genji/internal/database"
	"github.com/genjidb/genji/internal/expr"
	"github.com/genjidb/genji/internal/sql/scanner"
	"github.com/genjidb/genji/internal/stream"
	"github.com/genjidb/genji/internal/stringutil"
)

// SelectStmt holds SELECT configuration.
type SelectStmt struct {
	TableName        string
	Distinct         bool
	WhereExpr        expr.Expr
	GroupByExpr      expr.Expr
	OrderBy          expr.Path
	OrderByDirection scanner.Token
	OffsetExpr       expr.Expr
	LimitExpr        expr.Expr
	ProjectionExprs  []expr.Expr
}

func (stmt *SelectStmt) Run(tx *database.Transaction, params []expr.Param) (Result, error) {
	var res Result

	s, err := stmt.ToStream()
	if err != nil {
		return res, err
	}

	return s.Run(tx, params)
}

func (stmt *SelectStmt) IsReadOnly() bool {
	return true
}

func (stmt *SelectStmt) ToStream() (*StreamStmt, error) {
	var s *stream.Stream

	if stmt.TableName != "" {
		s = stream.New(stream.SeqScan(stmt.TableName))
	}

	if stmt.WhereExpr != nil {
		s = s.Pipe(stream.Filter(stmt.WhereExpr))
	}

	// when using GROUP BY, only aggregation functions or GroupByExpr can be selected
	if stmt.GroupByExpr != nil {
		// add Group node
		s = s.Pipe(stream.GroupBy(stmt.GroupByExpr))

		var invalidProjectedField expr.Expr
		var aggregators []expr.AggregatorBuilder

		for _, pe := range stmt.ProjectionExprs {
			ne, ok := pe.(*expr.NamedExpr)
			if !ok {
				invalidProjectedField = pe
				break
			}
			e := ne.Expr

			// check if the projected expression is an aggregation function
			if agg, ok := e.(expr.AggregatorBuilder); ok {
				aggregators = append(aggregators, agg)
				continue
			}

			// check if this is the same expression as the one used in the GROUP BY clause
			if expr.Equal(e, stmt.GroupByExpr) {
				continue
			}

			// otherwise it's an error
			invalidProjectedField = ne
			break
		}

		if invalidProjectedField != nil {
			return nil, stringutil.Errorf("field %q must appear in the GROUP BY clause or be used in an aggregate function", invalidProjectedField)
		}

		// add Aggregation node
		s = s.Pipe(stream.HashAggregate(aggregators...))
	} else {
		// if there is no GROUP BY clause, check if there are any aggregation function
		// and if so add an aggregation node
		var aggregators []expr.AggregatorBuilder

		for _, pe := range stmt.ProjectionExprs {
			ne, ok := pe.(*expr.NamedExpr)
			if !ok {
				continue
			}
			e := ne.Expr

			// check if the projected expression is an aggregation function
			if agg, ok := e.(expr.AggregatorBuilder); ok {
				aggregators = append(aggregators, agg)
			}
		}

		// add Aggregation node
		if len(aggregators) > 0 {
			s = s.Pipe(stream.HashAggregate(aggregators...))
		}
	}

	// If there is no FROM clause ensure there is no wildcard or path
	if stmt.TableName == "" {
		var err error

		for _, e := range stmt.ProjectionExprs {
			expr.Walk(e, func(e expr.Expr) bool {
				switch e.(type) {
				case expr.Path, expr.Wildcard:
					err = errors.New("no tables specified")
					return false
				default:
					return true
				}
			})
			if err != nil {
				return nil, err
			}
		}
	}

	s = s.Pipe(stream.Project(stmt.ProjectionExprs...))

	if stmt.Distinct {
		s = s.Pipe(stream.Distinct())
	}

	if stmt.OrderBy != nil {
		if stmt.OrderByDirection == scanner.DESC {
			s = s.Pipe(stream.SortReverse(stmt.OrderBy))
		} else {
			s = s.Pipe(stream.Sort(stmt.OrderBy))
		}
	}

	if stmt.OffsetExpr != nil {
		v, err := stmt.OffsetExpr.Eval(&expr.Environment{})
		if err != nil {
			return nil, err
		}

		if !v.Type.IsNumber() {
			return nil, stringutil.Errorf("offset expression must evaluate to a number, got %q", v.Type)
		}

		v, err = v.CastAsInteger()
		if err != nil {
			return nil, err
		}

		s = s.Pipe(stream.Skip(v.V.(int64)))
	}

	if stmt.LimitExpr != nil {
		v, err := stmt.LimitExpr.Eval(&expr.Environment{})
		if err != nil {
			return nil, err
		}

		if !v.Type.IsNumber() {
			return nil, stringutil.Errorf("limit expression must evaluate to a number, got %q", v.Type)
		}

		v, err = v.CastAsInteger()
		if err != nil {
			return nil, err
		}

		s = s.Pipe(stream.Take(v.V.(int64)))
	}

	return &StreamStmt{
		Stream:   s,
		ReadOnly: true,
	}, nil
}