package resolvemcp

// Cursor-based pagination adapted from pkg/resolvespec/cursor.go.

import (
	"fmt"
	"strings"

	"github.com/bitechdev/ResolveSpec/pkg/common"
	"github.com/bitechdev/ResolveSpec/pkg/logger"
)

type cursorDirection int

const (
	cursorForward  cursorDirection = 1
	cursorBackward cursorDirection = -1
)

// getCursorFilter generates a SQL EXISTS subquery for cursor-based pagination.
// expandJoins is an optional map[alias]string of JOIN clauses for join-column sort support.
func getCursorFilter(
	tableName string,
	pkName string,
	modelColumns []string,
	options common.RequestOptions,
	expandJoins map[string]string,
) (string, error) {
	fullTableName := tableName
	if strings.Contains(tableName, ".") {
		tableName = strings.SplitN(tableName, ".", 2)[1]
	}

	cursorID, direction := getActiveCursor(options)
	if cursorID == "" {
		return "", fmt.Errorf("no cursor provided for table %s", tableName)
	}

	sortItems := options.Sort
	if len(sortItems) == 0 {
		return "", fmt.Errorf("no sort columns defined")
	}

	var whereClauses []string
	joinSQL := ""
	reverse := direction < 0

	for _, s := range sortItems {
		col := strings.TrimSpace(s.Column)
		if col == "" {
			continue
		}

		parts := strings.Split(col, ".")
		field := strings.TrimSpace(parts[len(parts)-1])
		prefix := strings.Join(parts[:len(parts)-1], ".")

		desc := strings.EqualFold(s.Direction, "desc")
		if reverse {
			desc = !desc
		}

		cursorCol, targetCol, isJoin, err := resolveCursorColumn(field, prefix, tableName, modelColumns)
		if err != nil {
			logger.Warn("Skipping invalid sort column %q: %v", col, err)
			continue
		}

		if isJoin {
			if expandJoins != nil {
				if joinClause, ok := expandJoins[prefix]; ok {
					jSQL, cRef := rewriteCursorJoin(joinClause, tableName, prefix)
					joinSQL = jSQL
					cursorCol = cRef + "." + field
					targetCol = prefix + "." + field
				}
			}
			if cursorCol == "" {
				logger.Warn("Skipping cursor sort column %q: join alias %q not in expandJoins", col, prefix)
				continue
			}
		}

		op := "<"
		if desc {
			op = ">"
		}
		whereClauses = append(whereClauses, fmt.Sprintf("%s %s %s", cursorCol, op, targetCol))
	}

	if len(whereClauses) == 0 {
		return "", fmt.Errorf("no valid sort columns after filtering")
	}

	orSQL := buildCursorPriorityChain(whereClauses)

	query := fmt.Sprintf(`EXISTS (
  SELECT 1
  FROM %s cursor_select
  %s
  WHERE cursor_select.%s = %s
    AND (%s)
)`,
		fullTableName,
		joinSQL,
		pkName,
		cursorID,
		orSQL,
	)

	return query, nil
}

func getActiveCursor(options common.RequestOptions) (id string, direction cursorDirection) {
	if options.CursorForward != "" {
		return options.CursorForward, cursorForward
	}
	if options.CursorBackward != "" {
		return options.CursorBackward, cursorBackward
	}
	return "", 0
}

func resolveCursorColumn(field, prefix, tableName string, modelColumns []string) (cursorCol, targetCol string, isJoin bool, err error) {
	if strings.Contains(field, "->") {
		return "cursor_select." + field, tableName + "." + field, false, nil
	}

	if modelColumns != nil {
		for _, col := range modelColumns {
			if strings.EqualFold(col, field) {
				return "cursor_select." + field, tableName + "." + field, false, nil
			}
		}
	} else {
		return "cursor_select." + field, tableName + "." + field, false, nil
	}

	if prefix != "" && prefix != tableName {
		return "", "", true, nil
	}

	return "", "", false, fmt.Errorf("invalid column: %s", field)
}

func rewriteCursorJoin(joinClause, mainTable, alias string) (joinSQL, cursorAlias string) {
	joinSQL = strings.ReplaceAll(joinClause, mainTable+".", "cursor_select.")
	cursorAlias = "cursor_select_" + alias
	joinSQL = strings.ReplaceAll(joinSQL, " "+alias+" ", " "+cursorAlias+" ")
	joinSQL = strings.ReplaceAll(joinSQL, " "+alias+".", " "+cursorAlias+".")
	return joinSQL, cursorAlias
}

func buildCursorPriorityChain(clauses []string) string {
	var or []string
	for i := 0; i < len(clauses); i++ {
		and := strings.Join(clauses[:i+1], "\n    AND ")
		or = append(or, "("+and+")")
	}
	return strings.Join(or, "\n  OR ")
}