// Package sqlr ("squealer") provides some convenience wrappers around
// database/sql in the spirit of encoding/json.
package sqlr

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

// Scan calls rows.Scan to unpack a single row into the fields of v.
func Scan(rows *sql.Rows, v any) error {
	rv := reflect.ValueOf(v)
	if rv.Kind() != reflect.Pointer || rv.Elem().Kind() != reflect.Struct {
		return fmt.Errorf("Scan needs a pointer to a struct")
	}
	inner := rv.Elem()
	fields := reflect.VisibleFields(inner.Type())

	cols, err := rows.Columns()
	if err != nil {
		return err
	}
	dest := make([]any, len(cols))
Cols:
	for i, c := range cols {
		for _, f := range fields {
			if !f.IsExported() {
				continue
			}
			tag := f.Tag.Get("sql")
			if tag == "" {
				tag = f.Name
			}
			if tag == c {
				dest[i] = inner.FieldByIndex(f.Index).Addr().Interface()
				continue Cols
			}
		}
		return fmt.Errorf("no field with tag %q", c)
	}

	return rows.Scan(dest...)
}

// Querier is the underlying type used to query the database,
// usually *sql.DB or *sql.Tx.
type Querier interface {
	QueryContext(context.Context, string, ...any) (*sql.Rows, error)
}

// Query returns an iterator over the rows matched by a query.
func Query[Row any](ctx context.Context, querier Querier, query string, args ...any) iter.Seq2[Row, error] {
	return func(yield func(Row, error) bool) {
		rows, err := querier.QueryContext(ctx, query, args...)
		if err != nil {
			var zero Row
			yield(zero, err)
			return
		}
		defer rows.Close()
		for rows.Next() {
			var row Row
			if err := Scan(rows, &row); err != nil {
				yield(row, err)
				return
			}
			if !yield(row, nil) {
				return
			}
		}
		if err := rows.Err(); err != nil {
			var zero Row
			yield(zero, err)
		}
	}
}