elliotchance/gedcom

package q

import (
    "errors"
    "reflect"
)

// OnlyExpr is a function. See Evaluate.
type OnlyExpr struct{}

// Evaluate returns a new slice that only contains the entities that have
// returned true from the condition.
func (e *OnlyExpr) Evaluate(engine *Engine, input interface{}, args []*Statement) (interface{}, error) {
    in := reflect.ValueOf(input)

    if len(args) != 1 {
        return nil, errors.New("function Only() must take a single argument")
    }

    if in.Kind() != reflect.Slice {
        return nil, nil
    }

    inputSliceType := TypeOfSliceElement(input)
    results := reflect.MakeSlice(reflect.SliceOf(inputSliceType), 0, 0)

    condition := args[0]
    for i := 0; i < in.Len(); i++ {
        result, err := condition.Evaluate(engine, in.Index(i).Interface())
        if err != nil {
            return nil, err
        }

        if shouldAppend, ok := result.(bool); ok && shouldAppend {
            results = reflect.Append(results, in.Index(i))
        }
    }

    return results.Interface(), nil
}