elliotchance/gedcom

package q

import (
    "fmt"
    "reflect"
    "strconv"
)

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

// Evaluate returns up to the number of elements in a slice.
//
// If the input value is not a slice then it is converted into a slice of one
// element before evaluating. This means that the result will always be a slice.
// The only exception to this is if the input is nil, then the result will also
// be nil.
//
// There must be exactly one argument and it must be 0 or greater. If the number
// is greater than the length of the slice all elements are returned.
func (e *FirstExpr) Evaluate(engine *Engine, input interface{}, args []*Statement) (interface{}, error) {
    in := reflect.ValueOf(input)

    if len(args) != 1 {
        return nil, fmt.Errorf("function First() must take a single argument")
    }

    if input == nil {
        return nil, nil
    }

    // Convert into a slice if needed.
    if in.Kind() != reflect.Slice {
        s := reflect.MakeSlice(reflect.SliceOf(in.Type()), 1, 1)
        s.Index(0).Set(in)
        in = reflect.ValueOf(s.Interface())
    }

    if in.IsNil() {
        return nil, nil
    }

    result, err := args[0].Evaluate(engine, input)
    if err != nil {
        return nil, err
    }

    max, err := strconv.Atoi(fmt.Sprintf("%v", result))
    if err != nil {
        return nil, err
    }

    if len := in.Len(); max >= len {
        max = len
    }

    return in.Slice(0, max).Interface(), nil
}