package template

import (
	"errors"
	"fmt"

	"gopkg.in/Knetic/govaluate.v3"
)

// ErrNonFloatValue is returned when the result of the expression is not a float64.
var ErrNonFloatValue = errors.New("failed to cast expression result for float64")

// Value describes a measurement or dimension in the templates.
type Value string

// Evaluate a Value as [govaluate.EvaluateExpression] in combination with the provided parameters.
func (v *Value) Evaluate(parameters govaluate.MapParameters,
	funcs map[string]govaluate.ExpressionFunction,
) (float64, error) {
	if v == nil {
		return 0, nil
	}

	expression, err := govaluate.NewEvaluableExpressionWithFunctions(string(*v), funcs)
	if err != nil {
		return 0, fmt.Errorf("create new evaluable expression for %q: %w", *v, err)
	}

	result, err := expression.Evaluate(parameters)
	if err != nil {
		return 0, fmt.Errorf("evaluable expression for %q: %w", *v, err)
	}

	f, ok := result.(float64)
	if !ok {
		return 0, fmt.Errorf("cast %v to float: %w", result, ErrNonFloatValue)
	}

	return f, nil
}