check_length.go

package bynom

import "context"

// RequireLen runs all parsers noms and if the finished without errors tests
// if the amount of bytes they "ate" equals n.
// If the amount of bytes does not equal n the function will return ErrRequirementNotMet.
func RequireLen(n int, noms ...Nom) Nom {
	const funcName = "RequireLen"

	return func(ctx context.Context, p Plate) (err error) {
		var startPos int
		if startPos, err = p.TellPosition(ctx); err != nil {
			return WrapBreadcrumb(err, funcName, -1)
		}

		for _, nom := range noms {
			if err = nom(ctx, p); err != nil {
				return WrapBreadcrumb(err, funcName, -1)
			}
		}

		var endPos int
		if endPos, err = p.TellPosition(ctx); err != nil {
			return WrapBreadcrumb(err, funcName, -1)
		}

		var l = endPos - startPos
		if l != n {
			return WrapBreadcrumb(
				ErrRequirementNotMet{
					Expected: n,
					Have:     l,
					Msg:      "invalid length",
				},
				funcName,
				-1,
			)
		}

		return
	}
}