Error if the requested words exceeds the word list size (#15)

Also fix some things in the README

Author: sethvargo

README.md

@@ -1,7 +1,7 @@
 ## Golang Diceware Generator
 
 [![GoDoc](https://img.shields.io/badge/go-documentation-blue.svg?style=flat-square)](https://pkg.go.dev/github.com/sethvargo/go-diceware/diceware)
-[![GitHub Actions](https://img.shields.io/github/workflow/status/sethvargo/go-diceware/Test?style=flat-square)](https://github.com/sethvargo/go-diceware/actions?query=workflow%3ATest)
+[![GitHub Actions](https://img.shields.io/github/actions/workflow/status/sethvargo/go-diceware/test.yml?style=flat-square)](https://github.com/sethvargo/go-diceware/actions?query=workflow%3ATest)
 
 This library implements the [Diceware](https://en.wikipedia.org/wiki/Diceware)
 algorithm in pure Golang. The algorithm is most-commonly used when generating
@@ -60,7 +60,7 @@ information.
 As a CLI:
 
 ```sh
-$ GO111MODULE=off go get github.com/sethvargo/go-diceware/cmd/diceware
+$ go install github.com/sethvargo/go-diceware/cmd/diceware@latest
 ```
 
 ```sh

diceware/generate.go

@@ -2,6 +2,7 @@ package diceware
 
 import (
 	"crypto/rand"
+	"fmt"
 	"io"
 	"math"
 	"math/big"
@@ -67,6 +68,13 @@ func NewGenerator(i *GeneratorInput) (*Generator, error) {
 // non-overlapping words, use a single invocation of the function and split the
 // resulting string list.
 func (g *Generator) Generate(numWords int) ([]string, error) {
+	if typ, ok := g.wordList.(WordListNumWordser); ok {
+		if l := typ.NumWords(); numWords > l {
+			return nil, fmt.Errorf("number of requested words (%d) cannot exceed the size of the wordlist (%d)",
+				numWords, l)
+		}
+	}
+
 	list := make([]string, 0, numWords)
 	seen := make(map[string]struct{}, numWords)
 

diceware/word_list.go

@@ -12,7 +12,15 @@ type WordList interface {
 	WordAt(int) string
 }
 
+// WordListNumWordser is an auxillary interface that returns the number of words
+// in the list. This is a separate interface for backwards compatability.
+type WordListNumWordser interface {
+	// NumWords returns the total number of words in the list.
+	NumWords() int
+}
+
 var _ WordList = (*wordListInternal)(nil)
+var _ WordListNumWordser = (*wordListInternal)(nil)
 
 type wordListInternal struct {
 	digits int
@@ -26,3 +34,7 @@ func (w *wordListInternal) Digits() int {
 func (w *wordListInternal) WordAt(i int) string {
 	return w.words[i]
 }
+
+func (w *wordListInternal) NumWords() int {
+	return len(w.words)
+}