3 files changed, 66 insertions, 0 deletions
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..e8b747f
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+roll
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..c68aa93
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module bnbl.io/roll
+
+go 1.13
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..dd83503
--- /dev/null
+++ b/main.go
@@ -0,0 +1,62 @@
+package main
+
+import (
+	"crypto/rand"
+	"fmt"
+	"math/big"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Printf("usage: %s <die[+die...]>\n", os.Args[0])
+		os.Exit(1)
+	}
+	dice := strings.Split(os.Args[1], "+")
+	var sum int64
+	for _, die := range dice {
+		if result, err := resolve(die); err != nil {
+			fmt.Printf("%v\n", err)
+			os.Exit(1)
+		} else {
+			sum += result
+		}
+	}
+	fmt.Printf("%d\n", sum)
+}
+
+func resolve(die string) (int64, error) {
+	// try parsing as a die e.g. 2d6
+	var (
+		num   int
+		sides int64
+	)
+	if fields, err := fmt.Sscanf(die, "%dd%d", &num, &sides); err == nil && fields == 2 {
+		var sum int64
+		for i := 0; i < num; i++ {
+			if r, err := roll(sides); err != nil {
+				return 0, err
+			} else {
+				sum += r
+			}
+		}
+		return sum, nil
+	}
+
+	// try just parsing as a number, e.g. +5
+	if x, err := strconv.Atoi(die); err != nil {
+		return 0, fmt.Errorf("unrecognized die: %s", die)
+	} else {
+		return int64(x), nil
+	}
+}
+
+func roll(sides int64) (int64, error) {
+	r, err := rand.Int(rand.Reader, big.NewInt(sides))
+	if err != nil {
+		return 0, err
+	}
+	return 1 + r.Int64(), nil
+}