From 4c04b4de8266caf326d8915f129d42e3e83e0575 Mon Sep 17 00:00:00 2001
From: Luke Shumaker <lukeshu@sbcglobal.net>
Date: Thu, 26 Jan 2017 03:18:10 -0500
Subject: Add preroll for statistics about rolls.

---
 .gitignore |  1 +
 Makefile   |  1 +
 preroll.go | 92 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 94 insertions(+)
 create mode 100644 preroll.go

diff --git a/.gitignore b/.gitignore
index 44ed24a..ef072ec 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,6 +8,7 @@ daemon
 hangman-helper
 maildups
 ord
+preroll
 roll
 tempmon
 urldecode
diff --git a/Makefile b/Makefile
index b2178d4..214a903 100644
--- a/Makefile
+++ b/Makefile
@@ -18,6 +18,7 @@ BINFILES = \
   hangman-helper \
   maildups \
   ord \
+  preroll \
   roll \
   tempmon \
   urldecode \
diff --git a/preroll.go b/preroll.go
new file mode 100644
index 0000000..ae9d5a3
--- /dev/null
+++ b/preroll.go
@@ -0,0 +1,92 @@
+/* Copyright (C) 2011, 2013-2014, 2017 Luke Shumaker <lukeshu@sbcglobal.net> */
+package main
+
+import (
+	"math"
+	"fmt"
+	"os"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+func usage() {
+	fmt.Printf("Usage: %s <SPECSLIST>...\n", os.Args[0])
+	fmt.Println("SPECLIST = [-]SPEC[<<+|->SPEC>...]")
+	fmt.Println("SPEC = [<COUNT>]d<SIZE> | <MOD>")
+}
+
+var parser = regexp.MustCompile("^(([0-9]*)d)?([0-9]+)$")
+
+func rollSpec(spec string) (mean float64, variance float64) {
+	parts := parser.FindStringSubmatch(spec)
+	if len(parts) < 4 {
+		usage()
+		os.Exit(1)
+	}
+
+	if parts[1] == "" {
+		mean, _ := strconv.Atoi(parts[3])
+		return float64(mean), 0
+	} else {
+		dice, _ := strconv.Atoi(parts[2])
+		die_size, _ := strconv.Atoi(parts[3])
+		if dice < 1 {
+			dice = 1
+		}
+
+		// mean is E(X)
+		mean = float64(1 + die_size)/2.0
+
+		// sum is Σi² for i=1..die_size
+		sum := (die_size*(die_size+1)*(2*die_size+1))/6
+		// mean2 is E(X²)
+		mean2 := float64(sum)/float64(die_size)
+		variance = mean2-(mean*mean)
+
+		mean *= float64(dice)
+		variance *= float64(dice)
+
+		return
+	}
+}
+
+func roll(speclist string) {
+	var mean, variance float64
+
+	neg := strings.HasPrefix(speclist, "-")
+	speclist = strings.TrimPrefix(speclist, "-")
+	for {
+		sep := strings.IndexAny(speclist, "+-")
+		var spec string
+		if sep < 0 {
+			spec = speclist
+		} else {
+			spec = speclist[:sep]
+		}
+
+		_mean, _variance := rollSpec(spec)
+		if neg {
+			_mean = -_mean
+		}
+		mean += _mean
+		variance += _variance
+
+		if sep < 0 {
+			break
+		}
+		neg = speclist[sep] == '-'
+		speclist = speclist[sep+1:]
+	}
+
+	fmt.Printf("µ=%v σ=%.2v\n", mean, math.Sqrt(variance))
+}
+
+func main() {
+	if len(os.Args) < 2 {
+		usage()
+	}
+	for _, arg := range os.Args[1:] {
+		roll(arg)
+	}
+}
-- 
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