Parse out arbitrary dates

This will parse out dates in the format of "mon dd", the month being a 3
letter month, and the date being just the day.

due mar 10
due sep 25
due oct 2

it will also "intelligently" guess the year, based upon which is the
shorter duration from time.Now(). for instance, if today's date is dec
25 and you schedule an event for jan 2, it will schedule it for the
following year.

1 files changed, 43 insertions, 13 deletions

diff --git a/todolist/parser.go b/todolist/parser.go
index f78ce38..e06545e 100644
--- a/todolist/parser.go
+++ b/todolist/parser.go
@@ -3,6 +3,7 @@ package todolist
 import (
 	"fmt"
 	"regexp"
+	"strconv"
 	"strings"
 	"time"
 
@@ -56,32 +57,61 @@ func (p *Parser) Due(input string, day time.Time) string {
 
 	res := r.FindString(input)
 	res = res[4:len(res)]
-	switch {
-	case res == "none":
+	switch res {
+	case "none":
 		return ""
-	case res == "today":
+	case "today":
 		return now.BeginningOfDay().Format("2006-01-02")
-	case res == "tomorrow" || res == "tom":
+	case "tomorrow", "tom":
 		return now.BeginningOfDay().AddDate(0, 0, 1).Format("2006-01-02")
-	case res == "monday" || res == "mon":
+	case "monday", "mon":
 		return p.monday(day)
-	case res == "tuesday" || res == "tue":
+	case "tuesday", "tue":
 		return p.tuesday(day)
-	case res == "wednesday" || res == "wed":
+	case "wednesday", "wed":
 		return p.wednesday(day)
-	case res == "thursday" || res == "thu":
+	case "thursday", "thu":
 		return p.thursday(day)
-	case res == "friday" || res == "fri":
+	case "friday", "fri":
 		return p.friday(day)
-	case res == "saturday" || res == "sat":
+	case "saturday", "sat":
 		return p.saturday(day)
-	case res == "sunday" || res == "sun":
+	case "sunday", "sun":
 		return p.sunday(day)
-	case res == "next week":
+	case "next week":
 		n := now.BeginningOfDay()
 		return now.New(n).Monday().AddDate(0, 0, 7).Format("2006-01-02")
 	}
-	return time.Now().Format("2006-01-02")
+	return p.parseArbitraryDate(res, time.Now())
+}
+
+func (p *Parser) parseArbitraryDate(_date string, pivot time.Time) string {
+	d1 := p.parseArbitraryDateWithYear(_date, pivot.Year())
+
+	var diff1 time.Duration
+	if d1.After(time.Now()) {
+		diff1 = d1.Sub(pivot)
+	} else {
+		diff1 = pivot.Sub(d1)
+	}
+	d2 := p.parseArbitraryDateWithYear(_date, pivot.Year()+1)
+	if d2.Sub(pivot) > diff1 {
+		return d1.Format("2006-01-02")
+	} else {
+		return d2.Format("2006-01-02")
+	}
+}
+
+func (p *Parser) parseArbitraryDateWithYear(_date string, year int) time.Time {
+	res := strings.Join([]string{_date, strconv.Itoa(year)}, " ")
+	if date, err := time.Parse("Jan 2 2006", res); err == nil {
+		return date
+	}
+
+	if date, err := time.Parse("2 Jan 2006", res); err == nil {
+		return date
+	}
+	panic(fmt.Errorf("Could not parse the date you gave me: '%s'", _date))
 }
 
 func (p *Parser) monday(day time.Time) string {