Parse Durations much faster

Without using a regexp.

Code inspired by Go standard library `time.ParseDuration`, but
simplified and supporting different units.

Signed-off-by: Bryan Boreham <[email protected]>

model/time.go

@@ -18,7 +18,6 @@ import (
 	"errors"
 	"fmt"
 	"math"
-	"regexp"
 	"strconv"
 	"strings"
 	"time"
@@ -183,54 +182,78 @@ func (d *Duration) Type() string {
 	return "duration"
 }
 
-var durationRE = regexp.MustCompile("^(([0-9]+)y)?(([0-9]+)w)?(([0-9]+)d)?(([0-9]+)h)?(([0-9]+)m)?(([0-9]+)s)?(([0-9]+)ms)?$")
+func isdigit(c byte) bool { return c >= '0' && c <= '9' }
+
+// Units are required to go in order from biggest to smallest.
+// This guards against confusion from "1m1d" being 1 minute + 1 day, not 1 month + 1 day.
+var unitMap = map[string]struct {
+	pos  int
+	mult uint64
+}{
+	"ms": {7, uint64(time.Millisecond)},
+	"s":  {6, uint64(time.Second)},
+	"m":  {5, uint64(time.Minute)},
+	"h":  {4, uint64(time.Hour)},
+	"d":  {3, uint64(24 * time.Hour)},
+	"w":  {2, uint64(7 * 24 * time.Hour)},
+	"y":  {1, uint64(365 * 24 * time.Hour)},
+}
 
 // ParseDuration parses a string into a time.Duration, assuming that a year
 // always has 365d, a week always has 7d, and a day always has 24h.
-func ParseDuration(durationStr string) (Duration, error) {
-	switch durationStr {
+func ParseDuration(s string) (Duration, error) {
+	switch s {
 	case "0":
 		// Allow 0 without a unit.
 		return 0, nil
 	case "":
 		return 0, errors.New("empty duration string")
 	}
-	matches := durationRE.FindStringSubmatch(durationStr)
-	if matches == nil {
-		return 0, fmt.Errorf("not a valid duration string: %q", durationStr)
-	}
-	var dur time.Duration
 
-	// Parse the match at pos `pos` in the regex and use `mult` to turn that
-	// into ms, then add that value to the total parsed duration.
-	var overflowErr error
-	m := func(pos int, mult time.Duration) {
-		if matches[pos] == "" {
-			return
+	orig := s
+	var dur uint64
+	lastUnitPos := 0
+
+	for s != "" {
+		if !isdigit(s[0]) {
+			return 0, fmt.Errorf("not a valid duration string: %q", orig)
+		}
+		// Consume [0-9]*
+		i := 0
+		for ; i < len(s) && isdigit(s[i]); i++ {
+		}
+		v, err := strconv.ParseUint(s[:i], 10, 0)
+		if err != nil {
+			return 0, fmt.Errorf("not a valid duration string: %q", orig)
 		}
-		n, _ := strconv.Atoi(matches[pos])
+		s = s[i:]
 
+		// Consume unit.
+		for i = 0; i < len(s) && !isdigit(s[i]); i++ {
+		}
+		if i == 0 {
+			return 0, fmt.Errorf("not a valid duration string: %q", orig)
+		}
+		u := s[:i]
+		s = s[i:]
+		unit, ok := unitMap[u]
+		if !ok {
+			return 0, fmt.Errorf("unknown unit %q in duration %q", u, orig)
+		}
+		if unit.pos <= lastUnitPos { // Units must go in order from biggest to smallest.
+			return 0, fmt.Errorf("not a valid duration string: %q", orig)
+		}
+		lastUnitPos = unit.pos
 		// Check if the provided duration overflows time.Duration (> ~ 290years).
-		if n > int((1<<63-1)/mult/time.Millisecond) {
-			overflowErr = errors.New("duration out of range")
+		if v > 1<<63/unit.mult {
+			return 0, errors.New("duration out of range")
 		}
-		d := time.Duration(n) * time.Millisecond
-		dur += d * mult
-
-		if dur < 0 {
-			overflowErr = errors.New("duration out of range")
+		dur += v * unit.mult
+		if dur > 1<<63-1 {
+			return 0, errors.New("duration out of range")
 		}
 	}
-
-	m(2, 1000*60*60*24*365) // y
-	m(4, 1000*60*60*24*7)   // w
-	m(6, 1000*60*60*24)     // d
-	m(8, 1000*60*60)        // h
-	m(10, 1000*60)          // m
-	m(12, 1000)             // s
-	m(14, 1)                // ms
-
-	return Duration(dur), overflowErr
+	return Duration(dur), nil
 }
 
 func (d Duration) String() string {