Tidy go color interpolation.

It kinda-ish matches the cleaner JS version.  It is more awkward because it
has to do type conversions.

This is actually a little longer (9 lines), but I think its easier to
understand.

2 files changed, 74 insertions, 56 deletions

diff --git a/crtsh-pem2html.go b/crtsh-pem2html.go
index 0bad106..518c919 100644
--- a/crtsh-pem2html.go
+++ b/crtsh-pem2html.go
@@ -81,50 +81,59 @@ func getNow() time.Time {
 
 var now = getNow()
 
-type interpolation struct {
-	ta, tb time.Time
-	ba, bb byte
+type DateRange struct {
+	a, b time.Time
 }
 
-func (i interpolation) interpolate(tc time.Time) byte {
-	db := i.tb.Sub(i.ta)
-	dc := tc.Sub(i.ta)
+func (dr DateRange) ToPct(point time.Time) float64 {
+	dur_ab := dr.b.Sub(dr.a)
+	dur_ap := point.Sub(dr.a)
+	return float64(dur_ap) / float64(dur_ab)
+}
+
+type ByteRange struct {
+	a, b byte
+}
+
+func (br ByteRange) FromPct(pct float64) byte {
+	ab := int16(br.b) - int16(br.a)
+	ap := int16(pct * float64(ab))
+	return byte(int16(br.a) + ap)
+}
 
-	pct := float64(dc) / float64(db)
+func CapPct(pct float64) float64 {
 	if pct < 0 {
 		pct = 0
 	} else if pct > 1 {
 		pct = 1
 	}
-
-	sb := int16(i.bb) - int16(i.ba)
-	sc := int16(pct * float64(sb))
-
-	return byte(int16(i.ba) + sc)
-}
-
-var daysago = interpolation{
-	ta: now.AddDate(0, 0, -30),
-	tb: now,
-	ba: 0xF3,
-	bb: 0x00,
+	return pct
 }
 
-var daysuntil = interpolation{
-	ta: now,
-	tb: now.AddDate(0, 0, 30),
-	ba: 0x00,
-	bb: 0xF3,
+func mapRange(dr DateRange, br ByteRange, t time.Time) byte {
+	return br.FromPct(CapPct(dr.ToPct(t)))
 }
 
 func green(t time.Time) string {
-	b := daysago.interpolate(t)
-	return fmt.Sprintf("#%02X%02X%02X", b, 0xF3, b)
+	max := byte(0xF3)
+	// 30 days ago: 0 green
+	// now: max green
+	greenness := mapRange(
+		DateRange{now.AddDate(0, 0, -30), now},
+		ByteRange{0, max},
+		t)
+	return fmt.Sprintf("#%02X%02X%02X", max-greenness, max, max-greenness)
 }
 
 func red(t time.Time) string {
-	b := daysuntil.interpolate(t)
-	return fmt.Sprintf("#%02X%02X%02X", 0xF3, b, b)
+	max := byte(0xF3)
+	// now: max red
+	// 30 days from now: 0 red
+	redness := mapRange(
+		DateRange{now, now.AddDate(0, 0, 30)},
+		ByteRange{max, 0},
+		t)
+	return fmt.Sprintf("#%02X%02X%02X", max, max-redness, max-redness)
 }
 
 type Cert struct {
diff --git a/tls-pem2html.go b/tls-pem2html.go
index d339fc5..3b55f73 100644
--- a/tls-pem2html.go
+++ b/tls-pem2html.go
@@ -77,50 +77,59 @@ func getNow() time.Time {
 
 var now = getNow()
 
-type interpolation struct {
-	ta, tb time.Time
-	ba, bb byte
+type DateRange struct {
+	a, b time.Time
 }
 
-func (i interpolation) interpolate(tc time.Time) byte {
-	db := i.tb.Sub(i.ta)
-	dc := tc.Sub(i.ta)
+func (dr DateRange) ToPct(point time.Time) float64 {
+	dur_ab := dr.b.Sub(dr.a)
+	dur_ap := point.Sub(dr.a)
+	return float64(dur_ap) / float64(dur_ab)
+}
+
+type ByteRange struct {
+	a, b byte
+}
+
+func (br ByteRange) FromPct(pct float64) byte {
+	ab := int16(br.b) - int16(br.a)
+	ap := int16(pct * float64(ab))
+	return byte(int16(br.a) + ap)
+}
 
-	pct := float64(dc) / float64(db)
+func CapPct(pct float64) float64 {
 	if pct < 0 {
 		pct = 0
 	} else if pct > 1 {
 		pct = 1
 	}
-
-	sb := int16(i.bb) - int16(i.ba)
-	sc := int16(pct * float64(sb))
-
-	return byte(int16(i.ba) + sc)
-}
-
-var daysago = interpolation{
-	ta: now.AddDate(0, 0, -30),
-	tb: now,
-	ba: 0xF3,
-	bb: 0x00,
+	return pct
 }
 
-var daysuntil = interpolation{
-	ta: now,
-	tb: now.AddDate(0, 0, 30),
-	ba: 0x00,
-	bb: 0xF3,
+func mapRange(dr DateRange, br ByteRange, t time.Time) byte {
+	return br.FromPct(CapPct(dr.ToPct(t)))
 }
 
 func green(t time.Time) string {
-	b := daysago.interpolate(t)
-	return fmt.Sprintf("#%02X%02X%02X", b, 0xF3, b)
+	max := byte(0xF3)
+	// 30 days ago: 0 green
+	// now: max green
+	greenness := mapRange(
+		DateRange{now.AddDate(0, 0, -30), now},
+		ByteRange{0, max},
+		t)
+	return fmt.Sprintf("#%02X%02X%02X", max-greenness, max, max-greenness)
 }
 
 func red(t time.Time) string {
-	b := daysuntil.interpolate(t)
-	return fmt.Sprintf("#%02X%02X%02X", 0xF3, b, b)
+	max := byte(0xF3)
+	// now: max red
+	// 30 days from now: 0 red
+	redness := mapRange(
+		DateRange{now, now.AddDate(0, 0, 30)},
+		ByteRange{max, 0},
+		t)
+	return fmt.Sprintf("#%02X%02X%02X", max, max-redness, max-redness)
 }
 
 type Cert struct {