Added arm builds, improve database

- The program will now be cross-compiled and released for arm as well as x86
- What we previously called "cache" is not actually a cache, as it holds content that can't be always retrieved. Now we're stopping calling it a cache.
- Improved history merging
- Fixed the README to include the database as a volume, and to fix some errors

database.go

@@ -0,0 +1,110 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"time"
+)
+
+type HistoryEntry struct {
+	Date                       int64
+	GreenEnergyPercentage      float32
+	PolledSmartEnergySummation float32
+}
+type History []HistoryEntry
+
+func (config Config) updateHistory() {
+
+	greenEnergyPercentage, err := config.historyAverageAndConvertToGreen(config.Sensors.FossilPercentage, time.Now())
+	if err != nil {
+		return
+	}
+	historyPolledSmartEnergySummation, err := config.historyDelta(config.Sensors.PolledSmartEnergySummation, time.Now())
+	if err != nil {
+		return
+	}
+
+	config.db.Exec("INSERT OR REPLACE INTO cache(time,green_energy_percentage,energy_consumption) VALUES (?,?,?);", dayStart(time.Now()).Unix(), greenEnergyPercentage.Value, historyPolledSmartEnergySummation.Value)
+
+	cached, err := config.readHistory()
+	if err != nil {
+		return
+	}
+	if len(cached) != 8 && time.Now().Sub(config.HomeAssistant.InstallationDate) > 8*time.Hour*24 {
+		err := config.refreshCacheFromPast(time.Now().Add(-8 * time.Hour * 24))
+		if err != nil {
+			fmt.Println("Error refreshing cache", err.Error())
+			return
+		}
+	}
+
+}
+
+func (config Config) refreshCacheFromInstall() error {
+	return config.refreshCacheFromPast(config.HomeAssistant.InstallationDate)
+}
+
+func (config Config) refreshCacheFromPast(pastTime time.Time) error {
+
+	// in order to avoid querying and storing each day's data from 0001-01-01 in future versions
+	if config.HomeAssistant.InstallationDate.IsZero() {
+		return errors.New("installation date not set")
+	}
+
+	greenEnergyPercentage, err := config.historyBulkAverageAndConvertToGreen(config.Sensors.FossilPercentage, pastTime, time.Now())
+	if err != nil {
+		return err
+	}
+	historyPolledSmartEnergySummation, err := config.historyBulkDelta(config.Sensors.PolledSmartEnergySummation, pastTime, time.Now())
+	if err != nil {
+		return err
+	}
+
+	for key, day := range greenEnergyPercentage {
+
+		var action2 string
+		if greenEnergyPercentage[key].Value != 0 && historyPolledSmartEnergySummation[key].Value != 0 {
+			action2 = "REPLACE"
+		} else {
+			action2 = "IGNORE"
+		}
+
+		stmt, err := config.db.Prepare("INSERT OR " + action2 + " INTO cache(time, green_energy_percentage, energy_consumption) values(?,?,?)")
+		if err != nil {
+			return err
+		}
+		_, err = stmt.Exec(day.DayTime.Unix(), greenEnergyPercentage[key].Value, historyPolledSmartEnergySummation[key].Value)
+		if err != nil {
+			return err
+		}
+
+	}
+
+	return nil
+
+}
+
+func (config Config) readHistory() (History, error) {
+
+	start := dayStart(time.Now()).AddDate(0, 0, -8)
+
+	rows, err := config.db.Query("SELECT time, green_energy_percentage, energy_consumption FROM cache WHERE time > ?", start.Unix())
+	if err != nil {
+		return History{}, err
+	}
+	defer rows.Close()
+	var ret History
+
+	for rows.Next() {
+		var (
+			date                         int64
+			greenEnergyPercentage        float32
+			polledSmartEnergyConsumption float32
+		)
+		err = rows.Scan(&date, &greenEnergyPercentage, &polledSmartEnergyConsumption)
+		ret = append(ret, HistoryEntry{date, greenEnergyPercentage, polledSmartEnergyConsumption})
+	}
+
+	return ret, nil
+
+}