Add export to file (#58)

* Initial addition of a game exporter

* Fix snake state bug, remove test logs, fix final output line being empty

* Ignore test JSONL file

* Added explanation for design decision on the you key in SnakeResponse

* Adjust gitignore to be more generic

* Retain consistency in usage of pointer

* Re-word explanation to refer to requests instead of responses

* Remove unnecessary nil check

* Check error returned by WriteString

* Change file permissions for output file

* Initialise gameexporter regardless of whether output is requested

* Print error and exit if export to file fails

* Added another comment explaining reasoning around export checks

* Fixed broken test due to changed return type

cli/commands/output.go

@@ -0,0 +1,86 @@
+package commands
+
+import (
+	"encoding/json"
+	"fmt"
+	"log"
+	"os"
+
+	"github.com/BattlesnakeOfficial/rules/client"
+)
+
+type GameExporter struct {
+	game          client.Game
+	snakeRequests []client.SnakeRequest
+	winner        SnakeState
+	isDraw        bool
+}
+
+type result struct {
+	WinnerID   string `json:"winnerId"`
+	WinnerName string `json:"winnerName"`
+	IsDraw     bool   `json:"isDraw"`
+}
+
+func (ge *GameExporter) FlushToFile(filepath string, format string) error {
+	var formattedOutput []string
+	var formattingErr error
+
+	// TODO: Support more formats
+	if format == "JSONL" {
+		formattedOutput, formattingErr = ge.ConvertToJSON()
+	} else {
+		log.Fatalf("Invalid output format passed: %s", format)
+	}
+
+	if formattingErr != nil {
+		return formattingErr
+	}
+
+	f, err := os.OpenFile(filepath, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	for _, line := range formattedOutput {
+		_, err := f.WriteString(fmt.Sprintf("%s\n", line))
+		if err != nil {
+			return err
+		}
+	}
+
+	log.Printf("Written %d lines of output to file: %s\n", len(formattedOutput), filepath)
+
+	return nil
+}
+
+func (ge *GameExporter) ConvertToJSON() ([]string, error) {
+	output := make([]string, 0)
+	serialisedGame, err := json.Marshal(ge.game)
+	if err != nil {
+		return output, err
+	}
+	output = append(output, string(serialisedGame))
+	for _, board := range ge.snakeRequests {
+		serialisedBoard, err := json.Marshal(board)
+		if err != nil {
+			return output, err
+		}
+		output = append(output, string(serialisedBoard))
+	}
+	serialisedResult, err := json.Marshal(result{
+		WinnerID:   ge.winner.ID,
+		WinnerName: ge.winner.Name,
+		IsDraw:     ge.isDraw,
+	})
+	if err != nil {
+		return output, err
+	}
+	output = append(output, string(serialisedResult))
+	return output, nil
+}
+
+func (ge *GameExporter) AddSnakeRequest(snakeRequest client.SnakeRequest) {
+	ge.snakeRequests = append(ge.snakeRequests, snakeRequest)
+}