refactor: extract git diff logic to internal/git

This commit extracts the logic for retrieving and parsing git diff information from the root command into a new `internal/git` package.

This change improves code organization and modularity by separating concerns. The `internal/git` package now encapsulates all git-related operations, making the root command cleaner and more focused on its core responsibilities.
---
{"auto-commit-msg":{"language":"rust","version":"0.4.0-dev","model":"gemini-2.5-flash-lite","response_time":1.83,"execution_time":1.85}}

Author: sestrella

cmd/root.go

@@ -6,12 +6,10 @@ import (
 	"log"
 	"math"
 	"os"
-	"os/exec"
-	"regexp"
-	"strconv"
 	"strings"
 	"time"
 
+	"github.com/sestrella/auto-commit-msg/internal/git"
 	"github.com/sestrella/auto-commit-msg/internal/openai"
 	"github.com/spf13/cobra"
 	"github.com/spf13/viper"
@@ -78,40 +76,21 @@ var rootCmd = &cobra.Command{
 			return
 		}
 
-		gitDiff, err := exec.Command("git", "diff", "--cached").Output()
+		cachedGitDiff, err := git.DiffCached()
 		if err != nil {
 			cobra.CheckErr(err)
 		}
-		if len(gitDiff) == 0 {
+
+		if cachedGitDiff == "" {
 			cobra.CheckErr("git diff is empty")
 		}
 
-		shortStat, err := exec.Command("git", "diff", "--cached", "--shortstat").Output()
+		stats, err := git.DiffCachedStats()
 		if err != nil {
 			cobra.CheckErr(err)
 		}
-		shortStatStr := string(shortStat)
-
-		// TODO: Get insertions and deletions using a single regex, take into account num of files?
-		insertionMatches := regexp.MustCompile(`(\d+)\s+insertions?\(\+\)`).FindStringSubmatch(shortStatStr)
-		insertions := 0
-		if len(insertionMatches) > 1 {
-			insertions, err = strconv.Atoi(insertionMatches[1])
-			if err != nil {
-				cobra.CheckErr(err)
-			}
-		}
-
-		deletionMatches := regexp.MustCompile(`(\d+)\s+deletions?\(\-\)`).FindStringSubmatch(shortStatStr)
-		deletions := 0
-		if len(deletionMatches) > 1 {
-			deletions, err = strconv.Atoi(deletionMatches[1])
-			if err != nil {
-				cobra.CheckErr(err)
-			}
-		}
 
-		totalChanges := insertions + deletions
+		totalChanges := stats.Insertions + stats.Deletions
 		totalChangesThreshold := config.Diff.Threshold
 		var model string
 		if totalChanges < totalChangesThreshold {
@@ -151,7 +130,7 @@ var rootCmd = &cobra.Command{
 			},
 			{
 				Role:    "user",
-				Content: string(gitDiff),
+				Content: cachedGitDiff,
 			},
 		})
 		var responseDuration time.Duration

internal/git/root.go

@@ -0,0 +1,57 @@
+package git
+
+import (
+	"errors"
+	"os/exec"
+	"regexp"
+	"strconv"
+)
+
+type Stats struct {
+	FileChanged int
+	Insertions  int
+	Deletions   int
+}
+
+func DiffCached() (string, error) {
+	output, err := exec.Command("git", "diff", "--cached").Output()
+	if err != nil {
+		return "", err
+	}
+
+	return string(output), nil
+}
+
+func DiffCachedStats() (*Stats, error) {
+	output, err := exec.Command("git", "diff", "--cached", "--shortstat").Output()
+	if err != nil {
+		return nil, err
+	}
+
+	re := regexp.MustCompile(`\s+(\d+)\s+files? changed,\s+(\d+)\s+insertions\(\+\),\s+(\d+)\s+deletions\(\-\)`)
+	matches := re.FindStringSubmatch(string(output))
+	if len(matches) < 4 {
+		return nil, errors.New("TODO")
+	}
+
+	fileChanged, err := strconv.Atoi(matches[1])
+	if err != nil {
+		return nil, err
+	}
+
+	insertions, err := strconv.Atoi(matches[2])
+	if err != nil {
+		return nil, err
+	}
+
+	deletions, err := strconv.Atoi(matches[3])
+	if err != nil {
+		return nil, err
+	}
+
+	return &Stats{
+		FileChanged: fileChanged,
+		Insertions:  insertions,
+		Deletions:   deletions,
+	}, nil
+}