feat: Refactor diff threshold logic to use total changes

This commit refactors the logic for determining which language model to use based on the size of the git diff. Instead of counting the number of lines in the diff, it now calculates the total number of insertions and deletions using `git diff --shortstat`.

The `diff.threshold` default value has also been adjusted from 500 to 200 to better reflect the new calculation method.

auto-commit-msg{Version:0.4.0-dev Model:gemini-2.5-flash-lite ResponseTime:1.427025958s ExecutionTime:1.435464833s}

Author: sestrella

cmd/root.go

@@ -5,6 +5,8 @@ import (
 	"log"
 	"os"
 	"os/exec"
+	"regexp"
+	"strconv"
 	"strings"
 	"time"
 
@@ -77,22 +79,45 @@ var rootCmd = &cobra.Command{
 			cobra.CheckErr("git diff is empty")
 		}
 
-		gitDiffStr := string(gitDiff)
-		gitDiffLoc := strings.Count(gitDiffStr, "\n")
-		diffThreshold := config.Diff.Threshold
+		shortStat, err := exec.Command("git", "diff", "--cached", "--shortstat").Output()
+		if err != nil {
+			cobra.CheckErr(err)
+		}
+		shortStatStr := string(shortStat)
+
+		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
+		totalChangesThreshold := config.Diff.Threshold
 		var model string
-		if gitDiffLoc < diffThreshold {
+		if totalChanges < totalChangesThreshold {
 			model = config.Diff.ShortModel
 			if model == "" {
 				cobra.CheckErr("short_model cannot be empty")
 			}
-			log.Printf("git diff LOC %d under %d threshold, using model for short diffs: %s\n", gitDiffLoc, diffThreshold, model)
+			log.Printf("git diff total changes %d under %d threshold, using model for short diffs: %s\n", totalChanges, totalChangesThreshold, model)
 		} else {
 			model = config.Diff.LongModel
 			if model == "" {
 				cobra.CheckErr("long_model cannot be empty")
 			}
-			log.Printf("git diff LOC %d over %d threshold, using model for long diffs: %s\n", gitDiffLoc, diffThreshold, model)
+			log.Printf("git diff total changes %d over %d threshold, using model for long diffs: %s\n", totalChanges, totalChangesThreshold, model)
 		}
 		if config.Provider.ApiKey == "" {
 			cobra.CheckErr("api_key environment variable name cannot be empty")
@@ -118,7 +143,7 @@ var rootCmd = &cobra.Command{
 			},
 			{
 				Role:    "user",
-				Content: gitDiffStr,
+				Content: string(gitDiff),
 			},
 		})
 		var responseDuration time.Duration
@@ -195,7 +220,7 @@ func initConfig() {
 	viper.SetDefault("provider.api_key", "GEMINI_API_KEY")
 	viper.SetDefault("diff.short_model", "gemini-2.5-flash-lite")
 	viper.SetDefault("diff.long_model", "gemini-2.5-flash")
-	viper.SetDefault("diff.threshold", 500)
+	viper.SetDefault("diff.threshold", 200)
 	viper.AutomaticEnv() // read in environment variables that match
 
 	// If a config file is found, read it in.