Add Code Coverage Delta handling

.github/workflows/main.yml

@@ -71,6 +71,22 @@ jobs:
         token: ${{ secrets.CODECOV_TOKEN }}
         files: ./target/site/jacoco/jacoco.xml
         flags: unittests
+    - name: Upload coverage report as artifact
+      if: ${{ matrix.release_build && github.event_name != 'pull_request' }}
+      uses: actions/upload-artifact@v4
+      with:
+        name: jacoco-report
+        path: target/site/jacoco/jacoco.csv
+        retention-days: 30
+    - name: Download base branch coverage
+      if: ${{ matrix.release_build && github.event_name == 'pull_request' }}
+      uses: dawidd6/action-download-artifact@v6
+      continue-on-error: true
+      with:
+        workflow: main.yml
+        branch: ${{ github.event.pull_request.base.ref }}
+        name: jacoco-report
+        path: base-coverage/
     - name: Generate coverage summary
       if: ${{ matrix.release_build && github.event_name == 'pull_request' }}
       id: jacoco
@@ -84,31 +100,76 @@ jobs:
       if: ${{ matrix.release_build && github.event_name == 'pull_request' }}
       continue-on-error: true
       run: |
+        # Helper functions
+        parse_coverage() {
+          local csv_file=$1 col_missed=$2 col_covered=$3
+          awk -F',' -v m="$col_missed" -v c="$col_covered" \
+            'NR>1 && $1=="Jackson-databind" {
+              missed=$m; covered=$c;
+              if (missed + covered > 0) printf "%.1f", (covered * 100.0) / (missed + covered)
+            }' "$csv_file" | head -1
+        }
+
+        format_delta() {
+          local delta=$1
+          if awk -v d="$delta" 'BEGIN { exit (d >= 0) ? 0 : 1 }'; then
+            echo "+${delta}%|📈"
+          else
+            echo "${delta}%|📉"
+          fi
+        }
+
+        get_color() {
+          awk -v v="$1" 'BEGIN {
+            if (v >= 80) print "brightgreen"
+            else if (v >= 60) print "green"
+            else if (v >= 40) print "yellow"
+            else print "red"
+          }'
+        }
+
         # Convert decimal to percentage and round to 1 decimal place
         COVERAGE=$(awk -v cov="${{ steps.jacoco.outputs.coverage }}" 'BEGIN { printf "%.1f", cov * 100 }')
         BRANCHES=$(awk -v br="${{ steps.jacoco.outputs.branches }}" 'BEGIN { printf "%.1f", br * 100 }')
 
-        # Determine color for coverage badge using awk (more portable than bc)
-        COV_COLOR=$(awk -v cov="$COVERAGE" 'BEGIN {
-          if (cov >= 80) print "brightgreen"
-          else if (cov >= 60) print "green"
-          else if (cov >= 40) print "yellow"
-          else print "red"
-        }')
-
-        BR_COLOR=$(awk -v br="$BRANCHES" 'BEGIN {
-          if (br >= 80) print "brightgreen"
-          else if (br >= 60) print "green"
-          else if (br >= 40) print "yellow"
-          else print "red"
-        }')
+        # Check if base coverage artifact was downloaded and calculate deltas
+        HAS_DELTA=false
+        if [ -f "base-coverage/jacoco.csv" ]; then
+          echo "Found base branch coverage from artifact"
+          BASE_COVERAGE=$(parse_coverage "base-coverage/jacoco.csv" 4 5)
+          BASE_BRANCHES=$(parse_coverage "base-coverage/jacoco.csv" 6 7)
 
-        COMMENT_BODY="## :test_tube: Code Coverage Report
+          if [ -n "$BASE_COVERAGE" ] && [ -n "$BASE_BRANCHES" ]; then
+            COV_DELTA=$(awk -v curr="$COVERAGE" -v base="$BASE_COVERAGE" 'BEGIN { printf "%.1f", curr - base }')
+            BR_DELTA=$(awk -v curr="$BRANCHES" -v base="$BASE_BRANCHES" 'BEGIN { printf "%.1f", curr - base }')
 
-        | Metric | Coverage |
+            IFS='|' read -r COV_DELTA_STR COV_DELTA_EMOJI <<< "$(format_delta "$COV_DELTA")"
+            IFS='|' read -r BR_DELTA_STR BR_DELTA_EMOJI <<< "$(format_delta "$BR_DELTA")"
+
+            HAS_DELTA=true
+          fi
+        fi
+
+        # Determine badge colors
+        COV_COLOR=$(get_color "$COVERAGE")
+        BR_COLOR=$(get_color "$BRANCHES")
+
+        # Build coverage table with or without deltas
+        if [ "$HAS_DELTA" = "true" ]; then
+          COVERAGE_TABLE="| Metric | Coverage | Change |
+        |--------|----------|--------|
+        | **Instructions** | ![coverage](https://img.shields.io/badge/coverage-${COVERAGE}%25-${COV_COLOR}) **${COVERAGE}%** | ${COV_DELTA_EMOJI} **${COV_DELTA_STR}** |
+        | **Branches** | ![branches](https://img.shields.io/badge/branches-${BRANCHES}%25-${BR_COLOR}) **${BRANCHES}%** | ${BR_DELTA_EMOJI} **${BR_DELTA_STR}** |"
+        else
+          COVERAGE_TABLE="| Metric | Coverage |
         |--------|----------|
         | **Instructions** | ![coverage](https://img.shields.io/badge/coverage-${COVERAGE}%25-${COV_COLOR}) **${COVERAGE}%** |
-        | **Branches** | ![branches](https://img.shields.io/badge/branches-${BRANCHES}%25-${BR_COLOR}) **${BRANCHES}%** |
+        | **Branches** | ![branches](https://img.shields.io/badge/branches-${BRANCHES}%25-${BR_COLOR}) **${BRANCHES}%** |"
+        fi
+
+        COMMENT_BODY="## :test_tube: Code Coverage Report
+
+        $COVERAGE_TABLE
 
         > Coverage data generated from JaCoCo test results