test: cover filescan parallel helper

internal/filescan/parallel_test.go

@@ -0,0 +1,112 @@
+package filescan
+
+import (
+	"runtime"
+	"slices"
+	"strconv"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestParallelPreservesInputOrder(t *testing.T) {
+	paths := []string{"first", "second", "third", "fourth", "fifth"}
+	index := make(map[string]int, len(paths))
+	for i, path := range paths {
+		index[path] = i
+	}
+
+	got := Parallel(paths, func(path string) (string, bool) {
+		time.Sleep(time.Duration(len(paths)-index[path]) * time.Millisecond)
+		return path + "-done", true
+	})
+
+	want := []string{"first-done", "second-done", "third-done", "fourth-done", "fifth-done"}
+	if !slices.Equal(got, want) {
+		t.Fatalf("Parallel order = %v, want %v", got, want)
+	}
+}
+
+func TestParallelEmptyInputReturnsNilWithoutWork(t *testing.T) {
+	got := Parallel[int](nil, func(string) (int, bool) {
+		t.Fatal("callback should not run for empty input")
+		return 0, true
+	})
+
+	if got != nil {
+		t.Fatalf("Parallel(nil) = %#v, want nil", got)
+	}
+}
+
+func TestParallelDropsRejectedItems(t *testing.T) {
+	paths := []string{"keep-a", "drop", "keep-b", "skip", "keep-c"}
+
+	got := Parallel(paths, func(path string) (string, bool) {
+		switch path {
+		case "drop", "skip":
+			return "", false
+		default:
+			return path, true
+		}
+	})
+
+	want := []string{"keep-a", "keep-b", "keep-c"}
+	if !slices.Equal(got, want) {
+		t.Fatalf("Parallel filtered = %v, want %v", got, want)
+	}
+}
+
+func TestParallelSingleItem(t *testing.T) {
+	got := Parallel([]string{"only"}, func(path string) (string, bool) {
+		return path + "-value", true
+	})
+
+	want := []string{"only-value"}
+	if !slices.Equal(got, want) {
+		t.Fatalf("Parallel single item = %v, want %v", got, want)
+	}
+}
+
+func TestParallelLargeInputCompletesWithinConcurrencyLimit(t *testing.T) {
+	const n = 100
+	paths := make([]string, n)
+	for i := range paths {
+		paths[i] = strconv.Itoa(i)
+	}
+
+	var active atomic.Int64
+	var maxActive atomic.Int64
+
+	got := Parallel(paths, func(path string) (int, bool) {
+		current := active.Add(1)
+		recordMax(&maxActive, current)
+		defer active.Add(-1)
+
+		time.Sleep(time.Millisecond)
+		value, _ := strconv.Atoi(path)
+		return value, true
+	})
+
+	if len(got) != n {
+		t.Fatalf("Parallel large input returned %d items, want %d", len(got), n)
+	}
+	for i, value := range got {
+		if value != i {
+			t.Fatalf("Parallel large input[%d] = %d, want %d", i, value, i)
+		}
+	}
+
+	limit := min(max(runtime.NumCPU()*2, 4), len(paths))
+	if int(maxActive.Load()) > limit {
+		t.Fatalf("max concurrent callbacks = %d, want <= %d", maxActive.Load(), limit)
+	}
+}
+
+func recordMax(maxSeen *atomic.Int64, value int64) {
+	for {
+		seen := maxSeen.Load()
+		if value <= seen || maxSeen.CompareAndSwap(seen, value) {
+			return
+		}
+	}
+}