CPU patterns fallback

internal/video/generator.go

@@ -0,0 +1,61 @@
+package video
+
+import (
+	"errors"
+	"fmt"
+	"log"
+
+	"mxl-pattern-generator/internal/generator"
+)
+
+type generatorFactory func() (generator.FrameGenerator, error)
+
+func newFrameGenerator(cfg Config, pattern pattern) (generator.FrameGenerator, Backend, error) {
+	newGPU := func() (generator.FrameGenerator, error) {
+		return generator.NewWGPUGenerator(cfg.Width(), cfg.Height(), pattern.kernelSource)
+	}
+	newCPU := func() (generator.FrameGenerator, error) {
+		return generator.NewCPUPatternGenerator(cfg.Width(), cfg.Height(), cfg.Pattern)
+	}
+	return selectFrameGenerator(cfg.Backend, newGPU, newCPU)
+}
+
+func selectFrameGenerator(
+	backend Backend,
+	newGPU, newCPU generatorFactory,
+) (generator.FrameGenerator, Backend, error) {
+	switch backend {
+	case BackendGPU:
+		gen, err := newGPU()
+		if err != nil {
+			return nil, "", fmt.Errorf("initialize GPU video generator: %w", err)
+		}
+		return gen, BackendGPU, nil
+
+	case BackendCPU:
+		gen, err := newCPU()
+		if err != nil {
+			return nil, "", fmt.Errorf("initialize CPU video generator: %w", err)
+		}
+		return gen, BackendCPU, nil
+
+	case "", BackendAuto:
+		gpuGen, gpuErr := newGPU()
+		if gpuErr == nil {
+			return gpuGen, BackendGPU, nil
+		}
+		log.Printf("GPU video generator unavailable, falling back to CPU: %v", gpuErr)
+
+		cpuGen, cpuErr := newCPU()
+		if cpuErr == nil {
+			return cpuGen, BackendCPU, nil
+		}
+		return nil, "", errors.Join(
+			fmt.Errorf("initialize GPU video generator: %w", gpuErr),
+			fmt.Errorf("initialize CPU video generator: %w", cpuErr),
+		)
+
+	default:
+		return nil, "", fmt.Errorf("unsupported video backend %q", backend)
+	}
+}

internal/video/generator_test.go

@@ -0,0 +1,112 @@
+package video
+
+import (
+	"errors"
+	"strings"
+	"testing"
+
+	"mxl-pattern-generator/internal/generator"
+)
+
+type stubFrameGenerator struct{}
+
+func (*stubFrameGenerator) GenerateFrame([]byte, int) error { return nil }
+func (*stubFrameGenerator) Close() error                    { return nil }
+
+func TestSelectFrameGenerator(t *testing.T) {
+	gpuErr := errors.New("no GPU")
+	cpuErr := errors.New("no CPU pattern")
+	tests := []struct {
+		name          string
+		backend       Backend
+		gpuErr        error
+		cpuErr        error
+		wantBackend   Backend
+		wantGPUCalls  int
+		wantCPUCalls  int
+		wantErrSubstr []string
+	}{
+		{name: "explicit GPU", backend: BackendGPU, wantBackend: BackendGPU, wantGPUCalls: 1},
+		{name: "explicit CPU", backend: BackendCPU, wantBackend: BackendCPU, wantCPUCalls: 1},
+		{name: "auto prefers GPU", backend: BackendAuto, wantBackend: BackendGPU, wantGPUCalls: 1},
+		{name: "zero value is auto", wantBackend: BackendGPU, wantGPUCalls: 1},
+		{
+			name:         "auto falls back to CPU",
+			backend:      BackendAuto,
+			gpuErr:       gpuErr,
+			wantBackend:  BackendCPU,
+			wantGPUCalls: 1,
+			wantCPUCalls: 1,
+		},
+		{
+			name:          "auto reports both failures",
+			backend:       BackendAuto,
+			gpuErr:        gpuErr,
+			cpuErr:        cpuErr,
+			wantGPUCalls:  1,
+			wantCPUCalls:  1,
+			wantErrSubstr: []string{"GPU video generator", "CPU video generator"},
+		},
+		{
+			name:          "explicit GPU does not fall back",
+			backend:       BackendGPU,
+			gpuErr:        gpuErr,
+			wantGPUCalls:  1,
+			wantErrSubstr: []string{"GPU video generator"},
+		},
+		{
+			name:          "explicit CPU does not try GPU",
+			backend:       BackendCPU,
+			cpuErr:        cpuErr,
+			wantCPUCalls:  1,
+			wantErrSubstr: []string{"CPU video generator"},
+		},
+		{
+			name:          "invalid backend",
+			backend:       Backend("invalid"),
+			wantErrSubstr: []string{"unsupported video backend"},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			gpuCalls, cpuCalls := 0, 0
+			factory := func(calls *int, err error) generatorFactory {
+				return func() (generator.FrameGenerator, error) {
+					*calls++
+					if err != nil {
+						return nil, err
+					}
+					return &stubFrameGenerator{}, nil
+				}
+			}
+
+			gen, backend, err := selectFrameGenerator(
+				tc.backend,
+				factory(&gpuCalls, tc.gpuErr),
+				factory(&cpuCalls, tc.cpuErr),
+			)
+			if gpuCalls != tc.wantGPUCalls || cpuCalls != tc.wantCPUCalls {
+				t.Fatalf("factory calls GPU/CPU = %d/%d, want %d/%d",
+					gpuCalls, cpuCalls, tc.wantGPUCalls, tc.wantCPUCalls)
+			}
+			if len(tc.wantErrSubstr) == 0 {
+				if err != nil {
+					t.Fatalf("selectFrameGenerator: %v", err)
+				}
+				if gen == nil || backend != tc.wantBackend {
+					t.Fatalf("generator/backend = %v/%q, want non-nil/%q", gen, backend, tc.wantBackend)
+				}
+				return
+			}
+			if err == nil {
+				t.Fatal("selectFrameGenerator returned nil error")
+			}
+			for _, substring := range tc.wantErrSubstr {
+				if !strings.Contains(err.Error(), substring) {
+					t.Errorf("error = %q, want substring %q", err, substring)
+				}
+			}
+		})
+	}
+}

internal/video/runner.go

@@ -19,11 +19,11 @@ func Run(ctx context.Context, inst *mxl.Instance, cfg Config) (runErr error) {
 		return fmt.Errorf("unknown video pattern %q", cfg.Pattern)
 	}
 
-	// TODO: fall back to a CPU generator if GPU initialization fails.
-	gen, err := generator.NewWGPUGenerator(cfg.Width(), cfg.Height(), pattern.kernelSource)
+	gen, backend, err := newFrameGenerator(cfg, pattern)
 	if err != nil {
-		return fmt.Errorf("initialize wgpu video generator: %w", err)
+		return err
 	}
+	log.Printf("video generator backend: %s", backend)
 	defer func() {
 		if err := gen.Close(); err != nil {
 			runErr = errors.Join(runErr, fmt.Errorf("close video generator: %w", err))