1 files changed, 139 insertions, 0 deletions
diff --git a/cpu/nestest_test.go b/cpu/nestest_test.go
new file mode 100644
index 0000000..4786c5b
--- /dev/null
+++ b/cpu/nestest_test.go
@@ -0,0 +1,139 @@
+package cpu
+
+import (
+	"bufio"
+	"os"
+	"regexp"
+	"strconv"
+	"testing"
+)
+
+type nestestBus struct {
+	ram [0x0800]uint8
+	prg []uint8
+}
+
+func (b *nestestBus) Read(addr uint16) uint8 {
+	switch {
+	case addr < 0x2000:
+		return b.ram[addr%0x0800]
+	case addr >= 0x8000:
+		return b.prg[int(addr-0x8000)%len(b.prg)]
+	default:
+		return 0
+	}
+}
+
+func (b *nestestBus) Write(addr uint16, v uint8) {
+	if addr < 0x2000 {
+		b.ram[addr%0x0800] = v
+	}
+}
+
+func loadNestestPRG(path string) ([]uint8, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	prgBanks := int(data[4])
+	prgSize := prgBanks * 16 * 1024
+	hasTrainer := data[6]&0x04 != 0
+	offset := 16
+	if hasTrainer {
+		offset += 512
+	}
+	return data[offset : offset+prgSize], nil
+}
+
+type cpuState struct {
+	pc             uint16
+	a, x, y, p, sp uint8
+	cyc            uint64
+}
+
+var nestestLogRE = regexp.MustCompile(
+	`^([0-9A-F]{4}).*A:([0-9A-F]{2}) X:([0-9A-F]{2}) Y:([0-9A-F]{2}) P:([0-9A-F]{2}) SP:([0-9A-F]{2}).*CYC:\s*(\d+)`)
+
+func loadNestestLog(path string) ([]cpuState, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	var lines []cpuState
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		m := nestestLogRE.FindStringSubmatch(scanner.Text())
+		if m == nil {
+			continue
+		}
+		parseHex8 := func(s string) uint8 {
+			v, _ := strconv.ParseUint(s, 16, 8)
+			return uint8(v)
+		}
+		pc, _ := strconv.ParseUint(m[1], 16, 16)
+		cyc, _ := strconv.ParseUint(m[7], 10, 64)
+		lines = append(lines, cpuState{
+			pc:  uint16(pc),
+			a:   parseHex8(m[2]),
+			x:   parseHex8(m[3]),
+			y:   parseHex8(m[4]),
+			p:   parseHex8(m[5]),
+			sp:  parseHex8(m[6]),
+			cyc: cyc,
+		})
+	}
+	return lines, scanner.Err()
+}
+
+// TestNestest runs nestest.nes and diffs the state against a known good log before each instruction
+func TestNestest(t *testing.T) {
+	const romPath = "testdata/nestest.nes"
+	const logPath = "testdata/nestest.log"
+
+	if _, err := os.Stat(romPath); err != nil {
+		t.Skip("testdata/nestest.nes not present; add it and testdata/nestest.log to run this test")
+	}
+
+	prg, err := loadNestestPRG(romPath)
+	if err != nil {
+		t.Fatalf("loading ROM: %v", err)
+	}
+	want, err := loadNestestLog(logPath)
+	if err != nil {
+		t.Fatalf("loading log: %v", err)
+	}
+	if len(want) == 0 {
+		t.Fatal("no log lines parsed")
+	}
+
+	bus := &nestestBus{prg: prg}
+	c := NewCPU(bus)
+	c.PC = 0xC000
+	c.SP = 0xFD
+	c.Status = 0x24
+	c.Cycles = want[0].cyc
+
+	for i, w := range want {
+		got := cpuState{c.PC, c.A, c.X, c.Y, c.Status, c.SP, c.Cycles}
+		if got != w {
+			t.Fatalf("line %d: got %+v, want %+v", i+1, got, w)
+		}
+
+		stopped := func() (panicked bool) {
+			defer func() {
+				if r := recover(); r != nil {
+					t.Logf("stopped at line %d: %v", i+1, r)
+					panicked = true
+				}
+			}()
+			c.Step()
+			return false
+		}()
+		if stopped {
+			t.Logf("matched %d instructions before stopping", i+1)
+			return
+		}
+	}
+}