《互联网产品的服务端架构·内容、社交与实时互动》9.2 多端已读与未读计数

// file: internal/delivery/unread.go
// tier: L1
// verify: go test ./internal/delivery/

package delivery

import (
	"errors"
	"fmt"
)

// ErrInvalidLastSeq 表示调用方给出的会话最大序号为负，无法计算未读。
var ErrInvalidLastSeq = errors.New("delivery: lastSeq must be non-negative")

// ReadState 是某个用户在某个会话上、某一台设备报告的已读位置。
// 同一用户可能有多条记录：手机读到 88，桌面只读到 80。存储层不保证
// 合并，计算未读时必须先取同一用户同一会话的最大已读序号。
type ReadState struct {
	UserID      string
	ConvID      string
	LastReadSeq int64
}

// Count 返回 userID 在 convID 会话里的未读数。lastSeq 是该会话当前
// 最大消息序号，由消息存储给出。未读数是派生值：lastSeq 减去已读位置，
// 小于 0 时按 0 处理，因为客户端可能读过比服务端快照更新的消息。
//
// 没有已读记录等价于已读 0，也就是全部未读。记录里的负序号视为脏数据，
// 直接跳过，不能让一条错误记录把未读数抬高。lastSeq 为负是调用方错误，
// 返回 ErrInvalidLastSeq。
func Count(states []ReadState, convID string, lastSeq int64, userID string) (int64, error) {
	if lastSeq < 0 {
		return 0, fmt.Errorf("%w: got %d", ErrInvalidLastSeq, lastSeq)
	}

	var maxRead int64
	found := false
	for _, s := range states {
		if s.UserID != userID || s.ConvID != convID {
			continue
		}
		if s.LastReadSeq < 0 {
			continue
		}
		if !found || s.LastReadSeq > maxRead {
			maxRead = s.LastReadSeq
			found = true
		}
	}

	if !found {
		return lastSeq, nil
	}
	if maxRead >= lastSeq {
		return 0, nil
	}
	return lastSeq - maxRead, nil
}

// file: internal/delivery/unread_test.go
// tier: L1
// verify: go test ./internal/delivery/

package delivery

import (
	"errors"
	"testing"
)

func TestCountSubtractsLastRead(t *testing.T) {
	states := []ReadState{{UserID: "u1", ConvID: "c1", LastReadSeq: 88}}
	got, err := Count(states, "c1", 100, "u1")
	if err != nil {
		t.Fatalf("Count error = %v", err)
	}
	if got != 12 {
		t.Fatalf("unread = %d; want 12", got)
	}
}

func TestCountMultiDeviceTakesMax(t *testing.T) {
	states := []ReadState{
		{UserID: "u1", ConvID: "c1", LastReadSeq: 80},
		{UserID: "u1", ConvID: "c1", LastReadSeq: 95},
		{UserID: "u1", ConvID: "c1", LastReadSeq: 60},
	}
	got, err := Count(states, "c1", 100, "u1")
	if err != nil {
		t.Fatalf("Count error = %v", err)
	}
	if got != 5 {
		t.Fatalf("unread = %d; want 5", got)
	}
}

func TestCountNoRecordMeansAllUnread(t *testing.T) {
	got, err := Count(nil, "c1", 42, "u1")
	if err != nil {
		t.Fatalf("Count error = %v", err)
	}
	if got != 42 {
		t.Fatalf("unread = %d; want 42", got)
	}
}

func TestCountClampsWhenReadAhead(t *testing.T) {
	states := []ReadState{{UserID: "u1", ConvID: "c1", LastReadSeq: 120}}
	got, err := Count(states, "c1", 100, "u1")
	if err != nil {
		t.Fatalf("Count error = %v", err)
	}
	if got != 0 {
		t.Fatalf("unread = %d; want 0", got)
	}
}

func TestCountIgnoresOtherConversationAndUser(t *testing.T) {
	states := []ReadState{
		{UserID: "u1", ConvID: "c2", LastReadSeq: 99},
		{UserID: "u2", ConvID: "c1", LastReadSeq: 99},
	}
	got, err := Count(states, "c1", 100, "u1")
	if err != nil {
		t.Fatalf("Count error = %v", err)
	}
	if got != 100 {
		t.Fatalf("unread = %d; want 100", got)
	}
}

func TestCountSkipsNegativeStoredSeq(t *testing.T) {
	states := []ReadState{
		{UserID: "u1", ConvID: "c1", LastReadSeq: -5},
		{UserID: "u1", ConvID: "c1", LastReadSeq: 90},
	}
	got, err := Count(states, "c1", 100, "u1")
	if err != nil {
		t.Fatalf("Count error = %v", err)
	}
	if got != 10 {
		t.Fatalf("unread = %d; want 10", got)
	}
}

func TestCountNegativeLastSeqErrors(t *testing.T) {
	_, err := Count(nil, "c1", -1, "u1")
	if !errors.Is(err, ErrInvalidLastSeq) {
		t.Fatalf("err = %v; want ErrInvalidLastSeq", err)
	}
}

cd .local/checks/webapp-lab && go test ./internal/delivery/ -run 'TestCount' -count=1 -v

=== RUN   TestCountSubtractsLastRead
--- PASS: TestCountSubtractsLastRead (0.00s)
=== RUN   TestCountMultiDeviceTakesMax
--- PASS: TestCountMultiDeviceTakesMax (0.00s)
=== RUN   TestCountNoRecordMeansAllUnread
--- PASS: TestCountNoRecordMeansAllUnread (0.00s)
=== RUN   TestCountClampsWhenReadAhead
--- PASS: TestCountClampsWhenReadAhead (0.00s)
=== RUN   TestCountIgnoresOtherConversationAndUser
--- PASS: TestCountIgnoresOtherConversationAndUser (0.00s)
=== RUN   TestCountSkipsNegativeStoredSeq
--- PASS: TestCountSkipsNegativeStoredSeq (0.00s)
=== RUN   TestCountNegativeLastSeqErrors
--- PASS: TestCountNegativeLastSeqErrors (0.00s)
PASS
ok  	webapp/internal/delivery	0.303s