《互联网产品的服务端架构·内容、社交与实时互动》3.1 拉模式：读时聚合

// file: internal/feed/pull.go
// tier: L1
// verify: go test ./internal/feed/

package feed

import (
	"fmt"
	"sort"
)

// Item 是时间线上的一条候选内容。Score 由排序服务给出，
// At 是发布时间（Unix 秒），用于同分时保持新鲜度。
type Item struct {
	ID       string
	AuthorID string
	Score    float64
	At       int64
}

// Merge 把多个来源的条目合并成一条时间线。
// 先按 ID 去重，保留分数更高的一条；再按 Score 降序、At 降序排列，
// 最后截断到 limit。nil sources 表示调用方忘了传来源，直接报错。
func Merge(sources [][]Item, limit int) ([]Item, error) {
	if sources == nil {
		return nil, fmt.Errorf("feed: sources is nil")
	}
	if limit <= 0 {
		return nil, fmt.Errorf("feed: limit must be positive, got %d", limit)
	}

	all := make([]Item, 0)
	for _, src := range sources {
		all = append(all, src...)
	}

	out := ranked(all)
	if len(out) > limit {
		out = out[:limit]
	}
	return out, nil
}

// ranked 去重并按 Score 降序、At 降序排序，供 Merge 与 Mix 共用。
// 同分同时间时用 ID 兜底，避免 map 遍历顺序让输出在多次运行之间漂移。
func ranked(items []Item) []Item {
	best := make(map[string]Item, len(items))
	for _, it := range items {
		cur, ok := best[it.ID]
		if !ok || better(it, cur) {
			best[it.ID] = it
		}
	}

	out := make([]Item, 0, len(best))
	for _, it := range best {
		out = append(out, it)
	}
	sort.Slice(out, func(i, j int) bool {
		if out[i].Score != out[j].Score {
			return out[i].Score > out[j].Score
		}
		if out[i].At != out[j].At {
			return out[i].At > out[j].At
		}
		return out[i].ID < out[j].ID
	})
	return out
}

// better 定义同 ID 条目谁更值得保留：分数优先，同分时保留更新的。
func better(a, b Item) bool {
	if a.Score != b.Score {
		return a.Score > b.Score
	}
	return a.At > b.At
}

// file: internal/feed/pull_test.go
// tier: L1
// verify: go test ./internal/feed/

package feed

import "testing"

func TestMergeOrdersAndDeduplicates(t *testing.T) {
	sources := [][]Item{
		{
			{ID: "a", AuthorID: "u1", Score: 10, At: 100},
			{ID: "b", AuthorID: "u2", Score: 9, At: 50},
		},
		{
			{ID: "a", AuthorID: "u1", Score: 12, At: 90},
			{ID: "c", AuthorID: "u3", Score: 9, At: 70},
		},
	}

	got, err := Merge(sources, 10)
	if err != nil {
		t.Fatal(err)
	}
	if len(got) != 3 {
		t.Fatalf("got %d items, want 3: %+v", len(got), got)
	}
	want := []string{"a", "c", "b"}
	for i, id := range want {
		if got[i].ID != id {
			t.Fatalf("position %d = %s, want %s (all=%+v)", i, got[i].ID, id, got)
		}
	}
	if got[0].Score != 12 {
		t.Fatalf("duplicate kept score %.1f, want 12", got[0].Score)
	}
}

func TestMergeTieBreaksByNewerThenID(t *testing.T) {
	sources := [][]Item{{
		{ID: "old", Score: 5, At: 10},
		{ID: "new", Score: 5, At: 20},
		{ID: "also-new", Score: 5, At: 20},
	}}

	got, err := Merge(sources, 10)
	if err != nil {
		t.Fatal(err)
	}
	want := []string{"also-new", "new", "old"}
	for i, id := range want {
		if got[i].ID != id {
			t.Fatalf("position %d = %s, want %s (all=%+v)", i, got[i].ID, id, got)
		}
	}
}

func TestMergeKeepsHigherScoreAcrossSources(t *testing.T) {
	sources := [][]Item{
		{{ID: "x", Score: 3, At: 999}},
		{{ID: "x", Score: 8, At: 1}},
	}

	got, err := Merge(sources, 1)
	if err != nil {
		t.Fatal(err)
	}
	if len(got) != 1 {
		t.Fatalf("got %d items, want 1", len(got))
	}
	if got[0].Score != 8 || got[0].At != 1 {
		t.Fatalf("kept %+v, want score 8 at 1", got[0])
	}
}

func TestMergeRejectsInvalidInput(t *testing.T) {
	if _, err := Merge(nil, 10); err == nil {
		t.Fatal("want error for nil sources")
	}
	if _, err := Merge([][]Item{}, 0); err == nil {
		t.Fatal("want error for zero limit")
	}
	if _, err := Merge([][]Item{}, -3); err == nil {
		t.Fatal("want error for negative limit")
	}
}

func TestMergeHandlesEmptySources(t *testing.T) {
	got, err := Merge([][]Item{{}, nil}, 5)
	if err != nil {
		t.Fatal(err)
	}
	if len(got) != 0 {
		t.Fatalf("got %d items, want 0", len(got))
	}
}

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

=== RUN   TestMergeOrdersAndDeduplicates
--- PASS: TestMergeOrdersAndDeduplicates (0.00s)
=== RUN   TestMergeTieBreaksByNewerThenID
--- PASS: TestMergeTieBreaksByNewerThenID (0.00s)
=== RUN   TestMergeKeepsHigherScoreAcrossSources
--- PASS: TestMergeKeepsHigherScoreAcrossSources (0.00s)
=== RUN   TestMergeRejectsInvalidInput
--- PASS: TestMergeRejectsInvalidInput (0.00s)
=== RUN   TestMergeHandlesEmptySources
--- PASS: TestMergeHandlesEmptySources (0.00s)
PASS
ok  	webapp/internal/feed	0.286s