mobile: add Days, a text-free week projection for the calendar

A week through Days benchmarks at ~9-10ms/op, versus ~23ms for seven
full days through Day.

1 files changed, 83 insertions, 0 deletions

diff --git a/mobile/mobile_test.go b/mobile/mobile_test.go
index c7df63c..57e835a 100644
--- a/mobile/mobile_test.go
+++ b/mobile/mobile_test.go
@@ -2,6 +2,7 @@ package mobile
 
 import (
 	"encoding/json"
+	"strings"
 	"testing"
 )
 
@@ -40,3 +41,85 @@ func TestDayHasColourKeyButNoColourLabel(t *testing.T) {
 		t.Error("colour_label must not be returned: the app draws a swatch")
 	}
 }
+
+func decodeDays(t *testing.T, s string) []map[string]any {
+	t.Helper()
+	var a []map[string]any
+	if err := json.Unmarshal([]byte(s), &a); err != nil {
+		t.Fatalf("Days returned invalid JSON: %v\n%s", err, s)
+	}
+	return a
+}
+
+func TestDaysReturnsConsecutiveDates(t *testing.T) {
+	a := decodeDays(t, Days("2026-07-27", 7, "of", "pl"))
+	if len(a) != 7 {
+		t.Fatalf("got %d elements, want 7", len(a))
+	}
+	want := []string{"2026-07-27", "2026-07-28", "2026-07-29", "2026-07-30",
+		"2026-07-31", "2026-08-01", "2026-08-02"}
+	for i, w := range want {
+		if a[i]["date"] != w {
+			t.Errorf("element %d date = %v, want %v", i, a[i]["date"], w)
+		}
+	}
+}
+
+// Days must agree with Day about the same date -- it is a cheaper projection of
+// the same computation, not a second implementation.
+func TestDaysAgreesWithDay(t *testing.T) {
+	one := decodeDay(t, Day("2026-08-01", "of", "vul", "pl"))
+	week := decodeDays(t, Days("2026-08-01", 1, "of", "pl"))
+	if len(week) != 1 {
+		t.Fatalf("got %d elements, want 1", len(week))
+	}
+	row := week[0]
+	for _, k := range []string{"name", "colour", "rank", "rank_label"} {
+		if row[k] != one[k] {
+			t.Errorf("%s: Days = %v, Day = %v", k, row[k], one[k])
+		}
+	}
+	parts := row["parts"].([]any)
+	readings := one["readings"].([]any)
+	if len(parts) != len(readings) {
+		t.Fatalf("parts = %d, readings = %d", len(parts), len(readings))
+	}
+	for i := range parts {
+		p := parts[i].(map[string]any)
+		r := readings[i].(map[string]any)
+		if p["part"] != r["part"] || p["citation"] != r["citation"] {
+			t.Errorf("element %d: Days %v/%v vs Day %v/%v",
+				i, p["part"], p["citation"], r["part"], r["citation"])
+		}
+	}
+}
+
+// No reading text may cross the boundary: that is the whole point of Days.
+func TestDaysCarriesNoReadingText(t *testing.T) {
+	s := Days("2026-08-01", 7, "of", "pl")
+	if strings.Contains(s, `"text"`) {
+		t.Error("Days must not return reading text")
+	}
+	if strings.Contains(s, "colour_label") {
+		t.Error("Days must not return a colour label")
+	}
+}
+
+func TestDaysRejectsBadInput(t *testing.T) {
+	for _, c := range []struct {
+		start string
+		count int
+	}{
+		{"not-a-date", 7}, {"2026-08-01", 0}, {"2026-08-01", -1}, {"2026-08-01", 400},
+	} {
+		if got := Days(c.start, c.count, "of", "pl"); got != "[]" {
+			t.Errorf("Days(%q, %d) = %s, want []", c.start, c.count, got)
+		}
+	}
+}
+
+func BenchmarkDaysWeek(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Days("2026-07-27", 7, "of", "pl")
+	}
+}