feat: plan-fill resolution and overwrite/clear diff computation

@@ -0,0 +1,62 @@
+package day
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Change describes one slot transition for the confirmation table.
+type Change struct {
+	Index  int
+	FromID *string // nil = was empty
+	ToID   *string // nil = cleared
+}
+
+// ResolveLog computes the write set for a log command. activityID nil means
+// plan-fill: every target slot takes its plan block's activity, and any slot
+// without one aborts the whole command (all-or-nothing).
+func ResolveLog(targets []int, activityID *string, blocks, planBlocks map[int]string, blockSize int) (map[int]*string, []Change, error) {
+	assignments := make(map[int]*string, len(targets))
+	var overwrites []Change
+
+	if activityID == nil {
+		var missing []string
+		for _, i := range targets {
+			if _, ok := planBlocks[i]; !ok {
+				missing = append(missing, SlotLabel(i, blockSize))
+			}
+		}
+		if len(missing) > 0 {
+			return nil, nil, fmt.Errorf("no planned activity at %s — nothing painted", strings.Join(missing, ", "))
+		}
+	}
+
+	for _, i := range targets {
+		to := activityID
+		if to == nil {
+			v := planBlocks[i]
+			to = &v
+		}
+		assignments[i] = to
+		if from, ok := blocks[i]; ok && from != *to {
+			f := from
+			overwrites = append(overwrites, Change{Index: i, FromID: &f, ToID: to})
+		}
+	}
+	return assignments, overwrites, nil
+}
+
+// ResolveClear computes the write set for a clear command. Only slots that
+// are actually logged are touched; clearing an empty slot is a no-op.
+func ResolveClear(targets []int, blocks map[int]string) (map[int]*string, []Change) {
+	assignments := make(map[int]*string)
+	var cleared []Change
+	for _, i := range targets {
+		if from, ok := blocks[i]; ok {
+			f := from
+			assignments[i] = nil
+			cleared = append(cleared, Change{Index: i, FromID: &f, ToID: nil})
+		}
+	}
+	return assignments, cleared
+}

@@ -0,0 +1,70 @@
+package day
+
+import (
+	"strings"
+	"testing"
+)
+
+func sp(s string) *string { return &s }
+
+func TestResolveLogExplicitActivity(t *testing.T) {
+	blocks := map[int]string{28: "old"}
+	asg, over, err := ResolveLog([]int{28, 29}, sp("new"), blocks, nil, 30)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(asg) != 2 || *asg[28] != "new" || *asg[29] != "new" {
+		t.Fatalf("assignments %v", asg)
+	}
+	if len(over) != 1 || over[0].Index != 28 || *over[0].FromID != "old" || *over[0].ToID != "new" {
+		t.Fatalf("overwrites %v", over)
+	}
+}
+
+func TestResolveLogSameActivityNoOverwrite(t *testing.T) {
+	blocks := map[int]string{28: "same"}
+	_, over, err := ResolveLog([]int{28}, sp("same"), blocks, nil, 30)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(over) != 0 {
+		t.Fatalf("re-logging same activity should not count as overwrite: %v", over)
+	}
+}
+
+func TestResolveLogPlanFill(t *testing.T) {
+	plan := map[int]string{29: "planned-a", 30: "planned-b"}
+	asg, over, err := ResolveLog([]int{29, 30}, nil, map[int]string{}, plan, 30)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if *asg[29] != "planned-a" || *asg[30] != "planned-b" {
+		t.Fatalf("assignments %v", asg)
+	}
+	if len(over) != 0 {
+		t.Fatalf("overwrites %v", over)
+	}
+}
+
+func TestResolveLogPlanFillMissingPlanAllOrNothing(t *testing.T) {
+	plan := map[int]string{29: "planned-a"}
+	_, _, err := ResolveLog([]int{29, 30, 31}, nil, map[int]string{}, plan, 30)
+	if err == nil {
+		t.Fatal("want error for slots without plan")
+	}
+	msg := err.Error()
+	if !strings.Contains(msg, "15:00") || !strings.Contains(msg, "15:30") {
+		t.Fatalf("error should list offending slots (blockSize 30): %q", msg)
+	}
+}
+
+func TestResolveClear(t *testing.T) {
+	blocks := map[int]string{28: "a"}
+	asg, cleared := ResolveClear([]int{28, 29}, blocks)
+	if len(asg) != 1 || asg[28] != nil {
+		t.Fatalf("assignments %v", asg)
+	}
+	if len(cleared) != 1 || cleared[0].Index != 28 || *cleared[0].FromID != "a" || cleared[0].ToID != nil {
+		t.Fatalf("cleared %v", cleared)
+	}
+}