feat(inventory-go): build EXISTS conditions for spell filter params

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

go-services/inventory-go/spell_filter.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"net/url"
 	"sort"
 	"strings"
 )
@@ -58,3 +59,48 @@ func exactSpellID(spells map[int]map[string]any, q string) (int, bool) {
 	}
 	return best, best != -1
 }
+
+// spellExists renders the EXISTS clause for a set of spell IDs, binding each
+// ID as a positional arg. ids must be non-empty.
+func spellExists(ids []int, ab *argBuilder) string {
+	ph := make([]string, len(ids))
+	for i, id := range ids {
+		ph[i] = ab.add(id)
+	}
+	return "EXISTS (SELECT 1 FROM item_spells sp WHERE sp.item_id = db_item_id AND sp.spell_id IN (" + strings.Join(ph, ", ") + "))"
+}
+
+// spellFilterConds builds the WHERE conditions for has_spell, spell_contains,
+// and legendary_cantrips. Cantrips use AND semantics — one EXISTS per checked
+// cantrip (NOT the legacy COUNT>=N trick, which can false-positive when one
+// label matches two spells on an item while another matches none). Any
+// no-match filter yields the impossible condition, matching legacy intent.
+func spellFilterConds(q url.Values, spells map[int]map[string]any, ab *argBuilder) []string {
+	var conds []string
+	impossible := func() { conds = append(conds, "1 = 0") }
+
+	if v := q.Get("has_spell"); v != "" {
+		if id, ok := exactSpellID(spells, v); ok {
+			conds = append(conds, spellExists([]int{id}, ab))
+		} else {
+			impossible()
+		}
+	}
+	if v := q.Get("spell_contains"); v != "" {
+		if ids := spellIDsContaining(spells, v); len(ids) > 0 {
+			conds = append(conds, spellExists(ids, ab))
+		} else {
+			impossible()
+		}
+	}
+	if v := q.Get("legendary_cantrips"); v != "" {
+		for _, name := range splitNonEmpty(v) {
+			if ids := spellIDsForCantrip(spells, name); len(ids) > 0 {
+				conds = append(conds, spellExists(ids, ab))
+			} else {
+				impossible()
+			}
+		}
+	}
+	return conds
+}

go-services/inventory-go/spell_filter_test.go

@@ -50,3 +50,65 @@ func TestExactSpellID(t *testing.T) {
 		t.Error("partial name must not exact-match")
 	}
 }
+
+func TestSpellFilterConds_Cantrips(t *testing.T) {
+	ab := &argBuilder{}
+	q := map[string][]string{"legendary_cantrips": {"Legendary Invulnerability, Legendary Summoning Prowess"}}
+	conds := spellFilterConds(q, testSpells, ab)
+	want := []string{
+		"EXISTS (SELECT 1 FROM item_spells sp WHERE sp.item_id = db_item_id AND sp.spell_id IN ($1))",
+		"EXISTS (SELECT 1 FROM item_spells sp WHERE sp.item_id = db_item_id AND sp.spell_id IN ($2, $3))",
+	}
+	if !reflect.DeepEqual(conds, want) {
+		t.Errorf("conds = %v, want %v", conds, want)
+	}
+	// "Legendary Invulnerability" exact-contains only id 1; the Summoning
+	// label matches 3 and 5. AND semantics = one EXISTS per cantrip.
+	if wantArgs := []any{1, 3, 5}; !reflect.DeepEqual(ab.args, wantArgs) {
+		t.Errorf("args = %v, want %v", ab.args, wantArgs)
+	}
+}
+
+func TestSpellFilterConds_NoMatchIsImpossible(t *testing.T) {
+	ab := &argBuilder{}
+	q := map[string][]string{"legendary_cantrips": {"Legendary Frostbite"}}
+	if conds := spellFilterConds(q, testSpells, ab); !reflect.DeepEqual(conds, []string{"1 = 0"}) {
+		t.Errorf("unknown cantrip conds = %v, want [1 = 0]", conds)
+	}
+	ab = &argBuilder{}
+	q = map[string][]string{"spell_contains": {"frostbite"}}
+	if conds := spellFilterConds(q, testSpells, ab); !reflect.DeepEqual(conds, []string{"1 = 0"}) {
+		t.Errorf("unknown spell_contains conds = %v, want [1 = 0]", conds)
+	}
+}
+
+func TestSpellFilterConds_ContainsAndHasSpell(t *testing.T) {
+	ab := &argBuilder{}
+	q := map[string][]string{"spell_contains": {"invulnerability"}}
+	want := []string{"EXISTS (SELECT 1 FROM item_spells sp WHERE sp.item_id = db_item_id AND sp.spell_id IN ($1, $2))"}
+	if conds := spellFilterConds(q, testSpells, ab); !reflect.DeepEqual(conds, want) {
+		t.Errorf("spell_contains conds = %v, want %v", conds, want)
+	}
+
+	ab = &argBuilder{}
+	q = map[string][]string{"has_spell": {"Epic Invulnerability"}}
+	want = []string{"EXISTS (SELECT 1 FROM item_spells sp WHERE sp.item_id = db_item_id AND sp.spell_id IN ($1))"}
+	if conds := spellFilterConds(q, testSpells, ab); !reflect.DeepEqual(conds, want) {
+		t.Errorf("has_spell conds = %v, want %v", conds, want)
+	}
+	if _, ok := exactSpellID(testSpells, "No Such Spell"); ok {
+		t.Fatal("precondition")
+	}
+	ab = &argBuilder{}
+	q = map[string][]string{"has_spell": {"No Such Spell"}}
+	if conds := spellFilterConds(q, testSpells, ab); !reflect.DeepEqual(conds, []string{"1 = 0"}) {
+		t.Errorf("unknown has_spell conds = %v, want [1 = 0]", conds)
+	}
+}
+
+func TestSpellFilterConds_Empty(t *testing.T) {
+	ab := &argBuilder{}
+	if conds := spellFilterConds(map[string][]string{}, testSpells, ab); conds != nil {
+		t.Errorf("no params => nil conds, got %v", conds)
+	}
+}