Fix zero length queries WIP (#234)

* failing tests for zero-width queries

* support zero width timespans in splitArcs. Fixes one test, breaks a load more..

* fix fastGap

packages/core/pattern.mjs

@@ -63,6 +63,17 @@ export class Pattern {
     return new Pattern((state) => this.query(state.withSpan(func)));
   }
 
+  withQuerySpanMaybe(func) {
+    const pat = this;
+    return new Pattern((state) => {
+      const newState = state.withSpan(func);
+      if (!newState.span) {
+	return [];
+      }
+      return pat.query(newState);
+    });
+  }
+
   /**
    * As with {@link Pattern#withQuerySpan}, but the function is applied to both the
    * begin and end time of the query timespan.
@@ -744,12 +755,17 @@ export class Pattern {
     //     // there is no gap.. so maybe revert to _fast?
     //     return this._fast(factor)
     // }
+    // A bit fiddly, to drop zero-width queries at the start of the next cycle
     const qf = function (span) {
       const cycle = span.begin.sam();
-      const begin = cycle.add(span.begin.sub(cycle).mul(factor).min(1));
+      const bpos = span.begin.sub(cycle).mul(factor).min(1);
-      const end = cycle.add(span.end.sub(cycle).mul(factor).min(1));
+      const epos = span.end.sub(cycle).mul(factor).min(1);
-      return new TimeSpan(begin, end);
+      if (bpos >= 1) {
+	return undefined;
+      }
+      return new TimeSpan(cycle.add(bpos), cycle.add(epos));
     };
+    // Also fiddly, to maintain the right 'whole' relative to the part
     const ef = function (hap) {
       const begin = hap.part.begin;
       const end = hap.part.end;
@@ -765,7 +781,7 @@ export class Pattern {
           );
       return new Hap(newWhole, newPart, hap.value, hap.context);
     };
-    return this.withQuerySpan(qf)._withHap(ef)._splitQueries();
+    return this.withQuerySpanMaybe(qf)._withHap(ef)._splitQueries();
   }
   
   // Compress each cycle into the given timespan, leaving a gap

packages/core/test/pattern.test.mjs

@@ -137,6 +137,9 @@ describe('Pattern', () => {
     it('Can make a pattern', () => {
       expect(pure('hello').query(st(0.5, 2.5)).length).toBe(3);
     });
+    it('Supports zero-width queries', () => {
+      expect(pure('hello').queryArc(0,0).length).toBe(1);
+    });
   });
   describe('fmap()', () => {
     it('Can add things', () => {
@@ -430,6 +433,16 @@ describe('Pattern', () => {
       // mini('[c3 g3]/2 eb3') always plays [c3 eb3]
       // mini('eb3 [c3 g3]/2 ') always plays [c3 g3]
     });
+    it('Supports zero-length queries', () => {
+      expect(steady('a')._slow(1).queryArc(0,0)
+	    ).toStrictEqual(steady('a').queryArc(0,0))
+    });
+  });
+  describe('slow()', () => {
+    it('Supports zero-length queries', () => {
+      expect(steady('a').slow(1)._setContext({}).queryArc(0,0)
+	    ).toStrictEqual(steady('a')._setContext({}).queryArc(0,0))
+    });
   });
   describe('inside', () => {
     it('can rev inside a cycle', () => {

packages/core/timespan.mjs

@@ -18,6 +18,11 @@ export class TimeSpan {
     const end = this.end;
     const end_sam = end.sam();
 
+    // Support zero-width timespans
+    if (begin.equals(end)) {
+      return([new TimeSpan(begin, end)]);
+    }
+    
     while (end.gt(begin)) {
       // If begin and end are in the same cycle, we're done.
       if (begin.sam().equals(end_sam)) {